Selective Insurance (SIGI) 10-K/A 23 Feb 18 2017 FY Annual report (amended) Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 09, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Entity Registrant Name	SELECTIVE INSURANCE GROUP INC
Entity Central Index Key	230,557
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,859,898,742
Entity Common Stock, Shares Outstanding		58,717,701

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Fixed income securities, held-to-maturity – at carrying value (fair value: $44,100 – 2017; $105,211 – 2016)	$ 42,129,000	$ 101,556,000
Fixed income securities, available-for-sale – at fair value (amortized cost: $5,076,716 – 2017; $4,753,759 – 2016)	5,162,522,000	4,792,540,000
Equity securities, available-for-sale – at fair value (cost: $143,811 – 2017; $120,889 – 2016)	182,705,000	146,753,000
Short-term investments (at cost which approximates fair value)	165,555,000	221,701,000
Other investments	132,268,000	102,397,000
Total investments (Notes 5 and 7)	5,685,179,000	5,364,947,000
Cash	534,000	458,000
Interest and dividends due or accrued	40,897,000	40,164,000
Premiums receivable, net of allowance for uncollectible accounts of: $10,000 – 2017; $5,980 – 2016	747,029,000	681,611,000
Reinsurance recoverable, net of allowance for uncollectible accounts of: $4,600 – 2017; $5,500 – 2016 (Note 8)	594,832,000	621,537,000
Prepaid reinsurance premiums (Note 8)	153,493,000	146,282,000
Current federal income tax (Note 13)	3,243,000	2,486,000
Deferred federal income tax (Note 13)	31,990,000	84,840,000
Property and equipment – at cost, net of accumulated depreciation and amortization of: $213,227 – 2017; $198,729 – 2016	63,959,000	69,576,000
Deferred policy acquisition costs (Note 2)	235,055,000	222,564,000
Goodwill (Note 11)	7,849,000	7,849,000
Other assets	122,371,000	113,534,000
Total assets	7,686,431,000	7,355,848,000
LIABILITIES AND STOCKHOLDERS’ EQUITY
Reserve for loss and loss expense (Note 9)	3,771,240,000	3,691,719,000
Unearned premiums	1,349,644,000	1,262,819,000
Long-term debt (Note 10)	439,116,000	438,667,000
Accrued salaries and benefits	131,850,000	132,880,000
Other liabilities	281,624,000	298,393,000
Total liabilities	5,973,474,000	5,824,478,000
Stockholders’ Equity:
Preferred stock of $0 par value per share: Authorized shares: 5,000,000; no shares issued or outstanding	0	0
Common stock of $2 par value per share Authorized shares: 360,000,000 Issued: 102,284,564 - 2017; 101,620,436 - 2016	204,569,000	203,241,000
Additional paid-in capital	367,717,000	347,295,000
Retained earnings	1,698,613,000	1,568,881,000
Accumulated other comprehensive income (loss) (Note 6)	20,170,000	(15,950,000)
Treasury stock – at cost (shares: 43,789,442 – 2017; 43,653,237 – 2016)	(578,112,000)	(572,097,000)
Total stockholders’ equity	1,712,957,000	1,531,370,000
Commitments and contingencies (Notes 17 and 18)
Total liabilities and stockholders’ equity	$ 7,686,431,000	$ 7,355,848,000

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed income securities, held-to-maturity, fair value	$ 44,100	$ 105,211
Fixed income securities, available-for-sale, amortized cost	5,076,716	4,753,759
Equity securities, available-for-sale, cost	143,811	120,889
Premiums receivable, allowance for uncollectible accounts	10,000	5,980
Reinsurance recoverable, allowance for uncollectible accounts	4,600	5,500
Property and equipment, accumulated depreciation and amortization	$ 213,227	$ 198,729
Preferred stock, par value per share	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 2	$ 2
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	102,284,564	101,620,436
Treasury stock, shares	43,789,442	43,653,237

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Net premiums earned	$ 2,291,027	$ 2,149,572	$ 1,989,909
Net investment income earned	161,882	130,754	121,316
Net realized gains (losses):
Net realized investment gains	11,204	3,562	31,537
Other-than-temporary impairments	(4,809)	(8,509)	(18,366)
Other-than-temporary impairments on fixed income securities recognized in other comprehensive income	(36)	10	0
Total net realized gains (losses)	6,359	(4,937)	13,171
Other income	10,716	8,881	7,456
Total revenues	2,469,984	2,284,270	2,131,852
Expenses:
Loss and loss expense incurred	1,345,074	1,234,797	1,148,541
Amortization of deferred policy acquisition costs	469,236	450,328	399,436
Other insurance expenses	333,097	321,395	300,359
Interest expense	24,354	22,771	22,428
Corporate expenses	36,255	35,024	28,396
Total expenses	2,208,016	2,064,315	1,899,160
Income before federal income tax	261,968	219,955	232,692
Federal income tax expense:
Current	62,184	48,581	45,347
Deferred	30,958	12,879	21,484
Total federal income tax expense	93,142	61,460	66,831
Net income	$ 168,826	$ 158,495	$ 165,861
Earnings per share:
Basic net income	$ 2.89	$ 2.74	$ 2.90
Diluted net income	2.84	2.70	2.85
Dividends to stockholders	$ 0.66	$ 0.61	$ 0.57

Consolidation Statements of Com

Consolidation Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 168,826	$ 158,495	$ 165,861
Unrealized holding gains (losses) arising during year	43,015	(5,977)	(26,143)
Non-credit portion of other-than-temporary impairments recognized in other comprehensive income	23	(6)	0
Amounts reclassified into net income: Held-to-maturity securities	(116)	(92)	(377)
Amounts reclassified into net income: Non-credit other-than-temporary impairment	68	138	232
Amounts reclassified into net income: Realized losses (gains) on available for sale securities	(4,537)	3,064	(9,110)
Total unrealized gains (losses) on investment securities	38,453	(2,873)	(35,398)
Net actuarial (loss) gain	(3,700)	(7,852)	1,585
Amounts reclassified into net income: Net actuarial loss	1,367	4,200	4,600
Total defined benefit pension and post-retirement plans	(2,333)	(3,652)	6,185
Other comprehensive income (loss)	36,120	(6,525)	(29,213)
Comprehensive income	$ 204,946	$ 151,970	$ 136,648

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($)	Total	Common Stock [Member]	Additional paid-in capital [Member]	Retained earnings [Member]	Accumulated other comprehensive (loss) income [Member]	Treasury stock [Member]
Beginning of year at Dec. 31, 2014		$ 199,896,000	$ 305,385,000	$ 1,313,440,000	$ 19,788,000	$ (562,923,000)
Dividend reinvestment plan (shares: 28,607 – 2017; 38,741 – 2016; 50,013 – 2015)		100,000	1,374,000
Stock purchase and compensation plans (shares: 635,521 – 2017; 720,323 – 2016; 863,426 – 2015)		1,727,000	19,897,000
Net income	$ 165,861,000			165,861,000
Dividends to stockholders ($0.66 per share – 2017; $0.61 per share – 2016; $0.57 per share – 2015)				(33,109,000)
Other comprehensive income (loss)	(29,213,000)				(29,213,000)
Acquisition of treasury stock (shares: 136,205 – 2017; 152,595 – 2016; 147,461 – 2015)						(4,182,000)
End of year at Dec. 31, 2015	1,398,041,000	201,723,000	326,656,000	1,446,192,000	(9,425,000)	(567,105,000)
Dividend reinvestment plan (shares: 28,607 – 2017; 38,741 – 2016; 50,013 – 2015)		77,000	1,389,000
Stock purchase and compensation plans (shares: 635,521 – 2017; 720,323 – 2016; 863,426 – 2015)		1,441,000	19,250,000
Net income	158,495,000			158,495,000
Dividends to stockholders ($0.66 per share – 2017; $0.61 per share – 2016; $0.57 per share – 2015)				(35,806,000)
Other comprehensive income (loss)	(6,525,000)				(6,525,000)
Acquisition of treasury stock (shares: 136,205 – 2017; 152,595 – 2016; 147,461 – 2015)						(4,992,000)
End of year at Dec. 31, 2016	1,531,370,000	203,241,000	347,295,000	1,568,881,000	(15,950,000)	(572,097,000)
Dividend reinvestment plan (shares: 28,607 – 2017; 38,741 – 2016; 50,013 – 2015)		57,000	1,395,000
Stock purchase and compensation plans (shares: 635,521 – 2017; 720,323 – 2016; 863,426 – 2015)		1,271,000	19,027,000
Net income	168,826,000			168,826,000
Dividends to stockholders ($0.66 per share – 2017; $0.61 per share – 2016; $0.57 per share – 2015)				(39,094,000)
Other comprehensive income (loss)	36,120,000				36,120,000
Acquisition of treasury stock (shares: 136,205 – 2017; 152,595 – 2016; 147,461 – 2015)						(6,015,000)
End of year at Dec. 31, 2017	$ 1,712,957,000	$ 204,569,000	$ 367,717,000	$ 1,698,613,000	$ 20,170,000	$ (578,112,000)

Consolidated Statements of Sto7

Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividend reinvestment plan, shares	28,607	38,741	50,013
Stock purchase and compensation plans, shares	635,521	720,323	863,426
Dividends to stockholders, per share	$ 0.66	$ 0.61	$ 0.57
Acquisition of treasury stock, shares	136,205	152,595	147,461
Preferred stock, par value per share	$ 0	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000	5,000,000
Preferred stock, shares issued	0	0	0
Series A Preferred Stock [Member]
Preferred stock, par value per share	$ 0	$ 0	$ 0
Preferred stock, shares authorized	300,000	300,000	300,000

Consolidated Statements of Cash

Consolidated Statements of Cashflows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net income	$ 168,826	$ 158,495	$ 165,861
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	52,100	61,671	59,688
Stock-based compensation expense	12,089	10,449	8,973
Undistributed (gains) losses of equity method investments	(6,393)	(2,316)	1,889
Net realized (gains) losses	(6,359)	4,937	(13,171)
Loss on disposal of fixed assets	998	0	0
Changes in assets and liabilities:
Increase in reserves for loss and loss expense, net of reinsurance recoverables	106,226	114,422	59,438
Increase in unearned premiums, net of prepaid reinsurance	79,614	87,716	79,995
Decrease in net federal income taxes	30,918	11,150	25,004
Increase in premiums receivable	(65,418)	(66,447)	(56,386)
Increase in deferred policy acquisition costs	(12,491)	(9,405)	(27,551)
(Increase) decrease in interest and dividends due or accrued	(1,088)	(1,473)	407
(Decrease) increase in accrued salaries and benefits	(5,714)	(46,536)	11,392
Increase in other assets	(9,872)	(30,071)	(11,523)
Increase in other liabilities	27,297	9,191	77,564
Net cash provided by operating activities	370,733	301,783	381,580
Investing Activities
Purchase of fixed income securities, held-to-maturity	0	(4,235)	(3,316)
Purchase of fixed income securities, available-for-sale	(2,130,362)	(1,982,023)	(1,041,916)
Purchase of equity securities, available-for-sale	(61,931)	(35,490)	(195,720)
Purchase of other investments	(55,830)	(66,164)	(12,170)
Purchase of short-term investments	(4,280,553)	(3,499,380)	(1,602,327)
Sale of fixed income securities, available-for-sale	1,197,920	926,470	61,571
Sale of short-term investments	4,338,318	3,470,022	1,539,480
Redemption and maturities of fixed income securities, held-to-maturity	58,832	102,868	106,621
Redemption and maturities of fixed income securities, available-for-sale	555,216	641,524	567,445
Sale of equity securities, available-for-sale	37,960	119,617	172,561
Distributions from other investments	23,426	26,837	32,457
Purchase of property and equipment	(14,071)	(18,147)	(16,229)
Net cash used in investing activities	(331,075)	(318,101)	(391,543)
Financing Activities
Dividends to stockholders	(37,045)	(33,758)	(31,052)
Acquisition of treasury stock	(6,015)	(4,992)	(4,182)
Net proceeds from stock purchase and compensation plans	7,599	7,811	10,089
Proceeds from borrowings	84,000	165,000	15,000
Repayment of borrowings	(84,000)	(115,000)	0
Excess tax benefits from share-based payment arrangements	0	1,819	1,736
Repayment of capital lease obligations	(4,121)	(5,002)	(4,689)
Net cash (used in) provided by financing activities	(39,582)	15,878	(13,098)
Net increase (decrease) in cash	76	(440)	(23,061)
Cash, beginning of year	458	898	23,959
Cash, end of year	$ 534	$ 458	$ 898

Organization

Organization	12 Months Ended
	Dec. 31, 2017
Organization [Abstract]
Organization	Organization Selective Insurance Group, Inc., through its subsidiaries, (collectively referred to as “we,” “us,” or “our”) offers standard commercial, standard personal, and excess and surplus ("E&S") lines property and casualty insurance products. Selective Insurance Group, Inc. (referred to as the “Parent”) was incorporated in New Jersey in 1977 and its corporate headquarters is located in Branchville, New Jersey. The Parent’s common stock is publicly traded on the NASDAQ Global Select Market under the symbol “SIGI.” We have provided a glossary of terms as Exhibit 99.1 to this Form 10-K, which defines certain industry-specific and other terms that are used in this Form 10-K. We classify our business into four reportable segments, which are as follows: • Standard Commercial Lines - comprised of insurance products and services provided in the standard marketplace to commercial enterprises, which are typically businesses, non-profit organizations, and local government agencies. • Standard Personal Lines - comprised of insurance products and services, including flood insurance coverage, provided primarily to individuals acquiring coverage in the standard marketplace. • E&S Lines - comprised of insurance products and services provided to customers who have not obtained coverage in the standard marketplace. • Investments - invests the premiums collected by our insurance operations, as well as amounts generated through our capital management strategies, which may include the issuance of debt and equity securities.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Signifcant Accounting Policies	Summary of Significant Accounting Policies (a) Principles of Consolidation The accompanying consolidated financial statements (“Financial Statements”) include the accounts of the Parent and its subsidiaries, and have been prepared in conformity with: (i) U.S. generally accepted accounting principles ("GAAP"); and (ii) the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). All significant intercompany accounts and transactions are eliminated in consolidation. (b) Use of Estimates The preparation of our Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported financial statement balances, as well as the disclosure of contingent assets and liabilities. Actual results could differ from those estimates. (c) Reclassifications In 2017 , we reclassified certain line items within our Consolidated Statements of Income to enhance the ability to analyze our expenses. Specifically, we reclassified our insurance underwriting expenses into amortization of deferred policy acquisition costs and other insurance expenses. These expenses were previously included in policy acquisition costs and other expenses. In addition, all expenses of the Parent, which were previously included in other expenses, are now separately identifiable as corporate expenses on the Consolidated Statements of Income. All prior periods presented in this Form 10-K have been reclassified to reflect this change. (d) Investments Fixed income securities may include investment grade and below investment grade rated bonds, redeemable preferred stocks, non-redeemable preferred stocks with certain debt-like characteristics, mortgage-backed securities (“MBS”), collateralized loan obligations ("CLO"), and other asset-backed securities (“ABS”). MBS, CLO, and other ABS are jointly referred to as structured securities. Fixed income securities classified as available-for-sale (“AFS”) are reported at fair value. Those fixed income securities that we have the ability and positive intent to hold to maturity are classified as held-to-maturity (“HTM”) and are carried at either: (i) amortized cost; or (ii) market value at the date of transfer into the HTM category, adjusted for subsequent amortization. The amortized cost of fixed income securities is adjusted for the amortization of premiums and the accretion of discounts over the expected life of the security using the effective yield method. Premiums and discounts arising from the purchase of structured securities are amortized over the expected life of the security based on future principal payments, giving additional consideration to prepayments. These prepayments are estimated based on historical and projected cash flows. Prepayment assumptions are reviewed quarterly and adjusted to reflect actual prepayments and changes in expectations. Future amortization of any premium and/or discount is adjusted to reflect the revised assumptions. Interest income, as well as amortization and accretion, is included in "Net investment income earned" on our Consolidated Statements of Income. The amortized cost of a fixed income security is written down to fair value when a decline in value is considered to be other than temporary. See the discussion below on realized investment gains and losses for a description of the accounting for impairments. After-tax unrealized gains and losses on: (i) fixed income securities classified as AFS; and (ii) fixed income securities that were transferred into an HTM designation from an AFS designation, are included in accumulated other comprehensive income (loss) ("AOCI"). Equity securities, which are classified as AFS, may include common and non-redeemable preferred stocks. These securities are carried at fair value and the related dividend income is included in "Net investment income earned" on our Consolidated Statements of Income. The cost of equity securities is written down to fair value when a decline in value is considered to be other than temporary. See the discussion below on realized investment gains and losses for a description of the accounting for impairments. After-tax unrealized gains and losses are included in AOCI. Short-term investments may include certain money market instruments, savings accounts, commercial paper, and debt issues purchased with a maturity of less than one year. We also enter into reverse repurchase agreements that are included in short-term investments. These loans are fully collateralized with high quality, readily marketable instruments at a minimum of 102% of the loan principal. At maturity, we receive principal and interest income on these agreements. All short-term investments are carried at cost, which approximates fair value. The associated income is included in "Net investment income earned" on our Consolidated Statements of Income. Other investments may include alternative investments and other securities. Alternative investments are accounted for using the equity method. Our share of distributed and undistributed net income from alternative investments is included in "Net investment income earned" on our Consolidated Statements of Income. Other securities are primarily comprised of tax credit investments. Low income housing tax credits are accounted for under the proportional amortization method and all other tax credits are accounted for using the equity method. Under the proportional amortization method, our share of the investment’s performance is recorded in our Consolidated Statements of Income as a component of “Federal income tax expense.” Under the equity method, our share of distributed and undistributed net income is included in "Net investment income earned" on our Consolidated Statements of Income. For federal income tax credits accounted for under the equity method, we use the deferral method for recognizing the benefit of the tax credit with the related deferred revenue being recognized in our Consolidated Statements of Income as a component of "Federal income tax expense" proportionately over the life of the investment. We evaluate the alternative investments and tax credit investments included in our other investments portfolio to determine whether those investments are variable interest entities ("VIEs") and if so, whether consolidation is required. A VIE is an entity that either has equity investors that lack certain essential characteristics of a controlling financial interest or lack sufficient funds to finance its own activities without financial support provided by other entities. We consider several significant factors in determining if our investments are VIEs and if we are the primary beneficiary, including whether we have: (i) the power to direct activities of the VIE; (ii) the ability to remove the decision maker of the VIE; (iii) the ability to participate in making decisions that are significant to the VIE; and (iv) the obligation to absorb losses and the right to receive benefits that could potentially be significant to the VIE. We have reviewed our alternative and tax credit investments and have concluded that they are VIEs, but that we are not the primary beneficiary and therefore, consolidation is not required. Realized gains and losses on the sale of investments are determined on the basis of the cost of the specific investments sold and are credited or charged to income. Included in realized gains and losses are the other-than-temporary impairment ("OTTI") charges recognized in earnings, which are discussed below. On a quarterly basis, we review our investment portfolio for impairments that are other than temporary. Interest-related unrealized losses typically do not result in other-than-temporary impairments. The following provides information on this analysis for our fixed income securities and short-term investments, equity securities, and other investments. Fixed Income Securities and Short-Term Investments We review securities that are in an unrealized loss position to determine: (i) if we have the intent to sell the security; (ii) if it is more likely than not that we will be required to sell the debt security before its anticipated recovery; and (iii) if the decline is other than temporary. Broad changes in the overall market or interest rate environment generally will not lead to a write down. If we determine that we have either the intent or requirement to sell the security, we write down its amortized cost to its fair value through a charge to earnings as a component of realized losses. If we do not have either the intent or requirement to sell the security, our evaluation for OTTI may include, but is not limited to, evaluation of the following factors: • Whether the decline appears to be issuer or industry specific; • The degree to which the issuer is current or in arrears in making principal and interest payments on the fixed income security; • The issuer’s current financial condition and ability to make future scheduled principal and interest payments on a timely basis; • Evaluation of projected cash flows; • Buy/hold/sell recommendations published by outside investment advisors and analysts; and • Relevant rating history, analysis, and guidance provided by rating agencies and analysts. Non-redeemable preferred stocks that are classified as fixed income securities are evaluated under this OTTI method unless the security is below investment grade, at which time it is evaluated under the equity securities OTTI model discussed below. To determine if an impairment is other than temporary, we perform assessments that may include, but are not limited to, a discounted cash flow analysis ("DCF") to determine the security's present value of future cash flows. This analysis is also performed on all previously-impaired debt securities that continue to be held by us and all structured securities that were not of high credit quality at the date of purchase. Any shortfall in the expected present value of the future cash flows, based on the DCF, from the amortized cost basis of a security is considered a “credit impairment,” with the remaining decline in fair value of a security considered a “non-credit impairment.” Credit impairments are charged to earnings as a component of realized losses, while non-credit impairments are recorded to Other Comprehensive Income ("OCI") as a component of unrealized losses. The discount rate we use in a DCF is the effective interest rate implicit in the security at the date of acquisition for those structured securities that were not of high credit quality at acquisition. For all other securities, we use a discount rate that equals the current yield, excluding the impact of previous OTTI charges, used to accrete the beneficial interest. DCFs may include, but are not necessarily limited to: (i) generating cash flows for each tranche considering tranche-specific data, market data, and other pertinent information, such as the historical performance of the underlying collateral, including net operating income generated by underlying properties, conditional default rate assumptions, loan loss severity assumptions, consensus projections, prepayment projections, and actual pool and collateral information; (ii) identifying applicable benchmark yields; and (iii) applying market-based tranche specific spreads to determine an appropriate yield by incorporating collateral performance, tranche-level attributes, trades, bids, and offers. Equity Securities We review securities that are in an unrealized loss position to determine: (i) if we do not intend to hold the security to its forecasted recovery; or (ii) if the decline is other than temporary, which includes declines driven by market volatility for which we cannot assert the security will recover in the near term. If we determine either that we do not intend to hold a security, or the decline is other than temporary, we write down the security's cost to its fair value through a charge to earnings as a component of realized losses. If we intend to hold the security, our evaluation for OTTI may include, but is not limited to, an evaluation of the following factors: • Whether the decline appears to be issuer or industry specific; • The relationship of market prices per share to book value per share at the date of acquisition and date of evaluation; • The price-earnings ratio at the time of acquisition and date of evaluation; • The financial condition and near-term prospects of the issuer, including any specific events that may influence the issuer's operations, coupled with our intention to hold the securities in the near-term; • The recent income or loss of the issuer; • The independent auditors' report on the issuer's recent financial statements; • The dividend policy of the issuer at the date of acquisition and the date of evaluation; • Buy/hold/sell recommendations or price projections published by outside investment advisors; • Rating agency announcements; • The length of time and the extent to which the fair value has been, or is expected to be, less than its cost in the near term; and • Our expectation of when the cost of the security will be recovered. Other Investments Our evaluation for OTTI of an other investment (i.e., an alternative investment) may include, but is not limited to, conversations with the management of the alternative investment concerning the following: • The current investment strategy; • Changes made or future changes to be made to the investment strategy; • Emerging issues that may affect the success of the strategy; and • The appropriateness of the valuation methodology used regarding the underlying investments. If there is a decline in the fair value of an other investment that we do not intend to hold, or if we determine the decline is other than temporary, we write down the carry value of the investment and record the charge through earnings as a component of realized losses. (e) Fair Values of Financial Instruments Assets The fair values of our investments are generated using various valuation techniques and are placed into the fair value hierarchy considering the following: (i) the highest priority is given to quoted prices in active markets for identical assets (Level 1); (ii) the next highest priority is given to quoted prices in markets that are not active or inputs that are observable either directly or indirectly, including quoted prices for similar assets in markets that are not active and other inputs that can be derived principally from, or corroborated by, observable market data for substantially the full term of the assets (Level 2); and (iii) the lowest priority is given to unobservable inputs supported by little or no market activity and that reflect our assumptions about the exit price, including assumptions that market participants would use in pricing the asset (Level 3). An asset’s classification within the fair value hierarchy is based on the lowest level of significant input to its valuation. Transfers between levels in the fair value hierarchy are recognized at the end of the reporting period. The techniques used to value our financial assets are as follows: Level 1 Pricing Security Type Methodology Equity Securities; U.S. Treasury Notes Equity and U.S. Treasury Note prices are received from an independent pricing service that are based on observable market transactions. We validate these prices against a second external pricing service, and if established market value comparison thresholds are breached, further analysis is performed to determine the price to be used. Short-Term Investments Short-term investments are carried at cost, which approximates fair value. Given the liquid nature of our short-term investments, we generally validate their fair value by way of active trades within approximately one week of the financial statement close. Level 2 Pricing We utilize a market approach for our Level 2 securities, using primarily matrix pricing models prepared by external pricing services. Matrix pricing models use mathematical techniques to value debt securities by relying on the securities' relationship to other benchmark quoted securities, and not relying exclusively on quoted prices for specific securities, as the specific securities are not always frequently traded. As a matter of policy, we consistently use one pricing service as our primary source and secondary pricing services if prices are not available from the primary pricing service. Fixed income securities portfolio pricing is reviewed for reasonableness in the following ways: (i) comparing our pricing to other third-party pricing services as well as benchmark indexed pricing; (ii) comparing fair value fluctuations between months for reasonableness; and (iii) reviewing stale prices. If further analysis is needed, a challenge is sent to the pricing service for review and confirmation of the price. Further information on our Level 2 asset pricing is included in the following table: Security Type Methodology Corporate Securities including preferred stocks classified as Fixed Income Securities, and U.S. Government and Government Agencies Evaluations include obtaining relevant trade data, benchmark quotes and spreads and incorporating this information into either spread-based or price-based evaluations as determined by the observed market data. Spread-based evaluations include: (i) creating a range of spreads for relevant maturities of each issuer based on the new issue market, secondary trading, and dealer quotes; and (ii) incorporating option adjusted spreads for issues that have early redemption features. Based on the findings in (i) and (ii) above, final spreads are derived and added to benchmark curves. Price-based evaluations include matching each issue to its best-known market maker and contacting firms that transact in these securities. Obligations of States and Political Subdivisions Evaluations are based on yield curves that are developed based on factors such as: (i) benchmarks to issues with interest rates near prevailing market rates; (ii) established trading spreads over widely-accepted market benchmarks; (iii) yields on new issues; and (iv) market information from third-party sources such as reportable trades, broker-dealers, or issuers. Structured Securities (including CLO and other ABS, Commercial Mortgage-Backed Securities ("CMBS"), Residential Mortgage-Backed Securities ("RMBS")) Evaluations are based on a DCF, including: (i) generating cash flows for each tranche considering tranche-specific data, market data, and other pertinent information, such as historical performance of the underlying collateral, including net operating income generated by the underlying properties, conditional default rate assumptions, loan loss severity assumptions, consensus projections, prepayment projections, and actual pool and loan level collateral information; (ii) identifying applicable benchmark yields; and (iii) applying market-based tranche-specific spreads to determine an appropriate yield by incorporating collateral performance, tranche-level attributes, trades, bids, and offers. Foreign Government Evaluations are performed using a DCF model and by incorporating observed market yields of benchmarks as inputs, adjusting for varied maturities. Level 3 Pricing Less than 1% of our portfolio cannot be priced using our primary or secondary pricing service. At times, we may use non-binding broker quotes to value some of these securities. These prices are from various broker/dealers that use bid or ask prices, or benchmarks to indices, in measuring the fair value of a security. We review these fair value measurements for reasonableness. This review typically includes an analysis of price fluctuations between months with variances over established thresholds being analyzed further. Further information on our current Level 3 asset pricing is included in the following table: Security Type Methodology Corporate Securities These tax credit investments are priced internally using spread-based evaluations. Equity Securities These non-publicly traded stocks are valued by the issuer and reviewed internally. Liabilities The techniques used to value our notes payable are as follows: Level 1 Pricing Security Type Methodology 5.875% Senior Notes Based on the quoted market prices. Level 2 Pricing Security Type Methodology 7.25% Senior Notes; 6.70% Senior Notes Based on matrix pricing models prepared by external pricing services. Borrowings from Federal Home Loan Banks Evaluations are performed using a DCF model based on current borrowing rates provided by the Federal Home Loan Banks that are consistent with the remaining term of the borrowing. See Note 7. “Fair Value Measurements” for a summary table of the fair value and related carrying amounts of financial instruments. (f) Allowance for Doubtful Accounts We estimate an allowance for doubtful accounts on our premiums receivable. This allowance is based on historical write-off percentages adjusted for the effects of current and anticipated trends. An account is charged off when we believe it is probable that we will not collect a receivable. In making this determination, we consider information obtained from our efforts to collect amounts due directly or through collection agencies. (g) Share-Based Compensation Share-based compensation consists of all share-based payment transactions in which an entity acquires goods or services by issuing (or offering to issue) its shares, share units, share options, or other equity instruments. The cost resulting from all share-based payment transactions are recognized in the Financial Statements based on the fair value of both equity and liability awards. The fair value is measured at grant date for equity awards, whereas the fair value for liability awards are remeasured at each reporting period. The fair value of both equity and liability awards is recognized over the requisite service period. The requisite service period is typically the lesser of the vesting period or the period of time from the grant date to the date of retirement eligibility. The expense recognized for share-based awards, which, in some cases, contain performance criteria, is based on the number of shares or units expected to be issued at the end of the performance period. We repurchase the Parent’s stock from our employees in connection with tax withholding obligations, as permitted under our stock-based compensation plans. This activity is disclosed in our Consolidated Statements of Stockholders' Equity. (h) Reinsurance Reinsurance recoverables represent estimates of amounts that will be recovered from reinsurers under our various treaties. Generally, amounts recoverable from reinsurers are recognized as assets at the same time and in a manner consistent with the paid and unpaid losses associated with the reinsured policies. We require collateral to secure reinsurance recoverables primarily from our reinsurance carriers that are not authorized, otherwise approved, or certified to do business in one or more of our ten insurance subsidiaries' domiciliary states. Our ten insurance subsidiaries are collectively referred to as the "Insurance Subsidiaries." This collateral is typically in the form of a letter of credit or cash. An allowance for estimated uncollectible reinsurance is recorded based on an evaluation of balances due from reinsurers and other available information, such as each reinsurers' credit rating from A.M. Best Company ("A.M. Best") or Standard & Poor's Rating Services ("S&P"). We charge off reinsurance recoverables on paid losses when it becomes probable that we will not collect the balance. (i) Property and Equipment Property and equipment used in operations, including certain costs incurred to develop or obtain computer software for internal use, are capitalized and carried at cost less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the assets. The following estimated useful lives can be considered as general guidelines: Asset Category Years Computer hardware 3 Computer software 3 to 5 Internally developed software 5 to 10 Software licenses 3 to 5 Furniture and fixtures 10 Buildings and improvements 5 to 40 We recorded depreciation expense of $17.8 million , $17.4 million , and $16.4 million for 2017 , 2016 , and 2015 , respectively. (j) Deferred Policy Acquisition Costs Deferred policy acquisition costs are limited to costs directly related to the successful acquisition of insurance contracts. Costs meeting this definition typically include, among other things, sales commissions paid to our distribution partners, premium taxes, and the portion of employee salaries and benefits directly related to time spent on acquired contracts. These costs are deferred and amortized over the life of the contracts. Accounting guidance requires a premium deficiency analysis to be performed at the level an entity acquires, services, and measures the profitability of its insurance contracts. We currently perform three premium deficiency analyses for our insurance operations, consistent with our reportable segments of Standard Commercial Lines, Standard Personal Lines, and E&S Lines. A combined ratio of over 100% does not necessarily indicate a premium deficiency, as any year's combined ratio includes a portion of underwriting expenses that are expensed at policy inception and therefore are not covered by the remaining unearned premium. In addition, investment income is not contemplated in the combined ratio calculation. There were no premium deficiencies for any of the reported years, as the sum of the anticipated loss and loss expense, unamortized acquisition costs, policyholder dividends, and other expenses for each segment did not exceed that segment’s related unearned premium and anticipated investment income. The investment yields assumed in the premium deficiency assessment for each reporting period, which were based on our actual average investment yield before tax as of the September 30 calculation date, were 2.9% for 2017 , 2.4% for 2016 , and 2.5% for 2015 . (k) Goodwill Goodwill results from business acquisitions where the cost of assets and liabilities acquired exceeds the fair value of those assets and liabilities. A quantitative goodwill impairment analysis is performed if our quarterly qualitative analysis indicates that it is more likely than not that the fair value of a reporting unit is less than its carrying amount. Goodwill is allocated to the reporting units for purposes of these analyses. Based on our analysis at December 31, 2017, goodwill was not impaired. (l) Reserves for Loss and Loss Expense Reserves for loss and loss expense are comprised of both case reserves on individual claims and reserves for claims incurred but not reported ("IBNR"). Case reserves result from claims that have been reported to one or more of our Insurance Subsidiaries, and are estimated at the amount of the expected ultimate payment. IBNR reserves are established at more aggregated levels than case basis reserves, and in addition to reserves on claims that have been incurred but not reported, they include provisions for future emergence on known claims, as well as reopened claims. IBNR reserves are established based on the results of the Insurance Subsidiaries’ internal reserve analysis, supplemented with other internal and external information. The internal reserve analysis is performed quarterly, and relies upon generally accepted actuarial techniques. Such techniques assume that past experience, adjusted for the effects of current developments and anticipated trends, are an appropriate basis for predicting future events. Our analyses rely upon historical paid and case loss and loss expense experience organized by line of business, accident year, and maturity (i.e., “triangles”). Standard actuarial projection methods are applied to this history, producing a set of estimated ultimate loss and loss expenses. Ultimate loss and loss expenses are selected from the various methods, considering the strengths and weaknesses of the methods as they apply to the specific line and accident year. Certain types of exposures do not lend themselves to standard actuarial methods. Examples of these are: • Certain property catastrophe events may be low in frequency and high in severity. These events may affect many insureds simultaneously. Due to the unique nature of these events, ultimate liabilities are estimated for each event, based on surveys of our portfolio of exposures, in conjunction with individual claims estimates. While generally short-tailed, the liabilities associated with these events are subject to a higher degree of uncertainty. We maintain significant reinsurance protection that greatly limits the impact that these extreme events have on net loss and loss expenses. • Some insured events may span multiple years and trigger multiple policies, as in the case of asbestos and environmental claims, where the injury is deemed to occur over an extended period of time. These types of losses often do not lend themselves to traditional actuarial methods. Where we deem appropriate, our experience may be analyzed without differentiating by accident year, using alternative methods and metrics. In these cases, the associated selected ultimate loss and loss expenses are then allocated to the applicable accident years for reporting. • Another example of non-standard methods relate to loss expenses that cannot be attributed to a specific claim (referred to as “unallocated loss expenses”). These expenses are first allocated to line of business, and alternative projection methods are then applied to estimate expenses by calendar year, which are then allocated back to the applicable accident years for reporting. The selected ultimate losses and loss adjustment expenses are translated into indicated IBNR reserves, which are then compared to the recorded IBNR reserves. Management's judgment is applied in determining any required adjustments and the resulting adjustments are then recorded and assigned or allocated to accident year using the results of the actuarial analysis. While the reserve analysis is the primary basis for determining the recorded IBNR reserves, other internal and external factors are considered. Internal factors include: (i) supplemental data regarding claims reporting and settlement trends; (ii) exposure estimates for reported claims, along with recent development on those estimates with respect to individual large claims and the aggregate of all claims; (iii) the rate at which new large or complex claims are being reported; and (iv) additional trends observed by claims personnel or reported to them by defense counsel. External factors considered include: (i) legislative enactments; (ii) judicial decisions; (iii) legal developments in the determination of liability and the imposition of damages; and (iv) trends in general economic conditions, including the effects of inflation. Loss reserves are estimates, and as such, we also consider a range of possible loss and loss expense reserve estimates. This range is determined at the beginning of each year, using prior year-end data, and reflects the fact that there is no single precise method for estimating the required reserves, due to the many factors that may influence the amounts ultimately paid. Considering the reserve range along with all of the items described above, as well as current market conditions, IBNR estimates are then established and recorded. The combination of the IBNR estimates along with the case reserve estimates on individual claims results in our total reserves for loss and loss expense. These reserves are expected to be sufficient for settling losses and loss reserve obligations under our policies on unpaid claims, including changes in the volume of business written, claims frequency and severity, the mix of business, claims processing, and other items that management expects to affect our ultimate settlement of loss and loss expense. However, the ultimate claim settlements may be higher or lower than reserves established. As our experience emerges and other information develops, we revise our reserve estimates accordingly. The changes in these estimates, resulting from the continuous review process and the differences between estimates and ultimate payments, are reflected in the Consolidated Statements of Income for the period in which such estimates are changed. The associated impacts may be material to the results of operations in future periods. We do not discount to present value that portion of our losses and loss expense reserves expected to be paid in future periods. Our loss

Adoption of Accounting Pronounc

Adoption of Accounting Pronouncements Adoption of Accounting Pronouncements (Notes)	12 Months Ended
	Dec. 31, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Pronouncements	Adoption of Accounting Pronouncements In March 2016, the Financial Accounting Standards Board ("FASB") issued Accounting Standard Update ("ASU") 2016-09, Compensation - Stock Compensation: Improvements to Employee Share-based Payment Accounting (“ASU 2016-09”). ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions. We adopted this guidance in the first quarter of 2017, which resulted in the following impacts on our consolidated financial statements: Consolidated Statements of Income The new standard requires that the tax effects of share-based compensation be recognized in the income tax provision as discrete items outside of the annual estimated expected tax rate. In addition, all excess tax benefits and tax deficiencies should be recognized as income tax benefit or expense in the income statement. Previously, these amounts were recorded in additional paid-in capital. In addition, in calculating potential common shares used to determine diluted earnings per share, GAAP requires us to use the treasury stock method. The new standard requires that assumed proceeds under the treasury stock method be modified to exclude the amount of excess tax benefits that would have been recognized in additional paid-in capital. These changes were adopted on a prospective basis. As a result of adoption, we recognized an income tax benefit in the Consolidated Statements of Income of $4.3 million in 2017 related to stock grants that have vested this year. In recording share-based compensation expense, the standard allows companies to make a policy election as to whether they will include an estimate of awards expected to be forfeited or whether they will account for forfeitures as they occur. We have elected to include an estimate of forfeitures in the computation of our share-based compensation expense. As this treatment is consistent with previous guidance, this election had no impact on our consolidated financial statements. Consolidated Statements of Cash Flows ASU 2016-09 requires that excess tax benefits from share-based awards be reported as operating activities in the consolidated statement of cash flows. Previously, these cash flows were included in financing activities. We elected to apply this change on a prospective basis; therefore, no changes have been made to the prior periods disclosed in this report. ASU 2016-09 also requires that employee taxes paid when an employer withholds shares for tax-withholding purposes be reported as financing activities in the consolidated statement of cash flows. This requirement has no impact to us as we have historically reported these cash flows as part of financing activities. In October 2016, the FASB issued ASU 2016-17, Consolidation: Interests Held through Related Parties That Are under Common Control ("ASU 2016-17"). ASU 2016-17 changes how a decision maker considers indirect interests in a VIE held under common control in making the primary beneficiary determination. We adopted ASU 2016-17 in the first quarter of 2017. This adoption did not impact us, as we are not the decision maker in any of the VIEs in which we invest. In March 2017, the FASB issued ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs: Premium Amortization on Purchased Callable Debt Securities ("ASU 2017-08"). ASU 2017-08 revises the amortization period for certain callable debt securities held at a premium, requiring the premium to be amortized to the earliest call date. Under current GAAP, entities generally amortize the premium as an adjustment of yield over the contractual life of the instrument. ASU 2017-08 is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018 with early adoption permitted. We adopted this guidance in the fourth quarter of 2017 and the adoption did not impact us as we amortize premium on these callable debt securities to the earliest call date. In May 2017, the FASB issued ASU 2017-09, Compensation - Stock Compensation: Scope of Modification Accounting ("ASU 2017-09"). ASU 2017-09 provides clarification about which changes to the terms or conditions of a share-based payment award would require the application of modification accounting. ASU 2017-09 is effective for annual and interim periods beginning after December 15, 2017, with early adoption permitted. We adopted this guidance in the fourth quarter of 2017 and the adoption did not impact us, as we currently record modifications in accordance with this ASU. Pronouncements to be effective in the future In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”). ASU 2016-01 provides guidance to improve certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Specifically the guidance: (i) requires equity investments to be measured at fair value with changes in fair value recognized in earnings; (ii) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment; (iii) eliminates the requirement to disclose the methods and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost; (iv) requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes; and (v) clarifies that the need for a valuation allowance on a deferred tax asset related to an available-for-sale ("AFS") security should be evaluated with other deferred tax assets. ASU 2016-01 is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. Our adoption of this guidance will require a cumulative-effect adjustment between AOCI and retained earnings on the balance sheet for approximately $25 million , which represents the after-tax unrealized gain on our equity securities portfolio as of December 31, 2017. On a pre-tax basis, the unrealized gain on our equity securities portfolio increased $13 million during 2017 and, had this literature been in effect, we would have recognized additional after-tax net income of approximately $10 million , or $0.17 per diluted share, assuming a 21% corporate tax rate. In February 2016, the FASB issued ASU 2016-02, Leases (“ASU 2016-02”). ASU 2016-02 requires all lessees to recognize a lease liability and a right-of-use asset, measured at the present value of the future minimum lease payments, at the lease commencement date. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, including interim reporting periods within that fiscal year, with early adoption permitted. ASU 2016-02 requires the application of a modified retrospective approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. While we are currently evaluating ASU 2016-02, we do not expect a material impact on our financial condition or results of operations from the adoption of this guidance. In June 2016, the FASB issued ASU 2016-13 , Financial Instruments - Credit Losses (“ASU 2016-13”). ASU 2016-13 will change the way entities recognize impairment of financial assets by requiring immediate recognition of estimated credit losses expected to occur over the remaining life of many financial assets, including, among others, held-to-maturity debt securities, trade receivables, and reinsurance recoverables. ASU 2016-13 requires a valuation allowance to be calculated on these financial assets and that they be presented on the financial statements net of the valuation allowance. The valuation allowance is a measurement of expected losses that is based on relevant information about past events, including historical experience, current conditions, and reasonable and supportable forecasts that affect the collectability of the reported amount. This methodology is referred to as the current expected credit loss model. ASU 2016-13 is effective for fiscal years beginning after December 15, 2019, including interim periods within those annual periods. Early adoption is permitted, but no earlier than fiscal years beginning after December 15, 2018. We are currently evaluating the impact of this guidance on our financial condition and results of operations. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (“ASU 2016-15”). ASU 2016-15 adds or clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows, including, but not limited to: (i) debt prepayment or debt extinguishment costs; (ii) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; (iii) distributions received from equity method investees; and (iv) separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective, with retrospective adoption, for annual periods beginning after December 15, 2017, and interim periods within those fiscal years. We anticipate that the adoption of this guidance in 2018 will result in an increase to our 2017 and 2016 operating cash flows of approximately $2 million and $3 million , respectively, reflecting adjustments for distributions received from equity method investees. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash ("ASU 2016-18"). ASU 2016-18, requires that restricted cash and restricted cash equivalents be included with cash and cash equivalents in the reconciliation of beginning and ending cash on the statements of cash flows. This update also requires a reconciliation of the statement of the cash flows to the balance sheet if the balance sheet includes more than one line item containing cash, cash equivalents, and restricted cash. We currently have restricted cash associated with our participation in the National Flood Insurance Program ("NFIP") within "Other assets" on our consolidating balance sheets. This restricted cash amounted to $44.2 million , $36.9 million , and $11.9 million on December 31, 2017, 2016, and 2015, respectively. ASU 2016-18 is effective, with retrospective adoption, for annual periods beginning after December 15, 2017, and interim periods within those annual periods. We anticipate that the adoption of this guidance in 2018 will result in increases to operating cash flows of $7 million and $25 million for 2017 and 2016, respectively. The restricted cash balance will also be included in the reconciliation of beginning and ending cash balances. In January 2017, the FASB issued ASU 2017-04, Intangibles-Goodwill and Other: Simplifying the Test for Goodwill Impairment (“ASU 2017-04”). ASU 2017-04 eliminates the second step of the two part goodwill impairment test, which required entities to determine the fair value of individual assets and liabilities of a reporting unit to measure the goodwill impairment. Under the new guidance, a goodwill impairment is calculated as the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. The amendments in this update should be applied on a prospective basis for annual or interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. We intend to adopt this guidance in 2018, but do not expect it to impact our financial condition or results of operations. In March 2017, the FASB issued ASU 2017-07, Compensation-Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost ("ASU 2017-07"). ASU 2017-07 requires that an employer report a pension plan's service cost in the same line item or line items as other compensation costs arising from services rendered by pertinent employees during the period. ASU 2017-07 also requires that other components of net benefit cost be presented in the income statement separately from the service cost component. If a separate line item or items are not used, the line item or items used in the income statement to present the other components of net benefit cost must be disclosed. ASU 2017-07 is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods, with early adoption permitted at the beginning of an annual period. As our pension plan was frozen as of March 2016, we have ceased accruing additional service fee costs since that time. Therefore, the application of this guidance is not anticipated to impact our financial condition, results of operations, or disclosures.

Statements of Cash Flow

Statements of Cash Flow	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]
Statements of Cash Flow	Statements of Cash Flows Supplemental cash flow information for the years ended December 31, 2017, 2016, and 2015 is as follows: ($ in thousands) 2017 2016 2015 Cash paid during the period for: Interest $ 23,905 22,098 21,892 Federal income tax 62,000 46,405 39,500 Non-cash items: Exchange of fixed income securities, AFS 22,511 23,579 36,792 Exchange of fixed income securities, HTM — — 15,257 Corporate actions related to equity securities, AFS 1 4,725 3,263 4,239 Assets acquired under capital lease arrangements 278 3,151 6,760 Non-cash purchase of property and equipment — 78 — 1 Examples of such corporate actions include non-cash acquisitions and stock-splits. Included in "Other assets" on the Consolidated Balance Sheet was $44.2 million at December 31, 2017 and $36.9 million at December 31, 2016 of cash received from the NFIP, which is restricted to pay flood claims under the Write Your Own Program.

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments [Abstract]
Investments	Investments (a) Net unrealized gains on investments included in OCI by asset class were as follows for the years ended December 31, 2017, 2016, and 2015 : ($ in thousands) 2017 2016 2015 AFS securities: Fixed income securities $ 85,806 38,781 55,689 Equity securities 38,894 25,864 13,235 Total AFS securities 124,700 64,645 68,924 HTM securities: Fixed income securities (21 ) 159 300 Total HTM securities (21 ) 159 300 Total net unrealized gains 124,679 64,804 69,224 Deferred income tax (44,103 ) (22,681 ) (24,228 ) Net unrealized gains, net of deferred income tax 80,576 42,123 44,996 Increase (decrease) in net unrealized gains in OCI, net of deferred income tax $ 38,453 (2,873 ) (35,398 ) (b) The amortized cost, net unrealized gains and losses, carrying value, unrecognized holding gains and losses, and fair value of HTM fixed income securities were as follows: December 31, 2017 Net Unrealized Unrecognized Unrecognized Amortized Gains Carrying Holding Holding Fair ($ in thousands) Cost (Losses) Value Gains Losses Value Obligations of state and political subdivisions $ 25,154 84 25,238 1,023 — 26,261 Corporate securities 16,996 (105 ) 16,891 1,003 (55 ) 17,839 Total HTM fixed income securities $ 42,150 (21 ) 42,129 2,026 (55 ) 44,100 December 31, 2016 Net Unrealized Unrecognized Unrecognized Amortized Gains Carrying Holding Holding Fair ($ in thousands) Cost (Losses) Value Gains Losses Value Obligations of state and political subdivisions 77,466 317 77,783 2,133 — 79,916 Corporate securities 22,711 (143 ) 22,568 1,665 (158 ) 24,075 CMBS 1,220 (15 ) 1,205 15 — 1,220 Total HTM fixed income securities $ 101,397 159 101,556 3,813 (158 ) 105,211 Unrecognized holding gains and losses of HTM securities are not reflected in the Financial Statements, as they represent fair value fluctuations from the later of: (i) the date a security is designated as HTM either through purchase or transfer from AFS; or (ii) the date that an OTTI charge is recognized on an HTM security, through the date of the balance sheet. (c) The cost/amortized cost, unrealized gains and losses, and fair value of AFS securities were as follows: December 31, 2017 Cost/ Amortized Unrealized Unrealized Fair ($ in thousands) Cost Gains Losses Value AFS fixed income securities: U.S. government and government agencies $ 49,326 647 (233 ) 49,740 Foreign government 18,040 526 (11 ) 18,555 Obligations of states and political subdivisions 1,539,307 44,245 (582 ) 1,582,970 Corporate securities 1,588,339 30,891 (1,762 ) 1,617,468 CLO and other ABS 789,152 6,508 (202 ) 795,458 CMBS 382,727 1,563 (841 ) 383,449 RMBS 709,825 6,487 (1,430 ) 714,882 Total AFS fixed income securities 5,076,716 90,867 (5,061 ) 5,162,522 AFS equity securities: Common stock 129,696 38,287 (226 ) 167,757 Preferred stock 14,115 904 (71 ) 14,948 Total AFS equity securities 143,811 39,191 (297 ) 182,705 Total AFS securities $ 5,220,527 130,058 (5,358 ) 5,345,227 December 31, 2016 Cost/ Amortized Unrealized Unrealized Fair ($ in thousands) Cost Gains Losses Value AFS fixed income securities: U.S. government and government agencies $ 75,139 2,230 (36 ) 77,333 Foreign government 26,559 322 (16 ) 26,865 Obligations of states and political subdivisions 1,366,287 18,610 (5,304 ) 1,379,593 Corporate securities 1,976,556 27,057 (5,860 ) 1,997,753 CLO and other ABS 527,876 1,439 (355 ) 528,960 CMBS 256,356 1,514 (1,028 ) 256,842 RMBS 524,986 3,006 (2,798 ) 525,194 Total AFS fixed income securities 4,753,759 54,178 (15,397 ) 4,792,540 AFS equity securities: Common stock 104,663 26,250 (305 ) 130,608 Preferred stock 16,226 274 (355 ) 16,145 Total AFS equity securities 120,889 26,524 (660 ) 146,753 Total AFS securities $ 4,874,648 80,702 (16,057 ) 4,939,293 Unrealized gains and losses of AFS securities represent fair value fluctuations from the later of: (i) the date a security is designated as AFS; or (ii) the date that an OTTI charge is recognized on an AFS security, through the date of the balance sheet. These unrealized gains and losses are recorded in AOCI on the Consolidated Balance Sheets. (d) The severity of impairment on the securities in an unrealized/unrecognized loss position averaged 1% of amortized cost at December 31, 2017 and December 31, 2016 . Quantitative information regarding unrealized losses on our AFS portfolio is provided below. Our HTM portfolio had $0.1 million in unrealized/unrecognized losses at December 31, 2017 and no unrealized/unrecognized losses at December 31, 2016 . December 31, 2017 Less than 12 months 12 months or longer Total ($ in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Unrealized AFS fixed income securities: U.S. government and government agencies $ 23,516 (233 ) 250 — 23,766 (233 ) Foreign government 1,481 (11 ) — — 1,481 (11 ) Obligations of states and political subdivisions 107,514 (422 ) 14,139 (160 ) 121,653 (582 ) Corporate securities 238,326 (1,744 ) 3,228 (18 ) 241,554 (1,762 ) CLO and other ABS 74,977 (196 ) 1,655 (6 ) 76,632 (202 ) CMBS 154,267 (773 ) 5,214 (68 ) 159,481 (841 ) RMBS 269,485 (1,285 ) 11,200 (145 ) 280,685 (1,430 ) Total AFS fixed income securities 869,566 (4,664 ) 35,686 (397 ) 905,252 (5,061 ) AFS equity securities: Common stock 4,727 (226 ) — — 4,727 (226 ) Preferred stock 3,833 (71 ) — — 3,833 (71 ) Total AFS equity securities 8,560 (297 ) — — 8,560 (297 ) Total AFS securities $ 878,126 (4,961 ) 35,686 (397 ) 913,812 (5,358 ) December 31, 2016 Less than 12 months 12 months or longer Total ($ in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Unrealized AFS fixed income securities: U.S. government and government agencies $ 6,419 (36 ) — — $ 6,419 $ (36 ) Foreign government 13,075 (16 ) — — 13,075 (16 ) Obligations of states and political subdivisions 306,509 (5,304 ) — — 306,509 (5,304 ) Corporate securities 462,902 (5,771 ) 4,913 (89 ) 467,815 (5,860 ) CLO and other ABS 189,795 (354 ) 319 (1 ) 190,114 (355 ) CMBS 82,492 (1,021 ) 1,645 (7 ) 84,137 (1,028 ) RMBS 279,480 (2,489 ) 8,749 (309 ) 288,229 (2,798 ) Total AFS fixed income securities 1,340,672 (14,991 ) 15,626 (406 ) 1,356,298 (15,397 ) AFS equity securities: Common stock 11,271 (305 ) — — 11,271 (305 ) Preferred stock 6,168 (355 ) — — 6,168 (355 ) Total AFS equity securities 17,439 (660 ) — — 17,439 (660 ) Total AFS securities $ 1,358,111 (15,651 ) 15,626 (406 ) $ 1,373,737 $ (16,057 ) We do not intend to sell any of the securities in the tables above, nor do we believe we will be required to sell any of these securities. Additionally, we have reviewed these securities in accordance with our OTTI policy, as described in Note 2. “Summary of Significant Accounting Policies” of this Form 10-K and have concluded that they are temporarily impaired. This conclusion reflects our current judgment as to the financial position and future prospects of the entity that issued the security and underlying collateral. If our judgment about an individual security changes in the future, we may ultimately record a credit loss after having originally concluded that one did not exist, which could have a material impact on our net income and financial position in future periods. (e) Fixed income securities at December 31, 2017 , by contractual maturity are shown below. MBS are included in the maturity tables using the estimated average life of each security. Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Listed below are the contractual maturities of fixed income securities at December 31, 2017 : AFS HTM ($ in thousands) Fair Value Carrying Value Fair Value Due in one year or less $ 315,857 10,997 11,168 Due after one year through five years 2,099,529 23,035 24,235 Due after five years through 10 years 2,510,294 8,097 8,697 Due after 10 years 236,842 — — Total fixed income securities $ 5,162,522 42,129 44,100 (f) The following table summarizes our other investment portfolio by strategy: Other Investments December 31, 2017 December 31, 2016 ($ in thousands) Carrying Value Remaining Commitment Maximum Exposure to Loss 1 Carrying Value Remaining Maximum 1 Alternative Investments Private equity $ 52,251 99,026 151,277 41,135 76,774 117,909 Private credit 37,743 94,959 132,702 28,193 40,613 68,806 Real assets 25,379 27,014 52,393 14,486 22,899 37,385 Total alternative investments 115,373 220,999 336,372 83,814 140,286 224,100 Other securities 2 16,895 — 16,895 18,583 3,400 21,983 Total other investments $ 132,268 220,999 353,267 102,397 143,686 246,083 1 The maximum exposure to loss includes both the carrying value of these investments and the related unfunded commitments. In addition, tax credits that have been previously recognized in Other securities are subject to the risk of recapture, which we do not consider significant. 2 Other securities primarily consists of tax credit investments. We have reviewed various investments included in the table above and have concluded that they are VIEs, but that we are not the primary beneficiary and therefore, consolidation is not required. We do not have a future obligation to fund losses or debts on behalf of these investments; however, we are contractually committed to make additional investments up to the remaining commitment outlined above. We have not provided any non-contractual financial support at any time during 2017 or 2016. The following is a description of our alternative investment strategies: Our private equity strategy includes the following: • Primary Private Equity : This strategy makes private equity investments, primarily in established large and middle market companies across diverse industries globally. • Secondary Private Equity : This strategy purchases seasoned private equity funds from investors desiring liquidity prior to normal fund termination. Investments are made across all sectors of the private equity market, including leveraged buyouts ("LBO"), venture capital, distressed securities, mezzanine financing, real estate, and infrastructure. • Venture Capital : In general, these investments are made principally by investing in equity securities of privately-held corporations, for long-term capital appreciation. This strategy makes private equity investments in growth equity and buyout partnerships. Our private credit strategy includes the following: • Middle Market Lending : This strategy provides privately negotiated loans to U.S. middle market companies. Typically, these are floating rate, senior secured loans diversified across industries. Loans can be made to private equity sponsor-backed companies or non-sponsored companies to finance LBOs, recapitalizations, and acquisitions. • Mezzanine Financing : This strategy provides privately negotiated fixed income securities, generally with an equity component, to LBO firms and private and publicly traded large, mid, and small-cap companies to finance LBOs, recapitalizations, and acquisitions. • Distressed Debt : This strategy makes direct and indirect investments in debt and equity securities of companies that are experiencing financial and/or operational distress. Investments include buying indebtedness of bankrupt or financially troubled companies, small balance loan portfolios, special situations and capital structure arbitrage trades, commercial real estate mortgages, and similar non-U.S. securities and debt obligations. Our real assets strategy includes the following: • Energy & Power Generation : This strategy makes energy and power generation investments in cash flow generating infrastructure assets. Energy investments are made in a variety of industries including oil, natural gas, and coal. These investments are diversified across the energy supply chain and include assets in the exploration and production, pipeline, and refining sectors. Power generation includes investments in: (i) conventional power, such as natural gas and oil; (ii) renewable power, such as wind and solar; and (iii) electric transmission and distribution. • Real Estate : This strategy invests in real estate in North America, Europe, and Asia via direct property ownership, joint ventures, mortgages, and investments in equity and debt instruments. Our alternative investment strategies generally employ low or moderate levels of leverage and use hedging only to reduce foreign exchange or interest rate volatility. At this time, our alternative investment strategies do not include hedge funds. We cannot redeem our investments with the general partners of these investments; however, occasionally these partnerships can be traded on the secondary market. Once liquidation is triggered by clauses within the limited partnership agreements or at the funds’ stated end date, we will receive our final allocation of capital and any earned appreciation of the underlying investments, assuming we have not divested ourselves of our partnership interests prior to that time. We currently receive distributions from these alternative investments through the realization of the underlying investments in the limited partnerships. We anticipate that the general partners of these alternative investments will liquidate their underlying investment portfolios through 2032. The following tables set forth summarized financial information for our other investments portfolio, including the portion not owned by us. The investments are carried under the equity method of accounting. The last line in the income statement information table below reflects our share of the aggregate income, which is the portion included in our Financial Statements. As the majority of these investments report results to us on a one quarter lag, the summarized financial statement information is as of, and for the 12-month period ended, September 30: Balance Sheet Information September 30, ($ in millions) 2017 2016 Investments $ 21,046 11,244 Total assets 22,357 12,075 Total liabilities 4,767 1,802 Total partners’ capital 17,590 10,273 Income Statement Information 12 months ended September 30, ($ in millions) 2017 2016 2015 Net investment (loss) income $ (143 ) (44 ) 129 Realized gains 325 1,374 1,187 Net change in unrealized appreciation (depreciation) 2,894 (719 ) (1,364 ) Net income $ 3,076 611 (48 ) Insurance Subsidiaries' alternative investments income (loss) 12.7 3.1 (1.9 ) (g) We did not have exposure to any credit concentration risk of a single issuer greater than 10% of our stockholders' equity, other than certain U.S. government agencies, as of December 31, 2017 or December 31, 2016 . (h) We have pledged certain AFS fixed income securities as collateral related to our relationships with the Federal Home Loan Bank of Indianapolis ("FHLBI") and the Federal Home Loan Bank of New York ("FHLBNY"). In addition, certain securities were on deposit with various state and regulatory agencies at December 31, 2017 to comply with insurance laws. We retain all rights regarding securities pledged as collateral. The following table summarizes the market value of these securities at December 31, 2017 : ($ in millions) FHLBI Collateral FHLBNY Collateral State and Regulatory Deposits Total U.S. government and government agencies $ 3.0 — 22.6 25.6 Obligations of states and political subdivisions — — 3.1 3.1 CMBS 6.2 14.1 — 20.3 RMBS 56.3 59.6 — 115.9 Total pledged as collateral $ 65.5 73.7 25.7 164.9 (i) The components of pre-tax net investment income earned were as follows: ($ in thousands) 2017 2016 2015 Fixed income securities $ 153,230 129,306 123,230 Equity securities 6,442 7,368 9,161 Short-term investments 1,526 686 112 Other investments 12,871 2,940 (1,890 ) Investment expenses (12,187 ) (9,546 ) (9,297 ) Net investment income earned $ 161,882 130,754 121,316 (j) The following tables summarize OTTI by asset type for the periods indicated: 2017 Recognized in Earnings ($ in thousands) Gross Included in OCI AFS fixed income securities: U.S. government and government agencies $ 36 — 36 Obligations of states and political subdivisions 612 — 612 Corporate securities 587 — 587 CLO and other ABS 96 — 96 CMBS 670 — 670 RMBS 1,183 (36 ) 1,219 Total AFS fixed income securities 3,184 (36 ) 3,220 AFS equity securities: Common stock 1,435 — 1,435 Total AFS equity securities 1,435 — 1,435 Other investments $ 190 — 190 Total OTTI losses $ 4,809 (36 ) 4,845 2016 Recognized in Earnings ($ in thousands) Gross Included in OCI AFS fixed income securities: Obligations of states and political subdivisons $ 2,797 — 2,797 Corporate securities 1,880 — 1,880 CLO and other ABS 19 — 19 CMBS 220 — 220 RMBS 275 10 265 Total AFS fixed income securities 5,191 10 5,181 AFS equity securities: Common stock 3,316 — 3,316 Preferred stock 2 — 2 Total AFS equity securities 3,318 — 3,318 Total OTTI losses $ 8,509 10 8,499 2015 Recognized in Earnings ($ in thousands) Gross Included in OCI AFS fixed income securities: Corporate securities $ 2,188 — 2,188 RMBS 1 — 1 Total AFS fixed income securities 2,189 — 2,189 AFS equity securities: Common stock 15,996 — 15,996 Preferred stock 181 — 181 Total AFS equity securities 16,177 — 16,177 Total OTTI losses $ 18,366 — 18,366 The majority of the OTTI charges in both 2017 and 2016 were on securities for which we had the intent to sell to facilitate our fixed income strategy change to more actively manage the portfolio to maximize after-tax income and total return, while maintaining a similar level of credit quality and duration risk. Charges in 2015 related to equity securities for which we had the intent to sell in relation to our high-dividend yield strategy, with the remaining charges relating to securities that we did not believe would recover in the near term. (k) The components of net realized gains, excluding OTTI charges, were as follows: ($ in thousands) 2017 2016 2015 HTM fixed income securities Gains $ 44 3 5 Losses (1 ) (1 ) (1 ) AFS fixed income securities Gains 10,193 7,741 4,515 Losses (3,292 ) (11,411 ) (312 ) AFS equity securities Gains 5,829 8,108 29,168 Losses (1,200 ) (864 ) (1,347 ) Short-term investments Gains 2 — — Losses (6 ) (13 ) — Other investments Gains 494 3 162 Losses (859 ) (4 ) (653 ) Total net realized investment gains $ 11,204 3,562 31,537 Realized gains and losses on the sale of investments are determined on the basis of the cost of the specific investments sold. Proceeds from the sale of AFS securities were $1,235.9 million in 2017 , $1,046.1 million in 2016 , and $234.1 million in 2015 . Net realized gains in the table above were driven by the following: • 2017 : A higher trading volume in our fixed income securities portfolio related to a more active external investment management approach and opportunistic sales in our equity portfolio. • 2016 : A repositioning of our equity portfolio partially offset by net losses in our AFS fixed income portfolio related to the change in our strategy to more actively manage this portfolio. • 2015 : A change in our dividend strategy from a quantitative, model-driven stock selection strategy to a fundamentally-based stock selection approach that incorporates an assessment of the sustainability and growth rate of a company's dividends and future cash flow.

Comprehensive Income

Comprehensive Income	12 Months Ended
	Dec. 31, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Comprehensive Income	Comprehensive Income (a) The components of comprehensive income, both gross and net of tax, for 2017 , 2016 , and 2015 were as follows: 2017 ($ in thousands) Gross Tax Net Net income $ 261,968 93,142 168,826 Components of OCI: Unrealized gains (losses) on investment securities : Unrealized holding gains during the year 66,894 23,879 43,015 Non-credit portion of OTTI recognized in OCI 36 13 23 Amounts reclassified into net income: HTM securities (179 ) (63 ) (116 ) Non-credit OTTI 104 36 68 Realized gains on AFS securities (6,979 ) (2,442 ) (4,537 ) Net unrealized gains 59,876 21,423 38,453 Defined benefit pension and post-retirement plans: Net actuarial loss (4,684 ) (984 ) (3,700 ) Amounts reclassified into net income: Net actuarial loss 2,102 735 1,367 Defined benefit pension and post-retirement plans (2,582 ) (249 ) (2,333 ) Other comprehensive income 57,294 21,174 36,120 Comprehensive income $ 319,262 114,316 204,946 2016 ($ in thousands) Gross Tax Net Net income $ 219,955 61,460 158,495 Components of OCI: Unrealized (losses) gains on investment securities : Unrealized holding losses during the year (9,195 ) (3,218 ) (5,977 ) Non-credit portion of OTTI recognized in OCI (10 ) (4 ) (6 ) Amounts reclassified into net income: HTM securities (141 ) (49 ) (92 ) Non-credit OTTI 213 75 138 Realized losses on AFS securities 4,713 1,649 3,064 Net unrealized losses (4,420 ) (1,547 ) (2,873 ) Defined benefit pension and post-retirement plans: Net actuarial loss (12,079 ) (4,227 ) (7,852 ) Amounts reclassified into net income: Net actuarial loss 6,462 2,262 4,200 Defined benefit pension and post-retirement plans (5,617 ) (1,965 ) (3,652 ) Other comprehensive loss (10,037 ) (3,512 ) (6,525 ) Comprehensive income $ 209,918 57,948 151,970 2015 ($ in thousands) Gross Tax Net Net income $ 232,692 66,831 165,861 Components of OCI: Unrealized (losses) gains on investment securities : Unrealized holding losses during the year (40,221 ) (14,078 ) (26,143 ) Amounts reclassified into net income: HTM securities (580 ) (203 ) (377 ) Non-credit OTTI 357 125 232 Realized gains on AFS securities (14,016 ) (4,906 ) (9,110 ) Net unrealized losses (54,460 ) (19,062 ) (35,398 ) Defined benefit pension and post-retirement plans: Net actuarial gain 2,438 853 1,585 Amounts reclassified into net income: Net actuarial loss 7,077 2,477 4,600 Defined benefit pension and post-retirement plans 9,515 3,330 6,185 Other comprehensive loss (44,945 ) (15,732 ) (29,213 ) Comprehensive income $ 187,747 51,099 136,648 (b) The balances of, and changes in, each component of AOCI (net of taxes) as of December 31, 2017 and 2016 were as follows: Net Unrealized (Loss) Gain on Investment Securities ($ in thousands) OTTI Related HTM Related All Other Investments Subtotal Defined Benefit Pension and Post- retirement Plans Total AOCI Balance, December 31, 2015 $ (282 ) 194 45,083 44,995 (54,420 ) (9,425 ) OCI before reclassifications (6 ) — (5,977 ) (5,983 ) (7,852 ) (13,835 ) Amounts reclassified from AOCI 138 (92 ) 3,064 3,110 4,200 7,310 Net current period OCI 132 (92 ) (2,913 ) (2,873 ) (3,652 ) (6,525 ) Balance, December 31, 2016 (150 ) 102 42,170 42,122 (58,072 ) (15,950 ) OCI before reclassifications 23 — 43,015 43,038 (3,700 ) 39,338 Amounts reclassified from AOCI 68 (116 ) (4,537 ) (4,585 ) 1,367 (3,218 ) Net current period OCI 91 (116 ) 38,478 38,453 (2,333 ) 36,120 Balance, December 31, 2017 $ (59 ) (14 ) 80,648 80,575 (60,405 ) 20,170 The reclassifications out of AOCI are as follows: ($ in thousands) Year ended December 31, 2017 Year ended December 31, 2016 Affected Line Item in the Consolidated Statements of Income OTTI related Non-credit OTTI on disposed securities $ 104 213 Net realized gains (losses) 104 213 Income before federal income tax (36 ) (75 ) Total federal income tax expense 68 138 Net income HTM related Unrealized losses on HTM disposals 32 169 Net realized gains (losses) Amortization of net unrealized gains on HTM securities (211 ) (310 ) Net investment income earned (179 ) (141 ) Income before federal income tax 63 49 Total federal income tax expense (116 ) (92 ) Net income Realized (losses) gains on AFS Realized (losses) gains on AFS disposals (6,979 ) 4,713 Net realized gains (losses) (6,979 ) 4,713 Income before federal income tax 2,442 (1,649 ) Total federal income tax expense (4,537 ) 3,064 Net income Defined benefit pension and post-retirement life plans Net actuarial loss 450 1,486 Loss and loss expense incurred 1,652 4,976 Other insurance expenses Total defined benefit pension and post-retirement life 2,102 6,462 Income before federal income tax (735 ) (2,262 ) Total federal income tax expense 1,367 4,200 Net income Total reclassifications for the period $ (3,218 ) 7,310 Net income

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The following table presents the carrying amounts and estimated fair values of our financial instruments as of December 31, 2017 and 2016 : December 31, 2017 December 31, 2016 ($ in thousands) Carrying Amount Fair Value Carrying Amount Fair Value Financial Assets Fixed income securities: HTM $ 42,129 44,100 101,556 105,211 AFS 5,162,522 5,162,522 4,792,540 4,792,540 Equity securities, AFS 182,705 182,705 146,753 146,753 Short-term investments 165,555 165,555 221,701 221,701 Long-term debt: 7.25% Senior Notes 49,904 61,391 49,901 56,148 6.70% Senior Notes 99,446 116,597 99,430 108,333 5.875% Senior Notes 185,000 186,332 185,000 176,860 1.61% Borrowings from FHLBNY 25,000 24,270 25,000 24,286 1.56% Borrowings from FHLBNY 25,000 24,210 25,000 24,219 3.03% Borrowings from FHLBI 60,000 60,334 60,000 59,313 Subtotal long-term debt 444,350 473,134 444,331 449,159 Unamortized debt issuance costs (5,234 ) (5,664 ) Total long-term debt $ 439,116 438,667 For discussion regarding the fair value techniques of our financial instruments, refer to Note 2. "Summary of Significant Accounting Policies" in this Form 10-K. The following tables provide quantitative disclosures of our financial assets that were measured and recorded at fair value at December 31, 2017 and 2016 : December 31, 2017 Fair Value Measurements Using ($ in thousands) Assets Measured at Fair Value Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) 1 Significant Other Observable Inputs (Level 2) 1 Significant Unobservable Inputs (Level 3) Description Measured on a recurring basis: AFS fixed income securities: U.S. government and government agencies $ 49,740 24,652 25,088 — Foreign government 18,555 — 18,555 — Obligations of states and political subdivisions 1,582,970 — 1,582,970 — Corporate securities 1,617,468 — 1,617,468 — CLO and other ABS 795,458 — 795,458 — CMBS 383,449 — 376,895 6,554 RMBS 714,882 — 714,882 — Total AFS fixed income securities 5,162,522 24,652 5,131,316 6,554 AFS equity securities: Common stock 2 167,757 138,640 — 5,398 Preferred stock 14,948 14,948 — — Total AFS equity securities 182,705 153,588 — 5,398 Total AFS securities 5,345,227 178,240 5,131,316 11,952 Short-term investments 165,555 165,555 — — Total assets measured at fair value $ 5,510,782 343,795 5,131,316 11,952 December 31, 2016 Fair Value Measurements Using ($ in thousands) Assets Measured at Fair Value Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) 1 Significant Other Observable Inputs (Level 2) 1 Significant Unobservable Inputs (Level 3) Description Measured on a recurring basis: AFS fixed income securities: U.S. government and government agencies $ 77,333 27,520 49,813 — Foreign government 26,865 — 26,865 — Obligations of states and political subdivisions 1,379,593 — 1,379,593 — Corporate securities 1,997,753 — 1,997,753 — CLO and other ABS 528,960 — 528,960 — CMBS 256,842 — 256,842 — RMBS 525,194 — 525,194 — Total AFS fixed income securities 4,792,540 27,520 4,765,020 — AFS equity securities: Common stock 130,608 122,932 — 7,676 Preferred stock 16,145 16,145 — — Total AFS equity securities 146,753 139,077 — 7,676 Total AFS securities 4,939,293 166,597 4,765,020 7,676 Short-term investments 221,701 221,701 — — Total assets measured at fair value $ 5,160,994 388,298 4,765,020 7,676 1 There were no transfers of securities between Level 1 and Level 2. 2 In accordance with ASU 2015-07, investments amounting to $23.7 million at December 31, 2017, respectively, were measured at fair value using the net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value. The following table provides a summary of the changes in the fair value of securities measured using Level 3 inputs and related quantitative information during 2017 : 2017 ($ in thousands) CMBS Common Stock Fair value, December 31, 2016 $ — 7,676 Total net (losses) gains for the period included in: OCI 4 — Net income — — Purchases 6,550 3,780 Sales — (3,958 ) Issuances — — Settlements — — Transfers into Level 3 — — Transfers out of Level 3 — (2,100 ) Fair value, December 31, 2017 $ 6,554 $ 5,398 The following tables provide quantitative information regarding our financial assets and liabilities that were not measured, but were disclosed at fair value at December 31, 2017 and 2016 : December 31, 2017 Fair Value Measurements Using ($ in thousands) Assets/Liabilities Disclosed at Fair Value Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets HTM: Obligations of states and political subdivisions $ 26,261 — 26,261 — Corporate securities 17,839 — 12,306 5,533 Total HTM fixed income securities $ 44,100 — 38,567 5,533 Financial Liabilities Long-term debt: 7.25% Senior Notes $ 61,391 — 61,391 — 6.70% Senior Notes 116,597 — 116,597 — 5.875% Senior Notes 186,332 186,332 — — 1.61% Borrowings from FHLBNY 24,270 — 24,270 — 1.56% Borrowings from FHLBNY 24,210 — 24,210 — 3.03% Borrowings from FHLBI 60,334 — 60,334 — Total long-term debt $ 473,134 186,332 286,802 — December 31, 2016 Fair Value Measurements Using ($ in thousands) Assets/Liabilities Disclosed at Fair Value Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets HTM: Obligations of states and political subdivisions $ 79,916 — 79,916 — Corporate securities 24,075 — 16,565 7,510 CMBS 1,220 — 1,220 — Total HTM fixed income securities $ 105,211 — 97,701 7,510 Financial Liabilities Long-term debt: 7.25% Senior Notes $ 56,148 — 56,148 — 6.70% Senior Notes 108,333 — 108,333 — 5.875% Senior Notes 176,860 176,860 — — 1.61% Borrowings from FHLBNY 24,286 — 24,286 — 1.56% Borrowings from FHLBNY 24,219 — 24,219 — 3.03% Borrowings from FHLBI 59,313 — 59,313 — Total long-term debt $ 449,159 176,860 272,299 —

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance Our Financial Statements reflect the effects of assumed and ceded reinsurance transactions. Assumed reinsurance refers to the acceptance of certain insurance risks that other insurance entities have underwritten. Ceded reinsurance involves transferring certain insurance risks (along with the related written and earned premiums) that we have underwritten to other insurance companies that agree to share these risks. The primary purpose of ceded reinsurance is to protect the Insurance Subsidiaries from potential losses in excess of the amount that we are prepared to accept. Our major treaties covering property, property catastrophe, and casualty business are excess of loss contracts. In addition, we have an intercompany quota share pooling arrangement and other minor quota share treaties. As a Standard Commercial Lines and E&S Lines writer, we are subject to the Terrorism Risk Insurance Program Reauthorization Act ("TRIPRA"), which was extended by Congress to December 31, 2020. TRIPRA requires private insurers and the United States government to share the risk of loss on future acts of terrorism certified by the U.S. Secretary of the Treasury. Under TRIPRA, each participating insurer is responsible for paying a deductible of specified losses before federal assistance is available. This deductible is based on a percentage of the prior year’s applicable Standard Commercial Lines and E&S Lines premiums. In 2018 , our deductible is approximately $323 million . For losses above the deductible, the federal government will pay 82% of losses to an industry limit of $100 billion , and the insurer retains 18% . The federal share of losses will be reduced by 1% each year to 80% by 2020. The Insurance Subsidiaries remain liable to policyholders to the extent that any reinsurer becomes unable to meet their contractual obligations. In addition to this direct counterparty credit risk, we have indirect counterparty credit risk as our reinsurers often enter into their own reinsurance programs, or retrocessions, as part of managing their exposure to large losses. We evaluate and monitor the financial condition of our reinsurers under voluntary reinsurance arrangements to minimize our exposure to significant losses from reinsurer insolvencies. On an ongoing basis, we review amounts outstanding, length of collection period, changes in reinsurer credit ratings, and other relevant factors to determine collectability of reinsurance recoverables. The allowance for uncollectible reinsurance recoverables was $4.6 million at December 31, 2017 and $5.5 million at December 31, 2016 . The following table represents our total reinsurance balances segregated by reinsurer to depict our concentration of risk throughout our reinsurance portfolio: As of December 31, 2017 As of December 31, 2016 ($ in thousands) Reinsurance Balances % of Reinsurance Balance Reinsurance Balances % of Reinsurance Balance Total reinsurance recoverables $ 594,832 $ 621,537 Total prepaid reinsurance premiums 153,493 146,282 Total reinsurance balance 748,325 767,819 Federal and state pools 1 : NFIP 204,161 27 % 211,181 27 % New Jersey Unsatisfied Claim Judgment Fund 62,947 9 65,574 9 Other 3,634 — 3,227 — Total federal and state pools 270,742 36 279,982 36 Remaining reinsurance balance $ 477,583 64 $ 487,837 64 Munich Re Group (A.M. Best rated "A+") $ 117,460 16 $ 119,520 16 Hannover Ruckversicherungs AG (A.M. Best rated "A+") 101,652 14 106,298 13 AXIS Reinsurance Company (A.M. Best rated "A+") 62,396 8 59,737 8 Swiss Re Group (A.M. Best rated "A+") 40,772 5 50,494 7 Partner Reinsurance Company of the U.S. (A.M. Best rated “A”) 16,925 2 21,125 3 All other reinsurers 138,378 19 130,663 17 Total reinsurers 477,583 64 % 487,837 64 % Less: collateral 2 (122,413 ) (113,763 ) Reinsurers, net of collateral $ 355,170 $ 374,074 1 Considered to have minimal risk of default. 2 Includes letters of credit, trust funds, and funds held against reinsurance recoverables. Under our reinsurance arrangements, which are prospective in nature, reinsurance premiums ceded are recorded as prepaid reinsurance and amortized over the remaining contract period in proportion to the reinsurance protection provided, or recorded periodically, as per the terms of the contract, in a direct relationship to the gross premium recording. Reinsurance recoveries are recognized as gross losses are incurred. The following table contains a listing of direct, assumed, and ceded reinsurance amounts for premiums written, premiums earned, and loss and loss expense incurred: ($ in thousands) 2017 2016 2015 Premiums written: Direct $ 2,733,459 2,577,259 2,403,519 Assumed 26,685 28,779 23,848 Ceded (389,503 ) (368,750 ) (357,463 ) Net $ 2,370,641 2,237,288 2,069,904 Premiums earned: Direct $ 2,647,488 2,484,715 2,330,267 Assumed 25,831 28,214 23,209 Ceded (382,292 ) (363,357 ) (363,567 ) Net $ 2,291,027 2,149,572 1,989,909 Loss and loss expense incurred: Direct $ 1,570,678 1,560,356 1,274,872 Assumed 17,588 22,708 16,996 Ceded (243,192 ) (348,267 ) (143,327 ) Net $ 1,345,074 1,234,797 1,148,541 The ceded premiums and losses related to our participation in the NFIP, under which 100% of our flood premiums, and loss and loss expense are ceded to the NFIP, are as follows: Ceded to NFIP ($ in thousands) 2017 2016 2015 Ceded premiums written $ (241,345 ) (232,245 ) (228,907 ) Ceded premiums earned (235,088 ) (227,882 ) (233,940 ) Ceded loss and loss expense incurred (160,922 ) (239,891 ) (62,078 )

Reserve for Loss and Loss Expen

Reserve for Loss and Loss Expense	12 Months Ended
	Dec. 31, 2017
Insurance Loss Reserves [Abstract]
Reserve for Loss and Loss Expense	Reserve for Loss and Loss Expense (a) The table below provides a roll forward of reserves for loss and loss expense for beginning and ending reserve balances: ($ in thousands) 2017 2016 2015 Gross reserves for loss and loss expense, at beginning of year $ 3,691,719 3,517,728 3,477,870 Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year 611,200 551,019 571,978 Net reserves for loss and loss expense, at beginning of year 3,080,519 2,966,709 2,905,892 Incurred loss and loss expense for claims occurring in the: Current year 1,384,266 1,300,565 1,217,550 Prior years (39,192 ) (65,768 ) (69,009 ) Total incurred loss and loss expense 1,345,074 1,234,797 1,148,541 Paid loss and loss expense for claims occurring in the: Current year 497,486 450,811 446,550 Prior years 742,722 670,176 641,174 Total paid loss and loss expense 1,240,208 1,120,987 1,087,724 Net reserves for loss and loss expense, at end of year 3,185,385 3,080,519 2,966,709 Add: Reinsurance recoverable on unpaid loss and loss expense, at end of year 585,855 611,200 551,019 Gross reserves for loss and loss expense at end of year $ 3,771,240 3,691,719 3,517,728 Our net loss and loss expense reserves increased by $104.9 million in 2017 , $113.8 million in 2016 , and $60.8 million in 2015 . The loss and loss expense reserves are net of anticipated recoveries for salvage and subrogation claims, which amounted to $64.8 million for 2017 , $64.9 million for 2016 , and $62.1 million for 2015 . The changes in the net loss and loss expense reserves were the result of growth in exposures, anticipated loss trends, payments of claims, and normal reserve changes inherent in the uncertainty in establishing reserves for loss and loss expense. As additional information is collected in the loss settlement process, reserves are adjusted accordingly. These adjustments are reflected in the Consolidated Statements of Income in the period in which such adjustments are identified. These changes could have a material impact on the results of operations of future periods when the adjustments are made. In 2017 , we experienced overall net favorable prior year loss development of $39.2 million , compared to $ 65.8 million in 2016 and $69.0 million in 2015 . The following table summarizes the prior year development by line of business: (Favorable)/Unfavorable Prior Year Development ($ in millions) 2017 2016 2015 General Liability $ (48.3 ) (45.0 ) (51.0 ) Commercial Automobile 35.6 25.3 2.4 Workers Compensation (52.3 ) (56.0 ) (37.0 ) Businessowners' Policies 1.9 1.8 2.2 Commercial Property 8.7 0.3 (3.0 ) Homeowners 0.4 1.7 1.5 Personal Automobile 6.7 1.0 0.4 E&S Casualty Lines 10.0 6.0 16.0 Other (1.9 ) (0.9 ) (0.5 ) Total $ (39.2 ) (65.8 ) (69.0 ) The Insurance Subsidiaries had $39.2 million of favorable prior accident year development during 2017 , which included $48.6 million of net favorable casualty development and $9.4 million of unfavorable property development. The net favorable casualty reserve development was largely driven by the workers compensation and general liability lines of business, including products liability and excess liability. Partially offsetting this net favorable development was $36.0 million of unfavorable casualty development in the commercial auto line of business. In addition, our E&S casualty lines experienced unfavorable development of $10.0 million in 2017. The majority of the 2017 net favorable development was attributable to accident years 2016 and prior, driven by the general liability and workers compensation lines of business. This net favorable development was partially offset by unfavorable development in accident years 2015 and 2016 attributable to our commercial auto and E&S casualty lines of business. The unfavorable development in our commercial auto line of business was driven primarily by bodily injury liability for accident years 2012 through 2016, driven by higher than expected frequency and severity. The Insurance Subsidiaries had $65.8 million of favorable prior accident year development during 2016 , which included $ 69.0 million of net favorable casualty development and $ 3.2 million of unfavorable property development. The net favorable casualty reserve development was largely driven by the workers compensation and general liability lines of business, including products liability and excess liability. Partially offsetting this net favorable development was $25.0 million of unfavorable casualty development in the commercial automobile line of business. In addition, our E&S casualty lines experienced unfavorable development of $6.0 million in 2016. The majority of the 2016 net favorable development was attributable to accident years 2013 and prior, driven by the workers compensation and general liability lines of business. This net favorable development was partially offset by unfavorable development in accident years 2014 and 2015 attributable to our commercial auto and E&S casualty lines of business. The unfavorable development in our commercial auto line of business was driven primarily by bodily injury liability for accident years 2014 and 2015. The unfavorable development in accident year 2014 was driven by higher than expected severity, whereas accident year 2015 was driven by higher than expected frequency and severity. The Insurance Subsidiaries had $69.0 million of favorable prior accident year development during 2015 , which included $ 67.0 million of net favorable casualty development and $ 2.0 million of favorable property development. The net favorable casualty reserve development was largely driven by the workers compensation and general liability lines of business. Our E&S casualty lines experienced unfavorable development of $16.0 million in 2015. The majority of the 2015 net favorable development was attributable to accident years 2009 through 2013, driven by the workers compensation and general liability lines of business. This net favorable development was partially offset by unfavorable development in accident years 2012 through 2014 attributable to our E&S casualty lines of business. (b) Reserves established for liability insurance include exposure to asbestos and environmental claims. These claims have arisen primarily from insured exposures in municipal government, small non-manufacturing commercial risk, and homeowners policies. The emergence of these claims is slow and highly unpredictable. There are significant uncertainties in estimating our exposure to asbestos and environmental claims (for both case and IBNR reserves) resulting from lack of relevant historical data, the delayed and inconsistent reporting patterns associated with these claims, and uncertainty as to the number and identity of claimants and complex legal and coverage issues. Legal issues that arise in asbestos and environmental cases include federal or state venue, choice of law, causation, admissibility of evidence, allocation of damages and contribution among joint defendants, successor and predecessor liability, and whether direct action against insurers can be maintained. Coverage issues that arise in asbestos and environmental cases include the interpretation and application of policy exclusions, the determination and calculation of policy limits, the determination of the ultimate amount of a loss, the extent to which a loss is covered by a policy, if at all, the obligation of an insurer to defend a claim, and the extent to which a party can prove the existence of coverage. Courts have reached different and sometimes inconsistent conclusions on these legal and coverage issues. We do not discount to present value that portion of our losses and loss expense reserves expected to be paid in future periods. The following table details our loss and loss expense reserves for various asbestos and environmental claims: 2017 ($ in millions) Gross Net Asbestos $ 7.6 6.3 Landfill sites 12.4 7.7 Underground storage tanks 8.4 7.2 Total $ 28.4 21.2 Reserves for asbestos and environmental claims are highly uncertain. There are significant uncertainties associated with estimating critical assumptions, such as average clean-up costs, third-party costs, potentially responsible party shares, allocation of damages, litigation and coverage costs, and potential state and federal legislative changes. Estimating IBNR is challenging because of the delayed and inconsistent reporting patterns associated with these claims. Traditional actuarial approaches cannot be applied because past loss history is not necessarily indicative of future behavior. While certain alternative projection models can be applied, such models can produce significantly different results with small changes in assumptions. As a result, reserves for asbestos and environmental require a high degree of judgment. Because of the significant uncertainty in the estimate, we do not calculate an asbestos and environmental loss range. Historically, our asbestos and environmental claims have been significantly lower in volume than many other standard commercial lines carriers since, prior to the introduction of the absolute pollution exclusion endorsement in the mid-1980’s, we were primarily a Standard Personal Lines carrier and therefore do not have broad exposure to asbestos and environmental claims. Additionally, we are the primary insurance carrier on the majority of these exposures, which provides more certainty in our reserve position compared to other insurance carriers. The following table provides a roll forward of gross and net asbestos and environmental incurred loss and loss expense and related reserves thereon: 2017 2016 2015 ($ in thousands) Gross Net Gross Net Gross Net Asbestos Reserves for loss and loss expense at beginning of year $ 7,847 6,615 8,024 6,793 8,751 7,314 Incurred loss and loss expense — — 77 77 (428 ) (77 ) Less: loss and loss expense paid (270 ) (269 ) (254 ) (255 ) (299 ) (444 ) Reserves for loss and loss expense at the end of year $ 7,577 6,346 7,847 6,615 8,024 6,793 Environmental Reserves for loss and loss expense at beginning of year $ 22,115 16,101 22,387 16,368 21,902 15,680 Incurred loss and loss expense 126 — 1,406 1,303 3,396 3,397 Less: loss and loss expense paid (1,403 ) (1,235 ) (1,678 ) (1,570 ) (2,911 ) (2,709 ) Reserves for loss and loss expense at the end of year $ 20,838 14,866 22,115 16,101 22,387 16,368 Total Asbestos and Environmental Claims Reserves for loss and loss expense at beginning of year $ 29,962 22,716 30,411 23,161 30,653 22,994 Incurred loss and loss expense 126 — 1,483 1,380 2,968 3,320 Less: loss and loss expense paid (1,673 ) (1,504 ) (1,932 ) (1,825 ) (3,210 ) (3,153 ) Reserves for loss and loss expense at the end of year $ 28,415 21,212 29,962 22,716 30,411 23,161 (c) The following is information about incurred and paid claims development as of December 31, 2017 , net of reinsurance, as well as cumulative claim frequency and the total of IBNR liabilities. During the experience period, we implemented a series of claims-related initiatives and claims management changes. These initiatives focused on claims handling and reserving, medical claims costs, and loss adjustment expenses. As a result of these initiatives, several historical patterns have changed and may no longer be appropriate to use as the sole basis for projections. All Lines (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 957,247 988,584 990,931 964,862 947,306 936,975 927,958 931,785 926,017 923,978 41,791 85,338 2009 920,143 941,972 916,691 883,590 870,057 869,927 857,960 853,401 848,413 41,937 85,575 2010 950,114 973,742 977,959 956,600 943,118 922,404 915,131 907,074 50,293 94,258 2011 1,042,576 1,061,667 1,062,233 1,056,107 1,033,518 1,023,726 1,019,351 63,891 104,500 2012 1,065,437 1,071,290 1,020,655 998,028 973,089 973,644 77,542 103,745 2013 1,044,142 1,062,045 1,047,230 1,021,007 1,002,316 116,449 90,755 2014 1,107,513 1,133,798 1,146,990 1,124,014 171,913 94,375 2015 1,114,081 1,130,513 1,144,830 256,758 92,891 2016 1,188,608 1,203,634 416,010 92,191 2017 1,270,110 634,863 88,941 Total 10,417,364 All Lines (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 286,314 489,633 609,851 690,016 764,196 798,996 819,280 839,392 853,769 860,745 2009 277,275 442,417 540,982 634,902 695,249 736,100 760,589 775,885 784,713 2010 328,826 509,910 625,229 704,895 773,536 803,773 823,770 835,532 2011 391,944 585,867 692,730 782,655 852,202 901,801 924,111 2012 378,067 555,819 651,544 743,742 810,135 856,195 2013 335,956 518,872 644,475 748,758 833,823 2014 405,898 614,075 736,154 855,959 2015 376,641 581,203 725,385 2016 387,272 617,958 2017 433,440 Total 7,727,861 All outstanding liabilities before 2008, net of reinsurance 352,192 Liabilities for loss and loss adjustment expenses, net of reinsurance 3,041,694 General Liability (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 250,239 243,755 243,536 234,770 233,712 224,236 219,551 221,640 221,203 219,617 14,326 13,769 2009 237,913 241,625 233,530 223,146 212,947 211,243 206,387 205,741 201,568 17,629 13,841 2010 215,208 228,680 242,499 237,154 222,328 211,619 208,968 202,394 18,403 12,672 2011 227,769 228,720 239,480 230,785 217,256 211,196 212,011 25,729 11,579 2012 238,979 245,561 215,083 194,144 175,305 175,268 27,702 9,922 2013 250,609 251,421 239,776 225,709 210,785 53,014 10,226 2014 244,312 249,946 257,132 239,333 80,168 10,391 2015 254,720 245,710 246,990 124,639 9,987 2016 277,214 272,048 178,904 9,617 2017 293,747 249,085 8,194 Total 2,273,761 General Liability (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 16,397 45,595 82,421 113,088 151,055 166,394 176,873 186,896 194,257 198,360 2009 14,346 37,143 64,970 103,213 130,554 151,920 166,767 176,316 180,621 2010 15,726 46,201 80,018 113,050 143,360 161,487 172,394 178,179 2011 13,924 42,692 73,643 102,978 135,377 159,768 170,525 2012 13,030 35,241 56,580 89,008 109,448 130,866 2013 12,789 35,113 72,127 104,587 139,114 2014 14,901 46,825 79,972 121,969 2015 14,665 39,978 78,668 2016 15,684 46,549 2017 17,366 Total 1,262,217 All outstanding liabilities before 2008, net of reinsurance 80,514 Liabilities for loss and loss adjustment expenses, net of reinsurance 1,092,058 Workers Compensation (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 219,616 243,186 255,810 250,423 241,921 245,993 244,100 243,512 238,836 238,218 25,993 14,401 2009 197,504 215,946 213,036 210,109 210,756 216,992 212,536 208,611 208,142 22,575 12,217 2010 198,371 214,469 212,815 211,030 214,916 212,448 208,155 204,423 28,964 12,184 2011 205,238 218,973 214,743 215,114 210,591 205,708 200,674 32,881 11,845 2012 203,864 208,036 199,360 195,197 188,596 187,359 36,233 11,605 2013 199,794 194,318 187,658 173,160 166,662 34,776 11,366 2014 199,346 187,065 182,579 172,515 42,869 10,482 2015 193,729 194,639 183,604 42,287 10,530 2016 196,774 184,946 70,424 10,509 2017 195,202 112,086 10,182 Total 1,941,745 Workers Compensation (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 39,628 100,678 139,144 158,083 171,403 180,556 188,206 191,265 195,962 198,512 2009 37,885 87,299 117,019 133,116 145,417 154,726 160,529 164,336 167,894 2010 46,795 93,281 122,442 137,184 149,086 153,795 158,078 162,796 2011 42,941 90,836 118,847 134,646 139,232 149,269 154,320 2012 40,911 86,909 108,211 122,755 132,052 139,477 2013 36,829 74,568 96,376 109,739 118,669 2014 35,924 78,944 100,876 113,626 2015 33,857 77,320 98,195 2016 34,525 78,531 2017 40,375 Total 1,272,395 All outstanding liabilities before 2008, net of reinsurance 247,121 Liabilities for loss and loss adjustment expenses, net of reinsurance 916,471 Commercial Automobile (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 196,370 195,823 190,349 187,100 187,417 182,785 180,902 183,736 183,618 183,151 736 24,129 2009 199,541 191,079 182,724 169,858 166,682 162,911 161,251 161,923 161,300 897 24,652 2010 187,562 189,305 187,778 181,923 179,854 172,969 173,157 173,471 1,491 25,301 2011 174,006 183,044 182,325 178,421 172,617 174,882 174,514 2,937 25,272 2012 179,551 191,947 183,527 184,289 184,367 186,128 3,880 23,889 2013 188,289 205,282 209,197 207,994 210,410 8,258 25,392 2014 200,534 212,725 216,824 219,925 19,053 27,338 2015 220,994 240,958 253,074 36,137 28,818 2016 255,187 274,367 71,303 30,480 2017 301,274 132,814 30,187 Total 2,137,614 Commercial Automobile (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 69,053 104,711 130,857 151,741 166,487 173,795 175,244 180,779 181,779 181,979 2009 63,126 94,406 113,697 137,564 149,949 155,560 158,303 159,723 160,013 2010 68,098 99,254 128,015 146,913 163,513 167,227 169,100 169,793 2011 69,849 99,196 121,576 142,507 157,291 166,082 170,000 2012 73,316 105,371 127,235 148,669 168,114 176,656 2013 76,469 109,893 140,015 169,850 189,626 2014 80,810 117,169 148,884 180,701 2015 91,347 132,260 175,866 2016 106,022 155,720 2017 117,287 Total 1,677,641 All outstanding liabilities before 2008, net of reinsurance 4,158 Liabilities for loss and loss adjustment expenses, net of reinsurance 464,131 Businessowners' Policies (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 39,660 38,986 39,334 32,974 30,250 29,793 31,066 31,340 30,967 31,065 192 3,258 2009 48,535 51,762 46,645 43,828 43,553 44,938 44,299 44,273 43,933 272 3,474 2010 53,669 49,285 42,408 39,915 40,899 40,581 41,239 41,197 697 3,917 2011 54,469 57,083 51,047 58,242 59,256 58,966 58,456 1,080 4,959 2012 54,342 48,029 46,303 44,172 44,077 43,747 756 5,540 2013 49,617 42,618 41,005 40,624 41,369 3,192 3,479 2014 55,962 60,949 62,548 59,806 5,952 4,054 2015 52,871 53,768 57,245 10,256 3,913 2016 52,335 53,792 11,938 3,757 2017 46,624 15,252 3,462 Total 477,234 Businessowners' Policies (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 15,019 21,765 24,449 25,738 28,026 28,660 28,589 29,778 30,873 30,873 2009 18,915 29,612 32,689 36,073 40,052 42,895 43,358 43,448 43,547 2010 20,821 28,131 31,027 34,705 37,819 38,900 40,279 40,395 2011 27,884 37,362 41,011 46,444 52,114 55,856 57,045 2012 22,199 31,833 35,089 37,215 38,766 40,627 2013 17,412 26,592 30,845 34,760 37,993 2014 28,914 40,584 44,911 49,460 2015 24,189 36,014 42,710 2016 24,655 36,848 2017 21,865 Total 401,363 All outstanding liabilities before 2008, net of reinsurance 7,292 Liabilities for loss and loss adjustment expenses, net of reinsurance 83,163 Commercial Property (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 97,578 102,860 101,436 101,470 101,265 101,702 101,043 100,881 101,043 101,054 2 7,604 2009 82,619 82,124 82,025 82,014 80,774 80,455 80,558 80,545 80,416 4 7,009 2010 105,647 96,851 97,386 96,127 95,530 95,363 95,178 95,155 5 7,667 2011 136,954 131,667 130,942 131,282 131,353 131,113 131,049 4 9,036 2012 118,464 114,224 115,375 116,658 117,102 117,170 24 8,514 2013 88,101 90,639 90,103 90,005 90,436 42 5,709 2014 141,192 136,249 136,820 138,751 126 6,512 2015 110,270 109,513 111,750 261 6,398 2016 121,927 126,185 804 6,686 2017 138,773 8,794 6,358 Total 1,130,739 Commercial Property (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 68,211 98,921 100,465 99,288 100,213 100,752 100,908 100,868 101,034 101,032 2009 59,933 78,695 80,433 80,894 80,251 80,352 80,529 80,509 80,405 2010 69,543 91,918 94,602 95,111 95,270 95,147 95,156 95,150 2011 94,538 127,580 129,579 130,681 131,060 131,115 131,089 2012 81,528 108,834 111,503 114,699 116,291 116,625 2013 60,244 87,874 90,446 90,350 90,840 2014 101,131 132,909 136,634 137,883 2015 79,048 106,182 109,829 2016 83,966 118,789 2017 99,047 Total 1,080,689 All outstanding liabilities before 2008, net of reinsurance 250 Liabilities for loss and loss adjustment expenses, net of reinsurance 50,300 Personal Automobile (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 100,311 106,999 106,842 103,934 100,213 99,912 99,686 99,255 99,116 99,270 191 16,042 2009 93,808 103,319 105,033 103,908 104,734 103,866 103,393 103,412 103,348 191 17,346 2010 103,340 110,075 112,346 109,515 107,490 107,405 107,224 107,054 222 20,822 2011 113,232 116,164 113,686 112,993 114,241 113,830 113,988 284 22,700 2012 113,771 114,921 109,832 109,324 110,294 110,300 728 22,332 2013 108,417 109,620 106,225 106,703 107,759 851 22,371 2014 102,250 109,325 106,757 107,452 2,554 22,499 2015 96,387 99,698 100,214 6,541 20,840 2016 92,727 98,032 11,651 19,747 2017 101,880 22,284 19,818 Total 1,049,297 Personal Automobile (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 50,396 73,194 84,715 91,834 95,932 97,723 98,174 98,604 98,668 98,810 2009 51,039 71,911 86,431 96,229 100,566 102,187 102,322 102,437 103,009 2010 58,786 82,490 95,300 101,540 104,061 105,849 106,453 106,733 2011 61,323 82,102 93,878 105,068 111,085 112,732 113,551 2012 63,704 82,729 94,842 102,977 107,890 109,355 2013 61,384 80,861 92,637 100,528 105,131 2014 62,519 83,739 92,589 99,173 2015 58,725 76,470 87,163 2016 57,961 76,823 2017 62,854 Total 962,602 All outstanding liabilities before 2008, net of reinsurance 5,862 Liabilities for loss and loss adjustment expenses, net of reinsurance 92,557 Homeowners (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 41,224 41,747 39,342 39,203 38,062 38,410 38,111 38,042 38,045 38,038 56 5,139 2009 47,636 44,511 42,609 40,313 61,927 40,400 40,465 40,457 40,451 73 5,633 2010 68,373 67,525 63,285 97,761 62,462 62,402 62,339 62,392 84 9,131 2011 103,804 98,211 82,744 94,167 94,543 94,183 94,378 159 15,106 2012 87,260 82,745 86,560 86,667 86,271 86,330 180 16,936 2013 73,670 72,528 71,494 72,145 71,714 284 7,747 2014 80,111 82,461 83,637 83,844 1,146 8,762 2015 76,637 76,400 76,559 2,744 7,724 2016 60,105 60,931 2,067 6,820 2017 59,167 5,315 6,651 Total 673,804 Homeowners (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 21,277 33,535 36,271 37,086 37,763 37,837 37,933 37,939 37,930 37,928 2009 28,299 36,965 38,078 39,342 39,731 39,819 39,907 40,189 40,269 2010 43,699 58,638 60,295 61,106 62,155 62,227 62,241 62,272 2011 71,668 89,963 91,718 92,185 93,312 93,720 94,007 2012 69,056 79,584 82,720 84,250 85,196 85,562 2013 50,664 65,528 67,838 69,775 71,776 2014 61,561 76,007 79,751 81,664 2015 52,589 70,078 72,202 2016 42,252 57,333 2017 45,466 Total 648,479 All outstanding liabilities before 2008, net of reinsurance 5,403 Liabilities for loss and loss adjustment expenses, net of reinsurance 30,728 E&S Casualty Lines (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2011 2012 2013 2014 2015 2016 2017 2008 $ 92 169 146 119 52 (162 ) 119 — 35 2009 885 1,053 938 728 710 96 737 — 274 2010 3,294 4,106 3,369 4,299 3,831 3,055 4,932 — 804 2011 8,127 7,102 9,853 12,207 10,273 9,652 10,228 361 1,316 2012 42,367 42,621 43,175 46,149 46,165 45,988 7,313 2,006 2013 55,468 60,309 67,099 69,112 67,647 15,326 2,219 2014 55,316 63,505 69,929 71,719 18,224 1,987 2015 75,498 76,432 82,404 32,909 2,606 2016 94,451 96,416 65,489 2,454 2017 91,438 79,354 1,760 Total 471,628 E&S Casualty Lines (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2011 2012 2013 2014 2015 2016 2017 2008 $ — 24 70 80 79 92 97 2009 — 198 431 605 626 709 737 2010 — 1,218 2,570 3,574 4,078 4,513 4,610 2011 — 806 3,200 6,445 9,954 9,912 10,256 2012 3,722 7,914 16,430 25,064 32,343 36,278 2013 2,715 9,470 21,980 35,200 46,108 2014 2,353 12,234 25,571 43,877 2015 3,036 13,057 29,389 2016 3,720 16,195 2017 5,057 Total 192,604 All outstanding liabilities before 2008, net of reinsurance — Liabilities for loss and loss adjustment expenses, net of reinsurance 279,023 In 2011, the Parent purchased Mesa Underwriters Specialty Insurance Company ("MUSIC"), a wholly-owned E&S Lines subsidiary of Montpelier Re Holdings, Ltd. Under the terms of the purchase agreement, the Parent acquired net loss and loss adjustment reserves amounting to approximately $15 million . All development on this acquired business was fully reinsured as of the acquisition date. (d) The reconciliation of the net incurred and paid claims development tables to the liability for loss and loss adjustment expenses in the consolidated statement of financial position is as follows: (in thousands) December 31, 2017 Net outstanding liabilities: Standard Commercial Lines General liability 1,092,058 Workers compensation 916,471 Commercial automobile 464,131 Businessowners' policies 83,163 Commercial property 50,300 Other Standard Commercial Lines 10,560 Total Standard Commercial Lines net outstanding liabilities 2,616,683 Standard Personal Lines Personal automobile 92,557 Homeowners 30,728 Other Standard Personal Lines 9,184 Total Standard Personal Lines net outstanding liabilities 132,469 E&S Lines Casualty lines 279,023 Property lines 13,519 Total E&S Lines net outstanding liabilities 292,542 Total liabilities for unpaid loss and loss adjustment expenses, net of reinsurance 3,041,694 Reinsurance recoverable on unpaid claims: Standard Commercial Lines General liability 175,276 Workers compensation 218,024 Commercial automobile 16,745 Businessowners' policies 3,926 Commercial property 24,387 Other Standard Commercial Lines 2,287 Total Standard Commercial Lines reinsurance recoverable on unpaid loss 440,645 Standard Personal Lines Personal automobile 53,129 Homeowners 999 Other Standard Personal Lines 69,333 Total Standard Personal Lines reinsurance recoverable on unpaid loss 123,461 E&S Lines Casualty lines 21,360 Property lines 389 Total E&S Lines reinsurance recoverable on unpaid loss 21,749 Total reinsurance recoverable on unpaid loss 585,855 Unallocated loss adjustment expenses 143,691 Total gross liability for unpaid loss and loss adjustment expenses 3,771,240 (e) The table below reflects the historical average annual percentage payout of incurred claims by age. For example, the general liability line of business averages payout of 6.6% of its ultimate losses in the first year, 12.5% in the second year, and so forth. The following is supplementary information about average historical claims duration as of December 31, 2017 : Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 General liability 6.6% 12.5 15.4 16.7 14.9 10.1 6.0 4.6 2.2 1.3 Workers compensation 20.2 23.9 13.6 8.3 5.0 3.9 2.8 2.0 1.8 1.9 Commercial automobile 38.2 17.4 14.0 12.8 9.0 4.1 1.6 1.4 0.7 0.1 Businessowners’ policies 46.6 20.8 8.1 7.5 7.5 4.1 2.0 1.6 0.1 1.1 Commercial property 70.5 25.6 2.7 0.7 0.4 0.1 — — — — Personal automobile 56.1 18.9 10.7 7.4 4.2 1.5 0.4 0.1 0.2 — Homeowners 71.1 21.0 3.3 2.1 1.6 0.1 — 0.2 0.2 0.1 E&S Lines - casualty 4.5 12.5 18.2 20.4 15.5 8.3 4.0

Indebtedness

Indebtedness	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Indebtedness	Indebtedness The table below provides a summary of our outstanding debt at December 31, 2017 and 2016: Outstanding Debt 2017 Carry Value ($ in thousands) Issuance Date Maturity Date Interest Rate Original Amount Debt Discount and Unamortized Issuance Costs December 31, 2017 December 31, 2016 Description Long-term: (1) FHLBI 12/16/2016 12/16/2026 3.03 % $ 60,000 — 60,000 60,000 (2) FHLBNY 8/15/2016 8/16/2021 1.56 % 25,000 — 25,000 25,000 (2) FHLBNY 7/21/2016 7/21/2021 1.61 % 25,000 — 25,000 25,000 (3) Senior Notes 2/8/2013 2/9/2043 5.875 % 185,000 (4,570 ) 180,430 180,068 (4) Senior Notes 11/3/2005 11/1/2035 6.70 % 100,000 (989 ) 99,011 98,952 (5) Senior Notes 11/16/2004 11/15/2034 7.25 % 50,000 (325 ) 49,675 49,647 Total long-term debt $ 445,000 (5,884 ) 439,116 438,667 Short-term Debt Selective Insurance Company of America ("SICA") borrowed: (i) $64 million in short-term funds from the FHLBNY on February 28, 2017 at an interest rate of 0.75% , which it repaid on March 21, 2017; and (ii) $20 million in short-term funds from the FHLBNY on November 8, 2017 at an interest rate of 1.29% , which it repaid on November 15, 2017. The Parent's line of credit with Wells Fargo Bank, National Association, as administrative agent, and Branch Banking and Trust Company (BB&T) (referred to as our "Line of Credit"), was renewed effective December 1, 2015 , with a borrowing capacity of $30 million , which can be increased to $50 million with the approval of both lending partners. Our Line of Credit expires on December 1, 2020, and has an interest rate, which varies and is based on, among other factors, the Parent’s debt ratings. There were no balances outstanding under our Line of Credit at December 31, 2017 or at any time during 2017 . Our Line of Credit agreement contains representations, warranties, and covenants that are customary for credit facilities of this type, including, without limitation, financial covenants under which we are obligated to maintain a minimum consolidated net worth, minimum combined statutory surplus, maximum ratio of consolidated debt to total capitalization, and covenants limiting our ability to: (i) merge or liquidate; (ii) incur debt or liens; (iii) dispose of assets; (iv) make investments and acquisitions; and (v) engage in transactions with affiliates. The table below outlines information regarding certain of the covenants in the Line of Credit: Required as of Actual as of December 31, 2017 December 31, 2017 Consolidated net worth Not less than $1.2 billion $1.7 billion Statutory surplus Not less than $750 million $1.7 billion Debt-to-capitalization ratio 1 Not to exceed 35% 20.5% A.M. Best financial strength rating Minimum of A- A 1 Calculated in accordance with the Line of Credit agreement. In addition to the above requirements, the Line of Credit agreement contains a cross-default provision that provides that the Line of Credit will be in default if we fail to comply with any condition, covenant, or agreement (including payment of principal and interest when due on any debt with an aggregate principal amount of at least $20 million ), which causes or permits the acceleration of principal. Additionally, the Line of Credit limits borrowings from the FHLBI and the FHLBNY to 10% of the respective member company's admitted assets for the previous year. Long-term Debt (1) In the first quarter of 2009, Selective Insurance Company of South Carolina ("SICSC") and Selective Insurance Company of the Southeast ("SICSE"), which are collectively referred to as the "Indiana Subsidiaries" as they are domiciled in Indiana, joined, and invested in, the FHLBI, which provides them with access to additional liquidity. The Indiana Subsidiaries’ aggregate investment in the FHLBI was $2.8 million at December 31, 2017 and December 31, 2016 . Our investment provides us the ability to borrow approximately 20 times the total amount of the FHLBI common stock purchased with additional collateral, at comparatively low borrowing rates. The proceeds from the FHLBI borrowing on December 16, 2016 of $60 million were used to repay a $45 million borrowing from the FHLBI that was outstanding at the time, with the remaining $15 million used for general corporate purposes. All borrowings from the FHLBI require security. For information on investments that are pledged as collateral for these borrowings, see Note 5. "Investments" above. (2) In the fourth quarter of 2015, SICA and Selective Insurance Company of New York ("SICNY") joined, and invested in, the FHLBNY, which provides them with access to additional liquidity. The aggregate investment for both subsidiaries was $2.6 million at December 31, 2017 and $2.8 million at December 31, 2016 . Our investment provides us the ability to borrow approximately 20 times the total amount of the FHLBNY common stock purchased with additional collateral, at comparatively low borrowing rates. In 2016, SICA borrowed the following amounts from the FHLBNY: (i) $25 million in August 2016 at an interest rate of 1.56% , which is due on August 16, 2021; and (ii) $25 million from the FHLBNY at an interest rate of 1.61% , which is due on July 21, 2021. All borrowings from the FHLBNY require security. For information on investments that are pledged as collateral for these borrowings, see Note 5. "Investments" above. (3) In February 2013, we issued $185 million of 5.875% Senior Notes due 2043. The notes became callable by us on February 8, 2018, at a price equal to 100% of their principal outstanding amount, plus accrued and unpaid interest to, but excluding, the date of redemption. A portion of the proceeds from this debt issuance was used to fully redeem the $100 million aggregate principal amount of our 7.5% Junior Subordinated Notes due 2066. Of the remaining net proceeds, $57.1 million was used to make capital contributions to the Insurance Subsidiaries, while the balance was used for general corporate purposes. There are no financial debt covenants to which we are required to comply in regards to these Senior Notes. (4) In November 2005, we issued $100 million of 6.70% Senior Notes due 2035 . These notes were issued at a discount of $0.7 million resulting in an effective yield of 6.754% . Net proceeds of approximately $50 million were used to fund an irrevocable trust that subsequently funded certain payment obligations in respect of our outstanding debt. The remainder of the proceeds was used for general corporate purposes. The agreements covering these notes contain a standard default cross-acceleration provision that provides the 6.70% Senior Notes will enter a state of default upon the failure to pay principal when due or upon any event or condition that results in an acceleration of principal of any other debt instrument in excess of $10 million that we have outstanding concurrently with the 6.70% Senior Notes. There are no financial debt covenants to which we are required to comply in regards to these notes. (5) In November 2004, we issued $50 million of 7.25% Senior Notes due 2034 . These notes were issued at a discount of $0.1 million , resulting in an effective yield of 7.27% . We contributed $25 million of the bond proceeds to the Insurance Subsidiaries as capital. The remainder of the proceeds was used for general corporate purposes. The agreements covering these notes contain a standard default cross-acceleration provision that provides the 7.25% Senior Notes will enter a state of default upon the failure to pay principal when due or upon any event or condition that results in an acceleration of principal of any other debt instrument in excess of $10 million that we have outstanding concurrently with the 7.25% Senior Notes. There are no financial debt covenants to which we are required to comply in regards to these notes.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment Information	Segment Information The disaggregated results of our four reportable segments are used by senior management to manage our operations. These reportable segments are evaluated as follows: • Our Standard Commercial Lines, Standard Personal Lines, and E&S Lines are evaluated based on before and after-tax underwriting results (net premiums earned, incurred loss and loss expense, policyholders dividends, policy acquisition costs, and other underwriting expenses), and combined ratios. • Our Investments segment is evaluated based on after-tax net investment income and net realized gains and losses. In computing the results of each segment, we do not make adjustments for interest expense or corporate expenses. We do not maintain separate investment portfolios for the segments and therefore, do not allocate assets to the segments. Our combined insurance operations are subject to certain geographic concentrations, particularly in the Northeast and Mid-Atlantic regions of the country. In 2017 , approximately 20% of NPW were related to insurance policies written in New Jersey. The goodwill balance of $7.8 million at both December 31, 2017 and 2016 relates to our Standard Commercial Lines reporting unit. The following summaries present revenues (net investment income and net realized gains on investments in the case of the Investments segment) and pre-tax income for the individual segments: Revenue by Segment Years ended December 31, ($ in thousands) 2017 2016 2015 Standard Commercial Lines: Net premiums earned: Commercial automobile $ 442,818 398,942 358,909 Workers compensation 317,982 308,233 290,075 General liability 569,217 527,859 483,291 Commercial property 311,932 293,438 269,022 Businessowners’ policies 100,266 97,754 93,428 Bonds 29,086 23,227 20,350 Other 17,198 16,030 14,367 Miscellaneous income 9,488 7,782 6,343 Total Standard Commercial Lines revenue 1,797,987 1,673,265 1,535,785 Standard Personal Lines: Net premiums earned: Personal automobile 153,147 142,876 146,784 Homeowners 129,699 130,973 134,382 Other 6,855 6,758 6,968 Miscellaneous income 1,228 1,098 1,113 Total Standard Personal Lines revenue 290,929 281,705 289,247 E&S Lines: Net premiums earned: Casualty lines 157,366 151,638 126,064 Property lines 55,461 51,844 46,269 Miscellaneous income — 1 — Total E&S Lines revenue 212,827 203,483 172,333 Investments: Net investment income 161,882 130,754 121,316 Net realized investment gains (losses) 6,359 (4,937 ) 13,171 Total Investments revenues 168,241 125,817 134,487 Total revenues $ 2,469,984 2,284,270 2,131,852 Income Before Federal Income Tax Years ended December 31, ($ in thousands) 2017 2016 2015 Standard Commercial Lines: Underwriting gain, before federal income tax $ 149,514 146,435 164,496 Underwriting gain, after federal income tax 97,184 95,183 106,923 Combined ratio 91.6 % 91.2 % 89.2 % Standard Personal Lines: Underwriting gain, before federal income tax 11,104 12,419 1,336 Underwriting gain, after federal income tax 7,217 8,072 868 Combined ratio 96.2 % 95.6 % 99.5 % E&S Lines: Underwriting loss, before federal income tax (6,282 ) (6,921 ) (16,803 ) Underwriting loss, after federal income tax (4,083 ) (4,499 ) (10,922 ) Combined ratio 103.0 % 103.4 % 109.8 % Investments: Net investment income $ 161,882 130,754 121,316 Net realized investment gains (losses) 6,359 (4,937 ) 13,171 Total investment income, before federal income tax 168,241 125,817 134,487 Tax on investment income 45,588 30,621 32,090 Total investment income, after federal income tax $ 122,653 95,196 102,397 Reconciliation of Segment Results to Income Before Federal Income Tax Years ended December 31, ($ in thousands) 2017 2016 2015 Underwriting gain (loss) Standard Commercial Lines $ 149,514 146,435 164,496 Standard Personal Lines 11,104 12,419 1,336 E&S Lines (6,282 ) (6,921 ) (16,803 ) Investment income 168,241 125,817 134,487 Total all segments 322,577 277,750 283,516 Interest expense (24,354 ) (22,771 ) (22,428 ) Corporate expenses (36,255 ) (35,024 ) (28,396 ) Income, before federal income tax $ 261,968 219,955 232,692

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Share	Earnings per Share The following table provides a reconciliation of the numerators and denominators of basic and diluted earnings per share ("EPS"): 2017 Income Shares Per Share ($ in thousands, except per share amounts) (Numerator) (Denominator) Amount Basic EPS: Net income available to common stockholders $ 168,826 58,458 $ 2.89 Effect of dilutive securities: Stock compensation plans — 899 Diluted EPS: Net income available to common stockholders $ 168,826 59,357 $ 2.84 2016 Income Shares Per Share ($ in thousands, except per share amounts) (Numerator) (Denominator) Amount Basic EPS: Net income available to common stockholders $ 158,495 57,889 $ 2.74 Effect of dilutive securities: Stock compensation plans — 858 Diluted EPS: Net income available to common stockholders $ 158,495 58,747 $ 2.70 2015 Income Shares Per Share ($ in thousands, except per share amounts) (Numerator) (Denominator) Amount Basic EPS: Net income available to common stockholders $ 165,861 57,212 $ 2.90 Effect of dilutive securities: Stock compensation plans — 944 Diluted EPS: Net income available to common stockholders $ 165,861 58,156 $ 2.85

Federal Income Taxes

Federal Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal Income Taxes	Federal Income Taxes (a) On December 22, 2017, the Tax Cuts and Jobs Act of 2017 ("Tax Reform") was signed into law, which among other implications, will reduce our statutory corporate tax rate from 35% to 21% beginning with our 2018 tax year. We revalued our deferred tax inventory as of December 31, 2017 in anticipation of this reduction, which resulted in a $20.2 million charge to income as illustrated in the rate reconciliation table below. This charge included a $5.7 million benefit related to net unrealized gains on our investment portfolio and pension plan, which were originally recorded through AOCI. Our accounting for the impact of Tax Reform on our deferred tax assets and liabilities is complete with the exception of amounts related to loss reserve discounting. Prior to Tax Reform, we had elected to use our own loss reserve payment patterns in determining the factors that we use in our discounting calculation. Under Tax Reform, this election has been eliminated and we are required to use an industry experience approach that includes a discount rate based on a corporate bond yield curve for which the IRS has not yet issued any guidance. Considering this, we have recorded a $7.5 million provisional increase to our deferred tax asset that is based on the industry experience approach under the tax law that existed prior to Tax Reform. We believe this is a reasonable estimate for the elimination of the company experience method election. We have not estimated a provisional amount based on the revised Tax Reform industry experience approach. Based on a Tax Reform transition rule that allows for this type of change in accounting method to be amortized into expense over an eight-year period beginning in 2018, we have established an offsetting deferred tax liability of $7.5 million as of December 31, 2017. During 2018, we will obtain, prepare, and analyze the necessary information to complete the accounting for loss reserve discounting. (b) A reconciliation of federal income tax on income at the corporate rate to the effective tax rate is as follows: ($ in thousands) 2017 2016 2015 Tax at statutory rate of 35% $ 91,689 76,984 81,442 Tax-advantaged interest (11,510 ) (12,126 ) (13,164 ) Dividends received deduction (1,961 ) (1,114 ) (1,817 ) Stock based compensation (4,281 ) — — Tax reform rate change 20,205 — — Other (1,000 ) (2,284 ) 370 Federal income tax expense from continuing operations $ 93,142 61,460 66,831 In addition to the impact of Tax Reform discussed above, our rate reconciliation for 2017 was also impacted by the $4.3 million impact of new accounting literature requiring that the tax effects of share-based compensation be recognized in the income tax provision. Previously, these amounts were recorded in additional paid-in capital. See Note 3. "Adoption of Accounting Pronouncements" for additional information regarding this literature change. (c) The tax effects of the significant temporary differences that gave rise to deferred tax assets and liabilities were as follows: ($ in thousands) 2017 2016 Deferred tax assets: Net loss reserve discounting $ 38,771 70,065 Net unearned premiums 50,267 78,201 Employee benefits 8,606 17,881 Long-term incentive compensation plans 12,221 17,750 Temporary investment write-downs 1,044 2,475 Other investment related items, net — 1,484 Net operating loss 54 771 Other 5,784 8,344 Total deferred tax assets 116,747 196,971 Deferred tax liabilities: Deferred policy acquisition costs 47,484 75,310 Unrealized gains on investment securities 26,183 22,681 Other investment-related items, net 2,500 — Accelerated depreciation and amortization 8,590 14,140 Total deferred tax liabilities 84,757 112,131 Net deferred federal income tax asset $ 31,990 84,840 Net deferred federal income tax assets decreased by $52.9 million during 2017. As mentioned above, net deferred federal income tax assets were reduced by $20.2 million in relation to Tax Reform. In addition to this charge, net deferred assets decreased by $21.4 million resulting from additional unrealized gains generated during the year on our investment portfolio. After considering all evidence, both positive and negative, with respect to our federal tax loss carryback availability, expected levels of pre-tax financial statement income, and federal taxable income, we believe it is more likely than not that the existing deductible temporary differences will reverse during periods in which we generate net federal taxable income or have adequate federal carryback availability. As a result, we had no valuation allowance recognized for federal deferred tax assets at December 31, 2017 or 2016 . As of December 31, 2017 , we had federal tax net operating loss ("NOL") carryforwards of $0.3 million . These NOLs, which are subject to an annual limitation of $1.9 million , will expire between 2030 and 2031. Stockholders' equity reflects tax benefits related to compensation expense deductions for share-based compensation awards of $23.8 million at December 31, 2017 and December 31, 2016 , and $22.0 million at December 31, 2015 . As mentioned above, beginning in 2017, all excess tax benefits and tax deficiencies on share-based payment awards are recognized as income tax expense or benefit on the Consolidated Statements of Income. We have analyzed our tax positions in all open tax years, which as of December 31, 2017 were 2014 through 2016 , and we believe our tax positions will more likely than not be sustained upon examination, including related appeals or litigation. In the event we had a tax position that did not meet the more likely than not criteria, any tax, interest, and penalties incurred related to such a position would be reflected in "Total federal income tax expense" on our Consolidated Statements of Income. We are not currently under a federal income tax audit for any tax year.

Retirement Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Retirement Plans	Retirement Plans (a) Selective Insurance Retirement Savings Plan (“Retirement Savings Plan”) SICA offers a voluntary defined contribution 401(k) plan, which is available to most of our employees and is a tax-qualified retirement plan subject to the Employee Retirement Income Security Act of 1974 ("ERISA"). Expense recorded for this plan was $15.8 million in 2017 , $15.0 million in 2016 , and $14.1 million in 2015 . (b) Deferred Compensation Plan SICA offers a nonqualified deferred compensation plan ("Deferred Compensation Plan") to a group of management or highly compensated employees as a method of recognizing and retaining such employees. The Deferred Compensation Plan provides these employees the opportunity to elect to defer receipt of specified portions of compensation and to have such deferred amounts deemed to be invested in specified investment options. In addition to the employee deferrals, SICA may choose to make matching contributions to some or all of the participants in this plan to the extent the participant did not receive the maximum matching or non-elective contributions permissible under the Retirement Savings Plan due to limitations under the Internal Revenue Code or the Retirement Savings Plan. Expense recorded for these contributions was $0.2 million in 2017 , $0.3 million in 2016 , and $0.2 million in 2015 . (c) Retirement Income Plan and Retirement Life Plan SICA's primary pension plan is the Retirement Income Plan for Selective Insurance Company of America (the "Pension Plan"). This qualified, noncontributory defined benefit plan is closed to new entrants and existing participants ceased accruing benefits after March 31, 2016 . In addition to the Pension Plan, SICA also sponsors the Supplemental Excess Retirement Plan (the "Excess Plan") and a life insurance benefit plan (the "Retirement Life Plan"). Both of these plans are closed to new entrants and participants in the Excess Plan ceased accruing benefits after March 31, 2016 . The Retirement Life Plan does not accrue benefits and this plan applies only to retirees who terminated employment with SICA on or before March 31, 2009. These are both unfunded plans with benefit obligations as of December 31, 2017 and December 31, 2016 of $10.1 million and $9.1 million , respectively, for the Excess Plan and $6.4 million and $6.3 million , respectively, for the Retirement Life Plan. Expense recorded for the Excess Plan was $0.4 million in 2017 , $0.5 million in 2016 , and $0.8 million in 2015 . Expense recorded for the Retirement Life Plan was $0.3 million in 2017 , 2016 , and 2015 . The following tables provide details on the Pension Plan for 2017 and 2016 : December 31, Pension Plan ($ in thousands) 2017 2016 Change in Benefit Obligation: Benefit obligation, beginning of year $ 330,588 310,308 Service cost — 1,647 Interest cost 12,490 12,336 Actuarial losses 31,158 15,086 Benefits paid (9,825 ) (8,789 ) Benefit obligation, end of year $ 364,411 330,588 Change in Fair Value of Assets: Fair value of assets, beginning of year $ 316,515 249,700 Actual return on plan assets, net of expenses 46,983 21,079 Contributions by the employer to funded plans 10,000 54,525 Benefits paid (9,825 ) (8,789 ) Fair value of assets, end of year $ 363,673 316,515 Funded status $ (738 ) (14,073 ) Amounts Recognized in the Consolidated Balance Sheet: Liabilities $ (738 ) (14,073 ) Net pension liability, end of year $ (738 ) (14,073 ) Amounts Recognized in AOCI: Net actuarial loss $ 87,438 85,845 Total $ 87,438 85,845 Other Information as of December 31: Accumulated benefit obligation $ 364,411 330,588 Weighted-Average Liability Assumptions as of December 31: Discount rate 3.78 % 4.41 Pension Plan ($ in thousands) 2017 2016 2015 Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income: Net Periodic Benefit Cost: Service cost $ — 1,647 7,215 Interest cost 12,490 12,336 13,668 Expected return on plan assets (19,419 ) (17,309 ) (15,969 ) Amortization of unrecognized actuarial loss 2,001 6,299 6,831 Total net periodic cost $ (4,928 ) 2,973 11,745 Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income: Net actuarial loss (gain) $ 3,594 11,316 (1,425 ) Reversal of amortization of net actuarial loss (2,001 ) (6,299 ) (6,831 ) Total recognized in other comprehensive income $ 1,593 5,017 (8,256 ) Total recognized in net periodic benefit cost and other comprehensive income $ (3,335 ) 7,990 3,489 The estimated net actuarial loss for the Pension Plan that will be amortized from AOCI into net periodic benefit cost during the 2018 fiscal year is $2.0 million . Pension Plan 2017 2016 2015 Weighted-Average Expense Assumptions for the years ended December 31: Discount rate 4.41 % 4.69 4.29 Expected return on plan assets 6.24 6.37 6.27 Rate of compensation increase 1 — — 4.00 1 This assumption was 4.00% through March 31, 2016, the date after which benefits ceased accruing for all participants of the Pension Plan. Our latest measurement date was December 31, 2017 , at which time we increased our expected return on plan assets to 6.36% , reflecting a higher allocation to equity securities in the portfolio. When determining the most appropriate discount rate to be used in the valuation, we consider, among other factors, our expected payout patterns of the Pension Plan's obligations as well as our investment strategy and we ultimately select the rate that we believe best represents our estimate of the inherent interest rate at which our pension and post-retirement life benefits can be effectively settled. Effective January 1, 2016, the approach used to calculate the service and interest components of net periodic benefit cost for benefit plans was changed to provide a more precise measurement of service and interest costs. Prior to 2016, we calculated these service and interest components utilizing a single weighted-average discount rate derived from the yield curve used to measure the benefit obligation at the beginning of the period. Beginning in 2016, we elected to utilize an approach that discounts the individual expected cash flows using the applicable spot rates derived from the yield curve over the projected cash flow period. We have accounted for this change prospectively as a change in accounting estimate. The weighted average discount rate used to determine 2018 interest cost is 3.46% . Plan Assets Assets of the Pension Plan are invested to adequately support the liability associated with the Pension Plan's defined benefit obligation. Our return objective is to exceed the returns of the plan's policy benchmark, which is the return the plan would have earned if the assets were invested according to the target asset class weightings and earned index returns shown below. In 2018, we will continue to phase in adjustments to the asset allocation to steadily close the gap between the duration of the assets and the duration of the liabilities, provided certain improved funding targets are achieved. Over time, the target and actual asset allocations may change based on the funded status of the Pension Plan and market return expectations. The Pension Plan’s equity investments may not contain investments in any one security greater than 8% of the portfolio value without notification to our management investment committee, nor have more than 5% of the outstanding shares of any one corporation or other entity. The use of derivative instruments is permitted under certain circumstances, but shall not be used for unrelated speculative hedging or to apply leverage to portfolio positions. Within the alternative investments portfolio, some leverage is permitted as defined and limited by the partnership agreements. The plan’s target ranges, as well as the actual weighted average asset allocation by strategy, at December 31 were as follows: 2017 2016 Target Percentage 2 Actual Percentage Actual Percentage Return seeking assets 1 20% - 60% 58 % 50 % Liability hedging assets 40% - 80% 42 % 50 % Total 100 % 100 % 100 % 1 Includes limited partnerships. 2 Target percent allocations may change over time based on the funded status of the plan and market return expectations. The Pension Plan had no investments in the Parent’s common stock as of December 31, 2017 or 2016 . The techniques used to determine the fair value of the Pension Plan's invested assets that appear on the following page are as follows: • The long-duration fixed income mutual funds utilize a market approach wherein the quoted prices in the active market for identical assets are used. All of the mutual funds are traded in active markets at their net asset value per share. These investments are classified as Level 1 in the fair value hierarchy. • The investments in global equity collective investment funds utilize a market approach wherein the published prices in the active market for identical assets are used. These investments are traded at their net asset value per share. There are no restrictions as to the redemption of these investments nor do we have any contractual obligations for further investment. These investments are classified as Level 1 in the fair value hierarchy. • The investments in private equity limited partnerships are valued utilizing net asset value as a practical expedient for fair value. These investments are not classified in the fair value hierarchy. • The investments in other private equity securities are non-publicly traded stocks and are valued by the issuer and reviewed internally. These investments are classified as Level 3 in the fair value hierarchy. • Short-term investments are carried at cost, which approximates fair value. Given that these investments are listed on active exchanges, coupled with their liquid nature, these investments are classified as Level 1 in the fair value hierarchy. • The deposit administration contract is carried at cost, which approximates fair value. Given the liquid nature of the underlying investments in overnight cash deposits and other short-term duration products, we have determined that a correlation exists between the deposit administration contract and other short-term investments, such as money market funds. As such, this investment is classified as Level 2 in the fair value hierarchy. For discussion regarding the levels within the fair value hierarchy, see Note 2. "Summary of Significant Accounting Policies." In addition, refer to Note 5. "Investments" for discussion regarding the limited partnership investment strategies, excluding the middle market lending strategy, as these investments are not part of the Pension Plan. The hedge fund strategy is part of the overall private asset strategy and is only included in the Pension Plan assets. The Pension Plan invests in hedge funds with diversified exposure to a number of underlying systematic strategies that include arbitrage, macro-oriented and equity related strategies. These positions are expected to improve the risk-adjusted return of the portfolio given their lower volatility profile than public equities with returns that are generally uncorrelated to traditional asset classes over a complete market cycle. The following tables provide quantitative disclosures of the Pension Plan’s invested assets that are measured at fair value on a recurring basis: December 31, 2017 Fair Value Measurements at 12/31/17 Using ($ in thousands) Assets Measured at Fair Value At 12/31/17 Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Description Return seeking assets: Long-duration fixed income: Global asset allocation fund $ 41,309 41,309 — — Global equity: Non-U.S. equity 67,989 67,989 — — U.S. equity 66,353 66,353 — — Total global equity 134,342 134,342 — — Private assets: Limited partnerships (at net asset value) 1 : Real assets 16,305 — — — Private equity 1,096 — — — Private credit 460 — — — Hedge fund 15,192 — — — Total limited partnerships 33,053 — — — Other private assets 980 — — 980 Total private assets 34,033 — — 980 Total return seeking assets 209,684 175,651 — 980 Liability hedging assets: Long-duration fixed income: Extended duration fixed income 146,837 146,837 — — Cash and short-term investments: Short-term investments 4,939 4,939 — — Deposit administration contracts 1,615 — 1,615 — Total cash and short-term investments 6,554 4,939 1,615 — Total liability hedging assets 153,391 151,776 1,615 — Total invested assets $ 363,075 $ 327,427 $ 1,615 $ 980 December 31, 2016 Fair Value Measurements at 12/31/16 Using ($ in thousands) Assets Measured at Fair Value At 12/31/16 Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Description Return seeking assets: Long-duration fixed income: Global asset allocation fund $ 37,878 37,878 — — Global equity: Non-U.S. equity 48,836 48,836 — — U.S. equity 55,073 55,073 — — Total global equity 103,909 103,909 — — Private assets (limited partnerships, at net asset value) 1 : Real assets 15,466 — — — Private equity 1,615 — — — Private credit 1,108 — — — Total private assets 18,189 — — — Total return seeking assets 159,976 141,787 — — Liability hedging assets: Long-duration fixed income: Extended duration fixed income 131,457 131,457 — — Cash and short-term investments: Short-term investments 23,722 23,722 — — Deposit administration contracts 1,832 — 1,832 — Total cash and short-term investments 25,554 23,722 1,832 — Total liability hedging assets 157,011 155,179 1,832 — Total invested assets $ 316,987 $ 296,966 $ 1,832 $ — 1 In accordance with ASU 2015-07, certain investments that are measured at fair value using the net asset value per share (or its practical expedient) have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total Pension Plan invested assets. Contributions We presently do not anticipate contributing to the Pension Plan in 2018 , as we have no minimum required contribution amounts. Benefit Payments ($ in thousands) Pension Plan Benefits Expected to be Paid in Future Fiscal Years: 2018 $ 12,913 2019 12,936 2020 13,987 2021 15,093 2022 16,128 2023-2027 94,542

Share-Based Payments

Share-Based Payments	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Disclosure of Compensation Related Costs, Share-based Payments [Text Block]	Share-Based Payments Active Plans As of December 31, 2017 , the following four plans were available for the issuance of share-based payment awards: • The 2014 Omnibus Stock Plan (the "Stock Plan"); • The Cash Incentive Plan, amended and restated effective as of May 1, 2014 (the "Cash Plan"); • The Employee Stock Purchase Plan (2009) ("ESPP"); and • The Amended and Restated Stock Purchase Plan for Independent Insurance Agencies (the "Agent Plan"). The following table provides information regarding the approval of these plans: Plan Approvals Stock Plan Approved effective as of May 1, 2014 by stockholders on April 23, 2014. Cash Plan Approved effective April 1, 2005 by stockholders on April 27, 2005. ESPP Approved by stockholders on April 29, 2009 effective July 1, 2009. Agent Plan Approved by stockholders on April 26, 2006. The types of awards that can be issued under each of these plans are as follows: Plan Types of Share-Based Payments Issued Stock Plan Qualified and nonqualified stock options, stock appreciation rights ("SARs"), restricted stock, restricted stock units ("RSUs"), stock grants, and other awards valued in whole or in part by reference to the Parent's common stock. The maximum exercise period for an option grant under this plan is 10 years from the date of the grant. Dividend equivalent units ("DEUs") are earned during the vesting period on RSU grants. The DEUs are reinvested in the Parent's common stock at fair value on each dividend payment date. The requisite service period for grants to employees under this plan is the lesser of: (i) the stated vested date, which is typically three years from issuance; or (ii) the date the employee becomes eligible to retire. Cash Plan Cash incentive units (“CIUs”). The initial dollar value of each CIU will be adjusted to reflect the percentage increase or decrease in the total shareholder return on the Parent's common stock over a specified performance period. In addition, for certain grants, the number of CIUs granted will be increased or decreased to reflect our performance on specified performance indicators as compared to targeted peer companies. The requisite service period for grants under this plan is the lesser of: (i) the stated vested date, which is typically three years from issuance; or (ii) the date the employee becomes eligible to retire. ESPP Enables employees to purchase shares of the Parent’s common stock. The purchase price is the lower of: (i) 85% of the closing market price at the time the option is granted; or (ii) 85% of the closing price at the time the option is exercised. Shares are generally issued on June 30 and December 31 of each year. Agent Plan Quarterly offerings to purchase the Parent's common stock at a 10% discount with a one year restricted period during which the shares purchased cannot be sold or transferred. Only our independent retail insurance agencies and wholesale general agencies, and certain eligible persons associated with the agencies, are eligible to participate in this plan. Shares authorized and available for issuance as of December 31, 2017 are as follows: As of December 31, 2017 Authorized Available for Issuance Awards Outstanding Stock Plan 3,500,000 2,516,871 882,490 ESPP 1,500,000 499,629 — Agent Plan 3,000,000 1,817,493 — Retired Plans The following plans are closed for the issuance of new awards, although awards outstanding continue in effect according to the terms of the applicable award agreements: December 31, 2017 Types of Share-Based Payments Issued Reserve Shares Awards Outstanding 1 Plan 2005 Omnibus Stock Plan ("2005 Stock Plan") Qualified and nonqualified stock options, SARs, restricted stock, RSUs, phantom stock, stock bonuses, and other awards in such amounts and with such terms and conditions as it determined, subject to the provisions of the 2005 Stock Plan. The maximum exercise period for an option grant under this plan is 10 years from the date of the grant. DEUs are earned during the vesting period on RSU grants. The DEUs are reinvested in the Parent's common stock at fair value on each dividend payment date. 2,202,532 277,132 Parent's Stock Compensation Plan for Non-employee Directors ("Directors Stock Compensation Plan") Directors could elect to receive a portion of their annual compensation in shares of the Parent's common stock. 66,506 66,506 1 Awards outstanding under the 2005 Stock Plan consisted of 47,268 RSUs and 229,864 stock options. RSU Transactions A summary of the RSU transactions under our share-based payment plans is as follows: Number Weighted Unvested RSU awards at December 31, 2016 916,640 $ 26.20 Granted in 2017 321,928 42.66 Vested in 2017 (360,702 ) 22.78 Forfeited in 2017 (12,279 ) 32.09 Unvested RSU awards at December 31, 2017 865,587 $ 33.66 As of December 31, 2017 , total unrecognized compensation expense related to unvested RSU awards granted under our stock plans was $8.0 million . That expense is expected to be recognized over a weighted-average period of 1.8 years . The total intrinsic value of RSUs vested was $16.0 million for 2017 , $12.6 million for 2016 , and $10.3 million for 2015 . In connection with vested RSUs, the total value of the DEU shares that vested was $0.9 million during 2017 and $0.7 million in 2016 and 2015 . Option Transactions A summary of the stock option transactions under our share-based payment plans is as follows: Number Weighted Weighted Aggregate Outstanding at December 31, 2016 355,391 $ 16.87 Granted in 2017 — — Exercised in 2017 (120,496 ) 19.39 Forfeited or expired in 2017 (5,031 ) 24.54 Outstanding at December 31, 2017 229,864 $ 15.38 1.50 $ 9,958 Exercisable at December 31, 2017 229,864 $ 15.38 1.50 $ 9,958 The total intrinsic value of options exercised was $4.0 million in 2017 , $2.3 million in 2016 , and $2.2 million in 2015 . CIU Transactions The liability recorded in connection with our Cash Plan was $37.0 million at December 31, 2017 and $32.0 million at December 31, 2016 . The remaining cost associated with the CIUs is expected to be recognized over a weighted average period of 0.9 years . The CIU payments made were $14.2 million in 2017 , $14.3 million in 2016 , and $10.2 million in 2015 . ESPP and Agent Plan Transactions A summary of ESPP and Agent Plan share issuances is as follows: 2017 2016 2015 ESPP Issuances 75,093 88,432 100,944 Agent Plan Issuances 49,794 69,867 82,142 Fair Value Measurements The grant date fair value of RSUs is based on the market price of our common stock on the grant date, adjusted for the present value of our expected dividend payments. The expense recognized for share-based awards is based on the number of shares or units expected to be issued at the end of the performance period and the grant date fair value. The grant date fair value of each option award is estimated using the Black Scholes option valuation model ("Black Scholes"). The following are the significant assumptions used in applying Black Scholes: (i) the risk-free interest rate, which is the implied yield currently available on U.S. Treasury zero-coupon issues with an equal remaining term; (ii) the expected term, which is based on historical experience of similar awards; (iii) the dividend yield, which is determined by dividing the expected per share dividend during the coming year by the grant date stock price; and (iv) the expected volatility, which is based on the volatility of the Parent's stock price over a historical period comparable to the expected term. In applying Black Scholes, we use the weighted average assumptions illustrated in the following table: ESPP 2017 2016 2015 Risk-free interest rate 1.07 % 0.47 0.10 Expected term 6 months 6 months 6 months Dividend yield 1.3 % 1.7 2.0 Expected volatility 24 % 31 20 The weighted-average fair value of options and stock per share, including RSUs granted for the Parent's stock plans, during 2017 , 2016 , and 2015 was as follows: 2017 2016 2015 RSUs $ 42.66 32.53 25.22 ESPP: Six month option 2.73 2.63 1.26 Discount of grant date market value 7.06 5.23 4.16 Total ESPP 9.79 7.86 5.42 Agent Plan: Discount of grant date market value 5.04 3.79 2.94 The fair value of the CIU liability is remeasured at each reporting period through the settlement date of the awards, which is three years from the date of grant based on an amount expected to be paid. A Monte Carlo simulation is performed to approximate the projected fair value of the CIUs that, in accordance with the Cash Plan, is adjusted to reflect our performance on specified indicators as compared to targeted peer companies. Expense Recognition The following table provides share-based compensation expense in 2017 , 2016 , and 2015 : ($ in millions) 2017 2016 2015 Share-based compensation expense, pre-tax $ 31.2 30.3 23.8 Income tax benefit, including the benefit related to stock grants that have vested during the year (15.0 ) (10.3 ) (8.0 ) Share-based compensation expense, after-tax $ 16.2 20.0 15.8

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions William M. Rue, a Director of the Parent, is Chairman of, and owns more than 10% of the equity of, Chas. E. Rue & Son, Inc., t/a Rue Insurance, a general independent retail insurance agency ("Rue Insurance"). Rue Insurance is an appointed distribution partner of the Insurance Subsidiaries on terms and conditions similar to those of our other distribution partners, which includes the right to participate in the Agent Plan. Mr. Rue’s son is President, and an employee, of Rue Insurance and Mr. Rue’s daughter is an employee of Rue Insurance. Our relationship with Rue Insurance has existed since 1928. Rue Insurance placed insurance policies with the Insurance Subsidiaries for its customers and itself. Direct premiums written associated with these policies were $11.1 million in 2017 , $10.4 million in 2016 , and $9.6 million in 2015 . In return, the Insurance Subsidiaries paid standard market commissions, including supplemental commissions, to Rue Insurance of $2.3 million in 2017 , $2.1 million in 2016 , and $1.7 million in 2015 . Amounts due to Rue Insurance at December 31, 2017 and December 31, 2016 were $0.6 million and $0.7 million , respectively. In 2005, we established a private foundation, now named The Selective Insurance Group Foundation (the "Foundation"), under Section 501(c)(3) of the Internal Revenue Code. The Board of Directors of the Foundation is comprised of some of the Parent's officers. We made less than $0.1 million of contributions and no contributions to the Foundation in 2017 and 2016 , respectively. We made contributions to the Foundation in the amount of $1.0 million in 2015 . BlackRock, Inc., a leading publicly traded investment management firm (“BlackRock”), has purchased our common shares in the ordinary course of its investment business and has previously filed Schedules 13G/A with the SEC. On January 19, 2018, BlackRock filed a Schedule 13G/A reporting beneficial ownership as of December 31, 2017, of 12.8% of our common stock. In connection with purchasing our common shares, BlackRock filed the necessary filings with insurance regulatory authorities. On the basis of those filings, BlackRock is deemed not to be a controlling person for the purposes of applicable insurance law. We are required to disclose related party information for our transactions with BlackRock. BlackRock is highly regulated, serves its clients as a fiduciary, and has a diverse platform of active (alpha) and index (beta) investment strategies across asset classes that enables it to tailor investment outcomes and asset allocation solutions for clients. BlackRock also offers the BlackRock Solutions ® investment and risk management technology platform, Aladdin ® , risk analytics, advisory, and technology services and solutions to a broad base of institutional and wealth management investors. In 2017 and 2016, we incurred expenses related to BlackRock of $2.0 million and $0.4 million , respectively, for services rendered. Amounts payable for such services at December 31, 2017 and December 31, 2016, were $0.5 million and $0.4 million , respectively. All contracts with BlackRock were consummated in the ordinary course of business on an arm's-length basis.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies (a) We purchase annuities from life insurance companies to fulfill obligations under claim settlements that provide for periodic future payments to claimants. As of December 31, 2017 , we had purchased such annuities with a present value of $18.5 million for settlement of claims on a structured basis for which we are contingently liable. To our knowledge, there are no material defaults from any of the issuers of such annuities. (b) We have various operating leases for office space, equipment, and fleet vehicles. Such lease agreements, which expire at various times, are generally renewed or replaced by similar leases. Rental expense under these leases amounted to $10.8 million in 2017 , $12.3 million in 2016 , and $11.7 million in 2015 . We also lease computer hardware and software under capital lease agreements expiring at various dates through 2019 . See item (p) of Note 2. "Summary of Significant Accounting Policies" in this Form 10-K for information on our accounting policy regarding leases. In addition, certain of these leases are non-cancelable, and liability for payment will continue even though the leased asset may no longer be in use. At December 31, 2017 , the total future minimum rental commitments under non-cancelable leases were as follows: ($ in millions) Capital Leases Operating Leases Total 2018 $ 2.3 10.0 12.3 2019 0.1 7.5 7.6 2020 — 6.0 6.0 2021 — 3.7 3.7 2022 — 2.1 2.1 After 2022 — 2.6 2.6 Total minimum payment required $ 2.4 31.9 34.3 (c) As of December 31, 2017 , we had contractual obligations that expire at various dates through 2032 to invest up to an additional $221 million in alternative and other investments. There is no certainty that any such additional investment will be required. For additional information regarding these investments, see item (f) of Note 5. "Investments" in this Form 10-K. In addition, as of December 31, 2017 , we had contractual obligations that expire in 2023 to invest $16.3 million in a non-publicly traded common stock within our available-for-sale portfolio. We expect to have the capacity to repay and/or refinance these obligations as they become due.

Litigation

Litigation	12 Months Ended
	Dec. 31, 2017
Loss Contingency, Information about Litigation Matters [Abstract]
Litigation	Litigation In the ordinary course of conducting business, we are named as defendants in various legal proceedings. Most of these proceedings are claims litigation involving our Insurance Subsidiaries as either: (i) liability insurers defending or providing indemnity for third-party claims brought against our customers; or (ii) insurers defending first-party coverage claims brought against them. We account for such activity through the establishment of unpaid losses and loss expense reserves. We expect that any potential ultimate liability in such ordinary course claims litigation will not be material to our consolidated financial condition, results of operations, or cash flows after consideration of provisions made for potential losses and costs of defense. From time to time, our Insurance Subsidiaries also are named as defendants in other legal actions, some of which assert claims for substantial amounts. These actions include, among others, putative class actions seeking certification of a state or national class. Such putative class actions have alleged, for example, improper reimbursement of medical providers paid under workers compensation and personal and commercial automobile insurance policies. Similarly, our Insurance Subsidiaries are also named from time-to-time in individual actions seeking extra-contractual damages, punitive damages, or penalties, some of which allege bad faith in the handling of insurance claims. We believe that we have valid defenses to these cases. We expect that any potential ultimate liability in any such lawsuit will not be material to our consolidated financial condition, after consideration of provisions made for estimated losses. Nonetheless, given the inherent unpredictability of litigation and the large or indeterminate amounts sought in certain of these actions, an adverse outcome in certain matters could possibly have a material adverse effect on our consolidated results of operations or cash flows in particular quarterly or annual periods. As of December 31, 2017 , we do not believe the Company was involved in any legal action that could have a material adverse effect on our consolidated financial condition, results of operations, or cash flows.

Statutory Financial Information

Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Statutory Financial Information, Capital Requirments, and Retrictions on Dividends and Transfers of Funds	Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (a) Statutory Financial Information The Insurance Subsidiaries prepare their statutory financial statements in accordance with accounting principles prescribed or permitted by the various state insurance departments of domicile. Prescribed statutory accounting principles include state laws, regulations, and general administrative rules, as well as a variety of publications of the National Association of Insurance Commissioners (“NAIC"). Permitted statutory accounting principles encompass all accounting principles that are not prescribed; such principles differ from state to state, may differ from company to company within a state and may change in the future. The Insurance Subsidiaries do not utilize any permitted statutory accounting principles that materially affect the determination of statutory surplus, statutory net income, or risk-based capital (“RBC”). As of December 31, 2017 , the various state insurance departments of domicile have adopted the March 2017 version of the NAIC Accounting Practices and Procedures manual in its entirety, as a component of prescribed or permitted practices. The following table provides statutory data for each of our Insurance Subsidiaries: State of Domicile Unassigned Surplus Statutory Surplus Statutory Net Income ($ in millions) 2017 2016 2017 2016 2017 2016 2015 SICA New Jersey $ 455.5 414.4 609.7 568.6 84.6 72.2 69.6 Selective Way Insurance Company ("SWIC") New Jersey 276.1 260.5 325.1 309.5 43.6 41.2 42.3 SICSC Indiana 112.9 110.6 144.1 141.9 17.9 17.4 15.9 SICSE Indiana 86.2 83.5 111.8 109.1 14.7 13.4 12.1 SICNY New York 78.8 74.1 106.5 101.8 13.4 12.9 12.7 Selective Insurance Company of New England ("SICNE") New Jersey 16.1 13.6 46.3 43.7 6.3 5.9 5.5 Selective Auto Insurance Company of New Jersey ("SAICNJ") New Jersey 42.1 36.9 84.9 79.8 11.4 11.5 10.8 MUSIC New Jersey 21.4 16.7 89.9 85.2 10.3 9.7 9.5 Selective Casualty Insurance Company ("SCIC") New Jersey 34.5 26.6 109.0 101.0 13.4 12.6 12.1 Selective Fire and Casualty Insurance Company ("SFCIC") New Jersey 13.7 11.3 45.6 43.2 5.6 5.5 5.3 Total $ 1,137.3 1,048.2 1,672.9 1,583.8 221.2 202.3 195.8 (b) Capital Requirements The Insurance Subsidiaries are required to maintain certain minimum amounts of statutory surplus to satisfy the requirements of their various state insurance departments of domicile. RBC requirements for property and casualty insurance companies are designed to assess capital adequacy and to raise the level of protection that statutory surplus provides for policyholders. The Insurance Subsidiaries' combined total adjusted capital exceeded the authorized control level RBC, as defined by the NAIC based on their 2017 statutory financial statements. In addition to statutory capital requirements, we are impacted by various rating agency requirements related to certain rating levels. These required capital levels may be more than statutory requirements. (c) Restrictions on Dividends and Transfers of Funds Our ability to declare and pay dividends on the Parent's common stock is dependent on liquidity at the Parent coupled with the ability of the Insurance Subsidiaries to declare and pay dividends, if necessary, and/or the availability of other sources of liquidity to the Parent. As of December 31, 2017 , the Parent had an aggregate of $114.5 million in investments and cash available to fund future dividends and interest payments. These amounts are not subject to any regulatory restrictions other than standard state insolvency restrictions, whereas our consolidated retained earnings of $1.7 billion is predominately restricted due to the regulation associated with our Insurance Subsidiaries. In 2018 , the Insurance Subsidiaries have the ability to provide for $211.0 million in annual dividends to the Parent; however, as regulated entities, these dividends are subject to certain restrictions, which are further discussed below. The Parent also has available to it other potential sources of liquidity, such as: (i) borrowings from our Indiana Subsidiaries; (ii) debt issuances; (iii) common stock issuances; and (iv) borrowings under our Line of Credit. Borrowings from our Indiana Subsidiaries are governed by approved intercompany lending agreements with the Parent that provide for additional capacity of $70.5 million as of December 31, 2017 , based on restrictions in these agreements that limit borrowings to 10% of the admitted assets of the Indiana Subsidiaries. For additional restrictions on the Parent's debt, see Note 10. "Indebtedness" in this Form 10-K. Insurance Subsidiaries Dividend Restrictions As noted above, the restriction on our net assets and retained earnings is predominantly driven by our Insurance Subsidiaries' ability to pay dividends to the Parent under applicable law and regulations. Under the insurance laws of the domiciliary states of the Insurance Subsidiaries, New Jersey, Indiana, and New York, an insurer can potentially make an ordinary dividend payment if its statutory surplus following such dividend is reasonable in relation to its outstanding liabilities, is adequate to its financial needs, and the dividend does not exceed the insurer's unassigned surplus. In general, New Jersey defines an ordinary dividend as a dividend whose fair market value, together with other dividends made within the preceding 12 months, is less than the greater of 10% of the insurer's statutory surplus as of the preceding December 31, or the insurer's net income (excluding capital gains) for the 12-month period ending on the preceding December 31 . Indiana's ordinary dividend calculation is consistent with New Jersey's, except that it does not exclude capital gains from net income. In general, New York defines an ordinary dividend as a dividend whose fair market value, together with other dividends made within the preceding 12 months, is less than the lesser of 10% of the insurer's statutory surplus, or 100% of adjusted net investment income. New Jersey and Indiana require notice of the declaration of any ordinary dividend distribution. During the notice period, the relevant state regulatory authority may disallow all or part of the proposed dividend if it determines that the dividend is not appropriate given the above considerations. New York does not require notice of ordinary dividends. Dividend payments exceeding ordinary dividends are referred to as extraordinary dividends and require review and approval by the applicable domiciliary insurance regulatory authority prior to payment. The following table provides quantitative data regarding all Insurance Subsidiaries' dividends paid to the Parent in 2017 for debt service, shareholder dividends, and general operating purposes: Dividends Twelve Months ended December 31, 2017 ($ in millions) State of Domicile Ordinary Dividends Paid SICA New Jersey $ 28.0 SWIC New Jersey 19.0 SICSC Indiana 10.0 SICSE Indiana 7.5 SICNY New York 4.5 SICNE New Jersey 2.0 SAICNJ New Jersey 2.5 MUSIC New Jersey 2.1 SCIC New Jersey 3.0 SFCIC New Jersey 1.5 Total $ 80.1 Based on the 2017 statutory financial statements, the maximum ordinary dividends that can be paid to the Parent by the Insurance Subsidiaries in 2018 are as follows: 2018 ($ in millions) State of Domicile Maximum Ordinary Dividends SICA New Jersey $ 77.6 SWIC New Jersey 43.3 SICSC Indiana 17.9 SICSE Indiana 14.7 SICNY New York 10.7 SICNE New Jersey 6.2 SAICNJ New Jersey 11.3 MUSIC New Jersey 10.3 SCIC New Jersey 13.4 SFCIC New Jersey 5.6 Total $ 211.0

Quarterly Financial Information

Quarterly Financial Information	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	Quarterly Financial Information (unaudited, $ in thousands, First Quarter Second Quarter Third Quarter Fourth Quarter except per share data) 2017 2016 2017 2016 2017 2016 2017 1 2016 Net premiums earned $ 560,854 522,458 568,030 531,932 572,055 542,429 590,088 552,753 Net investment income earned 37,419 30,769 41,430 31,182 40,446 33,375 42,587 35,428 Net realized (losses) gains (1,045 ) (2,704 ) 1,734 1,765 6,798 3,688 (1,128 ) (7,686 ) Other income 3,241 951 3,291 3,868 1,994 2,199 2,190 1,863 Total revenues 600,469 551,474 614,485 568,747 621,293 581,691 633,737 582,358 Income before federal income taxes 67,574 51,875 58,929 62,311 67,315 55,443 68,150 50,326 Net income 50,440 37,032 41,426 43,601 46,718 38,502 30,242 39,360 Net income per share: Basic 0.87 0.64 0.71 0.75 0.80 0.66 0.52 0.68 Diluted 0.85 0.63 0.70 0.74 0.79 0.66 0.51 0.67 1 Results for the fourth quarter of 2017 include the impact of the $20.2 million write off of deferred tax assets required with the implementation of Tax Reform. See Note 13. "Federal Income Taxes" above for additional information. The addition of all quarters may not agree to annual amounts on the Financial Statements due to rounding.

Subsequent Events (Notes)

Subsequent Events (Notes)	12 Months Ended
	Dec. 31, 2017
Subsequent Event [Line Items]
Subsequent Events [Text Block]	Subsequent to year-end and through the end of January 2018, our insurance operations experienced significant insured property losses, principally due to the deep freeze that impacted our footprint states during the month, the Property Claims Services ("PCS") named winter storm that occurred between January 3 and January 6, and a relatively large number of severe fire losses. For January 2018, non-catastrophe property losses amounted to $47 million and catastrophe losses, which we define as only those losses specifically attributable to a named PCS catastrophe, totaled $16 million . In total, the $63 million of insured property losses were approximately $30 million in excess of our property loss expectations for the month of January.

Schedule I - Summay of Investme

Schedule I - Summay of Investments - Other Than Investments in Related Parties	12 Months Ended
	Dec. 31, 2017
Summary of Investments, Other than Investments in Related Parties [Abstract]
Schedule I - Summary of Investments- Other than Investments in Related Parties	Types of investment ($ in thousands) Amortized Cost or Cost Fair Value Carrying Amount Fixed income securities: Held-to-maturity: Obligations of states and political subdivisions $ 25,154 26,261 25,238 Public utilities 7,466 7,956 7,443 All other corporate securities 9,530 9,883 9,448 Total fixed income securities, held-to-maturity 42,150 44,100 42,129 Available-for-sale: U.S. government and government agencies 49,326 49,740 49,740 Foreign government 18,040 18,555 18,555 Obligations of states and political subdivisions 1,539,307 1,582,970 1,582,970 Public utilities 50,071 51,035 51,035 All other corporate securities 1,538,268 1,566,433 1,566,433 Collateralized loan obligation securities and other asset-backed securities 789,152 795,458 795,458 Commercial mortgage-backed securities 382,727 383,449 383,449 Residential mortgage-backed securities 709,825 714,882 714,882 Total fixed income securities, available-for-sale 5,076,716 5,162,522 5,162,522 Equity securities: Common stock: Public utilities 5,957 6,156 6,156 Banks, trusts and insurance companies 34,301 40,510 40,510 Industrial, miscellaneous and all other 89,438 121,091 121,091 Total common stock, available-for-sale 129,696 167,757 167,757 Preferred stock: Banks, trusts and insurance companies 14,115 14,948 14,948 Total preferred stock, available-for-sale 14,115 14,948 14,948 Total equity securities, available-for-sale 143,811 182,705 182,705 Short-term investments 165,555 165,555 165,555 Other investments 132,268 132,268 Total investments $ 5,560,500 5,685,179 See accompanying Report of Independent Registered Public Accounting Firm in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K.

Schedule II - Parent Corporatio

Schedule II - Parent Corporation Finanical Statements	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule II- Parent Corporation Financial Statements	SCHEDULE II SELECTIVE INSURANCE GROUP, INC. (Parent Corporation) Balance Sheets December 31, ($ in thousands, except share amounts) 2017 2016 Assets: Fixed income securities, available-for-sale – at fair value (amortized cost: $89,799 – 2017; $73,471 – 2016) $ 89,872 73,509 Short-term investments 24,080 17,777 Cash 534 458 Investment in subsidiaries 2,013,304 1,845,410 Current federal income tax 22,266 19,766 Deferred federal income tax 13,239 19,562 Other assets 871 840 Total assets $ 2,164,166 1,977,322 Liabilities: Long-term debt $ 329,116 328,667 Intercompany notes payable 78,443 79,324 Accrued long-term stock compensation 37,017 32,029 Other liabilities 6,633 5,932 Total liabilities $ 451,209 445,952 Stockholders’ Equity: Preferred stock at $0 par value per share: Authorized shares 5,000,000; no shares issued or outstanding $ — — Common stock of $2 par value per share: Authorized shares: 360,000,000 Issued: 102,284,564 – 2017; 101,620,436 – 2016 204,569 203,241 Additional paid-in capital 367,717 347,295 Retained earnings 1,698,613 1,568,881 Accumulated other comprehensive income (loss) 20,170 (15,950 ) Treasury stock – at cost (shares: 43,789,442 – 2017; 43,653,237 – 2016) (578,112 ) (572,097 ) Total stockholders’ equity 1,712,957 1,531,370 Total liabilities and stockholders’ equity $ 2,164,166 1,977,322 See accompanying Report of Independent Registered Public Accounting Firm. Information should be read in conjunction with the Notes to Consolidated Financial Statements of Selective Insurance Group, Inc. and its subsidiaries. Both items are in Item 8. “Financial Statements and Supplementary Data.” of this Form 10-K. SCHEDULE II (continued) SELECTIVE INSURANCE GROUP, INC. (Parent Corporation) Statements of Income Year ended December 31, ($ in thousands) 2017 2016 2015 Revenues: Dividends from subsidiaries $ 80,096 61,014 57,752 Net investment income earned 2,044 1,259 852 Net realized losses (15 ) (220 ) — Total revenues 82,125 62,053 58,604 Expenses: Interest expense 24,721 24,030 24,057 Other expenses 36,251 35,020 28,393 Total expenses 60,972 59,050 52,450 Income before federal income tax 21,153 3,003 6,154 Federal income tax (benefit) expense: Current (22,187 ) (17,924 ) (16,609 ) Deferred 6,311 (2,143 ) (1,603 ) Total federal income tax benefit (15,876 ) (20,067 ) (18,212 ) Net income before equity in undistributed income of subsidiaries 37,029 23,070 24,366 Equity in undistributed income of subsidiaries, net of tax 131,797 135,425 141,495 Net income $ 168,826 158,495 165,861 See accompanying Report of Independent Registered Public Accounting Firm. Information should be read in conjunction with the Notes to Consolidated Financial Statements of Selective Insurance Group, Inc. and its subsidiaries. Both items are in Item 8. “Financial Statements and Supplementary Data.” of this Form 10-K. SCHEDULE II (continued) SELECTIVE INSURANCE GROUP, INC. (Parent Corporation) Statements of Cash Flows Year ended December 31, ($ in thousands) 2017 2016 2015 Operating Activities: Net income $ 168,826 158,495 165,861 Adjustments to reconcile net income to net cash provided by operating activities: Equity in undistributed income of subsidiaries, net of tax (131,797 ) (135,425 ) (141,495 ) Stock-based compensation expense 12,089 10,449 8,973 Net realized losses 15 220 — Amortization – other 678 648 740 Changes in assets and liabilities: Increase in accrued long-term stock compensation 4,988 5,564 4,575 Decrease (increase) in net federal income taxes 3,811 (3,612 ) (3,052 ) Decrease in other assets (60 ) (202 ) (12 ) Increase (decrease) in other liabilities 714 80 (202 ) Net cash provided by operating activities 59,264 36,217 35,388 Investing Activities: Purchase of fixed income securities, available-for-sale (58,832 ) (45,789 ) (33,717 ) Redemption and maturities of fixed income securities, available-for-sale 10,465 14,983 21,578 Sale of fixed income securities, available-for-sale 31,819 18,768 — Purchase of short-term investments (185,590 ) (119,501 ) (106,933 ) Sale of short-term investments 179,292 130,841 94,422 Net cash used in investing activities (22,846 ) (698 ) (24,650 ) Financing Activities: Dividends to stockholders (37,045 ) (33,758 ) (31,052 ) Acquisition of treasury stock (6,015 ) (4,992 ) (4,182 ) Net proceeds from stock purchase and compensation plans 7,599 7,811 10,089 Excess tax benefits from share-based payment arrangements — 1,819 1,736 Principal payment on borrowings from subsidiaries (881 ) (6,839 ) (2,798 ) Net cash used in financing activities (36,342 ) (35,959 ) (26,207 ) Net increase (decrease) in cash 76 (440 ) (15,469 ) Cash, beginning of year 458 898 16,367 Cash, end of year $ 534 458 898 See accompanying Report of Independent Registered Public Accounting Firm. Information should be read in conjunction with the Notes to Consolidated Financial Statements of Selective Insurance Group, Inc. and its subsidiaries. Both items are in Item 8. “Financial Statements and Supplementary Data.” of this Form 10-K.

Schedule III - Supplementary In

Schedule III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
Schedule III- Supplementary Insurance Information	($ in thousands) Deferred policy acquisition costs Reserve for loss and loss expense Unearned premiums Net premiums earned Net investment income 1 Loss and loss expense incurred Amortization of deferred policy acquisition costs Other operating expenses 2 Net premiums written Standard Commercial Lines Segment $ 193,408 3,165,217 956,173 1,788,499 — 1,008,150 387,552 243,283 1,858,735 Standard Personal Lines Segment 16,952 263,166 295,435 289,701 — 189,294 32,542 56,761 296,775 E&S Lines Segment 24,695 342,857 98,036 212,827 — 147,630 49,142 22,337 215,131 Investments Segment — — — — 168,241 — — — — Total $ 235,055 3,771,240 1,349,644 2,291,027 168,241 1,345,074 469,236 322,381 2,370,641 1 Includes “Net investment income earned” and “Net realized investment gains” on the Consolidated Statements of Income. 2 “Other operating expenses” of $322,381 reconciles to the Consolidated Statements of Income as follows: Other insurance expenses $ 333,097 Other income (10,716 ) Total $ 322,381 See accompanying Report of Independent Registered Public Accounting Firm in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K. Year ended December 31, 2016 ($ in thousands) Deferred policy acquisition costs Reserve for loss and loss expense Unearned premiums Net premiums earned Net investment income 1 Loss and loss expense incurred Amortization of deferred policy acquisition costs Other operating expenses 2 Net premiums written Standard Commercial Lines Segment $ 181,193 3,098,554 884,976 1,665,483 — 913,506 367,813 237,729 1,745,782 Standard Personal Lines Segment 16,664 286,081 282,111 280,607 — 177,749 34,105 56,334 281,822 E&S Lines Segment 24,707 307,084 95,732 203,482 — 143,542 48,410 18,451 209,684 Investments Segment — — — — 125,817 — — — — Total $ 222,564 3,691,719 1,262,819 2,149,572 125,817 1,234,797 450,328 312,514 2,237,288 1 Includes “Net investment income earned” and “Net realized investment gains” on the Consolidated Statements of Income. 2 “Other operating expenses” of $312,514 reconciles to the Consolidated Statements of Income as follows: Other insurance expenses $ 321,395 Other income (8,881 ) Total $ 312,514 See accompanying Report of Independent Registered Public Accounting Firm in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K. SCHEDULE III (continued) SELECTIVE INSURANCE GROUP, INC. AND CONSOLIDATED SUBSIDIARIES SUPPLEMENTARY INSURANCE INFORMATION Year ended December 31, 2015 ($ in thousands) Deferred policy acquisition costs Reserve for loss and loss expense Unearned premiums Net premiums earned Net investment income 1 Loss and loss expense incurred Amortization of deferred policy acquisition costs Other operating expenses 2 Net premiums written Standard Commercial Lines Segment $ 171,476 2,998,749 803,648 1,529,442 — 819,573 323,754 221,619 1,596,965 Standard Personal Lines Segment 17,258 265,054 276,533 288,134 — 200,237 33,638 52,923 283,926 E&S Lines Segment 24,425 253,925 89,529 172,333 — 128,731 42,044 18,361 189,013 Investments Segment — — — — 134,487 — — — — Total $ 213,159 3,517,728 1,169,710 1,989,909 134,487 1,148,541 399,436 292,903 2,069,904 1 Includes “Net investment income earned” and “Net realized investment gains” on the Consolidated Statements of Income. 2 “Other operating expenses” of $292,903 reconciles to the Consolidated Statements of Income as follows: Other insurance expenses $ 300,359 Other income (7,456 ) Total $ 292,903 See accompanying Report of Independent Registered Public Accounting Firm in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K.

Schedule IV - Reinsurance

Schedule IV - Reinsurance	12 Months Ended
	Dec. 31, 2017
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Schedule IV - Reinsurance	($ thousands) Direct Amount Assumed from Other Companies Ceded to Other Companies Net Amount % of Amount Assumed to Net 2017 Premiums earned: Accident and health insurance $ 24 — 24 — — Property and liability insurance 2,647,464 25,831 382,268 2,291,027 1 % Total premiums earned 2,647,488 25,831 382,292 2,291,027 1 % 2016 Premiums earned: Accident and health insurance $ 32 — — 32 — Property and liability insurance 2,484,683 28,214 363,357 2,149,540 1 % Total premiums earned 2,484,715 28,214 363,357 2,149,572 1 % 2015 Premiums earned: Accident and health insurance $ 37 — 37 — — Property and liability insurance 2,330,230 23,209 363,530 1,989,909 1 % Total premiums earned 2,330,267 23,209 363,567 1,989,909 1 % See accompanying Report of Independent Registered Public Accounting Firm in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K.

Schedule V - Allowance for Unco

Schedule V - Allowance for Uncollectible Premiums and Other Receivables	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule V - Allowance for Uncollectible Premiums and Other Receivables	($ in thousands) 2017 2016 2015 Balance, January 1 $ 11,480 10,122 11,037 Additions 6,414 4,669 3,604 Deductions (3,294 ) (3,311 ) (4,519 ) Balance, December 31 $ 14,600 11,480 10,122 See accompanying Report of Independent Registered Public Accounting Firm in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K.

Summary of Significant Accoun35

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements (“Financial Statements”) include the accounts of the Parent and its subsidiaries, and have been prepared in conformity with: (i) U.S. generally accepted accounting principles ("GAAP"); and (ii) the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). All significant intercompany accounts and transactions are eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of our Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported financial statement balances, as well as the disclosure of contingent assets and liabilities. Actual results could differ from those estimates.
Reclassifications	Reclassifications In 2017 , we reclassified certain line items within our Consolidated Statements of Income to enhance the ability to analyze our expenses. Specifically, we reclassified our insurance underwriting expenses into amortization of deferred policy acquisition costs and other insurance expenses. These expenses were previously included in policy acquisition costs and other expenses. In addition, all expenses of the Parent, which were previously included in other expenses, are now separately identifiable as corporate expenses on the Consolidated Statements of Income. All prior periods presented in this Form 10-K have been reclassified to reflect this change.
Investments	Investments Fixed income securities may include investment grade and below investment grade rated bonds, redeemable preferred stocks, non-redeemable preferred stocks with certain debt-like characteristics, mortgage-backed securities (“MBS”), collateralized loan obligations ("CLO"), and other asset-backed securities (“ABS”). MBS, CLO, and other ABS are jointly referred to as structured securities. Fixed income securities classified as available-for-sale (“AFS”) are reported at fair value. Those fixed income securities that we have the ability and positive intent to hold to maturity are classified as held-to-maturity (“HTM”) and are carried at either: (i) amortized cost; or (ii) market value at the date of transfer into the HTM category, adjusted for subsequent amortization. The amortized cost of fixed income securities is adjusted for the amortization of premiums and the accretion of discounts over the expected life of the security using the effective yield method. Premiums and discounts arising from the purchase of structured securities are amortized over the expected life of the security based on future principal payments, giving additional consideration to prepayments. These prepayments are estimated based on historical and projected cash flows. Prepayment assumptions are reviewed quarterly and adjusted to reflect actual prepayments and changes in expectations. Future amortization of any premium and/or discount is adjusted to reflect the revised assumptions. Interest income, as well as amortization and accretion, is included in "Net investment income earned" on our Consolidated Statements of Income. The amortized cost of a fixed income security is written down to fair value when a decline in value is considered to be other than temporary. See the discussion below on realized investment gains and losses for a description of the accounting for impairments. After-tax unrealized gains and losses on: (i) fixed income securities classified as AFS; and (ii) fixed income securities that were transferred into an HTM designation from an AFS designation, are included in accumulated other comprehensive income (loss) ("AOCI"). Equity securities, which are classified as AFS, may include common and non-redeemable preferred stocks. These securities are carried at fair value and the related dividend income is included in "Net investment income earned" on our Consolidated Statements of Income. The cost of equity securities is written down to fair value when a decline in value is considered to be other than temporary. See the discussion below on realized investment gains and losses for a description of the accounting for impairments. After-tax unrealized gains and losses are included in AOCI. Short-term investments may include certain money market instruments, savings accounts, commercial paper, and debt issues purchased with a maturity of less than one year. We also enter into reverse repurchase agreements that are included in short-term investments. These loans are fully collateralized with high quality, readily marketable instruments at a minimum of 102% of the loan principal. At maturity, we receive principal and interest income on these agreements. All short-term investments are carried at cost, which approximates fair value. The associated income is included in "Net investment income earned" on our Consolidated Statements of Income. Other investments may include alternative investments and other securities. Alternative investments are accounted for using the equity method. Our share of distributed and undistributed net income from alternative investments is included in "Net investment income earned" on our Consolidated Statements of Income. Other securities are primarily comprised of tax credit investments. Low income housing tax credits are accounted for under the proportional amortization method and all other tax credits are accounted for using the equity method. Under the proportional amortization method, our share of the investment’s performance is recorded in our Consolidated Statements of Income as a component of “Federal income tax expense.” Under the equity method, our share of distributed and undistributed net income is included in "Net investment income earned" on our Consolidated Statements of Income. For federal income tax credits accounted for under the equity method, we use the deferral method for recognizing the benefit of the tax credit with the related deferred revenue being recognized in our Consolidated Statements of Income as a component of "Federal income tax expense" proportionately over the life of the investment. We evaluate the alternative investments and tax credit investments included in our other investments portfolio to determine whether those investments are variable interest entities ("VIEs") and if so, whether consolidation is required. A VIE is an entity that either has equity investors that lack certain essential characteristics of a controlling financial interest or lack sufficient funds to finance its own activities without financial support provided by other entities. We consider several significant factors in determining if our investments are VIEs and if we are the primary beneficiary, including whether we have: (i) the power to direct activities of the VIE; (ii) the ability to remove the decision maker of the VIE; (iii) the ability to participate in making decisions that are significant to the VIE; and (iv) the obligation to absorb losses and the right to receive benefits that could potentially be significant to the VIE. We have reviewed our alternative and tax credit investments and have concluded that they are VIEs, but that we are not the primary beneficiary and therefore, consolidation is not required. Realized gains and losses on the sale of investments are determined on the basis of the cost of the specific investments sold and are credited or charged to income. Included in realized gains and losses are the other-than-temporary impairment ("OTTI") charges recognized in earnings, which are discussed below. On a quarterly basis, we review our investment portfolio for impairments that are other than temporary. Interest-related unrealized losses typically do not result in other-than-temporary impairments. The following provides information on this analysis for our fixed income securities and short-term investments, equity securities, and other investments. Fixed Income Securities and Short-Term Investments We review securities that are in an unrealized loss position to determine: (i) if we have the intent to sell the security; (ii) if it is more likely than not that we will be required to sell the debt security before its anticipated recovery; and (iii) if the decline is other than temporary. Broad changes in the overall market or interest rate environment generally will not lead to a write down. If we determine that we have either the intent or requirement to sell the security, we write down its amortized cost to its fair value through a charge to earnings as a component of realized losses. If we do not have either the intent or requirement to sell the security, our evaluation for OTTI may include, but is not limited to, evaluation of the following factors: • Whether the decline appears to be issuer or industry specific; • The degree to which the issuer is current or in arrears in making principal and interest payments on the fixed income security; • The issuer’s current financial condition and ability to make future scheduled principal and interest payments on a timely basis; • Evaluation of projected cash flows; • Buy/hold/sell recommendations published by outside investment advisors and analysts; and • Relevant rating history, analysis, and guidance provided by rating agencies and analysts. Non-redeemable preferred stocks that are classified as fixed income securities are evaluated under this OTTI method unless the security is below investment grade, at which time it is evaluated under the equity securities OTTI model discussed below. To determine if an impairment is other than temporary, we perform assessments that may include, but are not limited to, a discounted cash flow analysis ("DCF") to determine the security's present value of future cash flows. This analysis is also performed on all previously-impaired debt securities that continue to be held by us and all structured securities that were not of high credit quality at the date of purchase. Any shortfall in the expected present value of the future cash flows, based on the DCF, from the amortized cost basis of a security is considered a “credit impairment,” with the remaining decline in fair value of a security considered a “non-credit impairment.” Credit impairments are charged to earnings as a component of realized losses, while non-credit impairments are recorded to Other Comprehensive Income ("OCI") as a component of unrealized losses. The discount rate we use in a DCF is the effective interest rate implicit in the security at the date of acquisition for those structured securities that were not of high credit quality at acquisition. For all other securities, we use a discount rate that equals the current yield, excluding the impact of previous OTTI charges, used to accrete the beneficial interest. DCFs may include, but are not necessarily limited to: (i) generating cash flows for each tranche considering tranche-specific data, market data, and other pertinent information, such as the historical performance of the underlying collateral, including net operating income generated by underlying properties, conditional default rate assumptions, loan loss severity assumptions, consensus projections, prepayment projections, and actual pool and collateral information; (ii) identifying applicable benchmark yields; and (iii) applying market-based tranche specific spreads to determine an appropriate yield by incorporating collateral performance, tranche-level attributes, trades, bids, and offers. Equity Securities We review securities that are in an unrealized loss position to determine: (i) if we do not intend to hold the security to its forecasted recovery; or (ii) if the decline is other than temporary, which includes declines driven by market volatility for which we cannot assert the security will recover in the near term. If we determine either that we do not intend to hold a security, or the decline is other than temporary, we write down the security's cost to its fair value through a charge to earnings as a component of realized losses. If we intend to hold the security, our evaluation for OTTI may include, but is not limited to, an evaluation of the following factors: • Whether the decline appears to be issuer or industry specific; • The relationship of market prices per share to book value per share at the date of acquisition and date of evaluation; • The price-earnings ratio at the time of acquisition and date of evaluation; • The financial condition and near-term prospects of the issuer, including any specific events that may influence the issuer's operations, coupled with our intention to hold the securities in the near-term; • The recent income or loss of the issuer; • The independent auditors' report on the issuer's recent financial statements; • The dividend policy of the issuer at the date of acquisition and the date of evaluation; • Buy/hold/sell recommendations or price projections published by outside investment advisors; • Rating agency announcements; • The length of time and the extent to which the fair value has been, or is expected to be, less than its cost in the near term; and • Our expectation of when the cost of the security will be recovered. Other Investments Our evaluation for OTTI of an other investment (i.e., an alternative investment) may include, but is not limited to, conversations with the management of the alternative investment concerning the following: • The current investment strategy; • Changes made or future changes to be made to the investment strategy; • Emerging issues that may affect the success of the strategy; and • The appropriateness of the valuation methodology used regarding the underlying investments. If there is a decline in the fair value of an other investment that we do not intend to hold, or if we determine the decline is other than temporary, we write down the carry value of the investment and record the charge through earnings as a component of realized losses.
Fair Value of Financial Instruments	Fair Values of Financial Instruments Assets The fair values of our investments are generated using various valuation techniques and are placed into the fair value hierarchy considering the following: (i) the highest priority is given to quoted prices in active markets for identical assets (Level 1); (ii) the next highest priority is given to quoted prices in markets that are not active or inputs that are observable either directly or indirectly, including quoted prices for similar assets in markets that are not active and other inputs that can be derived principally from, or corroborated by, observable market data for substantially the full term of the assets (Level 2); and (iii) the lowest priority is given to unobservable inputs supported by little or no market activity and that reflect our assumptions about the exit price, including assumptions that market participants would use in pricing the asset (Level 3). An asset’s classification within the fair value hierarchy is based on the lowest level of significant input to its valuation. Transfers between levels in the fair value hierarchy are recognized at the end of the reporting period. The techniques used to value our financial assets are as follows: Level 1 Pricing Security Type Methodology Equity Securities; U.S. Treasury Notes Equity and U.S. Treasury Note prices are received from an independent pricing service that are based on observable market transactions. We validate these prices against a second external pricing service, and if established market value comparison thresholds are breached, further analysis is performed to determine the price to be used. Short-Term Investments Short-term investments are carried at cost, which approximates fair value. Given the liquid nature of our short-term investments, we generally validate their fair value by way of active trades within approximately one week of the financial statement close. Level 2 Pricing We utilize a market approach for our Level 2 securities, using primarily matrix pricing models prepared by external pricing services. Matrix pricing models use mathematical techniques to value debt securities by relying on the securities' relationship to other benchmark quoted securities, and not relying exclusively on quoted prices for specific securities, as the specific securities are not always frequently traded. As a matter of policy, we consistently use one pricing service as our primary source and secondary pricing services if prices are not available from the primary pricing service. Fixed income securities portfolio pricing is reviewed for reasonableness in the following ways: (i) comparing our pricing to other third-party pricing services as well as benchmark indexed pricing; (ii) comparing fair value fluctuations between months for reasonableness; and (iii) reviewing stale prices. If further analysis is needed, a challenge is sent to the pricing service for review and confirmation of the price. Further information on our Level 2 asset pricing is included in the following table: Security Type Methodology Corporate Securities including preferred stocks classified as Fixed Income Securities, and U.S. Government and Government Agencies Evaluations include obtaining relevant trade data, benchmark quotes and spreads and incorporating this information into either spread-based or price-based evaluations as determined by the observed market data. Spread-based evaluations include: (i) creating a range of spreads for relevant maturities of each issuer based on the new issue market, secondary trading, and dealer quotes; and (ii) incorporating option adjusted spreads for issues that have early redemption features. Based on the findings in (i) and (ii) above, final spreads are derived and added to benchmark curves. Price-based evaluations include matching each issue to its best-known market maker and contacting firms that transact in these securities. Obligations of States and Political Subdivisions Evaluations are based on yield curves that are developed based on factors such as: (i) benchmarks to issues with interest rates near prevailing market rates; (ii) established trading spreads over widely-accepted market benchmarks; (iii) yields on new issues; and (iv) market information from third-party sources such as reportable trades, broker-dealers, or issuers. Structured Securities (including CLO and other ABS, Commercial Mortgage-Backed Securities ("CMBS"), Residential Mortgage-Backed Securities ("RMBS")) Evaluations are based on a DCF, including: (i) generating cash flows for each tranche considering tranche-specific data, market data, and other pertinent information, such as historical performance of the underlying collateral, including net operating income generated by the underlying properties, conditional default rate assumptions, loan loss severity assumptions, consensus projections, prepayment projections, and actual pool and loan level collateral information; (ii) identifying applicable benchmark yields; and (iii) applying market-based tranche-specific spreads to determine an appropriate yield by incorporating collateral performance, tranche-level attributes, trades, bids, and offers. Foreign Government Evaluations are performed using a DCF model and by incorporating observed market yields of benchmarks as inputs, adjusting for varied maturities. Level 3 Pricing Less than 1% of our portfolio cannot be priced using our primary or secondary pricing service. At times, we may use non-binding broker quotes to value some of these securities. These prices are from various broker/dealers that use bid or ask prices, or benchmarks to indices, in measuring the fair value of a security. We review these fair value measurements for reasonableness. This review typically includes an analysis of price fluctuations between months with variances over established thresholds being analyzed further. Further information on our current Level 3 asset pricing is included in the following table: Security Type Methodology Corporate Securities These tax credit investments are priced internally using spread-based evaluations. Equity Securities These non-publicly traded stocks are valued by the issuer and reviewed internally. Liabilities The techniques used to value our notes payable are as follows: Level 1 Pricing Security Type Methodology 5.875% Senior Notes Based on the quoted market prices. Level 2 Pricing Security Type Methodology 7.25% Senior Notes; 6.70% Senior Notes Based on matrix pricing models prepared by external pricing services. Borrowings from Federal Home Loan Banks Evaluations are performed using a DCF model based on current borrowing rates provided by the Federal Home Loan Banks that are consistent with the remaining term of the borrowing. See Note 7. “Fair Value Measurements” for a summary table of the fair value and related carrying amounts of financial instruments.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts We estimate an allowance for doubtful accounts on our premiums receivable. This allowance is based on historical write-off percentages adjusted for the effects of current and anticipated trends. An account is charged off when we believe it is probable that we will not collect a receivable. In making this determination, we consider information obtained from our efforts to collect amounts due directly or through collection agencies.
Share-Based Compensation	Share-Based Compensation Share-based compensation consists of all share-based payment transactions in which an entity acquires goods or services by issuing (or offering to issue) its shares, share units, share options, or other equity instruments. The cost resulting from all share-based payment transactions are recognized in the Financial Statements based on the fair value of both equity and liability awards. The fair value is measured at grant date for equity awards, whereas the fair value for liability awards are remeasured at each reporting period. The fair value of both equity and liability awards is recognized over the requisite service period. The requisite service period is typically the lesser of the vesting period or the period of time from the grant date to the date of retirement eligibility. The expense recognized for share-based awards, which, in some cases, contain performance criteria, is based on the number of shares or units expected to be issued at the end of the performance period. We repurchase the Parent’s stock from our employees in connection with tax withholding obligations, as permitted under our stock-based compensation plans. This activity is disclosed in our Consolidated Statements of Stockholders' Equity.
Reinsurance	Reinsurance Reinsurance recoverables represent estimates of amounts that will be recovered from reinsurers under our various treaties. Generally, amounts recoverable from reinsurers are recognized as assets at the same time and in a manner consistent with the paid and unpaid losses associated with the reinsured policies. We require collateral to secure reinsurance recoverables primarily from our reinsurance carriers that are not authorized, otherwise approved, or certified to do business in one or more of our ten insurance subsidiaries' domiciliary states. Our ten insurance subsidiaries are collectively referred to as the "Insurance Subsidiaries." This collateral is typically in the form of a letter of credit or cash. An allowance for estimated uncollectible reinsurance is recorded based on an evaluation of balances due from reinsurers and other available information, such as each reinsurers' credit rating from A.M. Best Company ("A.M. Best") or Standard & Poor's Rating Services ("S&P"). We charge off reinsurance recoverables on paid losses when it becomes probable that we will not collect the balance.
Property and Equipment	Property and Equipment Property and equipment used in operations, including certain costs incurred to develop or obtain computer software for internal use, are capitalized and carried at cost less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the assets. The following estimated useful lives can be considered as general guidelines: Asset Category Years Computer hardware 3 Computer software 3 to 5 Internally developed software 5 to 10 Software licenses 3 to 5 Furniture and fixtures 10 Buildings and improvements 5 to 40 We recorded depreciation expense of $17.8 million , $17.4 million , and $16.4 million for 2017 , 2016 , and 2015 , respectively.
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs Deferred policy acquisition costs are limited to costs directly related to the successful acquisition of insurance contracts. Costs meeting this definition typically include, among other things, sales commissions paid to our distribution partners, premium taxes, and the portion of employee salaries and benefits directly related to time spent on acquired contracts. These costs are deferred and amortized over the life of the contracts. Accounting guidance requires a premium deficiency analysis to be performed at the level an entity acquires, services, and measures the profitability of its insurance contracts. We currently perform three premium deficiency analyses for our insurance operations, consistent with our reportable segments of Standard Commercial Lines, Standard Personal Lines, and E&S Lines. A combined ratio of over 100% does not necessarily indicate a premium deficiency, as any year's combined ratio includes a portion of underwriting expenses that are expensed at policy inception and therefore are not covered by the remaining unearned premium. In addition, investment income is not contemplated in the combined ratio calculation. There were no premium deficiencies for any of the reported years, as the sum of the anticipated loss and loss expense, unamortized acquisition costs, policyholder dividends, and other expenses for each segment did not exceed that segment’s related unearned premium and anticipated investment income. The investment yields assumed in the premium deficiency assessment for each reporting period, which were based on our actual average investment yield before tax as of the September 30 calculation date, were 2.9% for 2017 , 2.4% for 2016 , and 2.5% for 2015 .
Goodwill	Goodwill Goodwill results from business acquisitions where the cost of assets and liabilities acquired exceeds the fair value of those assets and liabilities. A quantitative goodwill impairment analysis is performed if our quarterly qualitative analysis indicates that it is more likely than not that the fair value of a reporting unit is less than its carrying amount. Goodwill is allocated to the reporting units for purposes of these analyses. Based on our analysis at December 31, 2017, goodwill was not impaired.
Reserve for Loss and Loss Expense	Reserves for Loss and Loss Expense Reserves for loss and loss expense are comprised of both case reserves on individual claims and reserves for claims incurred but not reported ("IBNR"). Case reserves result from claims that have been reported to one or more of our Insurance Subsidiaries, and are estimated at the amount of the expected ultimate payment. IBNR reserves are established at more aggregated levels than case basis reserves, and in addition to reserves on claims that have been incurred but not reported, they include provisions for future emergence on known claims, as well as reopened claims. IBNR reserves are established based on the results of the Insurance Subsidiaries’ internal reserve analysis, supplemented with other internal and external information. The internal reserve analysis is performed quarterly, and relies upon generally accepted actuarial techniques. Such techniques assume that past experience, adjusted for the effects of current developments and anticipated trends, are an appropriate basis for predicting future events. Our analyses rely upon historical paid and case loss and loss expense experience organized by line of business, accident year, and maturity (i.e., “triangles”). Standard actuarial projection methods are applied to this history, producing a set of estimated ultimate loss and loss expenses. Ultimate loss and loss expenses are selected from the various methods, considering the strengths and weaknesses of the methods as they apply to the specific line and accident year. Certain types of exposures do not lend themselves to standard actuarial methods. Examples of these are: • Certain property catastrophe events may be low in frequency and high in severity. These events may affect many insureds simultaneously. Due to the unique nature of these events, ultimate liabilities are estimated for each event, based on surveys of our portfolio of exposures, in conjunction with individual claims estimates. While generally short-tailed, the liabilities associated with these events are subject to a higher degree of uncertainty. We maintain significant reinsurance protection that greatly limits the impact that these extreme events have on net loss and loss expenses. • Some insured events may span multiple years and trigger multiple policies, as in the case of asbestos and environmental claims, where the injury is deemed to occur over an extended period of time. These types of losses often do not lend themselves to traditional actuarial methods. Where we deem appropriate, our experience may be analyzed without differentiating by accident year, using alternative methods and metrics. In these cases, the associated selected ultimate loss and loss expenses are then allocated to the applicable accident years for reporting. • Another example of non-standard methods relate to loss expenses that cannot be attributed to a specific claim (referred to as “unallocated loss expenses”). These expenses are first allocated to line of business, and alternative projection methods are then applied to estimate expenses by calendar year, which are then allocated back to the applicable accident years for reporting. The selected ultimate losses and loss adjustment expenses are translated into indicated IBNR reserves, which are then compared to the recorded IBNR reserves. Management's judgment is applied in determining any required adjustments and the resulting adjustments are then recorded and assigned or allocated to accident year using the results of the actuarial analysis. While the reserve analysis is the primary basis for determining the recorded IBNR reserves, other internal and external factors are considered. Internal factors include: (i) supplemental data regarding claims reporting and settlement trends; (ii) exposure estimates for reported claims, along with recent development on those estimates with respect to individual large claims and the aggregate of all claims; (iii) the rate at which new large or complex claims are being reported; and (iv) additional trends observed by claims personnel or reported to them by defense counsel. External factors considered include: (i) legislative enactments; (ii) judicial decisions; (iii) legal developments in the determination of liability and the imposition of damages; and (iv) trends in general economic conditions, including the effects of inflation. Loss reserves are estimates, and as such, we also consider a range of possible loss and loss expense reserve estimates. This range is determined at the beginning of each year, using prior year-end data, and reflects the fact that there is no single precise method for estimating the required reserves, due to the many factors that may influence the amounts ultimately paid. Considering the reserve range along with all of the items described above, as well as current market conditions, IBNR estimates are then established and recorded. The combination of the IBNR estimates along with the case reserve estimates on individual claims results in our total reserves for loss and loss expense. These reserves are expected to be sufficient for settling losses and loss reserve obligations under our policies on unpaid claims, including changes in the volume of business written, claims frequency and severity, the mix of business, claims processing, and other items that management expects to affect our ultimate settlement of loss and loss expense. However, the ultimate claim settlements may be higher or lower than reserves established. As our experience emerges and other information develops, we revise our reserve estimates accordingly. The changes in these estimates, resulting from the continuous review process and the differences between estimates and ultimate payments, are reflected in the Consolidated Statements of Income for the period in which such estimates are changed. The associated impacts may be material to the results of operations in future periods. We do not discount to present value that portion of our losses and loss expense reserves expected to be paid in future periods. Our loss and loss expense reserves implicitly include anticipated recoveries for salvage and subrogation claims. Claims are counted at the occurrence, line of business, and policy level. For example, if a single occurrence (e.g. an auto accident) leads to a claim under an auto and an associated umbrella policy, they are each counted separately. Conversely, multiple claimants under the same occurrence/line/policy would contribute only a single count. The claim counts provided are on a reported basis. A claim is considered reported when a reserve is established or payment is made. Therefore, claims closed without payment are included in the count as long as there was an associated case reserve at some point in its life cycle. We also write a small amount of assumed reinsurance. Currently, this business is limited to our share of certain involuntary pools. Since the associated claims are not processed by us, they are not captured within our claims system. Therefore, the claim counts reported exclude this business.
Revenue Recognition	Revenue Recognition The Insurance Subsidiaries' net premiums written (“NPW”) include direct insurance policy writings, plus reinsurance assumed and estimates of premiums earned but unbilled on the workers compensation and general liability lines of insurance, less reinsurance ceded. The estimated premium on the workers compensation and general liability lines is referred to as audit premium. We estimate this premium, as it is anticipated to be either billed or returned on policies subsequent to expiration based on exposure levels (i.e. payroll or sales). Audit premium is based on historical trends adjusted for the uncertainty of future economic conditions. Economic instability could ultimately impact our estimates and assumptions, and changes in our estimate may be material to the results of operations in future periods. Premiums written are recognized as revenue over the period that coverage is provided using the semi-monthly pro-rata method. Unearned premiums and prepaid reinsurance premiums represent that portion of premiums written that are applicable to the unexpired terms of policies in force.
Dividends to Policyholders	Dividends to Policyholders We establish reserves for dividends to policyholders on certain policies, most significantly workers compensation policies. These dividends are based on the policyholders' loss experience. Dividend reserves are established based on past experience, adjusted for the effects of current developments and anticipated trends. The expense for these dividends is recognized over a period that begins at policy inception and ends with the payment of the dividend. We do not issue policies that entitle the policyholder to participate in the earnings or surplus of our Insurance Subsidiaries.
Federal Income Tax	Federal Income Tax We use the asset and liability method of accounting for income taxes. Current federal income taxes are recognized for the estimated taxes payable or refundable on tax returns for the current year. Deferred federal income taxes arise from the recognition of temporary differences between financial statement carrying amounts and the tax basis of assets and liabilities. We consider all evidence, both positive and negative, with respect to our federal tax loss carryback availability, expected levels of pre-tax financial statement income, and federal taxable income, when evaluating whether the temporary differences will be realized. In projecting future taxable income, we begin with budgeted pre-tax income adjusted for estimated non-taxable items. The assumptions about future taxable income require significant judgment and are consistent with the plans and estimates we use to manage our businesses. A valuation allowance is established when it is more likely than not that some portion of the deferred tax asset will not be realized. A liability for uncertain tax positions is recorded when it is more likely than not that a tax position will not be sustained upon examination by taxing authorities. The effect of a change in tax rates is recognized in the period of enactment. If we were to be levied interest and penalties by the Internal Revenue Service (“IRS”), the interest would be recognized as “Interest expense” and the penalties would be recognized as either “Other insurance expenses” or "Corporate expenses" on the Consolidated Statements of Income depending on the nature of what caused the occurrence of such an item. For information regarding the impact of the the recent tax reform, refer to Note 13. "Federal Income Taxes" of this Form 10-K.
Leases	Leases We have various operating leases for office space, equipment, and fleet vehicles. Rental expense for such leases is recorded on a straight-line basis over the lease term. If a lease has a fixed and determinable escalation clause, or periods of rent holidays, the difference between rental expense and rent paid is included in "Other liabilities" in the Consolidated Balance Sheets. In addition, we have various capital leases for computer hardware and software. These leases are accounted for as an acquisition of an asset with a corresponding obligation. Depreciation is calculated using the straight-line method over the shorter of the estimated useful life of the asset or the lease term.
Pension	Pension Our pension and post-retirement life benefit obligations and related costs are calculated using actuarial methods, within the framework of GAAP. Our pension benefit obligation is determined as the actuarial present value of the vested benefits to which the employee is currently entitled, based on the average life expectancy of the employee. Our funding policy provides that payments to our pension trust shall be equal to the minimum funding requirements of the Employee Retirement Income Security Act, plus additional amounts that the Board of Directors of Selective Insurance Company of America (“SICA”) may approve from time to time. Two key assumptions, the discount rate and the expected return on plan assets, are important elements of expense and/or liability measurement. We evaluate these key assumptions annually unless facts indicate that a more frequent review is required. The discount rate enables us to state expected future cash flows at their present value on the measurement date. The purpose of the discount rate is to determine the interest rates inherent in the price at which pension benefits could be effectively settled. Our discount rate selection is based on high-quality, long-term corporate bonds. To determine the expected long-term rate of return on the plan assets, we consider the current and expected asset allocation, as well as historical and expected returns on each plan asset class. Other assumptions involve demographic factors such as retirement age and mortality.

Summary of Significant Accoun36

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Estimated Useful Life for Property and Equipment	Asset Category Years Computer hardware 3 Computer software 3 to 5 Internally developed software 5 to 10 Software licenses 3 to 5 Furniture and fixtures 10 Buildings and improvements 5 to 40

Statements of Cash Flow (Tables

Statements of Cash Flow (Tables)	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]
Schedule of Cash Flow, Supplemental Disclosures	($ in thousands) 2017 2016 2015 Cash paid during the period for: Interest $ 23,905 22,098 21,892 Federal income tax 62,000 46,405 39,500 Non-cash items: Exchange of fixed income securities, AFS 22,511 23,579 36,792 Exchange of fixed income securities, HTM — — 15,257 Corporate actions related to equity securities, AFS 1 4,725 3,263 4,239 Assets acquired under capital lease arrangements 278 3,151 6,760 Non-cash purchase of property and equipment — 78 —

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investment [Line Items]
Schedule of Unrealized Gains On Investments	($ in thousands) 2017 2016 2015 AFS securities: Fixed income securities $ 85,806 38,781 55,689 Equity securities 38,894 25,864 13,235 Total AFS securities 124,700 64,645 68,924 HTM securities: Fixed income securities (21 ) 159 300 Total HTM securities (21 ) 159 300 Total net unrealized gains 124,679 64,804 69,224 Deferred income tax (44,103 ) (22,681 ) (24,228 ) Net unrealized gains, net of deferred income tax 80,576 42,123 44,996 Increase (decrease) in net unrealized gains in OCI, net of deferred income tax $ 38,453 (2,873 ) (35,398 )
Schedule of Held-to-maturity Securities	December 31, 2017 Net Unrealized Unrecognized Unrecognized Amortized Gains Carrying Holding Holding Fair ($ in thousands) Cost (Losses) Value Gains Losses Value Obligations of state and political subdivisions $ 25,154 84 25,238 1,023 — 26,261 Corporate securities 16,996 (105 ) 16,891 1,003 (55 ) 17,839 Total HTM fixed income securities $ 42,150 (21 ) 42,129 2,026 (55 ) 44,100 December 31, 2016 Net Unrealized Unrecognized Unrecognized Amortized Gains Carrying Holding Holding Fair ($ in thousands) Cost (Losses) Value Gains Losses Value Obligations of state and political subdivisions 77,466 317 77,783 2,133 — 79,916 Corporate securities 22,711 (143 ) 22,568 1,665 (158 ) 24,075 CMBS 1,220 (15 ) 1,205 15 — 1,220 Total HTM fixed income securities $ 101,397 159 101,556 3,813 (158 ) 105,211
Schedule of Available For Sale Securities	December 31, 2017 Cost/ Amortized Unrealized Unrealized Fair ($ in thousands) Cost Gains Losses Value AFS fixed income securities: U.S. government and government agencies $ 49,326 647 (233 ) 49,740 Foreign government 18,040 526 (11 ) 18,555 Obligations of states and political subdivisions 1,539,307 44,245 (582 ) 1,582,970 Corporate securities 1,588,339 30,891 (1,762 ) 1,617,468 CLO and other ABS 789,152 6,508 (202 ) 795,458 CMBS 382,727 1,563 (841 ) 383,449 RMBS 709,825 6,487 (1,430 ) 714,882 Total AFS fixed income securities 5,076,716 90,867 (5,061 ) 5,162,522 AFS equity securities: Common stock 129,696 38,287 (226 ) 167,757 Preferred stock 14,115 904 (71 ) 14,948 Total AFS equity securities 143,811 39,191 (297 ) 182,705 Total AFS securities $ 5,220,527 130,058 (5,358 ) 5,345,227 December 31, 2016 Cost/ Amortized Unrealized Unrealized Fair ($ in thousands) Cost Gains Losses Value AFS fixed income securities: U.S. government and government agencies $ 75,139 2,230 (36 ) 77,333 Foreign government 26,559 322 (16 ) 26,865 Obligations of states and political subdivisions 1,366,287 18,610 (5,304 ) 1,379,593 Corporate securities 1,976,556 27,057 (5,860 ) 1,997,753 CLO and other ABS 527,876 1,439 (355 ) 528,960 CMBS 256,356 1,514 (1,028 ) 256,842 RMBS 524,986 3,006 (2,798 ) 525,194 Total AFS fixed income securities 4,753,759 54,178 (15,397 ) 4,792,540 AFS equity securities: Common stock 104,663 26,250 (305 ) 130,608 Preferred stock 16,226 274 (355 ) 16,145 Total AFS equity securities 120,889 26,524 (660 ) 146,753 Total AFS securities $ 4,874,648 80,702 (16,057 ) 4,939,293
Schedule of Fair Value and Gross Pre-Tax Net Unrealized/Unrecognized Loss of Securities by Length of Time	December 31, 2017 Less than 12 months 12 months or longer Total ($ in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Unrealized AFS fixed income securities: U.S. government and government agencies $ 23,516 (233 ) 250 — 23,766 (233 ) Foreign government 1,481 (11 ) — — 1,481 (11 ) Obligations of states and political subdivisions 107,514 (422 ) 14,139 (160 ) 121,653 (582 ) Corporate securities 238,326 (1,744 ) 3,228 (18 ) 241,554 (1,762 ) CLO and other ABS 74,977 (196 ) 1,655 (6 ) 76,632 (202 ) CMBS 154,267 (773 ) 5,214 (68 ) 159,481 (841 ) RMBS 269,485 (1,285 ) 11,200 (145 ) 280,685 (1,430 ) Total AFS fixed income securities 869,566 (4,664 ) 35,686 (397 ) 905,252 (5,061 ) AFS equity securities: Common stock 4,727 (226 ) — — 4,727 (226 ) Preferred stock 3,833 (71 ) — — 3,833 (71 ) Total AFS equity securities 8,560 (297 ) — — 8,560 (297 ) Total AFS securities $ 878,126 (4,961 ) 35,686 (397 ) 913,812 (5,358 ) December 31, 2016 Less than 12 months 12 months or longer Total ($ in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Unrealized AFS fixed income securities: U.S. government and government agencies $ 6,419 (36 ) — — $ 6,419 $ (36 ) Foreign government 13,075 (16 ) — — 13,075 (16 ) Obligations of states and political subdivisions 306,509 (5,304 ) — — 306,509 (5,304 ) Corporate securities 462,902 (5,771 ) 4,913 (89 ) 467,815 (5,860 ) CLO and other ABS 189,795 (354 ) 319 (1 ) 190,114 (355 ) CMBS 82,492 (1,021 ) 1,645 (7 ) 84,137 (1,028 ) RMBS 279,480 (2,489 ) 8,749 (309 ) 288,229 (2,798 ) Total AFS fixed income securities 1,340,672 (14,991 ) 15,626 (406 ) 1,356,298 (15,397 ) AFS equity securities: Common stock 11,271 (305 ) — — 11,271 (305 ) Preferred stock 6,168 (355 ) — — 6,168 (355 ) Total AFS equity securities 17,439 (660 ) — — 17,439 (660 ) Total AFS securities $ 1,358,111 (15,651 ) 15,626 (406 ) $ 1,373,737 $ (16,057 )
Investments Classified by Contractual Maturity Date [Table Text Block]	AFS HTM ($ in thousands) Fair Value Carrying Value Fair Value Due in one year or less $ 315,857 10,997 11,168 Due after one year through five years 2,099,529 23,035 24,235 Due after five years through 10 years 2,510,294 8,097 8,697 Due after 10 years 236,842 — — Total fixed income securities $ 5,162,522 42,129 44,100
Schedule of Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy	Other Investments December 31, 2017 December 31, 2016 ($ in thousands) Carrying Value Remaining Commitment Maximum Exposure to Loss 1 Carrying Value Remaining Maximum 1 Alternative Investments Private equity $ 52,251 99,026 151,277 41,135 76,774 117,909 Private credit 37,743 94,959 132,702 28,193 40,613 68,806 Real assets 25,379 27,014 52,393 14,486 22,899 37,385 Total alternative investments 115,373 220,999 336,372 83,814 140,286 224,100 Other securities 2 16,895 — 16,895 18,583 3,400 21,983 Total other investments $ 132,268 220,999 353,267 102,397 143,686 246,083
Schedule of Aggregated Summarized Balance Sheet Financial Information For Partnerhips In Our Alternative Investment Portfolio	Balance Sheet Information September 30, ($ in millions) 2017 2016 Investments $ 21,046 11,244 Total assets 22,357 12,075 Total liabilities 4,767 1,802 Total partners’ capital 17,590 10,273
Schedule of Aggregated Summarized Income Statement Financial Information For Partnerhips In Our Alternative Investment Portfolio	Income Statement Information 12 months ended September 30, ($ in millions) 2017 2016 2015 Net investment (loss) income $ (143 ) (44 ) 129 Realized gains 325 1,374 1,187 Net change in unrealized appreciation (depreciation) 2,894 (719 ) (1,364 ) Net income $ 3,076 611 (48 ) Insurance Subsidiaries' alternative investments income (loss) 12.7 3.1 (1.9 )
Schedule Of Securities Pledged As Collateral	The following table summarizes the market value of these securities at December 31, 2017 : ($ in millions) FHLBI Collateral FHLBNY Collateral State and Regulatory Deposits Total U.S. government and government agencies $ 3.0 — 22.6 25.6 Obligations of states and political subdivisions — — 3.1 3.1 CMBS 6.2 14.1 — 20.3 RMBS 56.3 59.6 — 115.9 Total pledged as collateral $ 65.5 73.7 25.7 164.9
Schedule of Components of Pre-Tax Net Investment Income Earned	($ in thousands) 2017 2016 2015 Fixed income securities $ 153,230 129,306 123,230 Equity securities 6,442 7,368 9,161 Short-term investments 1,526 686 112 Other investments 12,871 2,940 (1,890 ) Investment expenses (12,187 ) (9,546 ) (9,297 ) Net investment income earned $ 161,882 130,754 121,316
Schedule of OTTI by Asset Type	2017 Recognized in Earnings ($ in thousands) Gross Included in OCI AFS fixed income securities: U.S. government and government agencies $ 36 — 36 Obligations of states and political subdivisions 612 — 612 Corporate securities 587 — 587 CLO and other ABS 96 — 96 CMBS 670 — 670 RMBS 1,183 (36 ) 1,219 Total AFS fixed income securities 3,184 (36 ) 3,220 AFS equity securities: Common stock 1,435 — 1,435 Total AFS equity securities 1,435 — 1,435 Other investments $ 190 — 190 Total OTTI losses $ 4,809 (36 ) 4,845 2016 Recognized in Earnings ($ in thousands) Gross Included in OCI AFS fixed income securities: Obligations of states and political subdivisons $ 2,797 — 2,797 Corporate securities 1,880 — 1,880 CLO and other ABS 19 — 19 CMBS 220 — 220 RMBS 275 10 265 Total AFS fixed income securities 5,191 10 5,181 AFS equity securities: Common stock 3,316 — 3,316 Preferred stock 2 — 2 Total AFS equity securities 3,318 — 3,318 Total OTTI losses $ 8,509 10 8,499 2015 Recognized in Earnings ($ in thousands) Gross Included in OCI AFS fixed income securities: Corporate securities $ 2,188 — 2,188 RMBS 1 — 1 Total AFS fixed income securities 2,189 — 2,189 AFS equity securities: Common stock 15,996 — 15,996 Preferred stock 181 — 181 Total AFS equity securities 16,177 — 16,177 Total OTTI losses $ 18,366 — 18,366
Schedule of Components of Net Realized Gains Excluding OTTI Charges	($ in thousands) 2017 2016 2015 HTM fixed income securities Gains $ 44 3 5 Losses (1 ) (1 ) (1 ) AFS fixed income securities Gains 10,193 7,741 4,515 Losses (3,292 ) (11,411 ) (312 ) AFS equity securities Gains 5,829 8,108 29,168 Losses (1,200 ) (864 ) (1,347 ) Short-term investments Gains 2 — — Losses (6 ) (13 ) — Other investments Gains 494 3 162 Losses (859 ) (4 ) (653 ) Total net realized investment gains $ 11,204 3,562 31,537

Comprehensive Income (Tables)

Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Schedule of Components of Comprehensive Income-Gross and Net of Tax	2017 ($ in thousands) Gross Tax Net Net income $ 261,968 93,142 168,826 Components of OCI: Unrealized gains (losses) on investment securities : Unrealized holding gains during the year 66,894 23,879 43,015 Non-credit portion of OTTI recognized in OCI 36 13 23 Amounts reclassified into net income: HTM securities (179 ) (63 ) (116 ) Non-credit OTTI 104 36 68 Realized gains on AFS securities (6,979 ) (2,442 ) (4,537 ) Net unrealized gains 59,876 21,423 38,453 Defined benefit pension and post-retirement plans: Net actuarial loss (4,684 ) (984 ) (3,700 ) Amounts reclassified into net income: Net actuarial loss 2,102 735 1,367 Defined benefit pension and post-retirement plans (2,582 ) (249 ) (2,333 ) Other comprehensive income 57,294 21,174 36,120 Comprehensive income $ 319,262 114,316 204,946 2016 ($ in thousands) Gross Tax Net Net income $ 219,955 61,460 158,495 Components of OCI: Unrealized (losses) gains on investment securities : Unrealized holding losses during the year (9,195 ) (3,218 ) (5,977 ) Non-credit portion of OTTI recognized in OCI (10 ) (4 ) (6 ) Amounts reclassified into net income: HTM securities (141 ) (49 ) (92 ) Non-credit OTTI 213 75 138 Realized losses on AFS securities 4,713 1,649 3,064 Net unrealized losses (4,420 ) (1,547 ) (2,873 ) Defined benefit pension and post-retirement plans: Net actuarial loss (12,079 ) (4,227 ) (7,852 ) Amounts reclassified into net income: Net actuarial loss 6,462 2,262 4,200 Defined benefit pension and post-retirement plans (5,617 ) (1,965 ) (3,652 ) Other comprehensive loss (10,037 ) (3,512 ) (6,525 ) Comprehensive income $ 209,918 57,948 151,970 2015 ($ in thousands) Gross Tax Net Net income $ 232,692 66,831 165,861 Components of OCI: Unrealized (losses) gains on investment securities : Unrealized holding losses during the year (40,221 ) (14,078 ) (26,143 ) Amounts reclassified into net income: HTM securities (580 ) (203 ) (377 ) Non-credit OTTI 357 125 232 Realized gains on AFS securities (14,016 ) (4,906 ) (9,110 ) Net unrealized losses (54,460 ) (19,062 ) (35,398 ) Defined benefit pension and post-retirement plans: Net actuarial gain 2,438 853 1,585 Amounts reclassified into net income: Net actuarial loss 7,077 2,477 4,600 Defined benefit pension and post-retirement plans 9,515 3,330 6,185 Other comprehensive loss (44,945 ) (15,732 ) (29,213 ) Comprehensive income $ 187,747 51,099 136,648
Schedule of Components of Accumulated Other Comprehensive Income	Net Unrealized (Loss) Gain on Investment Securities ($ in thousands) OTTI Related HTM Related All Other Investments Subtotal Defined Benefit Pension and Post- retirement Plans Total AOCI Balance, December 31, 2015 $ (282 ) 194 45,083 44,995 (54,420 ) (9,425 ) OCI before reclassifications (6 ) — (5,977 ) (5,983 ) (7,852 ) (13,835 ) Amounts reclassified from AOCI 138 (92 ) 3,064 3,110 4,200 7,310 Net current period OCI 132 (92 ) (2,913 ) (2,873 ) (3,652 ) (6,525 ) Balance, December 31, 2016 (150 ) 102 42,170 42,122 (58,072 ) (15,950 ) OCI before reclassifications 23 — 43,015 43,038 (3,700 ) 39,338 Amounts reclassified from AOCI 68 (116 ) (4,537 ) (4,585 ) 1,367 (3,218 ) Net current period OCI 91 (116 ) 38,478 38,453 (2,333 ) 36,120 Balance, December 31, 2017 $ (59 ) (14 ) 80,648 80,575 (60,405 ) 20,170
Schedule of Reclassification out of Accumulated Other Comprehensive Income	($ in thousands) Year ended December 31, 2017 Year ended December 31, 2016 Affected Line Item in the Consolidated Statements of Income OTTI related Non-credit OTTI on disposed securities $ 104 213 Net realized gains (losses) 104 213 Income before federal income tax (36 ) (75 ) Total federal income tax expense 68 138 Net income HTM related Unrealized losses on HTM disposals 32 169 Net realized gains (losses) Amortization of net unrealized gains on HTM securities (211 ) (310 ) Net investment income earned (179 ) (141 ) Income before federal income tax 63 49 Total federal income tax expense (116 ) (92 ) Net income Realized (losses) gains on AFS Realized (losses) gains on AFS disposals (6,979 ) 4,713 Net realized gains (losses) (6,979 ) 4,713 Income before federal income tax 2,442 (1,649 ) Total federal income tax expense (4,537 ) 3,064 Net income Defined benefit pension and post-retirement life plans Net actuarial loss 450 1,486 Loss and loss expense incurred 1,652 4,976 Other insurance expenses Total defined benefit pension and post-retirement life 2,102 6,462 Income before federal income tax (735 ) (2,262 ) Total federal income tax expense 1,367 4,200 Net income Total reclassifications for the period $ (3,218 ) 7,310 Net income

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments	December 31, 2017 December 31, 2016 ($ in thousands) Carrying Amount Fair Value Carrying Amount Fair Value Financial Assets Fixed income securities: HTM $ 42,129 44,100 101,556 105,211 AFS 5,162,522 5,162,522 4,792,540 4,792,540 Equity securities, AFS 182,705 182,705 146,753 146,753 Short-term investments 165,555 165,555 221,701 221,701 Long-term debt: 7.25% Senior Notes 49,904 61,391 49,901 56,148 6.70% Senior Notes 99,446 116,597 99,430 108,333 5.875% Senior Notes 185,000 186,332 185,000 176,860 1.61% Borrowings from FHLBNY 25,000 24,270 25,000 24,286 1.56% Borrowings from FHLBNY 25,000 24,210 25,000 24,219 3.03% Borrowings from FHLBI 60,000 60,334 60,000 59,313 Subtotal long-term debt 444,350 473,134 444,331 449,159 Unamortized debt issuance costs (5,234 ) (5,664 ) Total long-term debt $ 439,116 438,667
Schedule of Quantitative Disclosures of our Financial Assets that were Measured at Fair Value	December 31, 2017 Fair Value Measurements Using ($ in thousands) Assets Measured at Fair Value Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) 1 Significant Other Observable Inputs (Level 2) 1 Significant Unobservable Inputs (Level 3) Description Measured on a recurring basis: AFS fixed income securities: U.S. government and government agencies $ 49,740 24,652 25,088 — Foreign government 18,555 — 18,555 — Obligations of states and political subdivisions 1,582,970 — 1,582,970 — Corporate securities 1,617,468 — 1,617,468 — CLO and other ABS 795,458 — 795,458 — CMBS 383,449 — 376,895 6,554 RMBS 714,882 — 714,882 — Total AFS fixed income securities 5,162,522 24,652 5,131,316 6,554 AFS equity securities: Common stock 2 167,757 138,640 — 5,398 Preferred stock 14,948 14,948 — — Total AFS equity securities 182,705 153,588 — 5,398 Total AFS securities 5,345,227 178,240 5,131,316 11,952 Short-term investments 165,555 165,555 — — Total assets measured at fair value $ 5,510,782 343,795 5,131,316 11,952 December 31, 2016 Fair Value Measurements Using ($ in thousands) Assets Measured at Fair Value Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) 1 Significant Other Observable Inputs (Level 2) 1 Significant Unobservable Inputs (Level 3) Description Measured on a recurring basis: AFS fixed income securities: U.S. government and government agencies $ 77,333 27,520 49,813 — Foreign government 26,865 — 26,865 — Obligations of states and political subdivisions 1,379,593 — 1,379,593 — Corporate securities 1,997,753 — 1,997,753 — CLO and other ABS 528,960 — 528,960 — CMBS 256,842 — 256,842 — RMBS 525,194 — 525,194 — Total AFS fixed income securities 4,792,540 27,520 4,765,020 — AFS equity securities: Common stock 130,608 122,932 — 7,676 Preferred stock 16,145 16,145 — — Total AFS equity securities 146,753 139,077 — 7,676 Total AFS securities 4,939,293 166,597 4,765,020 7,676 Short-term investments 221,701 221,701 — — Total assets measured at fair value $ 5,160,994 388,298 4,765,020 7,676 1 There were no transfers of securities between Level 1 and Level 2. 2 In accordance with ASU 2015-07, investments amounting to $23.7 million at December 31, 2017, respectively, were measured at fair value using the net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.
Schedule of Summary of the Changes in Fair Value of Securities Using Level 3 Inputs	2017 ($ in thousands) CMBS Common Stock Fair value, December 31, 2016 $ — 7,676 Total net (losses) gains for the period included in: OCI 4 — Net income — — Purchases 6,550 3,780 Sales — (3,958 ) Issuances — — Settlements — — Transfers into Level 3 — — Transfers out of Level 3 — (2,100 ) Fair value, December 31, 2017 $ 6,554 $ 5,398
Schedule of Quantitative Information of our Financial Assets and Liabilities that were Disclosed at Fair Value	December 31, 2017 Fair Value Measurements Using ($ in thousands) Assets/Liabilities Disclosed at Fair Value Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets HTM: Obligations of states and political subdivisions $ 26,261 — 26,261 — Corporate securities 17,839 — 12,306 5,533 Total HTM fixed income securities $ 44,100 — 38,567 5,533 Financial Liabilities Long-term debt: 7.25% Senior Notes $ 61,391 — 61,391 — 6.70% Senior Notes 116,597 — 116,597 — 5.875% Senior Notes 186,332 186,332 — — 1.61% Borrowings from FHLBNY 24,270 — 24,270 — 1.56% Borrowings from FHLBNY 24,210 — 24,210 — 3.03% Borrowings from FHLBI 60,334 — 60,334 — Total long-term debt $ 473,134 186,332 286,802 — December 31, 2016 Fair Value Measurements Using ($ in thousands) Assets/Liabilities Disclosed at Fair Value Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets HTM: Obligations of states and political subdivisions $ 79,916 — 79,916 — Corporate securities 24,075 — 16,565 7,510 CMBS 1,220 — 1,220 — Total HTM fixed income securities $ 105,211 — 97,701 7,510 Financial Liabilities Long-term debt: 7.25% Senior Notes $ 56,148 — 56,148 — 6.70% Senior Notes 108,333 — 108,333 — 5.875% Senior Notes 176,860 176,860 — — 1.61% Borrowings from FHLBNY 24,286 — 24,286 — 1.56% Borrowings from FHLBNY 24,219 — 24,219 — 3.03% Borrowings from FHLBI 59,313 — 59,313 — Total long-term debt $ 449,159 176,860 272,299 —

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Schedule of Total Reinsurance Balances Segregated By Reinsurer	As of December 31, 2017 As of December 31, 2016 ($ in thousands) Reinsurance Balances % of Reinsurance Balance Reinsurance Balances % of Reinsurance Balance Total reinsurance recoverables $ 594,832 $ 621,537 Total prepaid reinsurance premiums 153,493 146,282 Total reinsurance balance 748,325 767,819 Federal and state pools 1 : NFIP 204,161 27 % 211,181 27 % New Jersey Unsatisfied Claim Judgment Fund 62,947 9 65,574 9 Other 3,634 — 3,227 — Total federal and state pools 270,742 36 279,982 36 Remaining reinsurance balance $ 477,583 64 $ 487,837 64 Munich Re Group (A.M. Best rated "A+") $ 117,460 16 $ 119,520 16 Hannover Ruckversicherungs AG (A.M. Best rated "A+") 101,652 14 106,298 13 AXIS Reinsurance Company (A.M. Best rated "A+") 62,396 8 59,737 8 Swiss Re Group (A.M. Best rated "A+") 40,772 5 50,494 7 Partner Reinsurance Company of the U.S. (A.M. Best rated “A”) 16,925 2 21,125 3 All other reinsurers 138,378 19 130,663 17 Total reinsurers 477,583 64 % 487,837 64 % Less: collateral 2 (122,413 ) (113,763 ) Reinsurers, net of collateral $ 355,170 $ 374,074 1 Considered to have minimal risk of default. 2 Includes letters of credit, trust funds, and funds held against reinsurance recoverables.
List Of Direct, Assumed, And Ceded Reinsurance Amounts	($ in thousands) 2017 2016 2015 Premiums written: Direct $ 2,733,459 2,577,259 2,403,519 Assumed 26,685 28,779 23,848 Ceded (389,503 ) (368,750 ) (357,463 ) Net $ 2,370,641 2,237,288 2,069,904 Premiums earned: Direct $ 2,647,488 2,484,715 2,330,267 Assumed 25,831 28,214 23,209 Ceded (382,292 ) (363,357 ) (363,567 ) Net $ 2,291,027 2,149,572 1,989,909 Loss and loss expense incurred: Direct $ 1,570,678 1,560,356 1,274,872 Assumed 17,588 22,708 16,996 Ceded (243,192 ) (348,267 ) (143,327 ) Net $ 1,345,074 1,234,797 1,148,541
Schedule of Ceded Premiums and Losses Related to Flood Operations	Ceded to NFIP ($ in thousands) 2017 2016 2015 Ceded premiums written $ (241,345 ) (232,245 ) (228,907 ) Ceded premiums earned (235,088 ) (227,882 ) (233,940 ) Ceded loss and loss expense incurred (160,922 ) (239,891 ) (62,078 )

Reserve for Loss and Loss Exp42

Reserve for Loss and Loss Expense (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance Loss Reserves [Abstract]
Schedule of Roll Forward of Reserve for Loss and Loss Expense	($ in thousands) 2017 2016 2015 Gross reserves for loss and loss expense, at beginning of year $ 3,691,719 3,517,728 3,477,870 Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year 611,200 551,019 571,978 Net reserves for loss and loss expense, at beginning of year 3,080,519 2,966,709 2,905,892 Incurred loss and loss expense for claims occurring in the: Current year 1,384,266 1,300,565 1,217,550 Prior years (39,192 ) (65,768 ) (69,009 ) Total incurred loss and loss expense 1,345,074 1,234,797 1,148,541 Paid loss and loss expense for claims occurring in the: Current year 497,486 450,811 446,550 Prior years 742,722 670,176 641,174 Total paid loss and loss expense 1,240,208 1,120,987 1,087,724 Net reserves for loss and loss expense, at end of year 3,185,385 3,080,519 2,966,709 Add: Reinsurance recoverable on unpaid loss and loss expense, at end of year 585,855 611,200 551,019 Gross reserves for loss and loss expense at end of year $ 3,771,240 3,691,719 3,517,728
Schedule of (Favorable)/ Unfavorable Prior Year Development	(Favorable)/Unfavorable Prior Year Development ($ in millions) 2017 2016 2015 General Liability $ (48.3 ) (45.0 ) (51.0 ) Commercial Automobile 35.6 25.3 2.4 Workers Compensation (52.3 ) (56.0 ) (37.0 ) Businessowners' Policies 1.9 1.8 2.2 Commercial Property 8.7 0.3 (3.0 ) Homeowners 0.4 1.7 1.5 Personal Automobile 6.7 1.0 0.4 E&S Casualty Lines 10.0 6.0 16.0 Other (1.9 ) (0.9 ) (0.5 ) Total $ (39.2 ) (65.8 ) (69.0 )
Schedule of Exposure to Various Asbestos and Environmental Claims	2017 ($ in millions) Gross Net Asbestos $ 7.6 6.3 Landfill sites 12.4 7.7 Underground storage tanks 8.4 7.2 Total $ 28.4 21.2
Schedule of Roll Forward of Gross and Net Asbestos and Environmental Incurred Losses and Loss Expenses and Related Reserves	2017 2016 2015 ($ in thousands) Gross Net Gross Net Gross Net Asbestos Reserves for loss and loss expense at beginning of year $ 7,847 6,615 8,024 6,793 8,751 7,314 Incurred loss and loss expense — — 77 77 (428 ) (77 ) Less: loss and loss expense paid (270 ) (269 ) (254 ) (255 ) (299 ) (444 ) Reserves for loss and loss expense at the end of year $ 7,577 6,346 7,847 6,615 8,024 6,793 Environmental Reserves for loss and loss expense at beginning of year $ 22,115 16,101 22,387 16,368 21,902 15,680 Incurred loss and loss expense 126 — 1,406 1,303 3,396 3,397 Less: loss and loss expense paid (1,403 ) (1,235 ) (1,678 ) (1,570 ) (2,911 ) (2,709 ) Reserves for loss and loss expense at the end of year $ 20,838 14,866 22,115 16,101 22,387 16,368 Total Asbestos and Environmental Claims Reserves for loss and loss expense at beginning of year $ 29,962 22,716 30,411 23,161 30,653 22,994 Incurred loss and loss expense 126 — 1,483 1,380 2,968 3,320 Less: loss and loss expense paid (1,673 ) (1,504 ) (1,932 ) (1,825 ) (3,210 ) (3,153 ) Reserves for loss and loss expense at the end of year $ 28,415 21,212 29,962 22,716 30,411 23,161
Short-duration Insurance Contracts, Claims Development [Table Text Block]	All Lines (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 957,247 988,584 990,931 964,862 947,306 936,975 927,958 931,785 926,017 923,978 41,791 85,338 2009 920,143 941,972 916,691 883,590 870,057 869,927 857,960 853,401 848,413 41,937 85,575 2010 950,114 973,742 977,959 956,600 943,118 922,404 915,131 907,074 50,293 94,258 2011 1,042,576 1,061,667 1,062,233 1,056,107 1,033,518 1,023,726 1,019,351 63,891 104,500 2012 1,065,437 1,071,290 1,020,655 998,028 973,089 973,644 77,542 103,745 2013 1,044,142 1,062,045 1,047,230 1,021,007 1,002,316 116,449 90,755 2014 1,107,513 1,133,798 1,146,990 1,124,014 171,913 94,375 2015 1,114,081 1,130,513 1,144,830 256,758 92,891 2016 1,188,608 1,203,634 416,010 92,191 2017 1,270,110 634,863 88,941 Total 10,417,364 All Lines (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 286,314 489,633 609,851 690,016 764,196 798,996 819,280 839,392 853,769 860,745 2009 277,275 442,417 540,982 634,902 695,249 736,100 760,589 775,885 784,713 2010 328,826 509,910 625,229 704,895 773,536 803,773 823,770 835,532 2011 391,944 585,867 692,730 782,655 852,202 901,801 924,111 2012 378,067 555,819 651,544 743,742 810,135 856,195 2013 335,956 518,872 644,475 748,758 833,823 2014 405,898 614,075 736,154 855,959 2015 376,641 581,203 725,385 2016 387,272 617,958 2017 433,440 Total 7,727,861 All outstanding liabilities before 2008, net of reinsurance 352,192 Liabilities for loss and loss adjustment expenses, net of reinsurance 3,041,694 General Liability (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 250,239 243,755 243,536 234,770 233,712 224,236 219,551 221,640 221,203 219,617 14,326 13,769 2009 237,913 241,625 233,530 223,146 212,947 211,243 206,387 205,741 201,568 17,629 13,841 2010 215,208 228,680 242,499 237,154 222,328 211,619 208,968 202,394 18,403 12,672 2011 227,769 228,720 239,480 230,785 217,256 211,196 212,011 25,729 11,579 2012 238,979 245,561 215,083 194,144 175,305 175,268 27,702 9,922 2013 250,609 251,421 239,776 225,709 210,785 53,014 10,226 2014 244,312 249,946 257,132 239,333 80,168 10,391 2015 254,720 245,710 246,990 124,639 9,987 2016 277,214 272,048 178,904 9,617 2017 293,747 249,085 8,194 Total 2,273,761 General Liability (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 16,397 45,595 82,421 113,088 151,055 166,394 176,873 186,896 194,257 198,360 2009 14,346 37,143 64,970 103,213 130,554 151,920 166,767 176,316 180,621 2010 15,726 46,201 80,018 113,050 143,360 161,487 172,394 178,179 2011 13,924 42,692 73,643 102,978 135,377 159,768 170,525 2012 13,030 35,241 56,580 89,008 109,448 130,866 2013 12,789 35,113 72,127 104,587 139,114 2014 14,901 46,825 79,972 121,969 2015 14,665 39,978 78,668 2016 15,684 46,549 2017 17,366 Total 1,262,217 All outstanding liabilities before 2008, net of reinsurance 80,514 Liabilities for loss and loss adjustment expenses, net of reinsurance 1,092,058 Workers Compensation (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 219,616 243,186 255,810 250,423 241,921 245,993 244,100 243,512 238,836 238,218 25,993 14,401 2009 197,504 215,946 213,036 210,109 210,756 216,992 212,536 208,611 208,142 22,575 12,217 2010 198,371 214,469 212,815 211,030 214,916 212,448 208,155 204,423 28,964 12,184 2011 205,238 218,973 214,743 215,114 210,591 205,708 200,674 32,881 11,845 2012 203,864 208,036 199,360 195,197 188,596 187,359 36,233 11,605 2013 199,794 194,318 187,658 173,160 166,662 34,776 11,366 2014 199,346 187,065 182,579 172,515 42,869 10,482 2015 193,729 194,639 183,604 42,287 10,530 2016 196,774 184,946 70,424 10,509 2017 195,202 112,086 10,182 Total 1,941,745 Workers Compensation (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 39,628 100,678 139,144 158,083 171,403 180,556 188,206 191,265 195,962 198,512 2009 37,885 87,299 117,019 133,116 145,417 154,726 160,529 164,336 167,894 2010 46,795 93,281 122,442 137,184 149,086 153,795 158,078 162,796 2011 42,941 90,836 118,847 134,646 139,232 149,269 154,320 2012 40,911 86,909 108,211 122,755 132,052 139,477 2013 36,829 74,568 96,376 109,739 118,669 2014 35,924 78,944 100,876 113,626 2015 33,857 77,320 98,195 2016 34,525 78,531 2017 40,375 Total 1,272,395 All outstanding liabilities before 2008, net of reinsurance 247,121 Liabilities for loss and loss adjustment expenses, net of reinsurance 916,471 Commercial Automobile (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 196,370 195,823 190,349 187,100 187,417 182,785 180,902 183,736 183,618 183,151 736 24,129 2009 199,541 191,079 182,724 169,858 166,682 162,911 161,251 161,923 161,300 897 24,652 2010 187,562 189,305 187,778 181,923 179,854 172,969 173,157 173,471 1,491 25,301 2011 174,006 183,044 182,325 178,421 172,617 174,882 174,514 2,937 25,272 2012 179,551 191,947 183,527 184,289 184,367 186,128 3,880 23,889 2013 188,289 205,282 209,197 207,994 210,410 8,258 25,392 2014 200,534 212,725 216,824 219,925 19,053 27,338 2015 220,994 240,958 253,074 36,137 28,818 2016 255,187 274,367 71,303 30,480 2017 301,274 132,814 30,187 Total 2,137,614 Commercial Automobile (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 69,053 104,711 130,857 151,741 166,487 173,795 175,244 180,779 181,779 181,979 2009 63,126 94,406 113,697 137,564 149,949 155,560 158,303 159,723 160,013 2010 68,098 99,254 128,015 146,913 163,513 167,227 169,100 169,793 2011 69,849 99,196 121,576 142,507 157,291 166,082 170,000 2012 73,316 105,371 127,235 148,669 168,114 176,656 2013 76,469 109,893 140,015 169,850 189,626 2014 80,810 117,169 148,884 180,701 2015 91,347 132,260 175,866 2016 106,022 155,720 2017 117,287 Total 1,677,641 All outstanding liabilities before 2008, net of reinsurance 4,158 Liabilities for loss and loss adjustment expenses, net of reinsurance 464,131 Businessowners' Policies (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 39,660 38,986 39,334 32,974 30,250 29,793 31,066 31,340 30,967 31,065 192 3,258 2009 48,535 51,762 46,645 43,828 43,553 44,938 44,299 44,273 43,933 272 3,474 2010 53,669 49,285 42,408 39,915 40,899 40,581 41,239 41,197 697 3,917 2011 54,469 57,083 51,047 58,242 59,256 58,966 58,456 1,080 4,959 2012 54,342 48,029 46,303 44,172 44,077 43,747 756 5,540 2013 49,617 42,618 41,005 40,624 41,369 3,192 3,479 2014 55,962 60,949 62,548 59,806 5,952 4,054 2015 52,871 53,768 57,245 10,256 3,913 2016 52,335 53,792 11,938 3,757 2017 46,624 15,252 3,462 Total 477,234 Businessowners' Policies (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 15,019 21,765 24,449 25,738 28,026 28,660 28,589 29,778 30,873 30,873 2009 18,915 29,612 32,689 36,073 40,052 42,895 43,358 43,448 43,547 2010 20,821 28,131 31,027 34,705 37,819 38,900 40,279 40,395 2011 27,884 37,362 41,011 46,444 52,114 55,856 57,045 2012 22,199 31,833 35,089 37,215 38,766 40,627 2013 17,412 26,592 30,845 34,760 37,993 2014 28,914 40,584 44,911 49,460 2015 24,189 36,014 42,710 2016 24,655 36,848 2017 21,865 Total 401,363 All outstanding liabilities before 2008, net of reinsurance 7,292 Liabilities for loss and loss adjustment expenses, net of reinsurance 83,163 Commercial Property (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 97,578 102,860 101,436 101,470 101,265 101,702 101,043 100,881 101,043 101,054 2 7,604 2009 82,619 82,124 82,025 82,014 80,774 80,455 80,558 80,545 80,416 4 7,009 2010 105,647 96,851 97,386 96,127 95,530 95,363 95,178 95,155 5 7,667 2011 136,954 131,667 130,942 131,282 131,353 131,113 131,049 4 9,036 2012 118,464 114,224 115,375 116,658 117,102 117,170 24 8,514 2013 88,101 90,639 90,103 90,005 90,436 42 5,709 2014 141,192 136,249 136,820 138,751 126 6,512 2015 110,270 109,513 111,750 261 6,398 2016 121,927 126,185 804 6,686 2017 138,773 8,794 6,358 Total 1,130,739 Commercial Property (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 68,211 98,921 100,465 99,288 100,213 100,752 100,908 100,868 101,034 101,032 2009 59,933 78,695 80,433 80,894 80,251 80,352 80,529 80,509 80,405 2010 69,543 91,918 94,602 95,111 95,270 95,147 95,156 95,150 2011 94,538 127,580 129,579 130,681 131,060 131,115 131,089 2012 81,528 108,834 111,503 114,699 116,291 116,625 2013 60,244 87,874 90,446 90,350 90,840 2014 101,131 132,909 136,634 137,883 2015 79,048 106,182 109,829 2016 83,966 118,789 2017 99,047 Total 1,080,689 All outstanding liabilities before 2008, net of reinsurance 250 Liabilities for loss and loss adjustment expenses, net of reinsurance 50,300 Personal Automobile (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 100,311 106,999 106,842 103,934 100,213 99,912 99,686 99,255 99,116 99,270 191 16,042 2009 93,808 103,319 105,033 103,908 104,734 103,866 103,393 103,412 103,348 191 17,346 2010 103,340 110,075 112,346 109,515 107,490 107,405 107,224 107,054 222 20,822 2011 113,232 116,164 113,686 112,993 114,241 113,830 113,988 284 22,700 2012 113,771 114,921 109,832 109,324 110,294 110,300 728 22,332 2013 108,417 109,620 106,225 106,703 107,759 851 22,371 2014 102,250 109,325 106,757 107,452 2,554 22,499 2015 96,387 99,698 100,214 6,541 20,840 2016 92,727 98,032 11,651 19,747 2017 101,880 22,284 19,818 Total 1,049,297 Personal Automobile (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 50,396 73,194 84,715 91,834 95,932 97,723 98,174 98,604 98,668 98,810 2009 51,039 71,911 86,431 96,229 100,566 102,187 102,322 102,437 103,009 2010 58,786 82,490 95,300 101,540 104,061 105,849 106,453 106,733 2011 61,323 82,102 93,878 105,068 111,085 112,732 113,551 2012 63,704 82,729 94,842 102,977 107,890 109,355 2013 61,384 80,861 92,637 100,528 105,131 2014 62,519 83,739 92,589 99,173 2015 58,725 76,470 87,163 2016 57,961 76,823 2017 62,854 Total 962,602 All outstanding liabilities before 2008, net of reinsurance 5,862 Liabilities for loss and loss adjustment expenses, net of reinsurance 92,557 Homeowners (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 41,224 41,747 39,342 39,203 38,062 38,410 38,111 38,042 38,045 38,038 56 5,139 2009 47,636 44,511 42,609 40,313 61,927 40,400 40,465 40,457 40,451 73 5,633 2010 68,373 67,525 63,285 97,761 62,462 62,402 62,339 62,392 84 9,131 2011 103,804 98,211 82,744 94,167 94,543 94,183 94,378 159 15,106 2012 87,260 82,745 86,560 86,667 86,271 86,330 180 16,936 2013 73,670 72,528 71,494 72,145 71,714 284 7,747 2014 80,111 82,461 83,637 83,844 1,146 8,762 2015 76,637 76,400 76,559 2,744 7,724 2016 60,105 60,931 2,067 6,820 2017 59,167 5,315 6,651 Total 673,804 Homeowners (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 21,277 33,535 36,271 37,086 37,763 37,837 37,933 37,939 37,930 37,928 2009 28,299 36,965 38,078 39,342 39,731 39,819 39,907 40,189 40,269 2010 43,699 58,638 60,295 61,106 62,155 62,227 62,241 62,272 2011 71,668 89,963 91,718 92,185 93,312 93,720 94,007 2012 69,056 79,584 82,720 84,250 85,196 85,562 2013 50,664 65,528 67,838 69,775 71,776 2014 61,561 76,007 79,751 81,664 2015 52,589 70,078 72,202 2016 42,252 57,333 2017 45,466 Total 648,479 All outstanding liabilities before 2008, net of reinsurance 5,403 Liabilities for loss and loss adjustment expenses, net of reinsurance 30,728 E&S Casualty Lines (in thousands, except for claim counts) Incurred Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 Accident Year Unaudited IBNR Cumulative Number of Reported Claims 2011 2012 2013 2014 2015 2016 2017 2008 $ 92 169 146 119 52 (162 ) 119 — 35 2009 885 1,053 938 728 710 96 737 — 274 2010 3,294 4,106 3,369 4,299 3,831 3,055 4,932 — 804 2011 8,127 7,102 9,853 12,207 10,273 9,652 10,228 361 1,316 2012 42,367 42,621 43,175 46,149 46,165 45,988 7,313 2,006 2013 55,468 60,309 67,099 69,112 67,647 15,326 2,219 2014 55,316 63,505 69,929 71,719 18,224 1,987 2015 75,498 76,432 82,404 32,909 2,606 2016 94,451 96,416 65,489 2,454 2017 91,438 79,354 1,760 Total 471,628 E&S Casualty Lines (in thousands) Cumulative Paid Loss and Allocated Loss Adjustment Expenses, Net of Reinsurance Accident Year Unaudited 2011 2012 2013 2014 2015 2016 2017 2008 $ — 24 70 80 79 92 97 2009 — 198 431 605 626 709 737 2010 — 1,218 2,570 3,574 4,078 4,513 4,610 2011 — 806 3,200 6,445 9,954 9,912 10,256 2012 3,722 7,914 16,430 25,064 32,343 36,278 2013 2,715 9,470 21,980 35,200 46,108 2014 2,353 12,234 25,571 43,877 2015 3,036 13,057 29,389 2016 3,720 16,195 2017 5,057 Total 192,604 All outstanding liabilities before 2008, net of reinsurance — Liabilities for loss and loss adjustment expenses, net of reinsurance 279,023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table Text Block]	(in thousands) December 31, 2017 Net outstanding liabilities: Standard Commercial Lines General liability 1,092,058 Workers compensation 916,471 Commercial automobile 464,131 Businessowners' policies 83,163 Commercial property 50,300 Other Standard Commercial Lines 10,560 Total Standard Commercial Lines net outstanding liabilities 2,616,683 Standard Personal Lines Personal automobile 92,557 Homeowners 30,728 Other Standard Personal Lines 9,184 Total Standard Personal Lines net outstanding liabilities 132,469 E&S Lines Casualty lines 279,023 Property lines 13,519 Total E&S Lines net outstanding liabilities 292,542 Total liabilities for unpaid loss and loss adjustment expenses, net of reinsurance 3,041,694 Reinsurance recoverable on unpaid claims: Standard Commercial Lines General liability 175,276 Workers compensation 218,024 Commercial automobile 16,745 Businessowners' policies 3,926 Commercial property 24,387 Other Standard Commercial Lines 2,287 Total Standard Commercial Lines reinsurance recoverable on unpaid loss 440,645 Standard Personal Lines Personal automobile 53,129 Homeowners 999 Other Standard Personal Lines 69,333 Total Standard Personal Lines reinsurance recoverable on unpaid loss 123,461 E&S Lines Casualty lines 21,360 Property lines 389 Total E&S Lines reinsurance recoverable on unpaid loss 21,749 Total reinsurance recoverable on unpaid loss 585,855 Unallocated loss adjustment expenses 143,691 Total gross liability for unpaid loss and loss adjustment expenses 3,771,240
Short-duration Insurance Contracts, Schedule of Historical Claims Duration [Table Text Block]	Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 General liability 6.6% 12.5 15.4 16.7 14.9 10.1 6.0 4.6 2.2 1.3 Workers compensation 20.2 23.9 13.6 8.3 5.0 3.9 2.8 2.0 1.8 1.9 Commercial automobile 38.2 17.4 14.0 12.8 9.0 4.1 1.6 1.4 0.7 0.1 Businessowners’ policies 46.6 20.8 8.1 7.5 7.5 4.1 2.0 1.6 0.1 1.1 Commercial property 70.5 25.6 2.7 0.7 0.4 0.1 — — — — Personal automobile 56.1 18.9 10.7 7.4 4.2 1.5 0.4 0.1 0.2 — Homeowners 71.1 21.0 3.3 2.1 1.6 0.1 — 0.2 0.2 0.1 E&S Lines - casualty 4.5 12.5 18.2 20.4 15.5 8.3 4.0

Indebtedness (Tables)

Indebtedness (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments [Table Text Block]	Outstanding Debt 2017 Carry Value ($ in thousands) Issuance Date Maturity Date Interest Rate Original Amount Debt Discount and Unamortized Issuance Costs December 31, 2017 December 31, 2016 Description Long-term: (1) FHLBI 12/16/2016 12/16/2026 3.03 % $ 60,000 — 60,000 60,000 (2) FHLBNY 8/15/2016 8/16/2021 1.56 % 25,000 — 25,000 25,000 (2) FHLBNY 7/21/2016 7/21/2021 1.61 % 25,000 — 25,000 25,000 (3) Senior Notes 2/8/2013 2/9/2043 5.875 % 185,000 (4,570 ) 180,430 180,068 (4) Senior Notes 11/3/2005 11/1/2035 6.70 % 100,000 (989 ) 99,011 98,952 (5) Senior Notes 11/16/2004 11/15/2034 7.25 % 50,000 (325 ) 49,675 49,647 Total long-term debt $ 445,000 (5,884 ) 439,116 438,667

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Schedule of Revenue from Continuing Operations By Segment	Revenue by Segment Years ended December 31, ($ in thousands) 2017 2016 2015 Standard Commercial Lines: Net premiums earned: Commercial automobile $ 442,818 398,942 358,909 Workers compensation 317,982 308,233 290,075 General liability 569,217 527,859 483,291 Commercial property 311,932 293,438 269,022 Businessowners’ policies 100,266 97,754 93,428 Bonds 29,086 23,227 20,350 Other 17,198 16,030 14,367 Miscellaneous income 9,488 7,782 6,343 Total Standard Commercial Lines revenue 1,797,987 1,673,265 1,535,785 Standard Personal Lines: Net premiums earned: Personal automobile 153,147 142,876 146,784 Homeowners 129,699 130,973 134,382 Other 6,855 6,758 6,968 Miscellaneous income 1,228 1,098 1,113 Total Standard Personal Lines revenue 290,929 281,705 289,247 E&S Lines: Net premiums earned: Casualty lines 157,366 151,638 126,064 Property lines 55,461 51,844 46,269 Miscellaneous income — 1 — Total E&S Lines revenue 212,827 203,483 172,333 Investments: Net investment income 161,882 130,754 121,316 Net realized investment gains (losses) 6,359 (4,937 ) 13,171 Total Investments revenues 168,241 125,817 134,487 Total revenues $ 2,469,984 2,284,270 2,131,852
Schedule of Income from Continuing Operations before Federal Income Tax	Income Before Federal Income Tax Years ended December 31, ($ in thousands) 2017 2016 2015 Standard Commercial Lines: Underwriting gain, before federal income tax $ 149,514 146,435 164,496 Underwriting gain, after federal income tax 97,184 95,183 106,923 Combined ratio 91.6 % 91.2 % 89.2 % Standard Personal Lines: Underwriting gain, before federal income tax 11,104 12,419 1,336 Underwriting gain, after federal income tax 7,217 8,072 868 Combined ratio 96.2 % 95.6 % 99.5 % E&S Lines: Underwriting loss, before federal income tax (6,282 ) (6,921 ) (16,803 ) Underwriting loss, after federal income tax (4,083 ) (4,499 ) (10,922 ) Combined ratio 103.0 % 103.4 % 109.8 % Investments: Net investment income $ 161,882 130,754 121,316 Net realized investment gains (losses) 6,359 (4,937 ) 13,171 Total investment income, before federal income tax 168,241 125,817 134,487 Tax on investment income 45,588 30,621 32,090 Total investment income, after federal income tax $ 122,653 95,196 102,397
Schedule of Reconciliation of Segment Results to Income From Continuing Operations, Before Federal Income Tax	Reconciliation of Segment Results to Income Before Federal Income Tax Years ended December 31, ($ in thousands) 2017 2016 2015 Underwriting gain (loss) Standard Commercial Lines $ 149,514 146,435 164,496 Standard Personal Lines 11,104 12,419 1,336 E&S Lines (6,282 ) (6,921 ) (16,803 ) Investment income 168,241 125,817 134,487 Total all segments 322,577 277,750 283,516 Interest expense (24,354 ) (22,771 ) (22,428 ) Corporate expenses (36,255 ) (35,024 ) (28,396 ) Income, before federal income tax $ 261,968 219,955 232,692

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Reconciliation of Earnings Per Share	2017 Income Shares Per Share ($ in thousands, except per share amounts) (Numerator) (Denominator) Amount Basic EPS: Net income available to common stockholders $ 168,826 58,458 $ 2.89 Effect of dilutive securities: Stock compensation plans — 899 Diluted EPS: Net income available to common stockholders $ 168,826 59,357 $ 2.84 2016 Income Shares Per Share ($ in thousands, except per share amounts) (Numerator) (Denominator) Amount Basic EPS: Net income available to common stockholders $ 158,495 57,889 $ 2.74 Effect of dilutive securities: Stock compensation plans — 858 Diluted EPS: Net income available to common stockholders $ 158,495 58,747 $ 2.70 2015 Income Shares Per Share ($ in thousands, except per share amounts) (Numerator) (Denominator) Amount Basic EPS: Net income available to common stockholders $ 165,861 57,212 $ 2.90 Effect of dilutive securities: Stock compensation plans — 944 Diluted EPS: Net income available to common stockholders $ 165,861 58,156 $ 2.85

Federal Income Taxes (Tables)

Federal Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of Federal Income Tax on Income at the Corporate Rate to the Effective Tax Rate	($ in thousands) 2017 2016 2015 Tax at statutory rate of 35% $ 91,689 76,984 81,442 Tax-advantaged interest (11,510 ) (12,126 ) (13,164 ) Dividends received deduction (1,961 ) (1,114 ) (1,817 ) Stock based compensation (4,281 ) — — Tax reform rate change 20,205 — — Other (1,000 ) (2,284 ) 370 Federal income tax expense from continuing operations $ 93,142 61,460 66,831
Schedule of Deferred Tax Assets and Liabilities	($ in thousands) 2017 2016 Deferred tax assets: Net loss reserve discounting $ 38,771 70,065 Net unearned premiums 50,267 78,201 Employee benefits 8,606 17,881 Long-term incentive compensation plans 12,221 17,750 Temporary investment write-downs 1,044 2,475 Other investment related items, net — 1,484 Net operating loss 54 771 Other 5,784 8,344 Total deferred tax assets 116,747 196,971 Deferred tax liabilities: Deferred policy acquisition costs 47,484 75,310 Unrealized gains on investment securities 26,183 22,681 Other investment-related items, net 2,500 — Accelerated depreciation and amortization 8,590 14,140 Total deferred tax liabilities 84,757 112,131 Net deferred federal income tax asset $ 31,990 84,840

Retirement Plans (Tables)

Retirement Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Plan Terms In Effect And Revisions Of Retirement Savings Plan	(a) Selective Insurance Retirement Savings Plan (“Retirement Savings Plan”) SICA offers a voluntary defined contribution 401(k) plan, which is available to most of our employees and is a tax-qualified retirement plan subject to the Employee Retirement Income Security Act of 1974 ("ERISA"). Expense recorded for this plan was $15.8 million in 2017 , $15.0 million in 2016 , and $14.1 million in 2015 .
Schedule of Funded Status Of Retirement Income Plan And Retirement Life Plan	December 31, Pension Plan ($ in thousands) 2017 2016 Change in Benefit Obligation: Benefit obligation, beginning of year $ 330,588 310,308 Service cost — 1,647 Interest cost 12,490 12,336 Actuarial losses 31,158 15,086 Benefits paid (9,825 ) (8,789 ) Benefit obligation, end of year $ 364,411 330,588 Change in Fair Value of Assets: Fair value of assets, beginning of year $ 316,515 249,700 Actual return on plan assets, net of expenses 46,983 21,079 Contributions by the employer to funded plans 10,000 54,525 Benefits paid (9,825 ) (8,789 ) Fair value of assets, end of year $ 363,673 316,515 Funded status $ (738 ) (14,073 )
Schedule of Amounts Recognized in Consolidated Balance Sheet	Amounts Recognized in the Consolidated Balance Sheet: Liabilities $ (738 ) (14,073 ) Net pension liability, end of year $ (738 ) (14,073 )
Schedule of Amounts recognized in Accumulated Other than Comprehensive Income	Amounts Recognized in AOCI: Net actuarial loss $ 87,438 85,845 Total $ 87,438 85,845
Schedule of Other Information as of December 31	Other Information as of December 31: Accumulated benefit obligation $ 364,411 330,588
Schedule of Components Of Net Periodic Benefit Cost And Other Amounts Recognized In Other Comprehensive Income	Pension Plan ($ in thousands) 2017 2016 2015 Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income: Net Periodic Benefit Cost: Service cost $ — 1,647 7,215 Interest cost 12,490 12,336 13,668 Expected return on plan assets (19,419 ) (17,309 ) (15,969 ) Amortization of unrecognized actuarial loss 2,001 6,299 6,831 Total net periodic cost $ (4,928 ) 2,973 11,745 Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income: Net actuarial loss (gain) $ 3,594 11,316 (1,425 ) Reversal of amortization of net actuarial loss (2,001 ) (6,299 ) (6,831 ) Total recognized in other comprehensive income $ 1,593 5,017 (8,256 ) Total recognized in net periodic benefit cost and other comprehensive income $ (3,335 ) 7,990 3,489
Schedule Of Weighted- Average Expense and Liability Assumptions	Pension Plan 2017 2016 2015 Weighted-Average Expense Assumptions for the years ended December 31: Discount rate 4.41 % 4.69 4.29 Expected return on plan assets 6.24 6.37 6.27 Rate of compensation increase 1 — — 4.00 Weighted-Average Liability Assumptions as of December 31: Discount rate 3.78 % 4.41
Schedule of Allocation of Plan Assets	2017 2016 Target Percentage 2 Actual Percentage Actual Percentage Return seeking assets 1 20% - 60% 58 % 50 % Liability hedging assets 40% - 80% 42 % 50 % Total 100 % 100 % 100 %
Schedule of Quantitative Disclosures of our Financial Assets that were Measured at Fair Value	December 31, 2017 Fair Value Measurements Using ($ in thousands) Assets Measured at Fair Value Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) 1 Significant Other Observable Inputs (Level 2) 1 Significant Unobservable Inputs (Level 3) Description Measured on a recurring basis: AFS fixed income securities: U.S. government and government agencies $ 49,740 24,652 25,088 — Foreign government 18,555 — 18,555 — Obligations of states and political subdivisions 1,582,970 — 1,582,970 — Corporate securities 1,617,468 — 1,617,468 — CLO and other ABS 795,458 — 795,458 — CMBS 383,449 — 376,895 6,554 RMBS 714,882 — 714,882 — Total AFS fixed income securities 5,162,522 24,652 5,131,316 6,554 AFS equity securities: Common stock 2 167,757 138,640 — 5,398 Preferred stock 14,948 14,948 — — Total AFS equity securities 182,705 153,588 — 5,398 Total AFS securities 5,345,227 178,240 5,131,316 11,952 Short-term investments 165,555 165,555 — — Total assets measured at fair value $ 5,510,782 343,795 5,131,316 11,952 December 31, 2016 Fair Value Measurements Using ($ in thousands) Assets Measured at Fair Value Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) 1 Significant Other Observable Inputs (Level 2) 1 Significant Unobservable Inputs (Level 3) Description Measured on a recurring basis: AFS fixed income securities: U.S. government and government agencies $ 77,333 27,520 49,813 — Foreign government 26,865 — 26,865 — Obligations of states and political subdivisions 1,379,593 — 1,379,593 — Corporate securities 1,997,753 — 1,997,753 — CLO and other ABS 528,960 — 528,960 — CMBS 256,842 — 256,842 — RMBS 525,194 — 525,194 — Total AFS fixed income securities 4,792,540 27,520 4,765,020 — AFS equity securities: Common stock 130,608 122,932 — 7,676 Preferred stock 16,145 16,145 — — Total AFS equity securities 146,753 139,077 — 7,676 Total AFS securities 4,939,293 166,597 4,765,020 7,676 Short-term investments 221,701 221,701 — — Total assets measured at fair value $ 5,160,994 388,298 4,765,020 7,676 1 There were no transfers of securities between Level 1 and Level 2. 2 In accordance with ASU 2015-07, investments amounting to $23.7 million at December 31, 2017, respectively, were measured at fair value using the net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.
Schedule of Expected Benefit Payments	Payments ($ in thousands) Pension Plan Benefits Expected to be Paid in Future Fiscal Years: 2018 $ 12,913 2019 12,936 2020 13,987 2021 15,093 2022 16,128 2023-2027 94,542
Retirement Income Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Schedule of Quantitative Disclosures of our Financial Assets that were Measured at Fair Value	December 31, 2017 Fair Value Measurements at 12/31/17 Using ($ in thousands) Assets Measured at Fair Value At 12/31/17 Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Description Return seeking assets: Long-duration fixed income: Global asset allocation fund $ 41,309 41,309 — — Global equity: Non-U.S. equity 67,989 67,989 — — U.S. equity 66,353 66,353 — — Total global equity 134,342 134,342 — — Private assets: Limited partnerships (at net asset value) 1 : Real assets 16,305 — — — Private equity 1,096 — — — Private credit 460 — — — Hedge fund 15,192 — — — Total limited partnerships 33,053 — — — Other private assets 980 — — 980 Total private assets 34,033 — — 980 Total return seeking assets 209,684 175,651 — 980 Liability hedging assets: Long-duration fixed income: Extended duration fixed income 146,837 146,837 — — Cash and short-term investments: Short-term investments 4,939 4,939 — — Deposit administration contracts 1,615 — 1,615 — Total cash and short-term investments 6,554 4,939 1,615 — Total liability hedging assets 153,391 151,776 1,615 — Total invested assets $ 363,075 $ 327,427 $ 1,615 $ 980 December 31, 2016 Fair Value Measurements at 12/31/16 Using ($ in thousands) Assets Measured at Fair Value At 12/31/16 Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Description Return seeking assets: Long-duration fixed income: Global asset allocation fund $ 37,878 37,878 — — Global equity: Non-U.S. equity 48,836 48,836 — — U.S. equity 55,073 55,073 — — Total global equity 103,909 103,909 — — Private assets (limited partnerships, at net asset value) 1 : Real assets 15,466 — — — Private equity 1,615 — — — Private credit 1,108 — — — Total private assets 18,189 — — — Total return seeking assets 159,976 141,787 — — Liability hedging assets: Long-duration fixed income: Extended duration fixed income 131,457 131,457 — — Cash and short-term investments: Short-term investments 23,722 23,722 — — Deposit administration contracts 1,832 — 1,832 — Total cash and short-term investments 25,554 23,722 1,832 — Total liability hedging assets 157,011 155,179 1,832 — Total invested assets $ 316,987 $ 296,966 $ 1,832 $ — 1 In accordance with ASU 2015-07, certain investments that are measured at fair value using the net asset value per share (or its practical expedient) have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total Pension Plan invested assets.

Share-Based Payments (Tables)

Share-Based Payments (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Share-Based Compensation Retired Plans	December 31, 2017 Types of Share-Based Payments Issued Reserve Shares Awards Outstanding 1 Plan 2005 Omnibus Stock Plan ("2005 Stock Plan") Qualified and nonqualified stock options, SARs, restricted stock, RSUs, phantom stock, stock bonuses, and other awards in such amounts and with such terms and conditions as it determined, subject to the provisions of the 2005 Stock Plan. The maximum exercise period for an option grant under this plan is 10 years from the date of the grant. DEUs are earned during the vesting period on RSU grants. The DEUs are reinvested in the Parent's common stock at fair value on each dividend payment date. 2,202,532 277,132 Parent's Stock Compensation Plan for Non-employee Directors ("Directors Stock Compensation Plan") Directors could elect to receive a portion of their annual compensation in shares of the Parent's common stock. 66,506 66,506 1 Awards outstanding under the 2005 Stock Plan consisted of 47,268 RSUs and 229,864 stock options.
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted	2017 2016 2015 ESPP Issuances 75,093 88,432 100,944 Agent Plan Issuances 49,794 69,867 82,142
Schedule of Share-Based Compensation Shares Authorized and Available for Issuance	As of December 31, 2017 Authorized Available for Issuance Awards Outstanding Stock Plan 3,500,000 2,516,871 882,490 ESPP 1,500,000 499,629 — Agent Plan 3,000,000 1,817,493 —
Schedule Of Share-Based Compensation Plan Approval	Plan Approvals Stock Plan Approved effective as of May 1, 2014 by stockholders on April 23, 2014. Cash Plan Approved effective April 1, 2005 by stockholders on April 27, 2005. ESPP Approved by stockholders on April 29, 2009 effective July 1, 2009. Agent Plan Approved by stockholders on April 26, 2006.
Schedule of Types of Share-Based Payments Issued	Plan Types of Share-Based Payments Issued Stock Plan Qualified and nonqualified stock options, stock appreciation rights ("SARs"), restricted stock, restricted stock units ("RSUs"), stock grants, and other awards valued in whole or in part by reference to the Parent's common stock. The maximum exercise period for an option grant under this plan is 10 years from the date of the grant. Dividend equivalent units ("DEUs") are earned during the vesting period on RSU grants. The DEUs are reinvested in the Parent's common stock at fair value on each dividend payment date. The requisite service period for grants to employees under this plan is the lesser of: (i) the stated vested date, which is typically three years from issuance; or (ii) the date the employee becomes eligible to retire. Cash Plan Cash incentive units (“CIUs”). The initial dollar value of each CIU will be adjusted to reflect the percentage increase or decrease in the total shareholder return on the Parent's common stock over a specified performance period. In addition, for certain grants, the number of CIUs granted will be increased or decreased to reflect our performance on specified performance indicators as compared to targeted peer companies. The requisite service period for grants under this plan is the lesser of: (i) the stated vested date, which is typically three years from issuance; or (ii) the date the employee becomes eligible to retire. ESPP Enables employees to purchase shares of the Parent’s common stock. The purchase price is the lower of: (i) 85% of the closing market price at the time the option is granted; or (ii) 85% of the closing price at the time the option is exercised. Shares are generally issued on June 30 and December 31 of each year. Agent Plan Quarterly offerings to purchase the Parent's common stock at a 10% discount with a one year restricted period during which the shares purchased cannot be sold or transferred. Only our independent retail insurance agencies and wholesale general agencies, and certain eligible persons associated with the agencies, are eligible to participate in this plan.
Schedule of Summary of Stock Option Transactions under Share Based Payment Plans	Number Weighted Weighted Aggregate Outstanding at December 31, 2016 355,391 $ 16.87 Granted in 2017 — — Exercised in 2017 (120,496 ) 19.39 Forfeited or expired in 2017 (5,031 ) 24.54 Outstanding at December 31, 2017 229,864 $ 15.38 1.50 $ 9,958 Exercisable at December 31, 2017 229,864 $ 15.38 1.50 $ 9,958
Schedule of Summary of Restricted Stock Units Transactions under Share-Based Payment Plans	Number Weighted Unvested RSU awards at December 31, 2016 916,640 $ 26.20 Granted in 2017 321,928 42.66 Vested in 2017 (360,702 ) 22.78 Forfeited in 2017 (12,279 ) 32.09 Unvested RSU awards at December 31, 2017 865,587 $ 33.66
Schedule of Weighted Average Assumptions for Employee Stock Purchase Plan	ESPP 2017 2016 2015 Risk-free interest rate 1.07 % 0.47 0.10 Expected term 6 months 6 months 6 months Dividend yield 1.3 % 1.7 2.0 Expected volatility 24 % 31 20
Schedule of Weighted-Average Fair Value of Options and Stock Per Share	2017 2016 2015 RSUs $ 42.66 32.53 25.22 ESPP: Six month option 2.73 2.63 1.26 Discount of grant date market value 7.06 5.23 4.16 Total ESPP 9.79 7.86 5.42 Agent Plan: Discount of grant date market value 5.04 3.79 2.94
Schedule of Compensation Cost for Share-based Payment Arrangements	($ in millions) 2017 2016 2015 Share-based compensation expense, pre-tax $ 31.2 30.3 23.8 Income tax benefit, including the benefit related to stock grants that have vested during the year (15.0 ) (10.3 ) (8.0 ) Share-based compensation expense, after-tax $ 16.2 20.0 15.8

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Commitments For Non-Cancelable Leases	($ in millions) Capital Leases Operating Leases Total 2018 $ 2.3 10.0 12.3 2019 0.1 7.5 7.6 2020 — 6.0 6.0 2021 — 3.7 3.7 2022 — 2.1 2.1 After 2022 — 2.6 2.6 Total minimum payment required $ 2.4 31.9 34.3

Statutory Financial Informati50

Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Schedule of Insurance Subsidiaries Statutory Surplus Data	State of Domicile Unassigned Surplus Statutory Surplus Statutory Net Income ($ in millions) 2017 2016 2017 2016 2017 2016 2015 SICA New Jersey $ 455.5 414.4 609.7 568.6 84.6 72.2 69.6 Selective Way Insurance Company ("SWIC") New Jersey 276.1 260.5 325.1 309.5 43.6 41.2 42.3 SICSC Indiana 112.9 110.6 144.1 141.9 17.9 17.4 15.9 SICSE Indiana 86.2 83.5 111.8 109.1 14.7 13.4 12.1 SICNY New York 78.8 74.1 106.5 101.8 13.4 12.9 12.7 Selective Insurance Company of New England ("SICNE") New Jersey 16.1 13.6 46.3 43.7 6.3 5.9 5.5 Selective Auto Insurance Company of New Jersey ("SAICNJ") New Jersey 42.1 36.9 84.9 79.8 11.4 11.5 10.8 MUSIC New Jersey 21.4 16.7 89.9 85.2 10.3 9.7 9.5 Selective Casualty Insurance Company ("SCIC") New Jersey 34.5 26.6 109.0 101.0 13.4 12.6 12.1 Selective Fire and Casualty Insurance Company ("SFCIC") New Jersey 13.7 11.3 45.6 43.2 5.6 5.5 5.3 Total $ 1,137.3 1,048.2 1,672.9 1,583.8 221.2 202.3 195.8
Schedule of Insurance Subsidiaries Dividend Paid To Parent	Dividends Twelve Months ended December 31, 2017 ($ in millions) State of Domicile Ordinary Dividends Paid SICA New Jersey $ 28.0 SWIC New Jersey 19.0 SICSC Indiana 10.0 SICSE Indiana 7.5 SICNY New York 4.5 SICNE New Jersey 2.0 SAICNJ New Jersey 2.5 MUSIC New Jersey 2.1 SCIC New Jersey 3.0 SFCIC New Jersey 1.5 Total $ 80.1
Schedule of Maximum Ordinary Dividends Paid to Parent by Insurance Subsidiaries	2018 ($ in millions) State of Domicile Maximum Ordinary Dividends SICA New Jersey $ 77.6 SWIC New Jersey 43.3 SICSC Indiana 17.9 SICSE Indiana 14.7 SICNY New York 10.7 SICNE New Jersey 6.2 SAICNJ New Jersey 11.3 MUSIC New Jersey 10.3 SCIC New Jersey 13.4 SFCIC New Jersey 5.6 Total $ 211.0

Quarterly Financial Informati51

Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	(unaudited, $ in thousands, First Quarter Second Quarter Third Quarter Fourth Quarter except per share data) 2017 2016 2017 2016 2017 2016 2017 1 2016 Net premiums earned $ 560,854 522,458 568,030 531,932 572,055 542,429 590,088 552,753 Net investment income earned 37,419 30,769 41,430 31,182 40,446 33,375 42,587 35,428 Net realized (losses) gains (1,045 ) (2,704 ) 1,734 1,765 6,798 3,688 (1,128 ) (7,686 ) Other income 3,241 951 3,291 3,868 1,994 2,199 2,190 1,863 Total revenues 600,469 551,474 614,485 568,747 621,293 581,691 633,737 582,358 Income before federal income taxes 67,574 51,875 58,929 62,311 67,315 55,443 68,150 50,326 Net income 50,440 37,032 41,426 43,601 46,718 38,502 30,242 39,360 Net income per share: Basic 0.87 0.64 0.71 0.75 0.80 0.66 0.52 0.68 Diluted 0.85 0.63 0.70 0.74 0.79 0.66 0.51 0.67 1 Results for the fourth quarter of 2017 include the impact of the $20.2 million write off of deferred tax assets required with the implementation of Tax Reform. See Note 13. "Federal Income Taxes" above for additional information. The addition of all quarters may not agree to annual amounts on the Financial Statements due to rounding.

Summary of Significant Accoun52

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended			12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Depreciation expense				$ 17.8	$ 17.4	$ 16.4
Investment yields before tax assumed in premium deficiency assessment	2.90%	2.40%	2.50%

Summary of Significant Accoun53

Summary of Significant Accounting Policies (Property and Equipment) (Details)	12 Months Ended
	Dec. 31, 2017
Computer hardware [Member]
Property, Plant and Equipment, Estimated Useful Lives	3
Furniture and fixtures [Member]
Property, Plant and Equipment, Estimated Useful Lives	10
Maximum [Member] | Computer software [Member]
Property, Plant and Equipment, Estimated Useful Lives	5
Maximum [Member] | Software licenses [Member]
Property, Plant and Equipment, Estimated Useful Lives	5
Maximum [Member] | Internally developed software [Member]
Property, Plant and Equipment, Estimated Useful Lives	10
Maximum [Member] | Building and improvements [Member]
Property, Plant and Equipment, Estimated Useful Lives	40
Minimum [Member] | Computer software [Member]
Property, Plant and Equipment, Estimated Useful Lives	3
Minimum [Member] | Software licenses [Member]
Property, Plant and Equipment, Estimated Useful Lives	3
Minimum [Member] | Internally developed software [Member]
Property, Plant and Equipment, Estimated Useful Lives	5
Minimum [Member] | Building and improvements [Member]
Property, Plant and Equipment, Estimated Useful Lives	5

Adoption of Accounting Pronou54

Adoption of Accounting Pronouncements(Other Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based compensation expense income tax benefit	$ (4,281)	$ 0	$ 0
Accumulated other comprehensive income (loss)	20,170	(15,950)
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax	$ 10,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share	$ 0.17
Proceeds from Equity Method Investment, Distribution	$ 2,000	3,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	7,000	25,000
Equity Securities [Member]
Accumulated other comprehensive income (loss)	25,000
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes	13,000
National Flood Insurance Program [Member]
Restricted cash	$ 44,200	$ 36,900	$ 11,900

Statements of Cash Flow (Cash F

Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest	$ 23,905	$ 22,098	$ 21,892
Federal income tax	62,000	46,405	39,500
Corporate actions related to equity securities, AFS	4,725	3,263	4,239
Assets acquired under capital lease arrangements	278	3,151	6,760
Non-cash purchase of property and equipment	0	78	0
AFS Fixed Income Securities [Member]
Exchange of fixed income securities	22,511	23,579	36,792
Held-to-maturity Securities [Member]
Exchange of fixed income securities	0	0	15,257
National Flood Insurance Program [Member]
Restricted cash	$ 44,200	$ 36,900	$ 11,900

Investments (Unrealized Gains o

Investments (Unrealized Gains on Investments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net unrealized gains	$ 124,679	$ 64,804	$ 69,224
Deferred income tax expense	(44,103)	(22,681)	(24,228)
Net unrealized gains, net of deferred income tax	80,576	42,123	44,996
Increase (decrease) in net unrealized gains in OCI, net of deferred income tax	38,453	(2,873)	(35,398)
AFS Fixed Income Securities [Member]
Net unrealized gains	85,806	38,781	55,689
Equity Securities [Member]
Net unrealized gains	38,894	25,864	13,235
Available-for-sale Securities [Member]
Net unrealized gains	124,700	64,645	68,924
Total HTM Securities [Member]
Net unrealized gains	$ (21)	$ 159	$ 300

Investments (Held-To-Maturity S

Investments (Held-To-Maturity Securities Disclosure) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost	$ 42,150	$ 101,397
Net Unrealized Gains (Losses)	(21)	159
Total HTM fixed income securities carrying value	42,129	101,556
Unrecognized Holding Gains	2,026	3,813
Unrecognized Holding Losses	(55)	(158)
Total HTM fixed income securities fair value	44,100	105,211
Obligations of States and Political Subdivisions [Member]
Amortized Cost	25,154	77,466
Net Unrealized Gains (Losses)	84	317
Total HTM fixed income securities carrying value	25,238	77,783
Unrecognized Holding Gains	1,023	2,133
Unrecognized Holding Losses	0	0
Total HTM fixed income securities fair value	26,261	79,916
Corporate Securities [Member]
Amortized Cost	16,996	22,711
Net Unrealized Gains (Losses)	(105)	(143)
Total HTM fixed income securities carrying value	16,891	22,568
Unrecognized Holding Gains	1,003	1,665
Unrecognized Holding Losses	(55)	(158)
Total HTM fixed income securities fair value	$ 17,839	24,075
Commercial Mortgage Backed Securities [Member]
Amortized Cost		1,220
Net Unrealized Gains (Losses)		(15)
Total HTM fixed income securities carrying value		1,205
Unrecognized Holding Gains		15
Unrecognized Holding Losses		0
Total HTM fixed income securities fair value		$ 1,220

Investments (Available-For-Sale

Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	$ 5,220,527	$ 4,874,648
Unrealized Gains	130,058	80,702
Unrealized Losses	(5,358)	(16,057)
Total AFS securities	5,345,227	4,939,293
AFS Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	5,076,716	4,753,759
Unrealized Gains	90,867	54,178
Unrealized Losses	(5,061)	(15,397)
Total AFS securities	5,162,522	4,792,540
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	49,326	75,139
Unrealized Gains	647	2,230
Unrealized Losses	(233)	(36)
Total AFS securities	49,740	77,333
AFS Fixed Income Securities [Member] | Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	18,040	26,559
Unrealized Gains	526	322
Unrealized Losses	(11)	(16)
Total AFS securities	18,555	26,865
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,539,307	1,366,287
Unrealized Gains	44,245	18,610
Unrealized Losses	(582)	(5,304)
Total AFS securities	1,582,970	1,379,593
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	1,588,339	1,976,556
Unrealized Gains	30,891	27,057
Unrealized Losses	(1,762)	(5,860)
Total AFS securities	1,617,468	1,997,753
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	789,152	527,876
Unrealized Gains	6,508	1,439
Unrealized Losses	(202)	(355)
Total AFS securities	795,458	528,960
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	382,727	256,356
Unrealized Gains	1,563	1,514
Unrealized Losses	(841)	(1,028)
Total AFS securities	383,449	256,842
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	709,825	524,986
Unrealized Gains	6,487	3,006
Unrealized Losses	(1,430)	(2,798)
Total AFS securities	714,882	525,194
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	143,811	120,889
Unrealized Gains	39,191	26,524
Unrealized Losses	(297)	(660)
Total AFS securities	182,705	146,753
Equity Securities [Member] | Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	129,696	104,663
Unrealized Gains	38,287	26,250
Unrealized Losses	(226)	(305)
Total AFS securities	167,757	130,608
Equity Securities [Member] | Nonredeemable Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost/Amortized Cost	14,115	16,226
Unrealized Gains	904	274
Unrealized Losses	(71)	(355)
Total AFS securities	$ 14,948	$ 16,145

Investments (Number of Securiti

Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Percentage Decline in Fair Value Percentage of Amortized Cost	1.00%	1.00%
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	$ 100	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	878,126	1,358,111
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4,961)	(15,651)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	35,686	15,626
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(397)	(406)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	913,812	1,373,737
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,358)	(16,057)
AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	869,566	1,340,672
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4,664)	(14,991)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	35,686	15,626
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(397)	(406)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	905,252	1,356,298
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,061)	(15,397)
Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	8,560	17,439
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(297)	(660)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	8,560	17,439
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(297)	(660)
U.S. Government and Government Agencies [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	23,516	6,419
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(233)	(36)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	250	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	23,766	6,419
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(233)	(36)
Foreign Government [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,481	13,075
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(11)	(16)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	1,481	13,075
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(11)	(16)
Obligations of States and Political Subdivisions [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	107,514	306,509
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(422)	(5,304)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	14,139	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(160)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	121,653	306,509
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(582)	(5,304)
Corporate Securities [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	238,326	462,902
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,744)	(5,771)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3,228	4,913
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(18)	(89)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	241,554	467,815
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,762)	(5,860)
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	74,977	189,795
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(196)	(354)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,655	319
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6)	(1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	76,632	190,114
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(202)	(355)
Commercial Mortgage Backed Securities [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	154,267	82,492
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(773)	(1,021)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,214	1,645
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(68)	(7)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	159,481	84,137
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(841)	(1,028)
Residential Mortgage Backed Securities [Member] | AFS Fixed Maturity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	269,485	279,480
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,285)	(2,489)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	11,200	8,749
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(145)	(309)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	280,685	288,229
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,430)	(2,798)
Common Stock [Member] | Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	4,727	11,271
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(226)	(305)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	4,727	11,271
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(226)	(305)
Nonredeemable Preferred Stock [Member] | Equity Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,833	6,168
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(71)	(355)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	3,833	6,168
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (71)	$ (355)

Investments (Fixed Maturity Sec

Investments (Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
AFS due in one year or less fair value	$ 315,857
AFS due after one year through five years fair value	2,099,529
AFS due after five years through 10 years fair value	2,510,294
AFS due after 10 years fair value	236,842
Fixed income securities, available-for-sale securities	5,162,522	$ 4,792,540
HTM due in one year or less carrying value	10,997
HTM due after one year through five years carrying value	23,035
HTM due after five years through 10 years carrying value	8,097
HTM due after 10 years carrying value	0
Total HTM fixed income securities carrying value	42,129	101,556
HTM due in one year or less fair value	11,168
HTM due after one year through five years fair value	24,235
HTM due after five years through 10 years fair value	8,697
HTM due after 10 years fair value	0
Total HTM fixed income securities fair value	$ 44,100	$ 105,211

Investments (Other Investment P

Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying Value	$ 132,268	$ 102,397
Other Investment Portfolio [Member]
Carrying Value	132,268	102,397
Remaining Commitment	220,999	143,686
Maximum Exposure to Loss	353,267	246,083
Alternative Investments [Member]
Carrying Value	115,373	83,814
Remaining Commitment	220,999	140,286
Maximum Exposure to Loss	336,372	224,100
Private Equity [Member] | Alternative Investments [Member]
Carrying Value	52,251	41,135
Remaining Commitment	99,026	76,774
Maximum Exposure to Loss	151,277	117,909
Private Credit Funds [Member] | Alternative Investments [Member]
Carrying Value	37,743	28,193
Remaining Commitment	94,959	40,613
Maximum Exposure to Loss	132,702	68,806
Real Assets [Member] | Alternative Investments [Member]
Carrying Value	25,379	14,486
Remaining Commitment	27,014	22,899
Maximum Exposure to Loss	52,393	37,385
Other Securities [Member]
Carrying Value	16,895	18,583
Remaining Commitment	0	3,400
Maximum Exposure to Loss	$ 16,895	$ 21,983

Investments (Aggregated Balance

Investments (Aggregated Balance Sheet Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - Investments Accounted For Under The Equity Method [Member] - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Investments	$ 21,046	$ 11,244
Total assets	22,357	12,075
Total liabilities	4,767	1,802
Partners' capital	$ 17,590	$ 10,273

Investments (Aggregated Income

Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Sep. 30, 2014
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 42,587	$ 40,446	$ 41,430	$ 37,419	$ 35,428	$ 33,375	$ 31,182	$ 30,769	$ 161,882	$ 130,754		$ 121,316
Investments Accounted For Under The Equity Method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment (loss) income											$ (143,000)		$ (44,000)	$ 129,000
Realized gains											325,000		1,374,000	1,187,000
Net change in unrealized appreciation (depreciation)											2,894,000		(719,000)	(1,364,000)
Net Income											$ 3,076,000		$ 611,000	$ (48,000)
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned									$ 12,700	$ 3,100		$ (1,900)

Investments (Credit Concentrati

Investments (Credit Concentration Risk) (Details)	Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Maximum exposure to credit concentration risk of the Company's stockholder's equity other than certain U.S. government agencies	10.00%	10.00%

Investments (Investments Pledge

Investments (Investments Pledged as Collateral) (Details) $ in Millions	Dec. 31, 2017USD ($)
Schedule of Available-for-sale Securities [Line Items]
Assets Held by Insurance Regulators	$ 25.7
Securities pledged as collateral for reinsurance obligations	164.9
U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held by Insurance Regulators	22.6
Securities pledged as collateral for reinsurance obligations	25.6
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	0
Assets Held by Insurance Regulators	3.1
Securities pledged as collateral for reinsurance obligations	3.1
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held by Insurance Regulators	0
Securities pledged as collateral for reinsurance obligations	20.3
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held by Insurance Regulators	0
Securities pledged as collateral for reinsurance obligations	115.9
Federal Home Loan Bank of New York [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	73.7
Federal Home Loan Bank of New York [Member] | U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	0
Federal Home Loan Bank of New York [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	14.1
Federal Home Loan Bank of New York [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	59.6
Federal Home Loan Bank of Indianapolis [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	65.5
Federal Home Loan Bank of Indianapolis [Member] | U.S. Government and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	3
Federal Home Loan Bank of Indianapolis [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	6.2
Federal Home Loan Bank of Indianapolis [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 56.3

Investments (Net Investment Inc

Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement [Line Items]
Net investment income earned	$ 42,587	$ 40,446	$ 41,430	$ 37,419	$ 35,428	$ 33,375	$ 31,182	$ 30,769	$ 161,882	$ 130,754	$ 121,316
Fixed Income Securities [Member]
Statement [Line Items]
Net investment income earned									153,230	129,306	123,230
Equity Securities [Member]
Statement [Line Items]
Net investment income earned									6,442	7,368	9,161
Short-term Investments [Member]
Statement [Line Items]
Net investment income earned									1,526	686	112
Other Investment Portfolio [Member]
Statement [Line Items]
Net investment income earned									12,871	2,940	(1,890)
Investment Expenses [Member]
Statement [Line Items]
Net investment income earned									$ (12,187)	$ (9,546)	$ (9,297)

Investments (OTTI by Asset Type

Investments (OTTI by Asset Type) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OTTI Losses Gross	$ 4,809	$ 8,509	$ 18,366
OTTI included in OCI	(36)	10	0
Total OTTI charges recognized in earnings	4,845	8,499	18,366
Other Investments [Member]
OTTI Losses Gross	190
OTTI included in OCI	0
Total OTTI charges recognized in earnings	190
AFS Fixed Income Securities [Member]
OTTI Losses Gross	3,184	5,191	2,189
OTTI included in OCI	(36)	10	0
Total OTTI charges recognized in earnings	3,220	5,181	2,189
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
OTTI Losses Gross	36
OTTI included in OCI	0
Total OTTI charges recognized in earnings	36
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
OTTI Losses Gross	612	2,797
OTTI included in OCI	0	0
Total OTTI charges recognized in earnings	612	2,797
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
OTTI Losses Gross	587	1,880	2,188
OTTI included in OCI	0	0	0
Total OTTI charges recognized in earnings	587	1,880	2,188
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member]
OTTI Losses Gross	96	19
OTTI included in OCI	0	0
Total OTTI charges recognized in earnings	96	19
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
OTTI Losses Gross	670	220
OTTI included in OCI	0	0
Total OTTI charges recognized in earnings	670	220
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
OTTI Losses Gross	1,183	275	1
OTTI included in OCI	(36)	10	0
Total OTTI charges recognized in earnings	1,219	265	1
Available-for-sale Securities [Member] | Equity Securities [Member]
OTTI Losses Gross	1,435	3,318	16,177
OTTI included in OCI	0	0	0
Total OTTI charges recognized in earnings	1,435	3,318	16,177
Common Stock [Member] | Available-for-sale Securities [Member] | Equity Securities [Member]
OTTI Losses Gross	1,435	3,316	15,996
OTTI included in OCI	0	0	0
Total OTTI charges recognized in earnings	$ 1,435	3,316	15,996
Nonredeemable Preferred Stock [Member] | Available-for-sale Securities [Member] | Equity Securities [Member]
OTTI Losses Gross		2	181
OTTI included in OCI		0	0
Total OTTI charges recognized in earnings		$ 2	$ 181

Investments (Components Of Net

Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
HTM fixed income securities gains									$ 44	$ 3	$ 5
HTM fixed income securities losses									(1)	(1)	(1)
Short Term Investments Realized Gains									(2)	0	0
Short Term Investments Realized Losses									(6)	(13)	0
Other investment securities gains									494	3	162
Other investment securities losses									(859)	(4)	(653)
Net realized investment gains	$ (1,128)	$ 6,798	$ 1,734	$ (1,045)	$ (7,686)	$ 3,688	$ 1,765	$ (2,704)	11,204	3,562	31,537
Proceeds from sale of available-for-sale securities									1,235,900	1,046,100	234,100
Fixed Income Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
AFS securities gains									10,193	7,741	4,515
AFS securities losses									(3,292)	(11,411)	(312)
Equity Securities [Member]
Schedule of Available-for-sale and Held-to-maturity Securities [Line Items]
AFS securities gains									5,829	8,108	29,168
AFS securities losses									$ (1,200)	$ (864)	$ (1,347)

Comprehensive Income (Component

Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income before federal income tax	$ 68,150	$ 67,315	$ 58,929	$ 67,574	$ 50,326	$ 55,443	$ 62,311	$ 51,875	$ 261,968	$ 219,955	$ 232,692
Income Tax Expense (Benefit)									93,142	61,460	66,831
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	168,826	158,495	165,861
Unrealized holding gains (losses) during the period, Gross									66,894	(9,195)	(40,221)
Unrealized holding gains (losses) during the period, Tax									23,879	(3,218)	(14,078)
Unrealized holding gains (losses) arising during period, Net									43,015	(5,977)	(26,143)
Non-credit portion of other-than-temporary impairment losses recognized in other comprehensive income, Gross									36	(10)
Non-Credit Portion of Other-Than-Temporary Impairment Losses Recognized In Other Comprehensive Income, Tax									13	(4)
Non-credit portion of other-than-temporary impairments recognized in other comprehensive income, Net									23	(6)
Amount reclassified into net income: HTM securities, Gross									(179)	(141)	(580)
Amounts reclassified into net income: HTM securities, Tax									(63)	(49)	(203)
Amount reclassified into net income: HTM securities, Net									(116)	(92)	(377)
Non-credit OTTI, Gross									104	213	357
Non-credit OTTI, Tax									36	75	125
Amounts reclassified into net income: Non-credit other-than-temporary impairment									68	138	232
Realized (gains) losses on AFS securities, Gross									(6,979)	4,713	(14,016)
Realized (gains) losses on AFS securities, Tax									(2,442)	1,649	(4,906)
Realized (gains) losses on available for sale securities									(4,537)	3,064	(9,110)
Net unrealized gains (losses), Gross									59,876	(4,420)	(54,460)
Net unrealized gains (losses), Tax									21,423	(1,547)	(19,062)
Total unrealized gains (losses) on investment securities									38,453	(2,873)	(35,398)
Net actuarial (loss) gain, Gross									(4,684)	(12,079)	2,438
Net actuarial (loss) gain, Tax									(984)	(4,227)	853
Net actuarial (loss) gain, Net									(3,700)	(7,852)	1,585
Amount reclassified into net i ncome: Net actuarial loss, Gross									2,102	6,462	7,077
Amount reclassified into Net Income: Net actuarial loss, Tax									735	2,262	2,477
Amount reclassified into net income: Net actuarial loss, Net									1,367	4,200	4,600
Defined pension and other post-retirement benefit plans, Gross									(2,582)	(5,617)	9,515
Defined pension and other post-retirement benefit plans, Tax									(249)	(1,965)	3,330
Total defined benefit pension and post-retirement plans									(2,333)	(3,652)	6,185
Other Comprehensive Income (Loss), Gross									57,294	(10,037)	(44,945)
Other Comprehensive Income (Loss), Tax									21,174	(3,512)	(15,732)
Other comprehensive income (loss)									36,120	(6,525)	(29,213)
Comprehensive Income, Before Tax									319,262	209,918	187,747
Comprehensive Income, Tax									114,316	57,948	51,099
Comprehensive income									$ 204,946	$ 151,970	$ 136,648

Comprehensive Income (Compone70

Comprehensive Income (Components of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	$ (15,950)
Amounts reclassified into net income: Non-credit other-than-temporary impairment	68	$ 138	$ 232
Amount reclassified from AOCI, HTM	(116)	(92)	(377)
OCI before reclassifications, Defined Benefit Pension and Post Retirement Plans	(3,700)	(7,852)	1,585
Amounts reclassified from AOCI,All other	(4,537)	3,064	(9,110)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	(2,333)	(3,652)	6,185
Other comprehensive income (loss)	36,120	(6,525)	(29,213)
Balance December,	20,170	(15,950)
Accumulated Other-than-Temporary Impairment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	(150)	(282)
OCI before reclassifications	23	(6)
Other comprehensive income (loss)	91	132
Balance December,	(59)	(150)	(282)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	(58,072)	(54,420)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	1,367	4,200
Balance December,	(60,405)	(58,072)	(54,420)
Accumulated other comprehensive income (loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	(15,950)	(9,425)
OCI before reclassifications	39,338	(13,835)
Amount reclassified from AOCI	(3,218)	7,310
Other comprehensive income (loss)	36,120	(6,525)	(29,213)
Balance December,	20,170	(15,950)	(9,425)
Held-to-maturity Securities [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	102	194
OCI before reclassifications	0	0
Other comprehensive income (loss)	(116)	(92)
Balance December,	(14)	102	194
Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	42,170	45,083
OCI before reclassifications	43,015	(5,977)
Other comprehensive income (loss)	38,478	(2,913)
Balance December,	80,648	42,170	45,083
Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance December,	42,122	44,995
OCI before reclassifications	43,038	(5,983)
Amount reclassified from AOCI	(4,585)	3,110
Other comprehensive income (loss)	38,453	(2,873)
Balance December,	$ 80,575	$ 42,122	$ 44,995

Comprehensive Income (Reclassif

Comprehensive Income (Reclassification out of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Realized investment gains (losses)									$ 6,359	$ (4,937)	$ 13,171
Net investment income	$ 42,587	$ 40,446	$ 41,430	$ 37,419	$ 35,428	$ 33,375	$ 31,182	$ 30,769	161,882	130,754	121,316
Income before federal income tax	68,150	67,315	58,929	67,574	50,326	55,443	62,311	51,875	261,968	219,955	232,692
Income Tax Expense (Benefit)									(93,142)	(61,460)	(66,831)
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	168,826	158,495	165,861
Loss and loss expense incurred									1,345,074	1,234,797	1,148,541
Policy acquisition costs									333,097	321,395	300,359
Total defined benefit pension and post-retirement plans									(2,333)	(3,652)	$ 6,185
Reclassification out of Accumulated Other Comprehensive Income [Member]
Net income									(3,218)	7,310
Reclassification out of Accumulated Other Comprehensive Income [Member] | Retirement Income Plan [Member]
Income before federal income tax									2,102	6,462
Income Tax Expense (Benefit)									(735)	(2,262)
Net income									1,367	4,200
Loss and loss expense incurred									450	1,486
Policy acquisition costs									1,652	4,976
Accumulated other-than-temporary impairments including portion attributable to noncontrolling interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Realized investment gains (losses)									104	213
Income before federal income tax									104	213
Income Tax Expense (Benefit)									(36)	(75)
Net income									68	138
Held-to-maturity Securities [Member] | Accumulated net investment gain (loss) including portion attributable to noncontrolling interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Realized investment gains (losses)									32	169
Net investment income									(211)	(310)
Income before federal income tax									(179)	(141)
Income Tax Expense (Benefit)									63	49
Net income									(116)	(92)
Available-for-sale Securities [Member] | Accumulated net investment gain (loss) including portion attributable to noncontrolling interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Realized investment gains (losses)									(6,979)	4,713
Income before federal income tax									(6,979)	4,713
Income Tax Expense (Benefit)									2,442	(1,649)
Net income									$ (4,537)	$ 3,064

Fair Value Measurements (Carryi

Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Held-to-maturity Securities	$ 42,129	$ 101,556
Available-for-sale Securities, Debt Securities	5,162,522	4,792,540
Equity securities, AFS	182,705	146,753
Long-term debt	439,116	438,667
Short-term investments	165,555	221,701
Seven Point Twenty Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	61,391	56,148
Six Point Seventy Percent Senior Notes [Member]
Long-term Debt, Fair Value	116,597	108,333
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	186,332	176,860
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Fair Value	24,270	24,286
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Fair Value	24,210	24,219
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term Debt, Fair Value	60,334	59,313
Carrying Amount [Member]
Held-to-maturity Securities	42,129	101,556
Available-for-sale Securities, Debt Securities	5,162,522	4,792,540
Equity securities, AFS	182,705	146,753
Long-term debt	439,116	438,667
Short-term investments	165,555	221,701
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Long-term debt	49,904	49,901
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member]
Long-term debt	99,446	99,430
Carrying Amount [Member] | Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Long-term debt	185,000	185,000
Carrying Amount [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term debt	25,000	25,000
Carrying Amount [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term debt	25,000	25,000
Carrying Amount [Member] | Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term debt	60,000	60,000
Carrying Amount [Member] | Long Term Debt Excluding Issuance Costs [Member]
Long-term debt	444,350	444,331
Carrying Amount [Member] | Unamortized Debt Issuance Costs [Member]
Unamortized Debt Issuance Costs Related to Long-Term Debt	(5,234)	(5,664)
Fair Value [Member]
Held-to-maturity Securities	44,100	105,211
Available-for-sale Securities, Debt Securities	5,162,522	4,792,540
Equity securities, AFS	182,705	146,753
Long-term Debt, Fair Value	473,134	449,159
Short-term investments	165,555	221,701
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	61,391	56,148
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member]
Long-term Debt, Fair Value	116,597	108,333
Fair Value [Member] | Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	186,332	176,860
Fair Value [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Fair Value	24,270	24,286
Fair Value [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Fair Value	24,210	24,219
Fair Value [Member] | Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term Debt, Fair Value	$ 60,334	$ 59,313

Fair Value Measurements (Quanti

Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments Net Asset Value	$ 23,700
Available-for-sale Securities, Debt Securities	5,162,522	$ 4,792,540
Equity securities, AFS	182,705	146,753
Total AFS securities	5,345,227	4,939,293
Short-term investments	165,555	221,701
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	5,162,522	4,792,540
Equity securities, AFS	182,705	146,753
Total AFS securities	5,345,227	4,939,293
Short-term investments	165,555	221,701
Total Assets	5,510,782	5,160,994
U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	49,740	77,333
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	18,555	26,865
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,582,970	1,379,593
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,617,468	1,997,753
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	795,458	528,960
Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	383,449	256,842
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	714,882	525,194
Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	167,757	130,608
Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	14,948	16,145
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	24,652	27,520
Equity securities, AFS	153,588	139,077
Total AFS securities	178,240	166,597
Short-term investments	165,555	221,701
Total Assets	343,795	388,298
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	24,652	27,520
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	138,640	122,932
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	14,948	16,145
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	5,131,316	4,765,020
Equity securities, AFS	0	0
Total AFS securities	5,131,316	4,765,020
Short-term investments	0	0
Total Assets	5,131,316	4,765,020
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	25,088	49,813
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	18,555	26,865
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,582,970	1,379,593
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	1,617,468	1,997,753
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	795,458	528,960
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	376,895	256,842
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	714,882	525,194
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	0	0
Significant Other Observable Inputs (Level 2) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	6,554	0
Equity securities, AFS	5,398	7,676
Total AFS securities	11,952	7,676
Short-term investments	0	0
Total Assets	11,952	7,676
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	6,554	0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt Securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	5,398	7,676
Significant Unobservable Inputs (Level 3) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, AFS	$ 0	$ 0

Fair Value Measurements (Change

Fair Value Measurements (Changes in Fair Value of Securities Using Level 3 Inputs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Beginning Balance	$ 7,676
Total net (losses) gains for the period included in OCI	0
Total net (losses) gains for the period included in net income	0
Purchases	3,780
Sales	(3,958)
Issuances	0
Settlements	0
Transfers into Level 3	0
Transfers out of Level 3	2,100
Fair Value, Ending Balance	5,398	$ 7,676
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Beginning Balance	$ 6,554	0
Total net (losses) gains for the period included in OCI		(4)
Total net (losses) gains for the period included in net income		0
Purchases		6,550
Sales		0
Issuances		0
Settlements		0
Transfers into Level 3		0
Transfers out of Level 3		0
Fair Value, Ending Balance		$ 6,554

Fair Value Measurements (Quan75

Fair Value Measurements (Quantitative Information of Our Financial Assets and Liabilities That Were Disclosed at Fair Value) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	$ 44,100	$ 105,211
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	0	0
Long-term Debt, Fair Value	186,332	176,860
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	38,567	97,701
Long-term Debt, Fair Value	286,802	272,299
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	5,533	7,510
Long-term Debt, Fair Value	0	0
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	26,261	79,916
Obligations of States and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	0	0
Obligations of States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	26,261	79,916
Obligations of States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	0	0
Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	17,839	24,075
Corporate Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	0	0
Corporate Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	12,306	16,565
Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value	5,533	7,510
Commercial Mortgage-backed Securities (CMBS) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value		1,220
Commercial Mortgage-backed Securities (CMBS) [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value		0
Commercial Mortgage-backed Securities (CMBS) [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value		1,220
Commercial Mortgage-backed Securities (CMBS) [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total HTM fixed income securities fair value		0
Seven Point Twenty Five Percent Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	61,391	56,148
Seven Point Twenty Five Percent Senior Notes [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Seven Point Twenty Five Percent Senior Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	61,391	56,148
Seven Point Twenty Five Percent Senior Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Six Point Seventy Percent Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	116,597	108,333
Six Point Seventy Percent Senior Notes [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Six Point Seventy Percent Senior Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	116,597	108,333
Six Point Seventy Percent Senior Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	186,332	176,860
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	186,332	176,860
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	24,270	24,286
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	24,270	24,286
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	24,210	24,219
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	24,210	24,219
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	60,334	59,313
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	60,334	59,313
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	0	0
Long-term Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	$ 473,134	$ 449,159

Reinsurance (Total Reinsurance

Reinsurance (Total Reinsurance Balances Segregated By Reinsurer) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Terrorism Risk Insurance Program Companys Estimated Deductible		$ 323,000
Terrorism Risk Insurance Program Reimbursed Percentage Maximum		82.00%
Terrorism Risk Insurance Program Annual Cap		$ 100,000,000
Terrorism Risk Insurance Program Insurer Retainer		18.00%
Annual Terrorism Risk Insurance Program Federal Share Reduction		1.00%
Terrorism Risk Insurance Program Reimbursed Percentage Minimum		80.00%
Allowance For Reinsurance Recoverable		$ 14,600	$ 11,480	$ 10,122	$ 11,037
Total reinsurance recoverables		594,832	621,537
Total prepaid reinsurance premiums		153,493	146,282
Net unsecured reinsurance balances		748,325	767,819
Less: collateral		(122,413)	(113,763)
National Flood Insurance Program [Member]
Net unsecured reinsurance balances	[1]	$ 204,161	$ 211,181
% of Net Unsecured Reinsurance	[1]	27.00%	27.00%
NJ Unsatisfied Claim Judgment Fund [Member]
Net unsecured reinsurance balances	[1]	$ 62,947	$ 65,574
% of Net Unsecured Reinsurance	[1]	9.00%	9.00%
Other Federal And State Pools [Member]
Net unsecured reinsurance balances	[1]	$ 3,634	$ 3,227
% of Net Unsecured Reinsurance	[1]	0.00%	0.00%
Total Federal And State Pools [Member]
Net unsecured reinsurance balances	[1]	$ 270,742	$ 279,982
% of Net Unsecured Reinsurance	[1]	36.00%	36.00%
Remaining Unsecured Reinsurance [Member]
Net unsecured reinsurance balances		$ 477,583	$ 487,837
% of Net Unsecured Reinsurance		64.00%	64.00%
Munich Re Group [Member]
Net unsecured reinsurance balances		$ 117,460	$ 119,520
% of Net Unsecured Reinsurance		16.00%	16.00%
Hanover Ruckversicherungs AG [Member]
Net unsecured reinsurance balances		$ 101,652	$ 106,298
% of Net Unsecured Reinsurance		14.00%	13.00%
AXIS Reinsurance Company [Member]
Net unsecured reinsurance balances		$ 62,396	$ 59,737
% of Net Unsecured Reinsurance		8.00%	8.00%
Swiss Re Group [Member]
Net unsecured reinsurance balances		$ 40,772	$ 50,494
% of Net Unsecured Reinsurance		5.00%	7.00%
Partner Reinsurance Company Of The US [Member]
Net unsecured reinsurance balances		$ 16,925	$ 21,125
% of Net Unsecured Reinsurance		2.00%	3.00%
All Other Reinsurers [Member]
Net unsecured reinsurance balances		$ 138,378	$ 130,663
% of Net Unsecured Reinsurance		19.00%	17.00%
Remaining Unsecured Reinsurance, Net of Collateral [Member]
Net unsecured reinsurance balances		$ 355,170	$ 374,074
Allowance for Reinsurance Recoverable [Member]
Allowance For Reinsurance Recoverable		$ 4,600	$ 5,500
[1]	Considered to have minimal risk of default.

Reinsurance (List of direct, as

Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Written:
Direct									$ 2,733,459	$ 2,577,259	$ 2,403,519
Assumed									26,685	28,779	23,848
Ceded premiums written									(389,503)	(368,750)	(357,463)
Net									2,370,641	2,237,288	2,069,904
Premiums Earned:
Direct									2,647,488	2,484,715	2,330,267
Assumed									25,831	28,214	23,209
Ceded premiums earned									(382,292)	(363,357)	(363,567)
Total Net Premiums Earned	$ 590,088	$ 572,055	$ 568,030	$ 560,854	$ 552,753	$ 542,429	$ 531,932	$ 522,458	2,291,027	2,149,572	1,989,909
Loss and loss expense incurred:
Direct									1,570,678	1,560,356	1,274,872
Assumed									17,588	22,708	16,996
Ceded loss and loss expense									(243,192)	(348,267)	(143,327)
Net									$ 1,345,074	$ 1,234,797	$ 1,148,541

Reinsurance (Ceded Premiums and

Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Percentage of NFIP claims ceded to the federal government	100.00%
Ceded premiums written	$ (389,503)	$ (368,750)	$ (357,463)
Ceded premiums earned	(382,292)	(363,357)	(363,567)
Ceded loss and loss expense	(243,192)	(348,267)	(143,327)
National Flood Insurance Program [Member]
Ceded premiums written	(241,345)	(232,245)	(228,907)
Ceded premiums earned	(235,088)	(227,882)	(233,940)
Ceded loss and loss expense	$ (160,922)	$ (239,891)	$ (62,078)

Reserve for Loss and Loss Exp79

Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gross reserve for loss and loss expense, at beginning of year	$ 3,691,719	$ 3,517,728	$ 3,477,870
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year	611,200	551,019	571,978
Net reserve for loss and loss expense, at beginning of year	3,080,519	2,966,709	2,905,892
Incurred loss and loss expense for claims occurring in the current year	1,384,266	1,300,565	1,217,550
Incurred loss and loss expense for claims occurring in the prior years	(39,192)	(65,768)	(69,009)
Total incurred loss and loss expense	1,345,074	1,234,797	1,148,541
Paid loss and loss expense for claims occurring in current year	497,486	450,811	446,550
Paid loss and loss expense for claims occurring in prior years	742,722	670,176	641,174
Total paid loss and loss expense	1,240,208	1,120,987	1,087,724
Net reserve for loss and loss expense, at end of year	3,185,385	3,080,519	2,966,709
Add: reinsurance recoverable on unpaid loss and loss expense, at end of year	585,855	611,200	551,019
Gross reserve for loss and loss expense, at end of year	3,771,240	3,691,719	3,517,728
Increase of net loss and loss expense reserves	104,900	113,800	60,800
Anticipated recoveries for salvage and subrogation claims	$ 64,800	$ 64,900	$ 62,100

Reserve for Loss and Loss Exp80

Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	$ 742,722	$ 670,176	$ 641,174
Favorable or adverse loss development	(39,192)	(65,768)	(69,009)
Favorable or unfavorable loss development [Member]
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	39,200	65,800	69,000
Favorable or adverse loss development	(39,200)	(65,800)	(69,000)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member]
Favorable or adverse loss development	48,600	69,000	67,000
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member]
Favorable or adverse loss development	9,400	3,200	2,000
Favorable or unfavorable loss development [Member] | General Liability [Member]
Favorable or adverse loss development	(48,300)	(45,000)	(51,000)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member]
Favorable or adverse loss development	35,600	25,300	2,400
Casualty Development	36,000	25,000
Favorable or unfavorable loss development [Member] | Workers Compensation [Member]
Favorable or adverse loss development	(52,300)	(56,000)	(37,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member]
Favorable or adverse loss development	1,900	1,800	2,200
Favorable or unfavorable loss development [Member] | Commercial Property [Member]
Favorable or adverse loss development	8,700	300	(3,000)
Favorable or unfavorable loss development [Member] | Homeowners [Member]
Favorable or adverse loss development	400	1,700	1,500
Favorable or unfavorable loss development [Member] | Personal Automobile [Member]
Favorable or adverse loss development	6,700	1,000	400
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member]
Favorable or adverse loss development	10,000	6,000	16,000
Casualty Development	10,000
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member]
Favorable or adverse loss development	$ (1,900)	$ (900)	$ (500)

Reserve for Loss and Loss Exp81

Reserve for Loss and Loss Expense (Exposure to Environmental Claims) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	$ 28,415	$ 29,962	$ 30,411	$ 30,653
Liability for Asbestos and Environmental Claims, Net	21,212	22,716	23,161	22,994
Asbestos Claim [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	7,600
Asbestos [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	7,577	7,847	8,024	8,751
Liability for Asbestos and Environmental Claims, Net	6,346	$ 6,615	$ 6,793	$ 7,314
Landfill Sites Related Claims [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	12,400
Liability for Asbestos and Environmental Claims, Net	7,700
Leaking Underground Storage Tanks Claims [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	8,400
Liability for Asbestos and Environmental Claims, Net	$ 7,200

Reserve for Loss and Loss Exp82

Reserve for Loss and Loss Expense (Roll Forward of Gross and Net Asbestos and Net Environmental Incurred Losses and Loss Expenses and Related Reserves) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Salvage and Subrogation Recoveries, Value	$ 64,800	$ 64,900	$ 62,100
Reserve for loss and loss expense at beginning of year, gross	29,962	30,411	30,653
Incurred loss and loss expense, gross	126	1,483	2,968
Less: loss and loss expense paid, gross	(1,673)	(1,932)	(3,210)
Reserve for loss and loss expense at the end of year, gross	28,415	29,962	30,411
Reserve for loss and loss expense at beginning of year, net	22,716	23,161	22,994
Incurred loss and loss expense, net	0	1,380	3,320
Less: loss and loss expense paid, net	(1,504)	(1,825)	(3,153)
Reserve for loss and loss expense at the end of year, net	21,212	22,716	23,161
Asbestos [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserve for loss and loss expense at beginning of year, gross	7,847	8,024	8,751
Incurred loss and loss expense, gross	0	77	428
Less: loss and loss expense paid, gross	(270)	(254)	(299)
Reserve for loss and loss expense at the end of year, gross	7,577	7,847	8,024
Reserve for loss and loss expense at beginning of year, net	6,615	6,793	7,314
Incurred loss and loss expense, net	0	77	(77)
Less: loss and loss expense paid, net	(269)	(255)	(444)
Reserve for loss and loss expense at the end of year, net	6,346	6,615	6,793
Enviromental [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserve for loss and loss expense at beginning of year, gross	22,115	22,387	21,902
Incurred loss and loss expense, gross	126	1,406	3,396
Less: loss and loss expense paid, gross	(1,403)	(1,678)	(2,911)
Reserve for loss and loss expense at the end of year, gross	20,838	22,115	22,387
Reserve for loss and loss expense at beginning of year, net	16,101	16,368	15,680
Incurred loss and loss expense, net	0	1,303	3,397
Less: loss and loss expense paid, net	(1,235)	(1,570)	(2,709)
Reserve for loss and loss expense at the end of year, net	$ 14,866	$ 16,101	$ 16,368

Reserve for Loss and Loss Exp83

Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Claims Development) (Details)	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Claims Development [Line Items]
Reserve for loss and loss expense	$ 3,771,240,000	$ 3,691,719,000
Incurred loss and loss expense for claims occurring in the prior years	(39,192,000)	(65,768,000)	$ (69,009,000)
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	7,727,861,000
Net outstanding liabilities	10,417,364,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,041,694,000
Short-duration Insurance Contracts, Accident Year 2008 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	860,745,000	853,769,000	839,392,000	$ 819,280,000	$ 798,996,000	$ 764,196,000	$ 690,016,000	$ 609,851,000	$ 489,633,000	$ 286,314,000
Net outstanding liabilities	923,978,000	926,017,000	931,785,000	927,958,000	936,975,000	947,306,000	964,862,000	990,931,000	988,584,000	957,247,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 41,791,000
Short-duration Insurance Contracts, Number of Reported Claims	85,338
Short-duration Insurance Contracts, Accident Year 2009 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 784,713,000	775,885,000	760,589,000	736,100,000	695,249,000	634,902,000	540,982,000	442,417,000	277,275,000
Net outstanding liabilities	848,413,000	853,401,000	857,960,000	869,927,000	870,057,000	883,590,000	916,691,000	941,972,000	920,143,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 41,937,000
Short-duration Insurance Contracts, Number of Reported Claims	85,575
Short-duration Insurance Contracts, Accident Year 2010 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 835,532,000	823,770,000	803,773,000	773,536,000	704,895,000	625,229,000	509,910,000	328,826,000
Net outstanding liabilities	907,074,000	915,131,000	922,404,000	943,118,000	956,600,000	977,959,000	973,742,000	950,114,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 50,293,000
Short-duration Insurance Contracts, Number of Reported Claims	94,258
Short-duration Insurance Contracts, Accident Year 2011 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 924,111,000	901,801,000	852,202,000	782,655,000	692,730,000	585,867,000	391,944,000
Net outstanding liabilities	1,019,351,000	1,023,726,000	1,033,518,000	1,056,107,000	1,062,233,000	1,061,667,000	1,042,576,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 63,891,000
Short-duration Insurance Contracts, Number of Reported Claims	104,500
Short-duration Insurance Contracts, Accident Year 2012 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 856,195,000	810,135,000	743,742,000	651,544,000	555,819,000	378,067,000
Net outstanding liabilities	973,644,000	973,089,000	998,028,000	1,020,655,000	1,071,290,000	1,065,437,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 77,542,000
Short-duration Insurance Contracts, Number of Reported Claims	103,745
Short-duration Insurance Contracts, Accident Year 2013 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 833,823,000	748,758,000	644,475,000	518,872,000	335,956,000
Net outstanding liabilities	1,002,316,000	1,021,007,000	1,047,230,000	1,062,045,000	1,044,142,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 116,449,000
Short-duration Insurance Contracts, Number of Reported Claims	90,755
Short-duration Insurance Contracts, Accident Year 2014 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 855,959,000	736,154,000	614,075,000	405,898,000
Net outstanding liabilities	1,124,014,000	1,146,990,000	1,133,798,000	1,107,513,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 171,913,000
Short-duration Insurance Contracts, Number of Reported Claims	94,375
Short-duration Insurance Contracts, Accident Year 2015 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 725,385,000	581,203,000	376,641,000
Net outstanding liabilities	1,144,830,000	1,130,513,000	1,114,081,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 256,758,000
Short-duration Insurance Contracts, Number of Reported Claims	92,891
Short-duration Insurance Contracts, Accident Year 2016 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 617,958,000	387,272,000
Net outstanding liabilities	1,203,634,000	1,188,608,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 416,010,000
Short-duration Insurance Contracts, Number of Reported Claims	92,191
Short-duration Insurance Contracts, Accident Year 2017 [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 433,440,000
Net outstanding liabilities	1,270,110,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 634,863,000
Short-duration Insurance Contracts, Number of Reported Claims	88,941
Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 352,192,000
Standard Commercial Lines [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,262,217,000
Net outstanding liabilities	2,273,761,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,092,058,000
Standard Commercial Lines [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,272,395,000
Net outstanding liabilities	1,941,745,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	916,471,000
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,677,641,000
Net outstanding liabilities	2,137,614,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	464,131,000
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	401,363,000
Net outstanding liabilities	477,234,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	83,163,000
Standard Commercial Lines [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,080,689,000
Net outstanding liabilities	1,130,739,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	50,300,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	198,360,000	194,257,000	186,896,000	176,873,000	166,394,000	151,055,000	113,088,000	82,421,000	45,595,000	16,397,000
Net outstanding liabilities	219,617,000	221,203,000	221,640,000	219,551,000	224,236,000	233,712,000	234,770,000	243,536,000	243,755,000	250,239,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 14,326,000
Short-duration Insurance Contracts, Number of Reported Claims	13,769,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 198,512,000	195,962,000	191,265,000	188,206,000	180,556,000	171,403,000	158,083,000	139,144,000	100,678,000	39,628,000
Net outstanding liabilities	238,218,000	238,836,000	243,512,000	244,100,000	245,993,000	241,921,000	250,423,000	255,810,000	243,186,000	219,616,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 25,993,000
Short-duration Insurance Contracts, Number of Reported Claims	14,401,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 181,979,000	181,779,000	180,779,000	175,244,000	173,795,000	166,487,000	151,741,000	130,857,000	104,711,000	69,053,000
Net outstanding liabilities	183,151,000	183,618,000	183,736,000	180,902,000	182,785,000	187,417,000	187,100,000	190,349,000	195,823,000	196,370,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 736,000
Short-duration Insurance Contracts, Number of Reported Claims	24,129
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 30,873,000	30,873,000	29,778,000	28,589,000	28,660,000	28,026,000	25,738,000	24,449,000	21,765,000	15,019,000
Net outstanding liabilities	31,065,000	30,967,000	31,340,000	31,066,000	29,793,000	30,250,000	32,974,000	39,334,000	38,986,000	39,660,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 192,000
Short-duration Insurance Contracts, Number of Reported Claims	3,258,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 101,032,000	101,034,000	100,868,000	100,908,000	100,752,000	100,213,000	99,288,000	100,465,000	98,921,000	68,211,000
Net outstanding liabilities	101,054,000	101,043,000	100,881,000	101,043,000	101,702,000	101,265,000	101,470,000	101,436,000	102,860,000	97,578,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,000
Short-duration Insurance Contracts, Number of Reported Claims	7,604,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 180,621,000	176,316,000	166,767,000	151,920,000	130,554,000	103,213,000	64,970,000	37,143,000	14,346,000
Net outstanding liabilities	201,568,000	205,741,000	206,387,000	211,243,000	212,947,000	223,146,000	233,530,000	241,625,000	237,913,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 17,629,000
Short-duration Insurance Contracts, Number of Reported Claims	13,841,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 167,894,000	164,336,000	160,529,000	154,726,000	145,417,000	133,116,000	117,019,000	87,299,000	37,885,000
Net outstanding liabilities	208,142,000	208,611,000	212,536,000	216,992,000	210,756,000	210,109,000	213,036,000	215,946,000	197,504,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22,575,000
Short-duration Insurance Contracts, Number of Reported Claims	12,217,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 160,013,000	159,723,000	158,303,000	155,560,000	149,949,000	137,564,000	113,697,000	94,406,000	63,126,000
Net outstanding liabilities	161,300,000	161,923,000	161,251,000	162,911,000	166,682,000	169,858,000	182,724,000	191,079,000	199,541,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 897,000
Short-duration Insurance Contracts, Number of Reported Claims	24,652
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 43,547,000	43,448,000	43,358,000	42,895,000	40,052,000	36,073,000	32,689,000	29,612,000	18,915,000
Net outstanding liabilities	43,933,000	44,273,000	44,299,000	44,938,000	43,553,000	43,828,000	46,645,000	51,762,000	48,535,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 272,000
Short-duration Insurance Contracts, Number of Reported Claims	3,474,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 80,405,000	80,509,000	80,529,000	80,352,000	80,251,000	80,894,000	80,433,000	78,695,000	59,933,000
Net outstanding liabilities	80,416,000	80,545,000	80,558,000	80,455,000	80,774,000	82,014,000	82,025,000	82,124,000	82,619,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4,000
Short-duration Insurance Contracts, Number of Reported Claims	7,009,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 178,179,000	172,394,000	161,487,000	143,360,000	113,050,000	80,018,000	46,201,000	15,726,000
Net outstanding liabilities	202,394,000	208,968,000	211,619,000	222,328,000	237,154,000	242,499,000	228,680,000	215,208,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 18,403,000
Short-duration Insurance Contracts, Number of Reported Claims	12,672,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 162,796,000	158,078,000	153,795,000	149,086,000	137,184,000	122,442,000	93,281,000	46,795,000
Net outstanding liabilities	204,423,000	208,155,000	212,448,000	214,916,000	211,030,000	212,815,000	214,469,000	198,371,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 28,964,000
Short-duration Insurance Contracts, Number of Reported Claims	12,184,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 169,793,000	169,100,000	167,227,000	163,513,000	146,913,000	128,015,000	99,254,000	68,098,000
Net outstanding liabilities	173,471,000	173,157,000	172,969,000	179,854,000	181,923,000	187,778,000	189,305,000	187,562,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,491,000
Short-duration Insurance Contracts, Number of Reported Claims	25,301
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 40,395,000	40,279,000	38,900,000	37,819,000	34,705,000	31,027,000	28,131,000	20,821,000
Net outstanding liabilities	41,197,000	41,239,000	40,581,000	40,899,000	39,915,000	42,408,000	49,285,000	53,669,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 697,000
Short-duration Insurance Contracts, Number of Reported Claims	3,917,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 95,150,000	95,156,000	95,147,000	95,270,000	95,111,000	94,602,000	91,918,000	69,543,000
Net outstanding liabilities	95,155,000	95,178,000	95,363,000	95,530,000	96,127,000	97,386,000	96,851,000	105,647,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5,000
Short-duration Insurance Contracts, Number of Reported Claims	7,667,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 170,525,000	159,768,000	135,377,000	102,978,000	73,643,000	42,692,000	13,924,000
Net outstanding liabilities	212,011,000	211,196,000	217,256,000	230,785,000	239,480,000	228,720,000	227,769,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 25,729,000
Short-duration Insurance Contracts, Number of Reported Claims	11,579,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 154,320,000	149,269,000	139,232,000	134,646,000	118,847,000	90,836,000	42,941,000
Net outstanding liabilities	200,674,000	205,708,000	210,591,000	215,114,000	214,743,000	218,973,000	205,238,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 32,881,000
Short-duration Insurance Contracts, Number of Reported Claims	11,845,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 170,000,000	166,082,000	157,291,000	142,507,000	121,576,000	99,196,000	69,849,000
Net outstanding liabilities	174,514,000	174,882,000	172,617,000	178,421,000	182,325,000	183,044,000	174,006,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,937,000
Short-duration Insurance Contracts, Number of Reported Claims	25,272
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 57,045,000	55,856,000	52,114,000	46,444,000	41,011,000	37,362,000	27,884,000
Net outstanding liabilities	58,456,000	58,966,000	59,256,000	58,242,000	51,047,000	57,083,000	54,469,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,080,000
Short-duration Insurance Contracts, Number of Reported Claims	4,959,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 131,089,000	131,115,000	131,060,000	130,681,000	129,579,000	127,580,000	94,538,000
Net outstanding liabilities	131,049,000	131,113,000	131,353,000	131,282,000	130,942,000	131,667,000	136,954,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4,000
Short-duration Insurance Contracts, Number of Reported Claims	9,036,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 130,866,000	109,448,000	89,008,000	56,580,000	35,241,000	13,030,000
Net outstanding liabilities	175,268,000	175,305,000	194,144,000	215,083,000	245,561,000	238,979,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 27,702,000
Short-duration Insurance Contracts, Number of Reported Claims	9,922,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 139,477,000	132,052,000	122,755,000	108,211,000	86,909,000	40,911,000
Net outstanding liabilities	187,359,000	188,596,000	195,197,000	199,360,000	208,036,000	203,864,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 36,233,000
Short-duration Insurance Contracts, Number of Reported Claims	11,605,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 176,656,000	168,114,000	148,669,000	127,235,000	105,371,000	73,316,000
Net outstanding liabilities	186,128,000	184,367,000	184,289,000	183,527,000	191,947,000	179,551,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3,880,000
Short-duration Insurance Contracts, Number of Reported Claims	23,889
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 40,627,000	38,766,000	37,215,000	35,089,000	31,833,000	22,199,000
Net outstanding liabilities	43,747,000	44,077,000	44,172,000	46,303,000	48,029,000	54,342,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 756,000
Short-duration Insurance Contracts, Number of Reported Claims	5,540,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 116,625,000	116,291,000	114,699,000	111,503,000	108,834,000	81,528,000
Net outstanding liabilities	117,170,000	117,102,000	116,658,000	115,375,000	114,224,000	118,464,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 24,000
Short-duration Insurance Contracts, Number of Reported Claims	8,514,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 139,114,000	104,587,000	72,127,000	35,113,000	12,789,000
Net outstanding liabilities	210,785,000	225,709,000	239,776,000	251,421,000	250,609,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 53,014,000
Short-duration Insurance Contracts, Number of Reported Claims	10,226,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 118,669,000	109,739,000	96,376,000	74,568,000	36,829,000
Net outstanding liabilities	166,662,000	173,160,000	187,658,000	194,318,000	199,794,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 34,776,000
Short-duration Insurance Contracts, Number of Reported Claims	11,366,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 189,626,000	169,850,000	140,015,000	109,893,000	76,469,000
Net outstanding liabilities	210,410,000	207,994,000	209,197,000	205,282,000	188,289,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 8,258,000
Short-duration Insurance Contracts, Number of Reported Claims	25,392
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 37,993,000	34,760,000	30,845,000	26,592,000	17,412,000
Net outstanding liabilities	41,369,000	40,624,000	41,005,000	42,618,000	49,617,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3,192,000
Short-duration Insurance Contracts, Number of Reported Claims	3,479,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 90,840,000	90,350,000	90,446,000	87,874,000	60,244,000
Net outstanding liabilities	90,436,000	90,005,000	90,103,000	90,639,000	88,101,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 42,000
Short-duration Insurance Contracts, Number of Reported Claims	5,709,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 121,969,000	79,972,000	46,825,000	14,901,000
Net outstanding liabilities	239,333,000	257,132,000	249,946,000	244,312,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 80,168,000
Short-duration Insurance Contracts, Number of Reported Claims	10,391,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 113,626,000	100,876,000	78,944,000	35,924,000
Net outstanding liabilities	172,515,000	182,579,000	187,065,000	199,346,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 42,869,000
Short-duration Insurance Contracts, Number of Reported Claims	10,482,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 180,701,000	148,884,000	117,169,000	80,810,000
Net outstanding liabilities	219,925,000	216,824,000	212,725,000	200,534,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 19,053,000
Short-duration Insurance Contracts, Number of Reported Claims	27,338
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 49,460,000	44,911,000	40,584,000	28,914,000
Net outstanding liabilities	59,806,000	62,548,000	60,949,000	55,962,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5,952,000
Short-duration Insurance Contracts, Number of Reported Claims	4,054,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 137,883,000	136,634,000	132,909,000	101,131,000
Net outstanding liabilities	138,751,000	136,820,000	136,249,000	141,192,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 126,000
Short-duration Insurance Contracts, Number of Reported Claims	6,512,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 78,668,000	39,978,000	14,665,000
Net outstanding liabilities	246,990,000	245,710,000	254,720,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 124,639,000
Short-duration Insurance Contracts, Number of Reported Claims	9,987,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 98,195,000	77,320,000	33,857,000
Net outstanding liabilities	183,604,000	194,639,000	193,729,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 42,287,000
Short-duration Insurance Contracts, Number of Reported Claims	10,530,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 175,866,000	132,260,000	91,347,000
Net outstanding liabilities	253,074,000	240,958,000	220,994,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 36,137,000
Short-duration Insurance Contracts, Number of Reported Claims	28,818
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 42,710,000	36,014,000	24,189,000
Net outstanding liabilities	57,245,000	53,768,000	52,871,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10,256,000
Short-duration Insurance Contracts, Number of Reported Claims	3,913,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 109,829,000	106,182,000	79,048,000
Net outstanding liabilities	111,750,000	109,513,000	110,270,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 261,000
Short-duration Insurance Contracts, Number of Reported Claims	6,398,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 46,549,000	15,684,000
Net outstanding liabilities	272,048,000	277,214,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 178,904,000
Short-duration Insurance Contracts, Number of Reported Claims	9,617,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 78,531,000	34,525,000
Net outstanding liabilities	184,946,000	196,774,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 70,424,000
Short-duration Insurance Contracts, Number of Reported Claims	10,509,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 155,720,000	106,022,000
Net outstanding liabilities	274,367,000	255,187,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 71,303,000
Short-duration Insurance Contracts, Number of Reported Claims	30,480
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 36,848,000	24,655,000
Net outstanding liabilities	53,792,000	52,335,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 11,938,000
Short-duration Insurance Contracts, Number of Reported Claims	3,757,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 118,789,000	83,966,000
Net outstanding liabilities	126,185,000	121,927,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 804,000
Short-duration Insurance Contracts, Number of Reported Claims	6,686,000,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 17,366,000
Net outstanding liabilities	293,747,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 249,085,000
Short-duration Insurance Contracts, Number of Reported Claims	8,194,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 40,375,000
Net outstanding liabilities	195,202,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 112,086,000
Short-duration Insurance Contracts, Number of Reported Claims	10,182,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 117,287,000
Net outstanding liabilities	301,274,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 132,814,000
Short-duration Insurance Contracts, Number of Reported Claims	30,187
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 21,865,000
Net outstanding liabilities	46,624,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15,252,000
Short-duration Insurance Contracts, Number of Reported Claims	3,462,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 99,047,000
Net outstanding liabilities	138,773,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 8,794,000
Short-duration Insurance Contracts, Number of Reported Claims	6,358,000,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | General Liability [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 80,514,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Workers Compensation [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	247,121,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Commercial Automobile [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	4,158,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Business Owners' Policies [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	7,292,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Commercial Property [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	250,000
Standard Personal Lines [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	962,602,000
Net outstanding liabilities	1,049,297,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	92,557,000
Standard Personal Lines [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	648,479,000
Net outstanding liabilities	673,804,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	30,728,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	98,810,000	98,668,000	98,604,000	98,174,000	97,723,000	95,932,000	91,834,000	84,715,000	73,194,000	50,396,000
Net outstanding liabilities	99,270,000	99,116,000	99,255,000	99,686,000	99,912,000	100,213,000	103,934,000	106,842,000	106,999,000	100,311,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 191,000
Short-duration Insurance Contracts, Number of Reported Claims	16,042
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 37,928,000	37,930,000	37,939,000	37,933,000	37,837,000	37,763,000	37,086,000	36,271,000	33,535,000	21,277,000
Net outstanding liabilities	38,038,000	38,045,000	38,042,000	38,111,000	38,410,000	38,062,000	39,203,000	39,342,000	41,747,000	$ 41,224,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 56,000
Short-duration Insurance Contracts, Number of Reported Claims	5,139,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 103,009,000	102,437,000	102,322,000	102,187,000	100,566,000	96,229,000	86,431,000	71,911,000	51,039,000
Net outstanding liabilities	103,348,000	103,412,000	103,393,000	103,866,000	104,734,000	103,908,000	105,033,000	103,319,000	93,808,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 191,000
Short-duration Insurance Contracts, Number of Reported Claims	17,346
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 40,269,000	40,189,000	39,907,000	39,819,000	39,731,000	39,342,000	38,078,000	36,965,000	28,299,000
Net outstanding liabilities	40,451,000	40,457,000	40,465,000	40,400,000	61,927,000	40,313,000	42,609,000	44,511,000	$ 47,636,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 73,000
Short-duration Insurance Contracts, Number of Reported Claims	5,633,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 106,733,000	106,453,000	105,849,000	104,061,000	101,540,000	95,300,000	82,490,000	58,786,000
Net outstanding liabilities	107,054,000	107,224,000	107,405,000	107,490,000	109,515,000	112,346,000	110,075,000	103,340,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 222,000
Short-duration Insurance Contracts, Number of Reported Claims	20,822
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 62,272,000	62,241,000	62,227,000	62,155,000	61,106,000	60,295,000	58,638,000	43,699,000
Net outstanding liabilities	62,392,000	62,339,000	62,402,000	62,462,000	97,761,000	63,285,000	67,525,000	$ 68,373,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 84,000
Short-duration Insurance Contracts, Number of Reported Claims	9,131,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 113,551,000	112,732,000	111,085,000	105,068,000	93,878,000	82,102,000	61,323,000
Net outstanding liabilities	113,988,000	113,830,000	114,241,000	112,993,000	113,686,000	116,164,000	113,232,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 284,000
Short-duration Insurance Contracts, Number of Reported Claims	22,700
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 94,007,000	93,720,000	93,312,000	92,185,000	91,718,000	89,963,000	71,668,000
Net outstanding liabilities	94,378,000	94,183,000	94,543,000	94,167,000	82,744,000	98,211,000	103,804,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 159,000
Short-duration Insurance Contracts, Number of Reported Claims	15,106,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 109,355,000	107,890,000	102,977,000	94,842,000	82,729,000	63,704,000
Net outstanding liabilities	110,300,000	110,294,000	109,324,000	109,832,000	114,921,000	113,771,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 728,000
Short-duration Insurance Contracts, Number of Reported Claims	22,332
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 85,562,000	85,196,000	84,250,000	82,720,000	79,584,000	69,056,000
Net outstanding liabilities	86,330,000	86,271,000	86,667,000	86,560,000	82,745,000	87,260,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 180,000
Short-duration Insurance Contracts, Number of Reported Claims	16,936,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 105,131,000	100,528,000	92,637,000	80,861,000	61,384,000
Net outstanding liabilities	107,759,000	106,703,000	106,225,000	109,620,000	108,417,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 851,000
Short-duration Insurance Contracts, Number of Reported Claims	22,371
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 71,776,000	69,775,000	67,838,000	65,528,000	50,664,000
Net outstanding liabilities	71,714,000	72,145,000	71,494,000	72,528,000	73,670,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 284,000
Short-duration Insurance Contracts, Number of Reported Claims	7,747,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 99,173,000	92,589,000	83,739,000	62,519,000
Net outstanding liabilities	107,452,000	106,757,000	109,325,000	102,250,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,554,000
Short-duration Insurance Contracts, Number of Reported Claims	22,499
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 81,664,000	79,751,000	76,007,000	61,561,000
Net outstanding liabilities	83,844,000	83,637,000	82,461,000	80,111,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1,146,000
Short-duration Insurance Contracts, Number of Reported Claims	8,762,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 87,163,000	76,470,000	58,725,000
Net outstanding liabilities	100,214,000	99,698,000	96,387,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 6,541,000
Short-duration Insurance Contracts, Number of Reported Claims	20,840
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 72,202,000	70,078,000	52,589,000
Net outstanding liabilities	76,559,000	76,400,000	76,637,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,744,000
Short-duration Insurance Contracts, Number of Reported Claims	7,724,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 76,823,000	57,961,000
Net outstanding liabilities	98,032,000	92,727,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 11,651,000
Short-duration Insurance Contracts, Number of Reported Claims	19,747
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 57,333,000	42,252,000
Net outstanding liabilities	60,931,000	60,105,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,067,000
Short-duration Insurance Contracts, Number of Reported Claims	6,820,000,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 62,854,000
Net outstanding liabilities	101,880,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22,284,000
Short-duration Insurance Contracts, Number of Reported Claims	19,818
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Homeowners [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 45,466,000
Net outstanding liabilities	59,167,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5,315,000
Short-duration Insurance Contracts, Number of Reported Claims	6,651,000,000
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Personal Automobile [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 5,862,000
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Homeowners [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	5,403,000
E&S Lines [Member]
Claims Development [Line Items]
Reserve for loss and loss expense						15,000,000
E&S Lines [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	192,604,000
Net outstanding liabilities	471,628,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	279,023,000
E&S Lines [Member] | Commercial Property [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	13,519,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	97,000	92,000	79,000	80,000	70,000	24,000	0
Net outstanding liabilities	119,000	(162,000)	52,000	119,000	146,000	169,000	92,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Number of Reported Claims	35,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 737,000	709,000	626,000	605,000	431,000	198,000	0
Net outstanding liabilities	737,000	96,000	710,000	728,000	938,000	1,053,000	885,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Number of Reported Claims	274,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 4,610,000	4,513,000	4,078,000	3,574,000	2,570,000	1,218,000	0
Net outstanding liabilities	4,932,000	3,055,000	3,831,000	4,299,000	3,369,000	4,106,000	3,294,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contracts, Number of Reported Claims	804,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 10,256,000	9,912,000	9,954,000	6,445,000	3,200,000	806,000	0
Net outstanding liabilities	10,228,000	9,652,000	10,273,000	12,207,000	9,853,000	7,102,000	$ 8,127,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 361,000
Short-duration Insurance Contracts, Number of Reported Claims	1,316,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 36,278,000	32,343,000	25,064,000	16,430,000	7,914,000	3,722,000
Net outstanding liabilities	45,988,000	46,165,000	46,149,000	43,175,000	42,621,000	$ 42,367,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7,313,000
Short-duration Insurance Contracts, Number of Reported Claims	2,006,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 46,108,000	35,200,000	21,980,000	9,470,000	2,715,000
Net outstanding liabilities	67,647,000	69,112,000	67,099,000	60,309,000	$ 55,468,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15,326,000
Short-duration Insurance Contracts, Number of Reported Claims	2,219,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 43,877,000	25,571,000	12,234,000	2,353,000
Net outstanding liabilities	71,719,000	69,929,000	63,505,000	$ 55,316,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 18,224,000
Short-duration Insurance Contracts, Number of Reported Claims	1,987,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 29,389,000	13,057,000	3,036,000
Net outstanding liabilities	82,404,000	76,432,000	$ 75,498,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 32,909,000
Short-duration Insurance Contracts, Number of Reported Claims	2,606,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 16,195,000	3,720,000
Net outstanding liabilities	96,416,000	$ 94,451,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 65,489,000
Short-duration Insurance Contracts, Number of Reported Claims	2,454,000,000
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 5,057,000
Net outstanding liabilities	91,438,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 79,354,000
Short-duration Insurance Contracts, Number of Reported Claims	1,760,000,000
E&S Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | General Liability [Member]
Claims Development [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 0

Reserve for Loss and Loss Exp84

Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities	$ 1,345,074	$ 1,234,797	$ 1,148,541
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,041,694
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	585,855	611,200	551,019	$ 571,978
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	143,691
Liability for Claims and Claims Adjustment Expense	3,771,240	$ 3,691,719	$ 3,517,728	$ 3,477,870
General Liability [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,092,058
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	175,276
General Liability [Member] | E&S Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	279,023
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	21,360
Workers Compensation [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	916,471
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	218,024
Commercial Automobile [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	464,131
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	16,745
Business Owners' Policies [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	83,163
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	3,926
Commercial Property [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	50,300
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	24,387
Commercial Property [Member] | E&S Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	13,519
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	389
Other Commercial [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	10,560
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	2,287
Total Standard Commercial Lines [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	2,616,683
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	440,645
Personal Automobile [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	92,557
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	53,129
Homeowners [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	30,728
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	999
Other Personal [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	9,184
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	69,333
Standard Personal Lines [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	132,469
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	123,461
Total E&S Lines [Member] | E&S Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	292,542
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 21,749

Reserve for Loss and Loss Exp85

Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Historical Claims Duration) (Details)	Dec. 31, 2017
General Liability [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	6.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	16.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	10.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.30%
General Liability [Member] | E&S Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	20.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	15.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.30%
Workers Compensation [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	20.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.90%
Commercial Automobile [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	38.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	17.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	12.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%
Business Owners' Policies [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	46.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	20.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.10%
Commercial Property [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	70.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	0.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Personal Automobile [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	56.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Homeowners [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	71.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	2.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%

Indebtedness Schedule of Long-T

Indebtedness Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	3.03%
Long-term Debt, Gross	$ 60,000
Long-term Debt	60,000	$ 60,000
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	1.56%
Long-term Debt, Gross	$ 25,000
Long-term Debt	25,000	25,000
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 0
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	1.61%
Long-term Debt, Gross	$ 25,000
Long-term Debt	25,000	25,000
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (4,570)
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	5.88%
Long-term Debt, Gross	$ 185,000
Long-term Debt	180,430	180,068
Six Point Seventy Percent Senior Notes [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (989)
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	6.70%
Long-term Debt, Gross	$ 100,000
Long-term Debt	99,011	98,952
Seven Point Twenty Five Percent Senior Notes [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (325)
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	7.25%
Long-term Debt, Gross	$ 50,000
Long-term Debt	49,675	49,647
Long-term Debt [Member]
Schedule of Debt Instruments [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(5,884)
Long-term Debt, Gross	445,000
Long-term Debt	$ 439,116	$ 438,667

Indebtedness (Narrative) (Detai

Indebtedness (Narrative) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2005USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2004USD ($)
Net proceeds from the issuance		$ 15,000
Borrowing threshold of FHLBI			20
Federal Home Loan Bank Advances		45,000
Five Point Eight Hundred Seventy Five Percent Senior Notes Due 2043 [Member]
Aggregate principal amount of notes				$ 185,000
Bond proceeds to Insurance Subsidiaries as capital				57,100
Long-term Debt, Gross			$ 185,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			5.88%
Redemption Of Seven Point Fifty Percent Junior Notes [Member]
Aggregate principal amount of notes				$ 100,000
Six Point Seventy Percent Senior Notes due 2035 [Member]
Aggregate principal amount of notes	$ 100,000
Discount for notes issued	$ 700
Effective yield	6.754%
Acceleration of principal			$ 10,000
Net proceeds from the issuance	$ 50,000
Long-term Debt, Gross			$ 100,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			6.70%
Seven Point Twenty Five Percent Senior Notes due 2034 [Member]
Aggregate principal amount of notes					$ 50,000
Discount for notes issued					$ 100
Effective yield					7.27%
Bond proceeds to Insurance Subsidiaries as capital					$ 25,000
Acceleration of principal			$ 10,000
Long-term Debt, Gross			$ 50,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			7.25%
Line of Credit expires Dec.1, 2020 [Member]
Acceleration of principal			$ 20,000
Line of credit borrowing capacity			30,000
Line of credit, maximum borrowing capacity			50,000
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Gross			$ 25,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			1.61%
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Gross			$ 25,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate			1.56%
Zero Point Seventy Five Percent Borrowing From Federal Home Loan Bank Due March 21, 2017 [Member]
Federal Home Loan Bank Advances			$ 64,000
Federal Home Loan Bank, Advances, Interest Rate			0.75%
One Point Two Nine Percent Borrowing From Federal Home Loan Bank Due November 15, 2017 [Member]
Federal Home Loan Bank Advances			$ 20,000
Federal Home Loan Bank, Advances, Interest Rate			1.29%
Federal Home Loan Bank of Indianapolis [Member]
Aggregate investment in FHLBI			$ 2,800
Federal Home Loan Bank of New York [Member]
Aggregate investment in FHLBI		$ 2,800	$ 2,600

Indebtedness (Covenants in the

Indebtedness (Covenants in the Line of Credit) (Details) - USD ($)		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Consolidated net worth, required		$ 1,200,000,000
Statutory surplus, required		$ 750,000,000
Debt-to-capitalization ratio, required	[1]	35.00%
Consolidated net worth, actual		$ 1,712,957,000	$ 1,531,370,000	$ 1,398,041,000
Statutory surplus, actual		$ 1,672,900,000	$ 1,583,800,000
Debt-to-capitalization ratio, actual	[1]	20.50%
[1]	Calculated in accordance with Line of Credit agreement.

Segment Information (Narrative)

Segment Information (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting [Abstract]
Number of Operating Segments	4
Goodwill balance for our Standard Commercial Lines reporting unit	$ 7,849	$ 7,849
Percentage of net premiums written related to insurance policies written in New Jersey	20.00%

Segment Information (Revenue Fr

Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Net premiums earned	$ 590,088	$ 572,055	$ 568,030	$ 560,854	$ 552,753	$ 542,429	$ 531,932	$ 522,458	$ 2,291,027	$ 2,149,572	$ 1,989,909
Other income	2,190	1,994	3,291	3,241	1,863	2,199	3,868	951	10,716	8,881	7,456
Net investment income	42,587	40,446	41,430	37,419	35,428	33,375	31,182	30,769	161,882	130,754	121,316
Realized investment gains (losses)									6,359	(4,937)	13,171
Total investment revenues									168,241	125,817	134,487
Total revenues	$ 633,737	$ 621,293	$ 614,485	$ 600,469	$ 582,358	$ 581,691	$ 568,747	$ 551,474	2,469,984	2,284,270	2,131,852
Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									10,716	8,881	7,456
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									442,818	398,942	358,909
Standard Commercial Lines [Member] | Workers Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned									317,982	308,233	290,075
Standard Commercial Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									569,217	527,859	483,291
Standard Commercial Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									311,932	293,438	269,022
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Revenue from External Customer [Line Items]
Net premiums earned									100,266	97,754	93,428
Standard Commercial Lines [Member] | Bonds Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned									29,086	23,227	20,350
Standard Commercial Lines [Member] | Other Commercial [Member]
Revenue from External Customer [Line Items]
Net premiums earned									17,198	16,030	14,367
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									9,488	7,782	6,343
Standard Commercial Lines [Member] | Total Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,797,987	1,673,265	1,535,785
Standard Personal Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,228	1,098	1,113
Standard Personal Lines [Member] | Personal Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									153,147	142,876	146,784
Standard Personal Lines [Member] | Homeowners [Member]
Revenue from External Customer [Line Items]
Net premiums earned									129,699	130,973	134,382
Standard Personal Lines [Member] | Other Personal [Member]
Revenue from External Customer [Line Items]
Net premiums earned									6,855	6,758	6,968
Standard Personal Lines [Member] | Standard Personal Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									290,929	281,705	289,247
E&S Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									157,366	151,638	126,064
E&S Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									55,461	51,844	46,269
E&S Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									0	1	0
E&S Lines [Member] | Total E&S Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									$ 212,827	$ 203,483	$ 172,333

Segment Information (Income Fro

Segment Information (Income From Continuing Operations, Before Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net investment income	$ 42,587	$ 40,446	$ 41,430	$ 37,419	$ 35,428	$ 33,375	$ 31,182	$ 30,769	$ 161,882	$ 130,754	$ 121,316
Realized investment gains (losses)									6,359	(4,937)	13,171
Total investment revenues									168,241	125,817	134,487
Tax on investment income									45,588	30,621	32,090
Total investment income, after federal income tax									122,653	95,196	102,397
Standard Commercial Lines [Member]
Underwriting gain (loss), before federal income tax									149,514	146,435	164,496
Underwriting gain (loss), after federal income tax									$ 97,184	$ 95,183	$ 106,923
Combined Ratio									91.60%	91.20%	89.20%
Standard Personal Lines [Member]
Underwriting gain (loss), before federal income tax									$ 11,104	$ 12,419	$ 1,336
Underwriting gain (loss), after federal income tax									$ 7,217	$ 8,072	$ 868
Combined Ratio									96.20%	95.60%	99.50%
Excess And Surplus Operations [Member]
Underwriting gain (loss), before federal income tax									$ (6,282)	$ (6,921)	$ (16,803)
Underwriting gain (loss), after federal income tax									$ (4,083)	$ (4,499)	$ (10,922)
Combined Ratio									103.00%	103.40%	109.80%

Segment Information (Reconcilia

Segment Information (Reconciliation of Segment Results to Income from Continuing Operations, before Federal Income Tax (Details)) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total investment revenues									$ 168,241	$ 125,817	$ 134,487
Interest expense									(24,354)	(22,771)	(22,428)
Corporate expenses									(36,255)	(35,024)	(28,396)
Income before federal income tax	$ 68,150	$ 67,315	$ 58,929	$ 67,574	$ 50,326	$ 55,443	$ 62,311	$ 51,875	261,968	219,955	232,692
Standard Commercial Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax									149,514	146,435	164,496
Standard Personal Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax									11,104	12,419	1,336
Excess And Surplus Operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax									(6,282)	(6,921)	(16,803)
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments									$ 322,577	$ 277,750	$ 283,516

Earnings Per Share (Reconciliat

Earnings Per Share (Reconciliation of EPS) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic EPS: [Abstract]
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	$ 168,826	$ 158,495	$ 165,861
Net income, basic shares									58,458	57,889	57,212
Basic net income per share	$ 0.52	$ 0.80	$ 0.71	$ 0.87	$ 0.68	$ 0.66	$ 0.75	$ 0.64	$ 2.89	$ 2.74	$ 2.90
Effect of dilutive securities: [Abstract]
Stock compensation plans, shares									899	858	944
Earnings Per Share, Diluted [Abstract]
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	$ 168,826	$ 158,495	$ 165,861
Shares net income, Diluted									59,357	58,747	58,156
Diluted net income per share	$ 0.51	$ 0.79	$ 0.70	$ 0.85	$ 0.67	$ 0.66	$ 0.74	$ 0.63	$ 2.84	$ 2.70	$ 2.85

Federal Income Taxes (Reconcili

Federal Income Taxes (Reconciliation of Federal Income Tax on Income at the Corporate Rate to the Effective Tax Rate) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Provisional Increase to Deferred Tax Asset	$ 7,500
Tax at statutory rate of 35%	91,689	$ 76,984	$ 81,442
Tax-advantaged interest	(11,510)	(12,126)	(13,164)
Dividends received deduction	(1,961)	(1,114)	(1,817)
Share-based compensation expense income tax benefit	(4,281)	0	0
Tax Reform Deferred Tax Asset Impact	20,205	0	0
Tax Benefit on Unrealized Gains	5,700
Other	(1,000)	(2,284)	370
Total federal income tax expense	$ 93,142	$ 61,460	$ 66,831

Federal Income Taxes (Deferred

Federal Income Taxes (Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Net Deferred Federal Income Tax Assets	$ 52,900
Net loss reserve discounting	38,771	$ 70,065
Net unearned premiums	50,267	78,201
Employee benefits	8,606	17,881
Long-term incentive compensation plans	12,221	17,750
Temporary investment write-downs	1,044	2,475
Other investment-related items, net	0	1,484
Net operating loss	54	771
Other	5,784	8,344
Total deferred tax assets	116,747	196,971
Deferred policy acquisition costs	47,484	75,310
Unrealized gains on investment securities	26,183	22,681
Other investment-related items, net	2,500	0
Accelerated depreciation and amortization	8,590	14,140
Total deferred tax liabilities	84,757	112,131
Tax benefits related to compensation expense deductions for share-based compensation awards	23,800	$ 23,800	$ 22,000
Investments [Member]
Change in Net Deferred Federal Income Tax Assets	$ 21,400

Federal Income Tax (Federal Tax

Federal Income Tax (Federal Tax Net Operating Loss Carryforwards) (Details) $ in Millions	Dec. 31, 2017USD ($)
Operating Loss Carryforwards [Line Items]
Annual Limitation Of Net Operating Loss Carryforwards	$ 1.9
Operating Loss Carryforwards	$ 0.3

Retirement Plans (Retirement Sa

Retirement Plans (Retirement Savings Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Savings Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 15.8	$ 15	$ 14.1

Retirement Plans (Deferred Comp

Retirement Plans (Deferred Compensation Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Plan Contributions By Employer	$ 0.2	$ 0.3	$ 0.2

Retirement Plans (Funded Status

Retirement Plans (Funded Status of Retirement Income Plan and Retirement Life Plan) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Contributions by Employer	$ 200	$ 300	$ 200
Net actuarial gain (loss), Gross	4,684	12,079	(2,438)
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	9,100
Benefit obligation, end of year	10,100	9,100
Retirement Income Plan [Member]
Defined Benefit Plan, Amortization of Gain (Loss)	(2,001)	(6,299)	(6,831)
Net actuarial gain (loss), Gross	3,594	11,316	(1,425)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	330,588	310,308
Service cost	0	1,647	7,215
Interest cost	12,490	12,336	13,668
Actuarial losses (gains)	31,158	15,086
Benefit obligation, end of year	364,411	330,588	310,308
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of assets, beginning of year	316,515	249,700
Actual return on plan assets, net of expenses	46,983	21,079
Contributions by the employer to funded plans	10,000	54,525
Benefits paid fair value	(9,825)	(8,789)
Fair value of assets, end of year	363,673	316,515	$ 249,700
Funded status	(738)	(14,073)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Liabilities	(738)	(14,073)
Net pension liability, end of year	(738)	(14,073)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial loss	87,438	85,845
Total	87,438	85,845
Weighted-Average Liability Asumptions
Accumulated benefit obligation	$ 364,411	$ 330,588
Discount rate	3.78%	4.41%
Retirement Life Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 6,300
Benefit obligation, end of year	$ 6,400	$ 6,300

Retirement Plans (Components of

Retirement Plans (Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net actuarial loss (gain)	$ 4,684	$ 12,079	$ (2,438)
Reversal of amortization of net actuarial loss	(2,102)	(6,462)	(7,077)
Retirement Income Plan [Member]
Service cost	0	1,647	7,215
Interest cost	12,490	12,336	13,668
Expected return on plan assets	(19,419)	(17,309)	(15,969)
Amortization of unrecognized actuarial loss	2,001	6,299	6,831
Total net periodic cost	(4,928)	2,973	11,745
Net actuarial loss (gain)	3,594	11,316	(1,425)
Reversal of amortization of net actuarial loss	(2,001)	(6,299)	(6,831)
Total recognized in other comprehensive income	1,593	5,017	(8,256)
Total recognized in net periodic benefit cost and OCI	(3,335)	7,990	3,489
Estimated net actuarial loss that will be amortized from AOCI into net periodic benefit cost	2,000
Retirement Life Plan [Member]
Total net periodic cost	300
Supplemental Employee Retirement Plan [Member]
Total net periodic cost	$ 400	$ 500	$ 800

Retirement Plans (Weighted-Aver

Retirement Plans (Weighted-Average Expense Assumptions) (Details)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Discount rate		3.46%
Expected return on plan assets		6.36%
Retirement Income Plan [Member]
Discount rate		4.41%	4.69%	4.29%
Expected return on plan assets		6.24%	6.37%	6.27%
Rate of compensation increase	[1]	0.00%	0.00%	4.00%
Discount rate used to value the liability		3.78%	4.41%
[1]	This assumption was 4.00% through March 31, 2016, the date after which benefits ceased accruing for all participants of the Pension Plan.

Retirement Plans (Plan's Alloca

Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Actual percentage	100.00%	100.00%
Maximum percentage of equity security value of total plans portfolio value that is allowed	8.00%
Maximum percentage of any corporation outstanding shares that is allowed for retirement income plan equity portfolio	5.00%
Return Seeking Assets [Member]
Actual percentage	58.00%	50.00%
Liability Hedging Assets [Member]
Actual percentage	42.00%	50.00%
Minimum [Member] | Return Seeking Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.2
Minimum [Member] | Liability Hedging Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.4
Maximum [Member] | Return Seeking Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.6
Maximum [Member] | Liability Hedging Assets [Member]
Defined Benefit Plan, Target Allocation Percentage	0.8

Retirement Plans (Quantitative

Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details) - Retirement Income Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total Assets	$ 363,673	$ 316,515	$ 249,700
Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	363,075	316,987
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	327,427	296,966
Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	1,615	1,832
Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	980	0
Global Asset Allocation Fund [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	41,309	37,878
Global Asset Allocation Fund [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	41,309	37,878
Global Asset Allocation Fund [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Global Asset Allocation Fund [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Extended Duration Fixed Income [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	146,837	131,457
Extended Duration Fixed Income [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	146,837	131,457
Extended Duration Fixed Income [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Extended Duration Fixed Income [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Short-term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	4,939	23,722
Short-term Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	4,939	23,722
Short-term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Short-term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Deposit Administration Contracts [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	1,615	1,832
Deposit Administration Contracts [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Deposit Administration Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	1,615	1,832
Deposit Administration Contracts [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Cash And Short Term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	6,554	25,554
Cash And Short Term Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	4,939	23,722
Cash And Short Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	1,615	1,832
Cash And Short Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Liability Hedging Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	153,391	157,011
Liability Hedging Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	151,776	155,179
Liability Hedging Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	1,615	1,832
Liability Hedging Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
International equity [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	67,989	48,836
International equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	67,989	48,836
International equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
International equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
US Global Equity [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	66,353	55,073
US Global Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	66,353	55,073
US Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
US Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Global Equity [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	134,342	103,909
Global Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	134,342	103,909
Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Real Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	16,305	15,466
Real Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Real Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Real Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Private Equity Funds [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	1,096	1,615
Private Equity Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Private Equity Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Private Equity Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Private Credit Funds [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	460	1,108
Private Credit Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Private Credit Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Private Credit Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Hedge Funds [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	15,192
Hedge Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Hedge Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Hedge Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Other Private Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	980
Other Private Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Other Private Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Other Private Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	980
Total Private Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	34,033
Total Private Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Total Private Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0
Total Private Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	980
Return Seeking Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	209,684	159,976
Return Seeking Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	175,651	141,787
Return Seeking Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Return Seeking Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	980	0
Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	33,053	18,189
Alternative Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Alternative Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	0	0
Alternative Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	$ 0	$ 0

Retirement Plans (Benefit Payme

Retirement Plans (Benefit Payments) (Details) - Retirement Income Plan [Member] $ in Thousands	Dec. 31, 2017USD ($)
2,018	$ 12,913
2,019	12,936
2,020	13,987
2,021	15,093
2,022	16,128
2023 - 2027	$ 94,542

Share-Based Payments (Summary o

Share-Based Payments (Summary of Shares Available for Issuance) (Details)	Dec. 31, 2017shares
Omnibus Stock Plan 2014 [Member]
Schedule of Share-Based Compensation Shares Available for Issuance [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	3,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	2,516,871
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	882,490
Employee Stock [Member]
Schedule of Share-Based Compensation Shares Available for Issuance [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	1,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	499,629
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	0
Agent Stock Purchase Plan [Member]
Schedule of Share-Based Compensation Shares Available for Issuance [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	3,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1,817,493
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	0

Shared-Based Payments (Summary

Shared-Based Payments (Summary of Retired Plans) (Details)	Dec. 31, 2017shares
Omnibus Stock Plan [Member]
Schedule of Share-Based Compensation Retired Plans [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	2,202,532
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	277,132
Stock Compensation Plan For Non Employee Directors [Member]
Schedule of Share-Based Compensation Retired Plans [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	66,506
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	66,506
Stock Option Share Based Compensation [Member] | Omnibus Stock Plan [Member]
Schedule of Share-Based Compensation Retired Plans [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	229,864
Restricted Stock Units (RSUs) [Member] | Omnibus Stock Plan [Member]
Schedule of Share-Based Compensation Retired Plans [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	47,268

Share-Based Payments (Summar107

Share-Based Payments (Summary of the Restricted Stock Unit Transactions under Share-Based Payment Plans) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Incentive Plan [Member]
Weighted average remaining expense life	11 months
Share Based Compensation Liability Awards Amount Of Liability	$ 37	$ 32
Cash incentive unit payments made	$ 14.2	$ 14.3	$ 10.2
Restricted Stock Units (RSUs) [Member]
Unvested RSU awards at December 31, 2016	916,640
Granted in 2017, number of shares	321,928
Vested in 2017, number of shares	(360,702)
Forfeited in 2017, number of shares	(12,279)
Unvested RSU awards at Decemeber 31, 2017, number of shares	865,587	916,640
Unvested RSU awards at December 31, 2016, weighted average grant date fair value	$ 26.20
Granted in 2017, weighted average grant date fair value	42.66	$ 32.53	$ 25.22
Vested in 2017, weighted average grant date value	22.78
Forfeited in 2017, weighted average grant date fair value	32.09
Unvested RSU awards at December 31, 2017, weighted average grant date fair value	$ 33.66	$ 26.20
Total unrecognized compensation cost related to unvested RSU awards	$ 8
Weighted average remaining expense life	1 year 9 months
Total intrinsic value of shares vested	$ 16	$ 12.6	$ 10.3
Dividend Equivalent Units [Member]
Total intrinsic value of shares vested	$ 0.9	$ 0.7	$ 0.7

Share-Based Payments (Summar108

Share-Based Payments (Summary of Stock Option Transactions under Share-Based Payment Plans) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Outstanding at December 31, 2016, number of shares	355,391
Granted in 2017, numbers of shares	0
Exercised in 2017, number of shares	(120,496)
Forfeited or expired 2017, number of shares	(5,031)
Outstanding at December 31, 2017, number of shares	229,864	355,391
Exercisable at December 31, 2017, number of shares	229,864
Outstanding at December 31, 2016, weighted average exercise price	$ 16.87
Granted in 2017, weighted average exercise price	0
Exercised in 2017, weighted average exercise price	19.39
Forfeited or expired 2017, weighted average exercise price	24.54
Outstanding at December 31, 2017, weighted average exercise price	15.38	$ 16.87
Exercisable as December 31, 2017, weighted average exercise price	$ 15.38
Outstanding at December 31, 2017, weighted average remaining contractual life in years	1 year 6 months
Exercisable at December 31, 2017, weighted average remaining contractual life in years	1 year 6 months
Outstanding at December 31, 2017, aggregate intrinsic value	$ 9,958
Exerciseable at December 31, 2017, aggregate intrinsic value	9,958
Total intrinsic value of options exercised	$ 4,000	$ 2,300	$ 2,200

Shared-Based Payments (Summa109

Shared-Based Payments (Summary of ESPP and ASPP Shares Issued) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted [Line Items]
Stock Issued During Period Shares	635,521	720,323	863,426
Employee Stock [Member]
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted [Line Items]
Stock Issued During Period Shares	75,093	88,432	100,944
Agent Stock Purchase Plan [Member]
Schedule of Share-Based Compensation ESPP and ASPP Shares Granted [Line Items]
Stock Issued During Period Shares	49,794	69,867	82,142

Share-Based Payments (Weighted

Share-Based Payments (Weighted Average Assumptions for Employee Stock Purchase Plan and Other Options) (Details) - Employee Stock Purchase Plan [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Risk-free interest rate	1.07%	0.47%	0.10%
Expected term	6 months	6 months	6 months
Dividend yield	1.30%	1.70%	2.00%
Expected volatility	24.00%	31.00%	20.00%

Share-Based Payments (Weighted-

Share-Based Payments (Weighted-Average Fair Value of Options and Stock Per Share) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units (RSUs) [Member]
Stock, weighted average grant date fair value	$ 42.66	$ 32.53	$ 25.22
Employee Stock Purchase Plan [Member]
Options, weighted average grant date fair value	2.73	2.63	1.26
15% of grant date market value	7.06	5.23	4.16
Total ESPP	9.79	7.86	5.42
Agent Stock Purchase Plan [Member]
Stock, weighted average grant date fair value	$ 5.04	$ 3.79	$ 2.94

Shared-Based Payments (Summa112

Shared-Based Payments (Summary of Share-Based Compensation Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Share-Based Compensation Expenses [Abstract]
Share-based compensation expense charged against net income, Before Tax	$ 31.2	$ 30.3	$ 23.8
Share-based compensation expense income tax benefit	(15)	(10.3)	(8)
Share-based Compensation Expense, Net of Tax	$ 16.2	$ 20	$ 15.8

Related Party Transactions (Nar

Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity ownership percentage of director of parent of general independent retail insurance agency	10.00%
Direct premiums written	$ 2,733,459	$ 2,577,259	$ 2,403,519
Due to Related Parties, Current	600	700
Contributions to the Selective Insurance Group Foundation	$ 100	0	1,000
Percentage Common Stock Owned by Related Party	12.80%
Costs and Expenses, Related Party	$ 2,000	400
Due to Related Parties	500	400
Related Party Insurance Agency [Member]
Direct premiums written	11,100	10,400	9,600
Commissions paid to Rue Insurance	$ 2,300	$ 2,100	$ 1,700

Commitments and Contingencie114

Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contractual Obligation Additional Investment in Alternative or Other Investments	$ 221,000
Purchase amount of annuities	18,500
Rental expense for operating leases for office space, equipment, and vehicles	$ 10,800	$ 12,300	$ 11,700
Capital Lease Agreements Expiration Period	2,019
Other Investments [Member]
Remaining Commitment	$ 220,999	$ 143,686

Commitments and Contingencie115

Commitments and Contingencies (Future Minimum Rental Commitments) (Details) $ in Millions	Dec. 31, 2017USD ($)
2018- Capital Leases	$ 2.3
2019- Capital Leases	0.1
2020- Capital Leases	0
2021- Capital Leases	0
2022- Capital Leases	0
After 2022- Capital Leases	0
Total minimum payment required- Capital Leases	2.4
2018- Operating Leases	10
2019- Operating Leases	7.5
2020- Operating Leases	6
2021- Operating Leases	3.7
2022- Operating Leases	2.1
After 2022- Operating Leases	2.6
Total minimum payment required- Operating Leases	31.9
2018- Total Leases	12.3
2019- Total Leases	7.6
2020- Total Leases	6
2021- Total Leases	3.7
2022- Total Leases	2.1
After 2022- Total Leases	2.6
Total minimum payment required	$ 34.3

Statutory Financial Informat116

Statutory Financial Information, Capital Requirements and Restrictions on Dividends and Transfers of Funds (Statutory Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory unassigned surplus	$ 1,137.3	$ 1,048.2
Statutory surplus	1,672.9	1,583.8
Statutory net income	221.2	202.3	$ 195.8
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	455.5	414.4
Statutory surplus	609.7	568.6
Statutory net income	84.6	72.2	69.6
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	276.1	260.5
Statutory surplus	325.1	309.5
Statutory net income	43.6	41.2	42.3
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	112.9	110.6
Statutory surplus	144.1	141.9
Statutory net income	17.9	17.4	15.9
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	86.2	83.5
Statutory surplus	111.8	109.1
Statutory net income	14.7	13.4	12.1
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	78.8	74.1
Statutory surplus	106.5	101.8
Statutory net income	13.4	12.9	12.7
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	16.1	13.6
Statutory surplus	46.3	43.7
Statutory net income	6.3	5.9	5.5
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	42.1	36.9
Statutory surplus	84.9	79.8
Statutory net income	11.4	11.5	10.8
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	21.4	16.7
Statutory surplus	89.9	85.2
Statutory net income	10.3	9.7	9.5
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	34.5	26.6
Statutory surplus	109	101
Statutory net income	13.4	12.6	12.1
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	13.7	11.3
Statutory surplus	45.6	43.2
Statutory net income	$ 5.6	$ 5.5	$ 5.3

Statutory Financial Informat117

Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Dividends) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total stockholders’ equity	$ 1,712,957,000	$ 1,531,370,000	$ 1,398,041,000
Ordinary dividends paid	80,100,000
Maximum ordinary dividends	$ 211,000,000
Maximum borrowing percentage	10.00%
New Jersey Dividend Restriction	10.00%
New York Dividend Restriction	10.00%
Dividend Restriction New York	100.00%
Selective Insurace Company of America (SICA) [Member]
Ordinary dividends paid	$ 28,000,000
Maximum ordinary dividends	77,600,000
Selective Way Insurance Company (SWIC) [Member]
Ordinary dividends paid	19,000,000
Maximum ordinary dividends	43,300,000
Selective Insurance Company of South Carolina (SICSC) [Member]
Ordinary dividends paid	10,000,000
Maximum ordinary dividends	17,900,000
Selective Insurance Company of the Southeast (SICSE) [Member]
Ordinary dividends paid	7,500,000
Maximum ordinary dividends	14,700,000
Selective Insurance Company of New York (SICNY) [Member]
Ordinary dividends paid	4,500,000
Maximum ordinary dividends	10,700,000
Selective Insurance Company of New England (SICNE) [Member]
Ordinary dividends paid	2,000,000
Maximum ordinary dividends	6,200,000
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Ordinary dividends paid	2,500,000
Maximum ordinary dividends	11,300,000
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Ordinary Dividends Paid To Parent	2,100,000
Maximum ordinary dividends	10,300,000
Selective Casualty Insurance Company (SCIC) [Member]
Ordinary dividends paid	3,000,000
Maximum ordinary dividends	13,400,000
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Ordinary dividends paid	1,500,000
Maximum ordinary dividends	5,600,000
Parent Company [Member]
Total stockholders’ equity	1,700,000,000
Maximum ordinary dividends	114,500,000
Additional Borrowing Capacity for Parent	$ 70,500,000

Quarterly Financial Informat118

Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net premiums earned	$ 590,088	$ 572,055	$ 568,030	$ 560,854	$ 552,753	$ 542,429	$ 531,932	$ 522,458	$ 2,291,027	$ 2,149,572	$ 1,989,909
Net investment income earned	42,587	40,446	41,430	37,419	35,428	33,375	31,182	30,769	161,882	130,754	121,316
Net realized (losses) gains	(1,128)	6,798	1,734	(1,045)	(7,686)	3,688	1,765	(2,704)	11,204	3,562	31,537
Other income	2,190	1,994	3,291	3,241	1,863	2,199	3,868	951	10,716	8,881	7,456
Total revenues	633,737	621,293	614,485	600,469	582,358	581,691	568,747	551,474	2,469,984	2,284,270	2,131,852
Income before federal income tax	68,150	67,315	58,929	67,574	50,326	55,443	62,311	51,875	261,968	219,955	232,692
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	$ 168,826	$ 158,495	$ 165,861
Basic net income per share	$ 0.52	$ 0.80	$ 0.71	$ 0.87	$ 0.68	$ 0.66	$ 0.75	$ 0.64	$ 2.89	$ 2.74	$ 2.90
Diluted net income per share	$ 0.51	$ 0.79	$ 0.70	$ 0.85	$ 0.67	$ 0.66	$ 0.74	$ 0.63	$ 2.84	$ 2.70	$ 2.85

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event [Member] $ in Millions	1 Months Ended
	Jan. 31, 2018USD ($)
Subsequent Event [Line Items]
Non Catastrophe Property Losses	$ 47
Loss from Catastrophes	16
Total Property Losses	63
Total Property Loss Estimate	$ 30

Schedule I - Summay of Inves120

Schedule I - Summay of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2017USD ($)
Amortized Cost or Cost	$ 5,560,500
Carrying Amount	5,685,179
Short-term Investments [Member]
Amortized Cost or Cost	165,555
Fair Value	165,555
Carrying Amount	165,555
Other Investments [Member]
Amortized Cost or Cost	132,268
Carrying Amount	132,268
Preferred Stock [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	14,115
Fair Value	14,948
Carrying Amount	14,948
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	25,154
Fair Value	26,261
Carrying Amount	25,238
Held-to-maturity Securities [Member] | Public Utilities
Amortized Cost or Cost	7,466
Fair Value	7,956
Carrying Amount	7,443
Held-to-maturity Securities [Member] | All other corporate securities
Amortized Cost or Cost	9,530
Fair Value	9,883
Carrying Amount	9,448
Held-to-maturity Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	42,150
Fair Value	44,100
Carrying Amount	42,129
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
Amortized Cost or Cost	49,326
Fair Value	49,740
Carrying Amount	49,740
AFS Fixed Income Securities [Member] | Foreign Government
Amortized Cost or Cost	18,040
Fair Value	18,555
Carrying Amount	18,555
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	1,539,307
Fair Value	1,582,970
Carrying Amount	1,582,970
AFS Fixed Income Securities [Member] | Public Utilities
Amortized Cost or Cost	50,071
Fair Value	51,035
Carrying Amount	51,035
AFS Fixed Income Securities [Member] | All other corporate securities
Amortized Cost or Cost	1,538,268
Fair Value	1,566,433
Carrying Amount	1,566,433
AFS Fixed Income Securities [Member] | CLO and Other ABS
Amortized Cost or Cost	789,152
Fair Value	795,458
Carrying Amount	795,458
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	382,727
Fair Value	383,449
Carrying Amount	383,449
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Amortized Cost or Cost	709,825
Fair Value	714,882
Carrying Amount	714,882
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	5,076,716
Fair Value	5,162,522
Carrying Amount	5,162,522
Equity Securities [Member] | Public Utilities
Amortized Cost or Cost	5,957
Fair Value	6,156
Carrying Amount	6,156
Equity Securities [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	34,301
Fair Value	40,510
Carrying Amount	40,510
Equity Securities [Member] | Industrial, miscellaneous and all other
Amortized Cost or Cost	89,438
Fair Value	121,091
Carrying Amount	121,091
Equity Securities [Member] | Common Stock [Member]
Amortized Cost or Cost	129,696
Fair Value	167,757
Carrying Amount	167,757
Equity Securities [Member] | Preferred Stock [Member]
Amortized Cost or Cost	14,115
Fair Value	14,948
Carrying Amount	14,948
Equity Securities [Member] | Equity Securities [Member]
Amortized Cost or Cost	143,811
Fair Value	182,705
Carrying Amount	$ 182,705

Parent Corporation Balance Shee

Parent Corporation Balance Sheet (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Fixed income securities, available-for-sale – at fair value (amortized cost: $89,799 – 2017; $73,471 – 2016)	$ 5,162,522,000	$ 4,792,540,000
Short-term investments	165,555,000	221,701,000
Cash	534,000	458,000	$ 898,000	$ 23,959,000
Current federal income tax	3,243,000	2,486,000
Deferred federal income tax	31,990,000	84,840,000
Other assets	122,371,000	113,534,000
Total assets	7,686,431,000	7,355,848,000
Liabilities:
Long-term debt	439,116,000	438,667,000
Other liabilities	281,624,000	298,393,000
Total liabilities	5,973,474,000	5,824,478,000
Stockholders’ Equity:
Preferred stock of $0 par value per share: Authorized shares: 5,000,000; no shares issued or outstanding	0	0
Common stock of $2 par value per share Authorized shares: 360,000,000 Issued: 102,284,564 - 2017; 101,620,436 - 2016	204,569,000	203,241,000
Additional paid-in capital	367,717,000	347,295,000
Retained earnings	1,698,613,000	1,568,881,000
Accumulated other comprehensive income (loss)	20,170,000	(15,950,000)
Treasury stock – at cost (shares: 43,789,442 – 2017; 43,653,237 – 2016)	(578,112,000)	(572,097,000)
Total stockholders’ equity	1,712,957,000	1,531,370,000	1,398,041,000
Total liabilities and stockholders’ equity	7,686,431,000	7,355,848,000
Parent Company [Member]
ASSETS
Fixed income securities, available-for-sale – at fair value (amortized cost: $89,799 – 2017; $73,471 – 2016)	89,872,000	73,509,000
Short-term investments	24,080,000	17,777,000
Cash	534,000	458,000	$ 898,000	$ 16,367,000
Investment in subsidiaries	2,013,304,000	1,845,410,000
Current federal income tax	22,266,000	19,766,000
Deferred federal income tax	13,239,000	19,562,000
Other assets	871,000	840,000
Total assets	2,164,166,000	1,977,322,000
Liabilities:
Long-term debt	329,116,000	328,667,000
Intercompany notes payable	78,443,000	79,324,000
Accrued long-term stock compensation	37,017,000	32,029,000
Other liabilities	6,633,000	5,932,000
Total liabilities	451,209,000	445,952,000
Stockholders’ Equity:
Preferred stock of $0 par value per share: Authorized shares: 5,000,000; no shares issued or outstanding	0	0
Common stock of $2 par value per share Authorized shares: 360,000,000 Issued: 102,284,564 - 2017; 101,620,436 - 2016	204,569,000	203,241,000
Additional paid-in capital	367,717,000	347,295,000
Retained earnings	1,698,613,000	1,568,881,000
Accumulated other comprehensive income (loss)	20,170,000	(15,950,000)
Treasury stock – at cost (shares: 43,789,442 – 2017; 43,653,237 – 2016)	(578,112,000)	(572,097,000)
Total stockholders’ equity	1,712,957,000	1,531,370,000
Total liabilities and stockholders’ equity	$ 2,164,166,000	$ 1,977,322,000

Parent Corporation Balance S122

Parent Corporation Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fixed income securities, available-for-sale, amortized cost	$ 5,076,716	$ 4,753,759
Preferred stock, par value per share	$ 0	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000	5,000,000
Preferred stock, shares issued	0	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 2	$ 2
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	102,284,564	101,620,436
Treasury stock, shares	43,789,442	43,653,237
Parent Company [Member]
Fixed income securities, available-for-sale, amortized cost	$ 89,799	$ 73,471
Preferred stock, par value per share	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 2	$ 2
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	102,284,564	101,620,436
Treasury stock, shares	43,789,442	43,653,237

Parent Corporation Statements o

Parent Corporation Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Dividends from subsidaries									$ (80,100)
Net investment income earned	$ 42,587	$ 40,446	$ 41,430	$ 37,419	$ 35,428	$ 33,375	$ 31,182	$ 30,769	161,882	$ 130,754	$ 121,316
Realized Investment Gains (Losses)									6,359	(4,937)	13,171
Total revenues	633,737	621,293	614,485	600,469	582,358	581,691	568,747	551,474	2,469,984	2,284,270	2,131,852
Expenses:
Interest expense									24,354	22,771	22,428
Total expenses									2,208,016	2,064,315	1,899,160
Federal income tax (benefit) expense:
Current									62,184	48,581	45,347
Deferred									30,958	12,879	21,484
Total federal income tax expense									93,142	61,460	66,831
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	168,826	158,495	165,861
Parent Company [Member]
Revenues:
Dividends from subsidaries									80,096	61,014	57,752
Net investment income earned									2,044	1,259	852
Realized Investment Gains (Losses)									(15)	(220)	0
Total revenues									82,125	62,053	58,604
Expenses:
Interest expense									24,721	24,030	24,057
Other expenses									36,251	35,020	28,393
Total expenses									60,972	59,050	52,450
Income before federal income tax									21,153	3,003	6,154
Federal income tax (benefit) expense:
Current									(22,187)	(17,924)	(16,609)
Deferred									6,311	(2,143)	(1,603)
Total federal income tax expense									(15,876)	(20,067)	(18,212)
Net income before equity in undistributed income of subsidiaries									37,029	23,070	24,366
Equity In Undistributed Income Of Continuing Subsidiaries, Net Of Tax									131,797	135,425	141,495
Net income									$ 168,826	$ 158,495	$ 165,861

Parent Corporation Statement124

Parent Corporation Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net income	$ 30,242	$ 46,718	$ 41,426	$ 50,440	$ 39,360	$ 38,502	$ 43,601	$ 37,032	$ 168,826	$ 158,495	$ 165,861
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense									12,089	10,449	8,973
Net realized losses									(6,359)	4,937	(13,171)
Amortization - other									52,100	61,671	59,688
Changes in assets and liabilities:
Decrease (increase) in net federal income taxes									30,918	11,150	25,004
Decrease in other assets									9,872	30,071	11,523
Increase (decrease) in other liabilities									27,297	9,191	77,564
Net cash provided by operating activities									370,733	301,783	381,580
Investing Activities
Purchase of fixed income securities, available-for-sale									(2,130,362)	(1,982,023)	(1,041,916)
Redemption and maturities of fixed income securities, available-for-sale									555,216	641,524	567,445
Sale of fixed income securities, available for sale									58,832	102,868	106,621
Purchase of short-term investments									(4,280,553)	(3,499,380)	(1,602,327)
Sale of short-term investments									4,338,318	3,470,022	1,539,480
Net cash used in investing activities									(331,075)	(318,101)	(391,543)
Financing Activities
Dividends to stockholders									(37,045)	(33,758)	(31,052)
Acquisition of treasury stock									(6,015)	(4,992)	(4,182)
Net proceeds from stock purchase and compensation plans									7,599	7,811	10,089
Excess tax benefits from share-based payment arrangements									0	1,819	1,736
Net cash used in financing activities									(39,582)	15,878	(13,098)
Cash, beginning of year				458				898	458	898	23,959
Cash, end of year	534				458				534	458	898
Parent Company [Member]
Operating Activities
Net income									168,826	158,495	165,861
Adjustments to reconcile net income to net cash provided by operating activities:
Equity In Undistributed Income Of Continuing Subsidiaries, Net Of Tax									(131,797)	(135,425)	(141,495)
Stock-based compensation expense									12,089	10,449	8,973
Net realized losses									15	220	0
Amortization - other									678	648	740
Changes in assets and liabilities:
Increase in accrued long-term stock compensation									4,988	5,564	4,575
Decrease (increase) in net federal income taxes									3,811	(3,612)	(3,052)
Decrease in other assets									(60)	(202)	(12)
Increase (decrease) in other liabilities									714	80	(202)
Net cash provided by operating activities									59,264	36,217	35,388
Investing Activities
Purchase of fixed income securities, available-for-sale									(58,832)	(45,789)	(33,717)
Redemption and maturities of fixed income securities, available-for-sale									10,465	14,983	21,578
Sale of fixed income securities, available for sale									31,819	18,768	0
Purchase of short-term investments									(185,590)	(119,501)	(106,933)
Sale of short-term investments									179,292	130,841	94,422
Net cash used in investing activities									(22,846)	(698)	(24,650)
Financing Activities
Dividends to stockholders									(37,045)	(33,758)	(31,052)
Acquisition of treasury stock									(6,015)	(4,992)	(4,182)
Net proceeds from stock purchase and compensation plans									7,599	7,811	10,089
Excess tax benefits from share-based payment arrangements									0	1,819	1,736
Principal payments of borrowings from subsidiaries									(881)	(6,839)	(2,798)
Net cash used in financing activities									(36,342)	(35,959)	(26,207)
Net increase (decrease) in cash									76	(440)	(15,469)
Cash, beginning of year				$ 458				$ 898	458	898	16,367
Cash, end of year	$ 534				$ 458				$ 534	$ 458	$ 898

Schedule III - Supplementary125

Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred policy acquisition costs	$ 235,055				$ 222,564				$ 235,055	$ 222,564	$ 213,159
Reserve for loss and loss expenses	3,771,240				3,691,719				3,771,240	3,691,719	3,517,728
Unearned premiums	1,349,644				1,262,819				1,349,644	1,262,819	1,169,710
Net premiums earned									2,291,027	2,149,572	1,989,909
Net investment income									168,241	125,817	134,487
Loss and loss expense incurred									1,345,074	1,234,797	1,148,541
Amortization of deferred policy acquisition costs									469,236	450,328	399,436
Other operating expenses									322,381	312,514	292,903
Net premiums written									2,370,641	2,237,288	2,069,904
Other insurance expenses									333,097	321,395	300,359
Other income	(2,190)	$ (1,994)	$ (3,291)	$ (3,241)	(1,863)	$ (2,199)	$ (3,868)	$ (951)	(10,716)	(8,881)	(7,456)
Standard Commercial Lines [Member]
Deferred policy acquisition costs	193,408				181,193				193,408	181,193	171,476
Reserve for loss and loss expenses	3,165,217				3,098,554				3,165,217	3,098,554	2,998,749
Unearned premiums	956,173				884,976				956,173	884,976	803,648
Net premiums earned									1,788,499	1,665,483	1,529,442
Net investment income									0	0	0
Loss and loss expense incurred									1,008,150	913,506	819,573
Amortization of deferred policy acquisition costs									387,552	367,813	323,754
Other operating expenses									243,283	237,729	221,619
Net premiums written									1,858,735	1,745,782	1,596,965
Standard Personal Lines [Member]
Deferred policy acquisition costs	16,952				16,664				16,952	16,664	17,258
Reserve for loss and loss expenses	263,166				286,081				263,166	286,081	265,054
Unearned premiums	295,435				282,111				295,435	282,111	276,533
Net premiums earned									289,701	280,607	288,134
Net investment income									0	0	0
Loss and loss expense incurred									189,294	177,749	200,237
Amortization of deferred policy acquisition costs									32,542	34,105	33,638
Other operating expenses									56,761	56,334	52,923
Net premiums written									296,775	281,822	283,926
Excess And Surplus Operations [Member]
Deferred policy acquisition costs	24,695				24,707				24,695	24,707	24,425
Reserve for loss and loss expenses	342,857				307,084				342,857	307,084	253,925
Unearned premiums	98,036				95,732				98,036	95,732	89,529
Net premiums earned									212,827	203,482	172,333
Net investment income									0	0	0
Loss and loss expense incurred									147,630	143,542	128,731
Amortization of deferred policy acquisition costs									49,142	48,410	42,044
Other operating expenses									22,337	18,451	18,361
Net premiums written									215,131	209,684	189,013
Investments Segment [Member]
Deferred policy acquisition costs	0				0				0	0	0
Reserve for loss and loss expenses	0				0				0	0	0
Unearned premiums	$ 0				$ 0				0	0	0
Net premiums earned									0	0	0
Net investment income									168,241	125,817	134,487
Loss and loss expense incurred									0	0	0
Amortization of deferred policy acquisition costs									0	0	0
Other operating expenses									0	0	0
Net premiums written									0	0	0
Miscellaneous Income [Member]
Other income									$ (10,716)	$ (8,881)	$ (7,456)

Schedule IV - Reinsurance (Deta

Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Direct Amount									$ 2,647,488	$ 2,484,715	$ 2,330,267
Assumed From Other Companies									25,831	28,214	23,209
Ceded to Other Companies									382,292	363,357	363,567
Total Net Premiums Earned	$ 590,088	$ 572,055	$ 568,030	$ 560,854	$ 552,753	$ 542,429	$ 531,932	$ 522,458	$ 2,291,027	$ 2,149,572	$ 1,989,909
Percentage of Amount Assumed to Net									1.00%	1.00%	1.00%
Accident and Health Insurance Segment [Member]
Direct Amount									$ 24	$ 32	$ 37
Assumed From Other Companies									0	0	0
Ceded to Other Companies									24	0	37
Total Net Premiums Earned									$ 0	$ 32	$ 0
Percentage of Amount Assumed to Net									0.00%	0.00%	0.00%
Property and liability insurance [Member]
Direct Amount									$ 2,647,464	$ 2,484,683	$ 2,330,230
Assumed From Other Companies									25,831	28,214	23,209
Ceded to Other Companies									382,268	363,357	363,530
Total Net Premiums Earned									$ 2,291,027	$ 2,149,540	$ 1,989,909
Percentage of Amount Assumed to Net									1.00%	1.00%	1.00%

Schedule V - Allowance for Unc

Schedule V - Allowance for Uncollectible Premiums and Other Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance, January 1	$ 11,480	$ 10,122	$ 11,037
Additions	6,414	4,669	3,604
Deductions	(3,294)	(3,311)	(4,519)
Balance, December 31	$ 14,600	$ 11,480	$ 10,122