Navigators (NAVG) 10-K 22 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Jan. 29, 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
Trading Symbol	NAVG
Entity Registrant Name	NAVIGATORS GROUP INC
Entity Central Index Key	793,547
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		29,531,368
Entity Public Float			$ 1,267,312,917

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments:
Fixed Maturities, available-for-sale, at fair value (amortized cost: 2017: $3,027,408; 2016: $2,628,225)	$ 3,057,054	$ 2,635,882
Equity Securities, available-for-sale, at fair value (cost: 2017: $224,159; 2016: $327,911)	235,981	349,142
Other Invested Assets	1,720	1,960
Short-Term Investments, at fair value (amortized cost: 2017: $127,125; 2016: $143,451)	127,128	143,539
Total Investments	3,421,883	3,130,523
Cash	67,084	64,643
Premiums Receivable	351,393	306,686
Prepaid Reinsurance Premiums	228,569	213,377
Reinsurance Recoverable on Paid Losses	72,494	82,582
Reinsurance Recoverable on Unpaid Losses and Loss Adjustment Expenses	809,765	779,276
Deferred Policy Acquisition Costs	135,249	119,660
Accrued Investment Income	19,480	17,315
Goodwill and Other Intangible Assets	6,596	6,451
Current Income Tax Receivable, Net	16,667	20,556
Deferred Income Tax, Net	22,271	20,938
Other Assets	73,171	52,030
Total Assets	5,224,622	4,814,037
Liabilities:
Reserves for Losses and Loss Adjustment Expenses	2,515,145	2,289,727
Unearned Premiums	987,681	887,344
Reinsurance Balances Payable	136,192	108,980
Senior Notes	263,885	263,728
Accounts Payable and Other Liabilities	95,754	86,070
Total Liabilities	3,998,657	3,635,849
Stockholders' Equity:
Preferred Stock, ($.10 par value, authorized 1,000 shares, none issued)	0	0
Common Stock, ($.10 par value, authorized 50,000 shares, issued 36,530 shares for 2017 and 36,147 shares for 2016)	3,650	3,612
Additional Paid-In Capital	376,868	373,983
Treasury Stock, at cost (7,023 shares for 2017 and 2016)	(155,801)	(155,801)
Retained Earnings	981,380	947,519
Accumulated Other Comprehensive Income	19,868	8,875
Total Stockholders' Equity	1,225,965	1,178,188
Total Liabilities and Stockholders' Equity	$ 5,224,622	$ 4,814,037

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost	$ 3,027,408	$ 2,628,225
Equity securities, available-for-sale, cost	224,159	327,911
Short-term investments, amortized cost	$ 127,125	$ 143,451
Preferred Stock, par value	$ 0.10	$ 0.10
Preferred Stock, authorized shares	1,000,000	1,000,000
Preferred Stock, issued shares	0	0
Common Stock, par value	$ 0.10	$ 0.10
Common Stock, authorized shares	50,000,000	50,000,000
Common Stock, issued shares	36,530,000	36,147,000
Treasury Stock, shares	7,023,000	7,023,000

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) 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
Income Statement [Abstract]
Gross Written Premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
Revenues:
Net Written Premiums	304,869	296,016	333,282	337,163	282,868	277,001	306,535	319,820	1,271,330	1,186,224	1,043,860
Change in Net Unearned Premiums	230	5,339	(39,447)	(51,032)	1,117	7,009	(38,543)	(55,462)	(84,910)	(85,879)	(59,773)
Net Earned Premiums	305,099	301,355	293,835	286,131	283,985	284,010	267,992	264,358	1,186,420	1,100,345	984,087
Net Investment Income	22,982	22,598	22,265	21,448	20,107	19,875	19,875	19,594	89,293	79,451	68,718
Total Other-Than-Temporary Impairment Losses	3	(957)	29	(1,077)	21	23	(162)	(109)	(2,002)	(227)	(1,870)
Portion of Loss Recognized in Other Comprehensive Income (Before Tax)	(3)	(15)	(29)	(16)	(21)	(23)	12	109	(62)	77	172
Net Other-Than-Temporary Impairment Losses Recognized In Earnings									(2,064)	(150)	(1,698)
Net Realized Gains	37,141	5,190	1,694	1,049	4,043	1,586	1,960	1,597	45,073	9,186	8,373
Other Income (Loss)	(3,201)	(1,699)	(411)	1,068	1,905	(183)	4,430	2,549	(4,243)	8,701	(491)
Total Revenues	362,021	326,472	317,383	308,603	310,040	305,288	294,107	288,098	1,314,479	1,197,533	1,058,989
Expenses:
Net Losses and Loss Adjustment Expenses	183,384	276,171	177,110	169,600	172,493	172,793	167,206	152,956	806,265	665,448	572,598
Commission Expenses	43,205	45,509	48,173	47,844	44,154	42,611	40,726	37,554	184,731	165,045	129,977
Other Operating Expenses	68,153	45,773	60,766	58,538	58,076	56,137	59,074	60,809	233,230	234,096	223,516
Interest Expense	3,863	3,862	3,861	3,861	3,860	3,859	3,858	3,858	15,447	15,435	15,424
Total Expenses	298,605	371,315	289,910	279,843	278,583	275,400	270,864	255,177	1,239,673	1,080,024	941,515
Income (Loss) Before Income Taxes	63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income Tax Expense	36,555	(16,864)	6,971	7,650	9,866	7,875	7,053	9,989	34,312	34,783	36,417
Net Income (Loss)	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Net Income Per Common Share:
Basic	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 0.74	$ 0.76	$ 0.56	$ 0.79	$ 1.38	$ 2.85	$ 2.82
Diluted	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 0.71	$ 0.73	$ 0.54	$ 0.77	$ 1.35	$ 2.75	$ 2.73
Average Common Shares Outstanding:
Basic									29,441	29,074	28,785
Diluted									30,071	30,032	29,651

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net Income	$ 40,494	$ 82,726	$ 81,057
Change in Net Unrealized Gains (Losses) on Investments:
Unrealized Gains (Losses) on Investments arising during the period, net of Deferred Tax of $(8,748), $3,730 and $2,624 in 2017, 2016 and 2015, respectively	23,123	(6,927)	(5,331)
Reclassification adjustment for Net Realized Gains included in Net Income net of Deferred Tax of $5,335, $(846) and $9,185 in 2017, 2016 and 2015, respectively	(14,100)	1,572	(17,058)
Change in Net Unrealized Gains (Losses) on Investments	9,023	(5,355)	(22,389)
Change in Other-Than-Temporary Impairments
Non Credit Other-Than-Temporary Impairments arising during the period, net of Deferred Tax of $(16), $27 and $60 in 2017, 2016 and 2015, respectively	40	(50)	(112)
Reclassification Adjustment for Other-Than-Temporary Impairment Credit Losses Recognized in Net Income net of Deferred Tax of $0, $0 and $(90) in 2017, 2016 and 2015, respectively	0	0	168
Change in Other-Than-Temporary Impairments	40	(50)	56
Change in Foreign Currency Translation Gains (Losses), net of Deferred Tax of $(2,874), $5,017 and $351 in 2017, 2016 and 2015, respectively	1,930	(9,324)	(622)
Other Comprehensive Income (Loss)	10,993	(14,729)	(22,955)
Comprehensive Income	$ 51,487	$ 67,997	$ 58,102

Consolidated Statements of Com6

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Unrealized Gains (Losses) on Investments arising during the period, Deferred Tax	$ (8,748)	$ 3,730	$ 2,624
Reclassification adjustment for Net Realized (Gains) Losses included in Net Income, Deferred Tax	5,335	(846)	9,185
Non Credit Other-Than-Temporary Impairments arising during the period, Deferred Tax	(16)	27	60
Reclassification Adjustment for Other-Than-Temporary Impairment Credit Losses Recognized in Net Income, Deferred Tax	0	0	(90)
Change in Foreign Currency Translation Gains (Losses), Deferred Tax	$ (2,874)	$ 5,017	$ 351

Consolidated Statement of Stock

Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands		Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2014		$ 1,027,224	$ 3,556	$ 345,244	$ (155,801)	$ 787,666	$ 46,559
Beginning Balance (in shares) at Dec. 31, 2014			35,586		7,023
Net Income		81,057				81,057
Changes in Comprehensive Income (Loss):
Change in Net Unrealized Gain (Loss) on Investments		(22,389)					(22,389)
Change in Net Non-Credit Other-Than- Temporary Impairment Losses		56					56
Change in Foreign Currency Translation Gain (Loss)		(622)					(622)
Total Comprehensive Income (Loss)		(22,955)					(22,955)
Shares Issued	[1]	(4,175)	$ 30	(4,205)
Shares Issued (in shares)	[1]		299
Share-Based Compensation		14,997		14,997
Ending Balance at Dec. 31, 2015		1,096,148	$ 3,586	356,036	$ (155,801)	868,723	23,604
Ending Balance (in shares) at Dec. 31, 2015			35,885		7,023
Net Income		82,726				82,726
Dividends Declared	[2]	(3,930)				(3,930)
Changes in Comprehensive Income (Loss):
Change in Net Unrealized Gain (Loss) on Investments		(5,355)					(5,355)
Change in Net Non-Credit Other-Than- Temporary Impairment Losses		(50)					(50)
Change in Foreign Currency Translation Gain (Loss)		(9,324)					(9,324)
Total Comprehensive Income (Loss)		(14,729)					(14,729)
Shares Issued	[1]	349	$ 26	323
Shares Issued (in shares)	[1]		262
Share-Based Compensation		17,624		17,624
Ending Balance at Dec. 31, 2016		$ 1,178,188	$ 3,612	373,983	$ (155,801)	947,519	8,875
Ending Balance (in shares) at Dec. 31, 2016		36,147	36,147		7,023
Net Income		$ 40,494				40,494
Dividends Declared	[2]	(6,633)				(6,633)
Changes in Comprehensive Income (Loss):
Change in Net Unrealized Gain (Loss) on Investments		9,023					9,023
Change in Net Non-Credit Other-Than- Temporary Impairment Losses		40					40
Change in Foreign Currency Translation Gain (Loss)		1,930					1,930
Total Comprehensive Income (Loss)		10,993					10,993
Shares Issued	[1]	(10,906)	$ 38	(10,944)
Shares Issued (in shares)	[1]		383
Share-Based Compensation		13,829		13,829
Ending Balance at Dec. 31, 2017		$ 1,225,965	$ 3,650	$ 376,868	$ (155,801)	$ 981,380	$ 19,868
Ending Balance (in shares) at Dec. 31, 2017		36,530	36,530		7,023
[1]	Includes shares issued under the Second Amended and Restated 2005 Stock Incentive Plan, to directors and the Employees Stock Purchase Plan.
[2]	Dividends Declared were $0.225 per share and $0.135 per share for the years ended December 31, 2017 and 2016, respectively. There were no dividends declared in 2015.

Consolidated Statement of Stoc8

Consolidated Statement of Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Stockholders Equity [Abstract]
Dividends declared	$ 0.225	$ 0.135	$ 0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net Income	$ 40,494	$ 82,726	$ 81,057
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Depreciation & Amortization	3,645	5,429	5,386
Share-Based Compensation	13,829	17,624	14,997
Deferred Income Taxes	(8,879)	(5,755)	7,004
Net Realized Gains	(45,073)	(9,186)	(8,373)
Net Other-Than-Temporary Impairments Recognized in Earnings	2,064	150	1,698
Changes in Assets And Liabilities:
Reinsurance Recoverable on Paid and Unpaid Losses and Loss Adjustment Expenses	(20,399)	(2,835)	43,822
Reserves for Losses and Loss Adjustment Expenses	225,132	87,083	43,010
Prepaid Reinsurance Premiums	(15,168)	19,211	5,264
Unearned Premiums	100,160	66,668	54,509
Premiums Receivable	(45,035)	(30,070)	65,863
Deferred Policy Acquisition Costs	(15,552)	(27,677)	(12,531)
Accrued Investment Income	(2,139)	(1,314)	(1,210)
Reinsurance Balances Payable	27,362	1,568	(45,124)
Current Income Tax Receivable, Net	3,966	(1,988)	(8,072)
Other	936	27,791	(13,654)
Net Cash Provided by Operating Activities	265,343	229,425	233,646
Fixed Maturities
Redemptions and Maturities	373,380	275,706	180,876
Sales	225,962	446,666	376,522
Purchases	(1,008,537)	(962,419)	(648,059)
Equity Securities
Sales	207,838	54,798	96,405
Purchases	(62,887)	(92,329)	(215,405)
Change in Net Payable for Securities	2,135	(1,517)	1,851
Purchase of Other Invested Assets	0	(1,960)	0
Net Change in Short-Term Investments	17,877	73,541	(38,468)
Purchase of Property and Equipment	(3,474)	(3,918)	(3,577)
Net Cash Used in Investing Activities	(247,706)	(211,432)	(249,855)
Financing Activities:
Proceeds from Employee Stock Purchase Plan	2,076	1,840	1,352
Proceeds of Stock Issued from Exercise of Stock Options	0	0	29
Payment of Employee Tax Withholding on Stock Compensation	(13,887)	(5,198)	(6,022)
Dividends Paid	(6,633)	(3,930)	0
Net Cash Used in Financing Activities	(18,444)	(7,288)	(4,641)
Effect of Exchange Rate on Cash	3,248	(15,963)	0
Change in Cash	2,441	(5,258)	(20,850)
Cash at Beginning of Year	64,643	69,901	90,751
Cash at End of Period	67,084	64,643	69,901
Supplemental Information:
Income Taxes Paid, Net	39,689	34,830	35,447
Interest Paid	15,238	15,238	15,238
Issuance of Stock to Directors	$ 578	$ 633	$ 563

Organization and Summary of Sig

Organization and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Organization and Summary of Significant Accounting Policies	NOTE 1. ORGANIZATION & SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation Unless the context requires otherwise, the terms “we,” “us,” “our,” or “our Company” are used to mean The Navigators Group, Inc., a Delaware holding company established in 1982, and its subsidiaries. The term “Parent Company” is used to mean The Navigators Group, Inc. without its subsidiaries. The accompanying consolidated financial statements of our Company have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP” or “U.S. GAAP”) and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). All significant intercompany transactions and balances have been eliminated in consolidation. The preparation of these financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported revenues and expenses during the reporting periods. Certain amounts for the prior year have been reclassified to conform with the current period presentation. Organization We are an international insurance company with a long-standing area of specialization in Marine insurance. We also offer Property and Casualty (“P&C”) insurance, which consists primarily of general liability coverage and umbrella & excess liability coverage to commercial enterprises through our Primary and Excess Casualty divisions. We have also developed niches in Professional Liability insurance, through our Directors & Officers (“D&O”) and Errors & Omissions (“E&O”) divisions, as well as assumed reinsurance products. We operate through various wholly-owned insurance and service companies. Our subsidiaries domiciled in the United States (“U.S.”) include two insurance companies, Navigators Insurance Company (“NIC”) and Navigators Specialty Insurance Company (“NSIC”), as well as our U.S. underwriting agency, Navigators Management Company, Inc. (“NMC”). NIC includes a branch in the United Kingdom (“U.K.”). We also have operations domiciled in the U.K., Hong Kong and Europe. Navigators International Insurance Company Ltd. (“NIIC”), Navigators Management (U.K.) Ltd. (“NMUK”) and Navigators Underwriting Ltd. (“NUL”) are domiciled in the U.K. and NUL includes European branches. Navigators Underwriting Agency Ltd. (“NUAL”), a Lloyd’s of London (“Lloyd’s”) underwriting agency, manages and provides the capital, through Navigators Corporate Underwriters Ltd. (“NCUL”), for our Lloyd’s Syndicate 1221 (“the Syndicate”), and is also domiciled in the U.K. We control 100% of the Syndicate’s stamp capacity. Effective January 1, 2017, we sold our underwriting agency operations in Sweden and Denmark. The transaction represented a 100% disposition of our Sweden and Denmark corporations, NUAL AB and Navigators A/S, respectively. This transaction did not have a material impact on our financial statements. Significant Accounting Policies Cash Cash includes cash on hand and demand deposits with banks, excluding such amounts held by the Syndicate included as Funds at Lloyd’s (“FAL”), which are classified as Short-Term Investments. Investments As of December 31, 2017 and 2016, all Fixed Maturity and Equity Securities held by our Company were carried at fair value and classified as available-for-sale. Available-for-sale securities are debt and equity securities not classified as either held-to-maturity securities or trading securities and are reported at fair value, with unrealized gains and losses excluded from earnings and reported in Accumulated Other Comprehensive Income (“AOCI”) as a separate component of Stockholders’ Equity. Fixed Maturity securities include bonds, mortgage-backed and asset-backed securities, and redeemable preferred stocks. Equity Securities consist of common stock, exchange traded funds and preferred stock. Other Invested Assets consist of investments our Company made in certain strategic companies which are accounted for using the equity method of accounting. In applying the equity method, these investments are initially recorded at cost and are subsequently adjusted based on our Company’s proportionate share of the net income or loss of the companies. Changes in the carrying value of such investments are recorded in Other Income. In applying the equity method, we use the most recently available financial information provided by the companies which is generally three months prior to the end of the reporting period. Short-Term Investments are carried at fair value. Short-Term Investments mature within one year from the purchase date. All prices for our Fixed Maturities, Equity Securities and Short-Term Investments are classified as Level 1, Level 2 or Level 3 under the fair value hierarchy, as defined in the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification 820 (“ASC 820”). Premiums and discounts on Fixed Maturity securities are amortized into interest income over the life of the security using the interest method. For Mortgage-Backed and Asset-Backed Securities, anticipated prepayments and expected maturities are utilized in applying the interest rate method. An effective yield is calculated based on projected principal cash flows at the time of original purchase. The effective yield is used to amortize the purchase price of the security over the security’s expected life. Book values are adjusted to reflect the amortization of premium or accretion of discount on a monthly basis. The projected principal cash flows are based on certain prepayment assumptions, which are generated using a prepayment model. The prepayment model uses a number of factors to estimate prepayment activity including the current levels of interest rates (refinancing incentive), time of year (seasonality), economic activity (including housing turnover) and term and age of the underlying collateral (burnout, seasoning). Prepayment assumptions associated with the Mortgage-Backed and Asset-Backed Securities are reviewed on a periodic basis. When changes in prepayment assumptions are deemed necessary as the result of actual prepayments differing from anticipated prepayments, securities are revalued based upon the new prepayment assumptions utilizing the retrospective adjustment method, whereby the effective yield is recalculated to reflect actual payments to date and anticipated future payments. The investment in such securities is adjusted to the amount that would have existed had the new effective yield been applied since the acquisition of the security. Such adjustments, if any, are included in Net Investment Income for the current period. Realized Gains and Losses on sales of investments are recognized when the related trades are executed and are determined on the basis of the specific identification method. Impairment of Invested Assets Management regularly reviews our Fixed Maturity and Equity Securities portfolios to evaluate the necessity of recording impairment losses for other-than-temporary declines in the fair value of securities. Our Company reviews the magnitude of a security’s unrealized loss compared to its cost/amortized cost and the length of time that the security has been impaired to determine if an unrealized loss is other-than-temporary. If warranted as a result of conditions relating to a particular security, our Company will also review securities with declines in fair value resulting from a headline news event involving the issuer, a headline news event involving the asset class, the advice of our external asset managers, or economic events that may impact the issuer to determine if an unrealized loss is other-than-temporary. The depth of analysis performed is dependent upon the nature and magnitude of the indicators of other-than-temporary impairment present in regards to each impaired security. For Equity Securities, our Company performs a fundamental analysis of the issuer, including an evaluation of the mean analysts’ target price, to assess the likelihood of recovery of our cost basis in the security. Management also assesses the likelihood of future cash flows, dividends and increases to dividends, all of which affect the securities eligible for our equity strategy and therefore our intent to hold the security. If an equity security is deemed to be other-than temporarily-impaired, the cost is written down to fair value with the loss recognized in earnings. For Fixed Maturities, our Company assesses the underlying fundamentals of each issuer to determine if there is a change in the amount or timing of expected cash flows. Management compares the amortized cost basis to the present value of the revised cash flows using the historical book yield to determine the credit loss portion of impairment which is recognized in earnings. All non-credit losses where we have the intent and ability to hold the security until recovery are recognized as changes in Other than Temporary Impairment (“ OTTI”) losses within AOCI. Specifically for structured Fixed Maturities, our Company analyzes projections provided by our investment managers with respect to an expected principal loss under a range of scenarios and utilizes the most likely outcomes. The analysis relies on actual collateral performance measures such as default rate, prepayment rate and loss severity. These assumptions are applied throughout the remaining term of the deal, incorporating the transaction structure and priority of payments, to generate loss adjusted cash flows. Results of the analysis will indicate whether the security is expected ultimately to incur a loss or whether there is a material impact on yield due to either a projected loss or a change in cash flow timing. A break-even default rate is also calculated. A comparison of the break-even default rate to the actual default rate provides an indication of the level of cushion or coverage to the first dollar principal loss. For securities in which a tranche loss is present and the net present value of loss adjusted cash flows is less than book value, credit impairment is recognized in earnings. The output data also includes a number of additional metrics such as average life remaining, original and current credit support, over 60 day delinquency and security rating. The significant inputs used to measure the amount of credit loss recognized in earnings are actual delinquency rates, default probability, severity and prepayment assumptions. Projected losses are a function of both loss severity and probability of default, which differ based on property type, vintage and the stress of the collateral . Foreign Currency Remeasurement and Translation The functional currency of each of our operations is generally the currency of the local operating environment, except for our Lloyd’s business which is United States Dollar (“USD”). Transactions in currencies other than an operation’s functional currency are remeasured into the functional currency and the resulting foreign exchange gains and losses are reflected in net Other Income (Expense) in the Consolidated Statements of Income, with the exception of those related to foreign-denominated available-for-sale investments. For these investments, exchange rate fluctuations represent an unrealized appreciation/depreciation in the value of the securities and are included in the related component of AOCI. Functional currency assets and liabilities of foreign operations are translated into USD using period end rates of exchange and the related translation adjustments are recorded as a separate component of AOCI. Consolidated Statements of Income amounts expressed in functional currencies are translated using average exchange rates. Premium Revenues Written premium is based on the insurance policies that have been reported to us and the policies that have been written by agents but not yet reported to us. We estimate the amount of written premium not yet reported based on judgments relative to current and historical trends of the business being written. Such estimates are regularly reviewed and updated and any resulting adjustments are included in the current year’s results. An unearned premium reserve is established to reflect the unexpired portion of each policy at the financial reporting date. Substantially all of our business is placed through agents and brokers. We record estimates for both unreported direct and assumed premiums. We also record the ceded portion of the estimated Gross Written Premiums and related acquisition costs. These estimates are mostly for our Marine and Energy & Engineering products written by our International Insurance (“Int’l Insurance”) reporting segment as well as our GlobalRe reporting segment. Such premium estimates are generally based on submission data received from brokers and agents and recorded when the insurance policy or reinsurance contract is written or bound. The earned gross, ceded and net premiums are calculated based on our earning methodology, which is generally pro-rata over the policy period or over the period of risk if the period of risk differs significantly from the contract period. Reinsurance Ceded In the normal course of business, we purchase reinsurance from insurers or reinsurers to reduce the amount of loss arising from claims. Management analyzes the reinsurance agreements to determine whether the reinsurance should be classified as prospective or retroactive based upon the terms of the reinsurance agreement and whether the reinsurer has assumed significant insurance risk to the extent that the reinsurer may realize a significant loss from the transaction. Prospective reinsurance is reinsurance in which an assuming company agrees to reimburse the ceding company for losses that may be incurred as a result of future insurable events covered under contracts subject to the reinsurance. Retroactive reinsurance is reinsurance in which an assuming company agrees to reimburse a ceding company for liabilities incurred as a result of past insurable events covered under contracts subject to the reinsurance. Ceded reinsurance premiums net of ceding commissions and ceded losses are reflected as reductions of the respective income or expense accounts over the terms of the reinsurance contracts. Prepaid Reinsurance Premiums represent the portion of premiums ceded to reinsurers applicable to the unexpired terms of the reinsurance contracts in force. Reinsurance reinstatement premiums (“RRPs”) are recognized in the same period as the loss event that gave rise to the reinstatement premiums. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policy. Ceded Unearned Premiums and estimates of amounts recoverable from reinsurers on paid and unpaid losses are reflected as assets. Provisions are made for estimated unrecoverable reinsurance. Deferred Policy Acquisition Costs Costs of acquiring business are deferred and amortized over the period that the related premiums are recognized as revenue. Such costs (e.g., Commission Expenses, Other Underwriting Expenses and premium taxes) are limited to the incremental direct costs related to the successful acquisition of new or renewal business. The method of computing Deferred Policy Acquisition Costs limits the deferral to their estimated net realizable value based on the related Unearned Premiums and takes into account anticipated Losses, Loss Adjustment Expense (“LAE”), Commission Expenses and Operating Expenses based on historical and current experience, as well as anticipated Investment Income. Reserves for Losses and Loss Adjustment Expenses Unpaid losses and LAE are determined on: (a) individual claims reported on direct business for insureds, (b) from reports received from ceding insurers for assumed business and (c) on estimates based on Company and industry experience for incurred but not reported (“IBNR”) claims and LAE. Indicated IBNR reserves for losses and LAE are calculated by our actuaries using several standard actuarial methodologies, including the paid and incurred loss development and the paid and incurred Bornhuetter-Ferguson loss methods. Frequency/severity analyses are performed for certain books of business. The Reserve for Losses and LAE has been established to cover the estimated unpaid cost of claims incurred. Such estimates are regularly compared to indicated reserves and updated and any resulting adjustments are included in the current year’s results. Management believes that the liability recognized for unpaid losses and LAE is a reasonable estimate of the ultimate unpaid claims incurred, however, no representation is made that the ultimate liability will not differ materially from the amounts recorded in the accompanying Consolidated Financial Statements. Losses and LAE are recorded on an undiscounted basis. Earnings per Share Basic earnings per share (“EPS”) is computed by dividing Net income by the weighted average number of common shares outstanding for the period. Diluted EPS reflects the basic EPS adjusted for the potential dilution that would occur if all issued stock options were exercised and all stock grants were fully vested. Share-Based Compensation Our Company applies a fair value based measurement method in accounting for its share-based payment arrangements with eligible employees and directors. The associated expense is estimated based on the fair value of the award at the grant date and is recognized in Net Income over the requisite service period with a corresponding increase in Shareholder’s Equity. Forfeitures of share-based payment awards are recognized as they occur. Depreciation and Amortization Depreciation of furniture and fixtures, electronic data processing equipment and computer software is provided over the estimated useful lives of the respective assets, ranging from three to seven years, using the straight-line method. Amortization of leasehold improvements are provided over the shorter of the useful lives of those improvements or the contractual terms of the leases, ranging from five to ten years, using the straight-line method. Goodwill and Other Intangible Assets Goodwill represents the excess of the cost of acquiring a business enterprise over the fair value of the net assets acquired. Our Company’s recorded indefinite lived intangible assets represent acquired stamp capacity in the Syndicate. Goodwill and indefinite lived intangible assets are reported at carrying value and are tested for impairment at least annually, and when permitted by the applicable accounting guidance, qualitative factors are assessed to determine whether it is necessary to calculate an asset’s fair value when testing an asset with an indefinite life for impairment. Goodwill and indefinite lived intangible assets are considered impaired if the estimated fair value is less than its carrying value and any impairment loss is measured as the difference between the implied fair value and the carrying value. Our Company did not recognize an impairment of goodwill or the indefinite lived intangible assets for any of the years ended December 31, 2017, 2016 and 2015. As of December 31, 2017, the carrying value of goodwill and indefinite lived intangible assets was $6.6 million, which is $0.1 million more than the carrying value as of December 31, 2016, $6.5 million. Changes in the carrying value of the goodwill and indefinite lived intangible assets are due to fluctuations in currency exchange rates between the USD and the GBP. Income Taxes We recognize deferred tax assets and liabilities based on the differences between the financial statement carrying amounts and the tax bases of assets and liabilities. Deferred tax assets are reduced by a valuation allowance if, based on the weight of available evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. All available evidence, both positive and negative, is considered to determine whether, based on the weight of that evidence, a valuation allowance is needed. Future realization of the tax benefit of an existing deferred tax asset ultimately depends on the existence of sufficient taxable income of the appropriate character within the carryback/carryforward period available under the tax law. In determining whether a valuation allowance is needed, management considers the timing of the reversal of each deferred tax asset as well as expected future levels of taxable income, amounts of taxable income in carryback years, and tax planning strategies. On December 22, 2017, the 2017 Tax Cuts and Jobs Act (the “Tax Act”) was enacted into law and the new legislation contains several key tax provisions that affected us, including a one-time mandatory Deemed Repatriation Transition Tax (“Transition Tax”) on accumulated foreign earnings and a reduction of the corporate income tax rate to 21% effective January 1, 2018, among others. We are required to recognize the effect of the tax law changes in the period of enactment, such as determining the Transition Tax, re-measuring our U.S. deferred tax assets and liabilities as well as reassessing the net realizability of our deferred tax assets and liabilities. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), Income Tax Accounting Implications of the Tax Act, which allows us to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. Since the Tax Act was passed late in the fourth quarter of 2017, and ongoing guidance and accounting interpretation is expected over the next 12 months, as of December 31, 2017, we have made a reasonable estimate of the effects on our deferred tax balances and in relation to the Transition Tax; however we will continue to gather additional information to more precisely compute these income tax impacts. We expect to complete our analysis within the measurement period in accordance with SAB 118. Additional information can be found in Note 9, Income Taxes New Accounting Standards Adopted in 2017 Share-Based Compensation Accounting Effective January 1, 2017, our Company adopted ASU 2016-09, “Improvements to Employee Share-Based Payment Accounting” issued by the FASB. This guidance requires all income tax effects of awards to be recognized in the income statement when the awards vest or are settled, eliminates the requirement that excess tax benefits be realized before companies can recognize them, requires the presentation of excess tax benefits as an operating activity on the statement of cash flows, allows employers to increase the amounts withheld to cover income taxes on share-based compensation awards without requiring liability classification, requires the presentation of employee taxes paid as a financing activity on the statement of cash flows, and requires companies to elect whether they will account for award forfeitures by recognizing forfeitures only as they occur or by estimating the number of awards expected to be forfeited. The requirement to recognize all income tax effects of awards in the income statement when the awards vest or are settled was adopted on a prospective basis. Previously, these amounts were recorded in Additional Paid-In Capital. This change also prospectively impacts the calculation of potential common shares used to determine Diluted Net Income per Common Share under 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 previously would have been recognized in Additional Paid-In Capital. We elected to account for forfeitures of share-based payment awards by recognizing forfeitures of awards as they occur, and upon adoption of this guidance, the payment of employee taxes was retrospectively adjusted on the Consolidated Statements of Cash Flows to be classified as a financing activity. All other changes in the ASU did not result in cumulative-effect or retrospective adjustments. Transition to Equity Method of Accounting Effective January 1, 2017, our Company adopted ASU 2016-07, “Simplifying the Transition to the Equity Method of Accounting” issued by the FASB. This guidance eliminates the requirement to retrospectively apply equity method accounting when an investment that had been accounted for by another method initially qualifies for the equity method. Our Company did not have any investments transitioning to the equity method of accounting during 2017. The adoption of this guidance did not impact our results of operations, financial condition or liquidity. Share-Based Compensation Modification Accounting During the second quarter of 2017, our Company early adopted ASU 2017-09, “Scope of Modification Accounting” issued by the FASB. This guidance clarifies when changes to the terms or conditions of a share-based award must be accounted for as modifications. Under the new guidance, modification accounting is required if the value, vesting conditions or classification of the award changes. This guidance will be applied prospectively to awards modified on or after the adoption date. The adoption of this guidance did not impact our results of operations, financial condition or liquidity. Recently Issued Accounting Standards Not Yet Adopted Revenue From Contracts With Customers In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers (Topic 606)”. This guidance affects any entity that either enters into contracts with customers to transfer goods or services or enters into contracts for the transfer of nonfinancial assets unless those contracts are within the scope of other standards (for example, insurance contracts are not in scope of the new guidance). The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In August 2015, the FASB delayed the effective date by one year through the issuance of ASU 2015-14, “Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date”. This guidance is effective for interim and annual reporting periods beginning after December 15, 2017. Early adoption was permitted for interim and annual reporting periods beginning after December 15, 2016. Substantially all of our revenue is not included in the scope of this guidance. We do generate an insignificant amount of fee income that is in the scope of this guidance, but will not be materially impacted by the adoption of this guidance. The adoption of this guidance will not have a material impact on our results of operations, financial condition or liquidity. Classification and Measurement of Financial Instruments In January 2016, the FASB issued ASU 2016-01 “Financial Instruments – Overall (Subtopic 825-10) – Recognition and Measurement of Financial Assets and Financial Liabilities” which requires equity investments (except those accounted for under the equity method of accounting, investments that are consolidated or those that meet a practicability exception) to be measured at fair value with changes in fair value recognized in net income, simplifies the impairment assessment of equity investments without readily determinable values by requiring a qualitative assessment to identify impairment, eliminates the requirement to disclose the methods and significant assumptions used to estimate the fair value for financial instruments measured at amortized cost, requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes, requires separate presentation in other comprehensive income of the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the organization has elected to measure the liabilities in accordance with the fair value option, requires the separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or the accompanying notes to the financial statements and clarifies that the reporting organization should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the organization’s other deferred tax assets. This guidance is effective for interim and annual reporting periods beginning after December 15, 2017 with early adoption permitted for certain of the amendments. The adoption of this guidance will result in a cumulative-effect adjustment of approximately $11.8 million relating to net unrealized gains in accumulated other comprehensive income for available-for-sale Equity Securities that will be measured at fair value with changes in fair value recognized in net income upon adoption of this guidance. The other aspects of this guidance impacting our Company relate to disclosures and will not impact our results of operations, financial condition or liquidity. Leases In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842)” which provides a new comprehensive model for lease accounting. The guidance will require a lessee to recognize a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. This guidance is effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. Our Company’s future minimum lease payments, which represent minimum annual rental commitments and will be subject to this new guidance, totaled $91.2 million at December 31, 2017. The calculation of the lease liability and right-of-use asset requires further analysis of the underlying leases. Adoption of this guidance will impact our Company’s consolidated balance sheets but is not expected to have a material impact on our Company’s results of operations, financial condition and liquidity. Our Company is still evaluating the impact of adopting this standard on our consolidated financial statements. Credit Losses In June 2016, the FASB issued ASU 2016-13, “Financial Instruments – Credit Losses (Topic 326) – Measurement of Credit Losses on Financial Instruments” which replaces the “incurred loss” impairment methodology with an approach based on “expected losses” to estimate credit losses on certain types of financial instruments and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The guidance requires financial assets measured at amortized cost to be presented at the net amount expected to be collected. The allowance for credit losses is a valuation account that is deducted from the amortized cost of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses. The guidance also provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. This guidance is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted for interim and annual periods beginning after December 15, 2018. The application of this guidance is not expected to significantly impact our Company’s available-for-sale debt investment portfolio, but will impact our Company’s other financial assets, including our reinsurance recoverables and premium receivable. Upon adoption of this guidance, any impairment losses on our available-for-sale debt securities will be recorded as an allowance, subject to reversal, rather than as a reduction in amortized cost. Our Company is currently evaluating the impact of this guidance on our results of operations, financial condition and liquidity. Cash Flows In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230) – Classification of Certain Cash Receipts and Cash Payments” which addresses diversity in practice in how eight specific cash receipts and cash payments should be presented and classified on the statement of cash flows. This guidance is effective for interim and annual periods beginning after December 15, 2017, with early adoptions permitted. The adoption of this guidance is not expected to impact our results of operations, financial condition or liquidity. In November 2016, the FASB issued ASU 2016

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment Information	NOTE 2. SEGMENT INFORMATION We report our results of operations consistent with the manner in which our Chief Operating Decision Maker reviews the business to assess performance through our reporting segments: U.S. Insurance, International Insurance (“Int’l Insurance”), Global Reinsurance (“GlobalRe”) and Corporate. We classify our business into three underwriting segments: U.S. Insurance, Int’l Insurance and GlobalRe. Both the U.S. Insurance and Int’l Insurance reporting segments are each comprised of three operating segments: Marine, P&C and Professional Liability. We evaluate the performance of each of the underwriting segments based on underwriting results. Underwriting results are measured based on Underwriting Profit or Loss and the related Combined Ratio, which are both measures of underwriting profitability. Underwriting Profit (Loss) is calculated from Net Earned Premiums less the sum of Net Losses and LAE, Commission Expenses, Other Operating Expenses and Other Underwriting Income (Expense). The Combined Ratio is derived by dividing the sum of Net Losses and LAE, Commission Expenses, Other Operating Expenses and Other Underwriting Income (Expense) by Net Earned Premiums. A Combined Ratio of less than 100% indicates an Underwriting Profit and greater than 100% indicates an Underwriting Loss. Our underwriting performance is evaluated separately from the rest of our operations. The performance of our investment portfolios, our liquidity and capital resource needs, our foreign currency exposure and our tax planning strategies are evaluated on a consolidated basis within our Corporate segment. We do not allocate our assets by underwriting segment as we evaluate the underwriting results of these segments separately from the results of our investments portfolio. The following tables set forth the financial data by segment for the years ended December 31, 2017, 2016 and 2015: Year Ended December 31, 2017 U.S. Int'l amounts in thousands Insurance Insurance GlobalRe Corporate (1) Total Net Earned Premiums $ 674,665 $ 333,792 $ 177,963 $ — $ 1,186,420 Net Losses and LAE (443,353 ) (229,601 ) (133,311 ) — (806,265 ) Commission Expenses (77,729 ) (68,824 ) (39,136 ) 958 (184,731 ) Other Operating Expenses (128,905 ) (83,464 ) (20,861 ) — (233,230 ) Other Underwriting Income (Expense) 461 — 565 (958 ) 68 Underwriting Profit (Loss) $ 25,139 $ (48,097 ) $ (14,780 ) $ — $ (37,738 ) Net Investment Income 89,293 89,293 Net Realized Gains 43,009 43,009 Interest Expense (15,447 ) (15,447 ) Other Loss (4,311 ) (4,311 ) Income (Loss) Before Income Taxes $ 25,139 $ (48,097 ) $ (14,780 ) $ 112,544 $ 74,806 Income Tax Expense (34,312 ) (34,312 ) Net Income $ 40,494 Losses and LAE ratio 65.7 % 68.8 % 74.9 % 68.0 % Commission Expense Ratio 11.5 % 20.6 % 22.0 % 15.6 % Other Operating Expense Ratio (2) 19.1 % 25.0 % 11.4 % 19.6 % Combined Ratio 96.3 % 114.4 % 108.3 % 103.2 % ( 1) - Includes Corporate segment intercompany eliminations. (2) - Includes Other Operating Expenses and Other Underwriting Income (Expense). Year Ended December 31, 2016 U.S. Int'l amounts in thousands Insurance Insurance GlobalRe Corporate (1) Total Net Earned Premiums $ 629,308 $ 307,416 $ 163,621 $ — $ 1,100,345 Net Losses and LAE (397,860 ) (178,284 ) (89,304 ) — (665,448 ) Commission Expenses (70,812 ) (61,703 ) (34,008 ) 1,478 (165,045 ) Other Operating Expenses (128,108 ) (86,395 ) (19,593 ) — (234,096 ) Other Underwriting Income (Expense) 1,092 — 522 (1,478 ) 136 Underwriting Profit (Loss) $ 33,620 $ (18,966 ) $ 21,238 $ — $ 35,892 Net Investment Income 79,451 79,451 Net Realized Gains 9,036 9,036 Interest Expense (15,435 ) (15,435 ) Other Income 8,565 8,565 Income (Loss) Before Income Taxes $ 33,620 $ (18,966 ) $ 21,238 $ 81,617 $ 117,509 Income Tax Expense (34,783 ) (34,783 ) Net Income $ 82,726 Losses and LAE ratio 63.2 % 58.0 % 54.6 % 60.5 % Commission Expense Ratio 11.3 % 20.1 % 20.8 % 15.0 % Other Operating Expense Ratio (2) 20.2 % 28.1 % 11.6 % 21.2 % Combined Ratio 94.7 % 106.2 % 87.0 % 96.7 % (1) - Includes Corporate segment intercompany eliminations. (2) - Includes Other Operating Expenses and Other Underwriting Income (Expense ). Year Ended December 31, 2015 U.S. Int'l amounts in thousands Insurance Insurance GlobalRe Corporate (1) Total Net Earned Premiums $ 555,836 $ 259,960 $ 168,291 $ — $ 984,087 Net Losses and LAE (343,497 ) (134,702 ) (94,399 ) — (572,598 ) Commission Expenses (56,319 ) (43,676 ) (32,240 ) 2,258 (129,977 ) Other Operating Expenses (131,407 ) (75,867 ) (16,242 ) — (223,516 ) Other Underwriting Income (Expense) 1,690 — 690 (2,258 ) 122 Underwriting Profit $ 26,303 $ 5,715 $ 26,100 $ — $ 58,118 Net Investment Income 68,718 68,718 Net Realized Gains 6,675 6,675 Interest Expense (15,424 ) (15,424 ) Other Loss (613 ) (613 ) Income Before Income Taxes $ 26,303 $ 5,715 $ 26,100 $ 59,356 $ 117,474 Income Tax Expense (36,417 ) (36,417 ) Net Income $ 81,057 Losses and LAE ratio 61.8 % 51.8 % 56.1 % 58.2 % Commission Expense Ratio 10.1 % 16.8 % 19.2 % 13.2 % Other Operating Expense Ratio (2) 23.4 % 29.2 % 9.2 % 22.7 % Combined Ratio 95.3 % 97.8 % 84.5 % 94.1 % (1) - Includes Corporate segment intercompany eliminations. (2) - Includes Other Operating Expenses and Other Underwriting Income (Expense ). The following tables provide additional financial data by operating segment for the years ended December 31, 2017, 2016 and 2015: Year Ended December 31, 2017 amounts in thousands Gross written premiums Ceded premiums Net written premiums Net earned premiums U.S. Insurance Marine $ 156,171 $ (72,431 ) $ 83,740 $ 86,605 P&C 713,539 (173,501 ) 540,038 495,260 Professional Liability 118,583 (21,061 ) 97,522 92,800 Total $ 988,293 $ (266,993 ) $ 721,300 $ 674,665 Int'l Insurance Marine $ 198,241 $ (42,018 ) $ 156,223 $ 152,396 P&C 159,123 (85,298 ) 73,825 88,517 Professional Liability 143,766 (37,962 ) 105,804 92,879 Total $ 501,130 $ (165,278 ) $ 335,852 $ 333,792 GlobalRe $ 223,842 $ (9,664 ) $ 214,178 $ 177,963 Total $ 1,713,265 $ (441,935 ) $ 1,271,330 $ 1,186,420 Year Ended December 31, 2016 amounts in thousands Gross written premiums Ceded written premiums Net written premiums Net earned premiums U.S. Insurance Marine $ 169,405 $ (70,858 ) $ 98,547 $ 100,132 P&C 631,562 (135,888 ) 495,674 453,673 Professional Liability 118,428 (29,081 ) 89,347 75,503 Total $ 919,395 $ (235,827 ) $ 683,568 $ 629,308 Int'l Insurance Marine $ 183,228 $ (40,092 ) $ 143,136 $ 141,593 P&C 181,094 (69,606 ) 111,488 89,455 Professional Liability 120,149 (28,806 ) 91,343 76,368 Total $ 484,471 $ (138,504 ) $ 345,967 $ 307,416 GlobalRe $ 165,045 $ (8,356 ) $ 156,689 $ 163,621 Total $ 1,568,911 $ (382,687 ) $ 1,186,224 $ 1,100,345 Year Ended December 31, 2015 amounts in thousands Gross written premiums Ceded written premiums Net written premiums Net earned premiums U.S. Insurance Marine $ 158,124 $ (61,916 ) $ 96,208 $ 96,082 P&C 596,673 (152,168 ) 444,505 401,408 Professional Liability 110,984 (54,691 ) 56,293 58,346 Total $ 865,781 $ (268,775 ) $ 597,006 $ 555,836 Int'l Insurance Marine $ 183,707 $ (36,515 ) $ 147,192 $ 149,256 P&C 130,729 (67,722 ) 63,007 55,320 Professional Liability 97,511 (29,768 ) 67,743 55,384 Total $ 411,947 $ (134,005 ) $ 277,942 $ 259,960 GlobalRe $ 175,774 $ (6,862 ) $ 168,912 $ 168,291 Total $ 1,453,502 $ (409,642 ) $ 1,043,860 $ 984,087 The following is a summary of our Company's Gross Written Premiums allocated based on the location of the insured's head office: Years Ended December 31, amounts in thousands 2017 2016 2015 United States $ 1,203,695 $ 1,101,337 $ 1,057,895 Worldwide (Excluding the United States) 509,570 467,574 395,607 Total $ 1,713,265 $ 1,568,911 $ 1,453,502 The assets of our Company are reviewed in total by management for purposes of decision making.

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Investments	NOTE 3. INVESTMENTS The following tables set forth our Company’s available-for-sale investments as of December 31, 2017 and 2016 and include OTTI securities recognized within AOCI: December 31, 2017 Gross Gross Unrealized Unrealized Amortized amounts in thousands Fair Value Gains Losses Cost Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 393,563 $ 2,081 $ (2,014 ) $ 393,496 States, Municipalities and Political Subdivisions 814,632 20,136 (1,423 ) 795,919 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities 407,619 2,352 (5,414 ) 410,681 Residential Mortgage Obligations 54,104 606 (79 ) 53,577 Asset-Backed Securities 328,753 2,138 (663 ) 327,278 Commercial Mortgage-Backed Securities 160,904 2,354 (1,182 ) 159,732 Subtotal $ 951,380 $ 7,450 $ (7,338 ) $ 951,268 Corporate Exposures (1) 897,479 14,491 (3,737 ) 886,725 Total Fixed Maturities $ 3,057,054 $ 44,158 $ (14,512 ) $ 3,027,408 Equity Securities: Common Stocks $ 52,439 $ 7,423 $ (112 ) $ 45,128 Preferred Stocks 183,542 6,071 (1,560 ) 179,031 Total Equity Securities $ 235,981 $ 13,494 $ (1,672 ) $ 224,159 Short-Term Investments 127,128 3 — 127,125 Total Investments $ 3,420,163 $ 57,655 $ (16,184 ) $ 3,378,692 (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks. December 31, 2016 Gross Gross Unrealized Unrealized Amortized amounts in thousands Fair Value Gains Losses Cost Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 273,776 $ 2,192 $ (5,128 ) $ 276,712 States, Municipalities and Political Subdivisions 547,415 11,542 (4,036 ) 539,909 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities 487,364 4,016 (6,585 ) 489,933 Residential Mortgage Obligations 20,530 453 (55 ) 20,132 Asset-Backed Securities 314,601 824 (1,178 ) 314,955 Commercial Mortgage-Backed Securities 154,139 2,859 (1,904 ) 153,184 Subtotal $ 976,634 $ 8,152 $ (9,722 ) $ 978,204 Corporate Exposures (1) 838,057 10,185 (5,528 ) 833,400 Total Fixed Maturities $ 2,635,882 $ 32,071 $ (24,414 ) $ 2,628,225 Equity Securities: Common Stocks $ 164,087 $ 24,677 $ (964 ) $ 140,374 Preferred Stocks 185,055 2,339 (4,821 ) 187,537 Total Equity Securities $ 349,142 $ 27,016 $ (5,785 ) $ 327,911 Short-Term Investments 143,539 88 — 143,451 Total Investments $ 3,128,563 $ 59,175 $ (30,199 ) $ 3,099,587 (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks. During 2016 our Company made strategic investments in certain companies which are reported as Other invested assets on the Consolidated Balance Sheet and accounted for using the equity method. In applying the equity method, these investments are initially recorded at cost and are subsequently adjusted based on our Company’s proportionate share of the net income or loss of the companies. Our initial purchase price of $2.0 million has a current carrying value of $1.7 million and $2.0 million at December 31, 2017 and 2016, respectively. As of December 31, 2017 and 2016, our Company did not have a concentration of greater than 5% of invested assets in a single non-U.S. government-backed issuer. As of December 31, 2017 and 2016, Fixed Maturities for which non-credit OTTI was previously recognized and included in AOCI are now in a net unrealized net gains position of $0.5 million and $0.4 million, respectively. The fair value of our Company’s investment portfolio may fluctuate significantly in response to various factors such as changes in interest rates, investment quality ratings, equity prices, foreign exchange rates and credit spreads. Our Company does not have the intent to sell, nor is it more likely than not that it will have to sell, Fixed Maturities in unrealized loss positions that are not other-than-temporarily impaired before recovery. For structured securities, default probability and severity assumptions differ based on property type, vintage and the stress of the collateral. Our Company does not intend to sell, and it is more likely than not that our Company will not be required to sell, these securities before the recovery of the amortized cost basis. For Equity Securities, our Company also considers our intent to hold securities as part of the process of evaluating whether a decline in fair value represents an other-than-temporary decline in value. Our Company may realize investment losses to the extent our liquidity needs require the disposition of Fixed Maturity Securities in unfavorable interest rate, liquidity or credit spread environments. Significant changes in the factors our Company considers when evaluating investments for impairment losses could result in a significant change in impairment losses reported in the Consolidated Financial Statements. The contractual maturity dates for Fixed Maturities categorized by the number of years until maturity as of December 31, 2017 are shown in the following table: December 31, 2017 Amortized amounts in thousands Fair Value Cost Due in One Year or Less $ 228,479 $ 228,255 Due after One Year Through Five Years 809,718 809,195 Due After Five Years Through Ten Years 359,745 352,524 Due After Ten Years 707,732 686,166 Mortgage-Backed and Asset-Backed Securities 951,380 951,268 Total $ 3,057,054 $ 3,027,408 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. Prepayment assumptions associated with the Mortgage-Backed and Asset-Backed Securities are reviewed on a periodic basis. When changes in prepayment assumptions are deemed necessary as the result of actual prepayments differing from anticipated prepayments, securities are revalued based upon the new prepayment assumptions utilizing the retrospective accounting method. Due to the periodic repayment of principal, the Mortgage-Backed and Asset-Backed Securities are estimated to have an effective maturity of approximately 5.0 years. The following tables summarize all securities in a gross unrealized loss position as of December 31, 2017 and 2016, showing the aggregate fair value and gross unrealized loss by the length of time those securities have continuously been in a gross unrealized loss position: December 31, 2017 Less than 12 months Greater than 12 months Total Gross Gross Gross Fair Unrealized Fair Unrealized Fair Unrealized amounts in thousands Value (Losses) Value (Losses) Value (Losses) Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 273,672 $ (1,502 ) $ 54,484 $ (512 ) $ 328,156 $ (2,014 ) States, Municipalities and Political Subdivisions 74,097 (503 ) 45,085 (920 ) 119,182 (1,423 ) Mortgage-Backed and Asset-Backed Securities: Agency Residential Mortgage-Backed Securities 87,496 (346 ) 236,745 (5,068 ) 324,241 (5,414 ) Residential Mortgage Obligations 12,418 (62 ) 546 (17 ) 12,964 (79 ) Asset-Backed Securities 85,877 (468 ) 24,733 (195 ) 110,610 (663 ) Commercial Mortgage-Backed Securities 20,482 (95 ) 22,903 (1,087 ) 43,385 (1,182 ) Subtotal $ 206,273 $ (971 ) $ 284,927 $ (6,367 ) $ 491,200 $ (7,338 ) Corporate Exposures (1) 295,433 (1,690 ) 121,410 (2,047 ) 416,843 (3,737 ) Total Fixed Maturities $ 849,475 $ (4,666 ) $ 505,906 $ (9,846 ) $ 1,355,381 $ (14,512 ) Equity Securities: Common Stocks $ 11,245 $ (81 ) $ 1,770 $ (31 ) $ 13,015 $ (112 ) Preferred Stocks 50,861 (1,524 ) 662 (36 ) 51,523 (1,560 ) Total Equity Securities $ 62,106 $ (1,605 ) $ 2,432 $ (67 ) $ 64,538 $ (1,672 ) Total Fixed Maturities and Equity Securities $ 911,581 $ (6,271 ) $ 508,338 $ (9,913 ) $ 1,419,919 $ (16,184 ) (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks. December 31, 2016 Less than 12 months Greater than 12 months Total Gross Gross Gross Fair Unrealized Fair Unrealized Fair Unrealized amounts in thousands Value (Losses) Value (Losses) Value (Losses) Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 150,891 $ (2,570 ) $ 16,819 $ (2,558 ) $ 167,710 $ (5,128 ) States, Municipalities and Political Subdivisions 137,731 (3,111 ) 13,255 (925 ) 150,986 (4,036 ) Mortgage-Backed and Asset-Backed Securities: Agency Residential Mortgage-Backed Securities 349,119 (6,155 ) 12,401 (430 ) 361,520 (6,585 ) Residential Mortgage Obligations 953 (18 ) 926 (37 ) 1,879 (55 ) Asset-Backed Securities 95,514 (970 ) 48,093 (208 ) 143,607 (1,178 ) Commercial Mortgage-Backed Securities 51,932 (1,164 ) 7,910 (740 ) 59,842 (1,904 ) Subtotal $ 497,518 $ (8,307 ) $ 69,330 $ (1,415 ) $ 566,848 $ (9,722 ) Corporate Exposures (1) 325,733 (5,086 ) 26,005 (442 ) 351,738 (5,528 ) Total Fixed Maturities $ 1,111,873 $ (19,074 ) $ 125,409 $ (5,340 ) $ 1,237,282 $ (24,414 ) Equity Securities: Common Stocks $ 31,272 $ (634 ) $ 1,471 $ (330 ) $ 32,743 $ (964 ) Preferred Stocks 113,742 (4,785 ) 523 (36 ) 114,265 (4,821 ) Total Equity Securities $ 145,014 $ (5,419 ) $ 1,994 $ (366 ) $ 147,008 $ (5,785 ) Total Fixed Maturities and Equity Securities $ 1,256,887 $ (24,493 ) $ 127,403 $ (5,706 ) $ 1,384,290 $ (30,199 ) (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks. Our Company analyzes impaired securities quarterly to determine if any impairments are other-than-temporary. The above securities with unrealized losses have been determined to be temporarily impaired based on our evaluation. At December 31, 2017, there were 454 Fixed Maturities and 22 Equity Securities in an unrealized loss position. In the above table, the gross unrealized loss for the greater than 12 months category consists primarily of Agency Residential Mortgage-Backed Securities and Corporate Bonds and is mostly due to an increase in interest rates since time or purchase. The gross unrealized loss for the less than 12 months category consists primarily of Corporate Bonds and Preferred Stocks, which are reported in Equity Securities, due to an increase in interest rates since time of purchase, as well as Foreign Government Bonds due to an unfavorable exchange rate movement in our Canadian portfolio. At December 31, 2016, there were 413 Fixed Maturities and 75 Equity Securities in an unrealized loss position. In the above table, the gross unrealized loss for the greater than 12 months category consists primarily of Agency and Foreign Government Bonds mostly due to an unfavorable foreign exchange movement in our Canadian portfolio. To a lesser extent the gross unrealized loss for the greater than 12 months category is driven by unrealized losses in our longer dated Municipal Securities which were impacted by rising interest rates and credit spreads widening. The gross unrealized loss for the less than 12 months category consists primarily of Agency Residential Mortgage Backed Securities, Corporate Bonds and Preferred Stocks, which are reported in Equity Securities, due to an increase in interest rates. As of December 31, 2017 and 2016, the largest unrealized loss by a non-government backed issuer in the investment portfolio was $0.7 million and $1.0 million, respectively. Our Company’s ability to hold securities is supported by sufficient cash flow from our operations and from maturities within our investment portfolio in order to meet our claims payments and other disbursement obligations arising from our underwriting operations without selling such investments. With respect to securities where the decline in value is determined to be temporary and the security's value is not written down, a subsequent decision may be made to sell that security and realize a loss. Subsequent decisions on security sales are made within the context of overall risk monitoring, changing information and market conditions. Our Company had four credit related OTTI loss totaling $2.1 million during the year ended December 31, 2017 from our Equity Securities portfolio. Our Company had one credit related OTTI loss totaling $0.2 million during the year ended December 31, 2016 from our Fixed Maturities portfolio. Our Company had three credit related OTTI losses totaling $1.7 million from our Equity Securities portfolio during the year ended December 31, 2015. The following table summarizes the cumulative amounts related to our Company’s credit loss portion of the OTTI losses on Fixed Maturities for the years ended December 31, 2017, 2016 and 2015. Years Ended December 31, amounts in thousands 2017 2016 2015 Beginning Balance $ 2,361 $ 2,361 $ 2,361 Additions for Credit Loss Impairments recognized in the current period on securities not previously impaired — 150 — Additions for Credit Loss Impairments recognized in the current period on securities previously impaired — — — Reductions for Credit Loss Impairments previously recognized on securities sold during the period — (150 ) — Ending Balance $ 2,361 $ 2,361 $ 2,361 Our Company’s Net Investment Income was derived from the following sources: Years Ended December 31, amounts in thousands 2017 2016 2015 Fixed Maturities $ 76,784 $ 67,772 $ 61,572 Equity Securities 15,108 14,271 9,813 Short-Term Investments 1,064 727 683 Total Investment Income $ 92,956 $ 82,770 $ 72,068 Investment Expenses (3,663 ) (3,319 ) (3,350 ) Net Investment Income $ 89,293 $ 79,451 $ 68,718 Realized Gains and Losses, excluding net OTTI losses recognized in earnings, for the periods indicated, were as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 Fixed Maturities: Gains $ 4,041 $ 5,681 $ 4,756 Losses (3,249 ) (4,271 ) (5,926 ) Fixed Maturities, net $ 792 $ 1,410 $ (1,170 ) Short-Term Investments: Gains $ 1,631 $ 890 $ 130 Losses (612 ) (1,552 ) (383 ) Short-Term Investments, Net $ 1,019 $ (662 ) $ (253 ) Equity Securities: Gains $ 44,783 $ 9,096 $ 14,331 Losses (1,521 ) (658 ) (4,535 ) Equity Securities, Net $ 43,262 $ 8,438 $ 9,796 Net Realized Gains (Losses) $ 45,073 $ 9,186 $ 8,373 Net Realized Gains of $45.1 million for the year ended December 31, 2017 is primarily due to the liquidation of our actively managed common stock equity securities portfolio in December 2017 as we reassess our common equity strategy.

Fair Value Measurement

Fair Value Measurement	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurement	NOTE 4. FAIR VALUE MEASUREMENT Fair value measurements are received from independent pricing service vendors, utilized by our outside investment manager whom we employ to assist us with investment accounting services. This manager utilizes a pricing committee, which oversees the use of one or more independent pricing service vendors. The pricing committee consists of five or more members of the investment management firm, one from senior management and one from the accounting group, with the remainder representing asset class specialists and client strategists. The pricing source for each security is determined in accordance with the pricing source procedures approved by the pricing committee. The investment manager receives supporting documentation from the independent pricing service vendor detailing the inputs, models and processes used in the vendors’ evaluation process to determine the appropriate fair value hierarchy. It is ultimately our responsibility to determine whether the values obtained from these service providers are representative of fair value. To validate the techniques or models used by pricing sources, our review process includes, but is not limited to: (i) A review of the validity of the fair market valuation of individual securities deemed as outliers (i.e., vendor price differed significantly from other vendor prices), securities with significant price movements from previous months, securities with stale prices and securities with negative yields. (ii) A comparison of the count of securities priced by certain vendors for significant movements in vendor CUSIP counts. (iii) A review of the results of back-testing, including the comparison of executed prices to the historical fair value estimates from the pricing service and documentation to support trades above certain variance thresholds. (iv) A review of the Statement on Standards for Attestation Engagements (“SSAE”) No.16 report of our outside investment managers for any exceptions. (v) Management also periodically independently prices the portfolio using alternative pricing vendors and investigates variances outside of the established thresholds. The fair value of our financial instruments is determined based on the following fair value hierarchy: Level 1 – Quoted prices for identical instruments in active markets. Examples are listed equity and fixed income securities traded on an exchange. U.S. Treasury securities are reported as Level 1 and are valued based on unadjusted quoted prices for identical assets in active markets that our Company can access. Level 2 – Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations in which all significant inputs and significant value drivers are observable in active markets. Examples are asset-backed and mortgage-backed securities that are similar to other asset-backed or mortgage-backed securities observed in the market. U.S. government agency securities are reported as Level 2 and are valued using yields and spreads that are observable in active markets. Level 3 – Valuations derived from valuation techniques in which one or more significant inputs or significant value drivers are unobservable. An example would be a private placement with minimal liquidity. The following tables present, for each of the fair value hierarchy levels as defined by the accounting guidance for fair value measurements and described below, our Company’s Fixed Maturities and Equity Securities by asset class that are measured at fair value on a recurring basis, as well as the fair value of the 5.75% Senior notes due October 15, 2023 (the “5.75% Senior notes”) carried at amortized cost as of December 31, 2017 and 2016: December 31, 2017 amounts in thousands Level 1 Level 2 Level 3 Total Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 178,251 $ 215,312 $ — $ 393,563 States, Municipalities and Political Subdivisions — 814,632 — 814,632 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities — 407,619 — 407,619 Residential Mortgage Obligations — 54,104 — 54,104 Asset-Backed Securities — 328,753 — 328,753 Commercial Mortgage-Backed Securities — 160,904 — 160,904 Subtotal $ — $ 951,380 $ — $ 951,380 Corporate Exposures — 897,479 — 897,479 Total Fixed Maturities $ 178,251 $ 2,878,803 $ — $ 3,057,054 Equity Securities: Common Stocks 52,439 — — 52,439 Preferred Stocks — 183,542 — 183,542 Total Equity Securities $ 52,439 $ 183,542 $ — $ 235,981 Short-Term Investments 127,128 — — 127,128 Total Assets Measured at Fair Value $ 357,818 $ 3,062,345 $ — $ 3,420,163 Senior Notes $ — $ 277,951 $ — $ 277,951 Total Liabilities Measured at Fair Value $ — $ 277,951 $ — $ 277,951 December 31, 2016 amounts in thousands Level 1 Level 2 Level 3 Total Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 47,704 $ 226,072 $ — $ 273,776 States, Municipalities and Political Subdivisions — 547,415 — 547,415 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities — 487,364 — 487,364 Residential Mortgage Obligations — 20,530 — 20,530 Asset-Backed Securities — 314,601 — 314,601 Commercial Mortgage-Backed Securities — 154,139 — 154,139 Subtotal $ — $ 976,634 $ — $ 976,634 Corporate Exposures — 838,057 — 838,057 Total Fixed Maturities $ 47,704 $ 2,588,178 $ — $ 2,635,882 Equity Securities: Common Stocks 164,087 — — 164,087 Preferred Stocks — 185,055 — 185,055 Total Equity Securities $ 164,087 $ 185,055 $ — $ 349,142 Short-Term Investments 143,539 — — 143,539 Total Assets Measured at Fair Value $ 355,330 $ 2,773,233 $ — $ 3,128,563 Senior Notes $ — $ 280,316 $ — $ 280,316 Total Liabilities Measured at Fair Value $ — $ 280,316 $ — $ 280,316 All other financial assets and liabilities including Cash, Premium Receivables, Reinsurance Recoverables and Reinsurance Balance Payables are carried at cost, which approximates fair value. Our Company did not have any significant transfers between the Level 1 and Level 2 classifications for the years ended December 31, 2017 and 2016. As of December 31, 2017 and 2016, our Company did not have any Level 3 assets.

Reserves for Losses and LAE

Reserves for Losses and LAE	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Reserves for Losses and LAE	NOTE 5. RESERVES FOR LOSSES AND LAE We establish reserves for the estimated unpaid ultimate liability for losses and LAE under the terms of our policies and agreements. The determination of reserves for losses and LAE is partially dependent upon the receipt of information from agents and brokers. Reserves include estimates for both claims that have been reported and for IBNR, and include estimates of expenses associated with processing and settling these claims. Reserves are recorded in Reserves for Losses and LAE in the Consolidated Balance Sheets. Our estimates and judgments may be revised as additional experience and other data become available and are reviewed, as new or improved methodologies are developed, or as laws change. Frequency/severity analyses are also performed for certain books of business. To the extent that reserves are found deficient or redundant, a strengthening or release is recognized as a charge or credit to earnings. The following table summarizes activity for our Company’s Reserves for Losses and LAE for the years ended December 31, 2017, 2016 and 2015: Years Ended December 31, amounts in thousands 2017 2016 2015 Net Reserves for Losses and LAE at Beginning of Year $ 1,510,451 $ 1,393,126 $ 1,308,136 Provision for Losses and LAE for Claims Occurring in the Current Year 771,955 693,976 637,267 Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years 34,310 (28,528 ) (64,669 ) Incurred Losses and LAE $ 806,265 $ 665,448 $ 572,598 Losses and LAE Paid for Claims Occurring During: Current Year (144,095 ) (119,485 ) (159,802 ) Prior Years (478,979 ) (412,544 ) (320,863 ) Losses and LAE Payments $ (623,074 ) $ (532,029 ) $ (480,665 ) Foreign Currency Adjustment 11,738 (16,094 ) (6,943 ) Net Reserves for Losses and LAE at End of Period 1,705,380 1,510,451 1,393,126 Reinsurance Recoverables on Unpaid Losses and LAE 809,765 779,276 809,518 Gross Reserves for Losses and LAE at End of Period $ 2,515,145 $ 2,289,727 $ 2,202,644 The reporting and operating segments breakdowns of prior period net reserve strengthening (releases) for the years ended December 31, 2017, 2016 and 2015 are as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 U.S. Insurance Marine $ 1,696 $ (10,122 ) $ (24,825 ) P&C 1,613 2,790 (706 ) Professional Liability 13,405 5,984 (3,788 ) Total $ 16,714 $ (1,348 ) $ (29,319 ) Int'l Insurance Marine $ 6,518 $ (22,068 ) $ (21,914 ) P&C 3,118 (7,051 ) (8,458 ) Professional Liability 5,198 4,100 4,156 Total $ 14,834 $ (25,019 ) $ (26,216 ) GlobalRe $ 2,762 $ (2,161 ) $ (9,134 ) Total Strengthening (Releases) $ 34,310 $ (28,528 ) $ (64,669 ) For the year ended December 31, 2017, our Incurred Losses and LAE increased $140.8 million as compared to the same period in 2016. The Provision for Losses and LAE for Claims Occurring in the Current Year increased primarily due to additional net current accident year (“AY”) loss activity related to a higher level of catastrophe (“CAT”) losses incurred during the year ended December 31, 2017, including Hurricanes Irma ($28.3 million), Maria ($23.4 million) and Harvey ($15.8 million), Typhoon Hato ($4.2 million) the Puebla Mexican Earthquake ($3.5 million), and $9.6 million of other CAT events, as compared to the same period in 2016, which included Alberta Wildfires ($11.7 million), Hurricane Matthew ($7.1 million), Ecuador Earthquake ($3.8 million) and Taiwan Earthquake ($3.3 million). Additionally, growth in Net Earned Premium over the prior year added to the increase in Provision for Losses and LAE for Claims Occurring in the Current Year. The following is a discussion of CAT losses incurred of $84.8 million for the year ended December 31, 2017: • Our U.S. Insurance reporting segment incurred $8.2 million of CAT losses, including $6.6 million and $1.6 million in our P&C and Marine operating segments, respectively, primarily due to Hurricanes Harvey, Irma and Maria. • Our Int’l Insurance reporting segment incurred $32.9 million of CAT losses, including $16.9 million and $16.0 million within our P&C and Marine operating segments, primarily due to Hurricanes Irma, Maria and Harvey, Typhoon Hato, and to a lesser extent, the California Wildfires and Puebla Mexican Earthquake. • Our GlobalRe reporting segment incurred $43.7 million of CAT losses primarily due to Hurricanes Irma, Maria and Harvey, and the Puebla Mexican Earthquake. For the year ended December 31, 2016, our Incurred Losses and LAE increased $92.9 million as compared to the same period in 2015. The Provision for Losses and LAE for Claims Occurring in the Current Year increased primarily due to additional net current AY loss activity related to higher level of catastrophe losses during the year ended December 31, 2016, including the Alberta Wildfires, ($11.7 million), Hurricane Matthew ($7.1 million), Ecuador Earthquake ($3.8 million) and Taiwan Earthquake ($3.3 million), as compared to the same period in 2015 which was impacted by large loss activity. The following is a discussion of CAT losses incurred of $26.8 million for the year ended December 31, 2016: • Our Int’l Insurance reporting segment incurred $17.4 million of CAT losses, primarily due to Hurricane Matthew and the Alberta Wildfires. • Our GlobalRe reporting segment incurred $9.4 million of CAT losses, primarily due to the Alberta Wildfires and the Ecuador Earthquake. Our December 31, 2017 Net Reserves for Losses and LAE include $18.5 million of net incurred but not reported (“IBNR”) claims activities for numerous catastrophe events. We caution that the magnitude and complexity of losses arising from these events inherently increases the level of uncertainty and therefore the level of management judgement involved in arriving at our estimated Net Reserves for Losses and LAE. As a result, our actual losses for these events may ultimately differ materially from our current estimates. The following is a discussion of the relevant factors related to the prior period net reserve strengthening of $34.3 million recorded for the year ended December 31, 2017: Our U.S. Insurance reporting segment recorded prior period net reserve strengthening of $16.7 million. The drivers are as follows: • Our Marine operating segment recorded prior period net reserve strengthening of $1.7 million driven by large loss activity within our Cargo product line. • Our P&C operating segment recorded prior period net reserve strengthening of $1.6 million driven by large loss activity within our Primary Casualty division, partially offset by ongoing favorable performance within our Excess Casualty, Environmental and Other P&C divisions. • Our Professional Liability operating segment recorded prior period net reserve strengthening of $13.4 million driven by large loss activity within our D&O division. Our Int’l Insurance reporting segment recorded prior period net reserve strengthening of $14.8 million. The drivers are as follows: • Our Marine operating segment recorded prior period net reserve strengthening of $6.5 million due to unfavorable loss development predominately from our Cargo, Specie and P&I products. • Our P&C operating segment recorded prior period net reserve strengthening of $3.1 million primarily due to unfavorable loss development within our Property division, partially offset by favorable loss emergence in our Energy & Engineering division. • Our Professional Liability operating segment recorded prior period net reserve strengthening of $5.2 million primarily due to large loss development within our E&O and D&O divisions. Our GlobalRe reporting segment recorded prior period net reserve strengthening of $2.8 million primarily driven by the settlement of a large claim in our A&H product, and unfavorable loss emergence within our P&C product. This strengthening was partially offset by reserve releases in our Agriculture and Marine products due to favorable loss emergence. The following is a discussion of the relevant factors related to the prior period net reserve releases of $28.5 million recorded for the year ended December 31, 2016: Our U.S. Insurance reporting segment recorded prior period net reserve releases of $1.3 million. The drivers are as follows: • Our Marine operating segment recorded prior period net reserve releases of $10.1 million driven by favorable claims development in the Craft, Marine Liability, Customs Bonds and Inland Marine products. • Our P&C operating segment recorded prior period net reserve strengthening of $2.8 million driven by large loss activity within our Primary Casualty division, partially offset by ongoing favorable performance within our Excess Casualty, Environmental and Other P&C divisions. • Our Professional Liability operating segment recorded prior period net reserve strengthening of $6.0 million driven by unfavorable development on a few large D&O claims. Our Int’l Insurance reporting segment recorded prior period net reserve releases of $25.0 million. The drivers are as follows: • Our Marine operating segment recorded prior period net reserve releases of $22.1 million due to favorable claims development predominately from our Marine Liability product. • Our P&C operating segment recorded prior period net reserve releases of $7.1 million primarily due to favorable claims development in our Energy & Engineering division, partially offset by large loss activity within our Property division. • Our Professional Liability operating segment recorded prior period net reserve strengthening of $4.1 million primarily due to a decline in expected recoveries from a large reinsurer resulting in an increase in bad debt reserve within our D&O division. Our GlobalRe reporting segment recorded prior period net reserve releases of $2.2 million primarily driven by favorable loss development in our A&H, P&C and Professional Liability products, partially offset by large loss activity in our Marine product. The following is a discussion of the relevant factors related to the prior period net reserve releases of $64.7 million recorded for the year ended December 31, 2015: Our U.S. Insurance reporting segment recorded prior period net reserve releases of $29.3 million. The drivers are as follows: • Our Marine operating segment recorded prior period net reserve releases of $24.8 million driven by favorable claims development in the Marine Liability, Inland Marine and Cargo products. • Our P&C operating segment recorded prior period net reserve releases of $0.7 million driven by ongoing favorable performance within our Excess Casualty, Environmental and Other P&C divisions, mostly offset by large loss activity within our Primary Casualty division. • Our Professional Liability operating segment recorded prior period net reserve releases of $3.8 million driven by favorable emergence in our E&O division, partially offset by unfavorable development in several large D&O claims. Our Int’l Insurance reporting segment recorded prior period net reserve releases of $26.2 million. The drivers are as follows: • Our Marine operating segment recorded prior period net reserve releases of $21.9 million primarily due to favorable claims development on the Marine Liability, Protection and Indemnity and Specie products. • Our P&C operating segment recorded prior period net reserve releases of $8.5 million due to favorable claims development on the Offshore Energy product within the Energy & Engineering division. • Our Professional Liability operating segment recorded prior period net reserve strengthening of $4.2 million driven by adverse development in our D&O division related to two large losses, partially offset by favorable claims development on our E&O division. Our GlobalRe reporting segment recorded prior period net reserve releases of $9.1 million primarily driven by favorable loss development in our Marine, P&C and A&H products. Supplemental tables related to Short Duration Contracts Effectively all property and casualty insurance contracts are described as “Short Duration Contracts” as opposed to life insurance, by the Securities and Exchange Commission. Our Company only issues short duration contracts. Below are two tables with reserve data for each reporting and operating segment. These tables are presented net of reinsurance. The first table reflects incurred claims development as of December 31, 2017, as well as the cumulative claims frequency and the total of Incurred But Not Reported (“IBNR”) liabilities and expected development on reported claims included within the net incurred claims amounts. The second table reflects the cumulative paid claims development as of December 31, 2017. The conversion of all non USD currencies amounts, for all accident years, is in USD as of December 31, 2017. The information provided in our supplemental tables related to short duration contracts includes estimates. Beginning in the 2016 calendar year, the Company implemented a process for the business underwritten through Syndicate 1221 that captures and retains more detailed incurred claim data to enable it to assign AY information as accurately and completely as possible using the data available. Prior to the 2016 calendar year, records for Syndicate 1221 were not available by individual AY, which impacts the presentation of our Int’l Insurance and, to a limited extent, our Global Re supplemental tables for 2015 and prior reporting years. For this 2017 calendar year report, we revised and improved the allocation process used in our 2016 presentation in relation to our Syndicate 1221 business. For the 2016 calendar year report, we applied an allocation process using inception to date gross paid and case information through 2016 to extrapolate the data for reporting years 2012 through 2015 in our supplemental tables. Upon further consideration, for 2017 reporting, the Company developed a process to allocate the Syndicate 1221 incurred claims to individual prior accident years using only data known as of the end of such calendar year. While the estimation process used in 2016 provided information regarding the systematic nature of loss emergence, the revised and improved process applied in the current calendar year more accurately reflects information on variability of loss emergence from period to period. We believe that this process is more aligned with the objectives of this disclosure, and aligns with the first table of this Note 5 In addition, due to the limitations of the data available for our Syndicate 1221 business, the supplemental tables below for our Int’l Insurance and GlobalRe reporting segments begin with the 2012 reporting year and will increase by one year until ten years are presented. Claims frequency is based on claims posted in our Company’s data system as reported. Generally, for most direct insurance and excess-of-loss assumed reinsurance, one claim count is posted for each claim. For assumed bordereau business and business written on binders, one claim count is posted for each bordereau received, which could account for multiple claims. Additionally, if the insured is covered for multiple years, a claim may be initially established in all potentially impacted policy years until the appropriate policy year is determined. The information about incurred claims development and cumulative paid claims and Allocated Loss Adjustment Expense (“ALAE”), net of reinsurance, for the years ended December 31, 2008 through 2016, is presented as unaudited supplementary information. Note that asbestos reserves are included in the U.S. Marine operating segment in years prior to 2007. IBNR values shown in the following tables are the difference between the case reserves for reported losses and the estimated future payments for claims arising from the specific accident period. Our Company has always applied prudent claim reserving practices. For a number of segments, this has resulted in case reserves for more mature accident years exceeding the future payments consistently. IBNR estimates will be negative where our history indicates such favorable future emergence is the best estimate. U.S. Marine Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 56,878 $ 67,421 $ 72,259 $ 73,734 $ 73,494 $ 68,393 $ 68,557 $ 66,717 $ 66,937 $ 66,849 $ 282 3,194 2009 62,556 59,278 56,052 50,501 48,366 49,571 51,783 50,874 51,537 205 2,323 2010 64,753 63,812 60,035 54,813 52,847 57,008 56,458 56,281 (224 ) 2,007 2011 70,219 69,293 67,640 63,099 61,912 60,329 59,750 630 1,910 2012 85,275 87,606 72,219 68,841 70,337 70,887 (619 ) 1,856 2013 61,356 57,865 51,748 54,381 56,319 144 1,814 2014 59,270 42,684 41,006 45,762 (377 ) 1,927 2015 61,106 51,864 50,331 5,148 2,187 2016 53,011 48,791 12,748 2,352 2017 54,061 23,327 1,986 Total $ 560,568 U.S. Marine Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 7,554 $ 36,397 $ 54,175 $ 56,460 $ 62,398 $ 62,700 $ 63,027 $ 63,140 $ 64,029 $ 64,433 2009 12,947 29,266 35,449 41,650 43,450 46,033 46,894 49,573 50,191 2010 16,582 32,323 39,825 42,896 48,114 51,887 55,096 55,249 2011 14,955 39,821 48,850 52,408 56,955 57,929 58,558 2012 27,254 46,308 54,673 60,734 67,220 69,059 2013 17,831 35,471 41,488 47,162 51,466 2014 10,717 23,415 34,899 41,021 2015 16,945 31,010 40,586 2016 11,610 25,154 2017 18,947 Total $ 474,664 Net reserves < 2008 15,187 Total Net reserves $ 101,091 U.S. Marine Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 U.S. Marine 27.7 % 31.8 % 16.7 % 8.4 % 7.7 % 3.3 % 2.2 % 1.9 % 1.3 % 0.6 % U.S. P&C Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 151,401 $ 152,659 $ 138,860 $ 135,255 $ 136,498 $ 138,499 $ 142,641 $ 147,763 $ 152,814 $ 154,169 $ 3,706 10,937 2009 127,237 129,613 130,860 135,984 141,186 146,321 147,036 150,294 149,622 6,143 8,515 2010 96,885 96,446 94,317 100,006 103,038 104,955 107,914 110,170 7,776 7,874 2011 79,832 78,093 86,557 89,666 90,613 98,238 100,461 12,051 6,450 2012 103,089 89,758 91,502 99,207 107,245 111,146 18,194 4,579 2013 139,517 125,952 129,100 137,298 149,409 20,569 3,898 2014 191,515 159,477 148,541 155,248 51,231 3,880 2015 244,311 218,973 214,547 76,700 3,672 2016 271,351 243,754 190,052 3,136 2017 295,386 263,648 2,527 Total $ 1,683,912 U.S. P&C Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 10,266 $ 32,307 $ 54,663 $ 78,024 $ 99,825 $ 111,420 $ 119,766 $ 128,886 $ 142,284 $ 146,295 2009 11,245 33,049 55,773 86,736 110,648 123,981 129,461 136,636 138,829 2010 11,985 26,482 46,478 65,094 77,302 85,740 90,625 97,125 2011 3,630 14,579 30,359 46,638 58,489 70,486 79,682 2012 4,761 17,311 27,513 47,005 67,502 82,906 2013 7,146 20,914 48,716 84,494 118,506 2014 5,229 13,264 44,468 74,979 2015 3,575 24,057 78,856 2016 5,831 29,131 2017 12,860 Total $ 859,169 Net reserves < 2008 24,266 Total Net reserves $ 849,009 U.S. P&C Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 U.S. P&C 5.0 % 10.9 % 17.1 % 18.6 % 15.7 % 10.0 % 5.7 % 5.5 % 5.1 % 2.6 % U.S. Professional Liability Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 34,346 $ 38,449 $ 39,130 $ 35,844 $ 37,603 $ 37,939 $ 37,497 $ 37,620 $ 41,766 $ 43,003 $ 875 770 2009 47,473 53,273 61,023 58,600 60,789 58,487 58,660 58,143 61,083 788 1,370 2010 52,302 64,020 68,375 75,619 74,535 86,787 85,348 86,164 932 1,431 2011 58,116 62,087 70,240 73,028 79,181 79,136 82,558 699 1,726 2012 61,196 54,480 57,097 55,917 58,344 58,563 2,254 2,175 2013 60,899 54,982 49,104 48,298 54,228 3,915 1,982 2014 50,020 35,870 39,826 42,060 5,255 2,017 2015 31,659 30,764 31,088 8,721 1,674 2016 41,673 36,650 12,608 1,806 2017 51,708 44,396 1,902 Total $ 547,105 U.S. Professional Liability Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 1,880 $ 15,288 $ 25,042 $ 30,581 $ 34,902 $ 35,206 $ 35,598 $ 37,175 $ 42,056 $ 42,046 2009 3,837 21,742 41,675 51,642 55,378 53,859 57,744 58,095 58,150 2010 2,981 23,147 46,868 54,019 59,765 82,013 84,511 84,529 2011 3,204 23,878 43,551 59,576 65,710 75,641 78,258 2012 3,855 22,635 35,681 44,028 49,675 54,728 2013 4,677 22,167 32,904 38,876 47,062 2014 2,304 14,371 29,392 35,114 2015 2,257 11,249 18,428 2016 2,382 18,687 2017 2,682 Total $ 439,684 Net reserves < 2008 881 Total Net reserves $ 108,302 U.S. Professional Liability Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 U.S. Professional Liability 5.8 % 30.6 % 25.9 % 13.7 % 9.2 % 8.9 % 3.3 % 1.4 % 5.7 % 0.0 % Int'l Marine Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 85,148 $ 80,793 $ 80,589 $ 84,122 $ 84,449 $ 83,962 (135 ) 5,733 2009 96,465 91,705 90,527 83,477 82,074 81,729 (265 ) 5,045 2010 132,080 130,102 119,156 114,940 112,259 108,523 (428 ) 5,129 2011 100,162 85,720 82,441 77,848 73,970 74,309 (500 ) 4,643 2012 110,316 132,245 107,636 100,029 93,052 92,983 (1,224 ) 4,908 2013 87,987 94,614 82,641 81,989 84,100 3,168 4,716 2014 104,397 117,974 116,899 119,767 (1,483 ) 4,986 2015 99,067 95,001 96,614 (1,913 ) 7,400 2016 88,388 95,584 8,335 9,114 2017 107,915 32,198 7,385 Total $ 945,486 Int'l Marine Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 71,355 $ 73,799 $ 75,255 $ 79,024 $ 80,185 $ 80,183 2009 66,388 70,553 73,321 76,783 77,487 78,772 2010 78,509 90,871 96,920 100,897 103,188 103,646 2011 45,896 59,341 63,778 68,702 69,672 70,022 2012 23,382 54,954 70,657 78,789 81,411 83,381 2013 23,731 46,552 58,687 64,859 67,730 2014 29,977 57,629 81,941 90,619 2015 22,862 54,550 76,044 2016 23,665 55,420 2017 29,082 Total $ 734,899 Net reserves < 2008 11,072 Total Net reserves $ 221,659 Int'l Marine Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 Int'l Marine 25.1 % 29.7 % 19.0 % 7.8 % 4.3 % 2.4 % 2.3 % 1.9 % 1.5 % 0.0 % Int'l P&C Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 37,256 $ 34,428 $ 34,495 $ 33,877 $ 41,536 $ 41,472 $ 67 913 2009 38,680 40,896 40,984 40,692 34,186 34,461 206 885 2010 43,096 42,527 41,922 42,196 42,822 42,613 309 1,089 2011 71,531 61,564 59,405 59,529 59,516 55,127 221 1,216 2012 45,693 37,520 36,444 32,472 32,622 33,075 152 1,358 2013 36,044 37,520 36,599 34,499 35,443 1,027 1,592 2014 27,829 22,787 20,997 16,449 (1,396 ) 1,300 2015 34,889 36,689 33,969 10 1,986 2016 79,393 92,108 18,665 3,556 2017 56,953 10,229 2,213 Total $ 441,670 Int'l P&C Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 38,275 $ 39,489 $ 39,869 $ 40,398 $ 40,249 $ 40,333 2009 28,919 33,223 33,685 33,317 33,405 33,494 2010 35,192 40,880 43,429 41,102 41,204 41,691 2011 38,548 47,851 50,753 53,318 54,239 52,225 2012 13,238 21,578 27,084 30,135 31,332 31,596 2013 7,160 18,441 25,840 28,682 30,279 2014 3,902 9,015 13,232 16,805 2015 6,845 18,436 29,624 2016 26,190 55,749 2017 15,019 Total $ 346,815 Net reserves < 2008 825 Total Net reserves $ 95,680 Int'l P&C Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 Int'l P&C 25.0 % 32.4 % 26.5 % 13.0 % 4.3 % -1.0 % -1.5 % 0.9 % -0.1 % 0.2 % Int'l Professional Liability Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 11,749 $ 13,524 $ 13,798 $ 13,829 $ 14,082 $ 14,463 $ 258 439 2009 16,661 17,225 17,365 17,016 16,020 15,988 758 624 2010 14,423 14,023 14,225 14,221 15,864 14,639 247 745 2011 15,390 18,125 19,429 8,923 6,989 12,823 1,769 665 2012 6,835 11,506 12,789 13,457 17,168 18,068 2,365 1,102 2013 10,218 4,099 14,532 15,748 16,842 1,607 1,174 2014 20,662 24,519 23,861 21,758 4,216 1,747 2015 28,319 30,539 35,586 9,550 2,370 2016 36,167 34,218 22,567 2,744 2017 47,697 41,954 1,522 Total $ 232,082 Int'l Professional Liability Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 12,198 $ 13,360 $ 13,754 $ 13,784 $ 14,071 $ 14,105 2009 8,954 10,901 12,983 14,008 14,331 14,918 2010 5,909 7,320 10,659 12,840 13,597 13,643 2011 1,221 2,038 2,994 4,087 4,677 16,642 2012 2,141 3,236 4,293 5,600 7,526 6,933 2013 733 2,285 3,078 9,478 13,906 2014 1,387 2,493 5,228 8,945 2015 3,457 5,939 17,502 2016 2,076 6,955 2017 2,024 Total $ 115,573 Net reserves < 2008 274 Total Net reserves $ 116,783 Int'l Professional Liability Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 Int'l Professional 6.7 % 8.8 % 16.6 % 20.8 % 15.2 % 5.4 % 35.0 % 0.8 % 2.8 % 0.2 % GlobalRe Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 14,467 $ 14,846 $ 14,198 $ 13,792 $ 13,487 $ 13,123 $ 529 306 2009 11,247 11,381 11,267 10,516 10,567 10,406 53 200 2010 13,150 13,110 14,142 11,908 12,631 13,159 (54 ) 201 2011 53,868 76,362 76,164 73,610 72,877 73,530 (438 ) 769 2012 107,189 99,116 92,730 88,367 115,058 119,771 (815 ) 1,456 2013 114,873 118,779 102,632 105,172 102,433 1,067 1,285 2014 119,379 142,058 121,716 118,255 5,062 1,070 2015 101,764 91,767 94,428 6,897 896 2016 88,543 90,755 15,972 749 2017 129,150 60,576 428 Total $ 765,010 GlobalRe Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 11,775 $ 11,799 $ 11,661 $ 11,640 $ 11,702 $ 11,797 2009 8,694 8,930 9,060 10,018 10,133 10,133 2010 10,005 10,349 10,529 11,154 11,207 11,272 2011 43,450 71,138 71,816 72,102 72,246 72,414 2012 37,746 77,227 83,408 84,801 111,962 117,782 2013 53,476 82,701 90,721 98,484 100,059 2014 66,286 119,249 105,648 108,794 2015 41,660 64,527 76,973 2016 36,352 66,277 2017 43,663 Total $ 619,164 Net reserves < 2008 2,504 Total Net reserves $ 148,350 GlobalRe Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 GlobalRe 39.3 % 34.0 % 3.2 % 3.8 % 8.2 % 3.3 % 3.3 % 0.5 % 0.2 % 0.7 % The reconciliation of the net incurred and paid claims development tables to the Reserve for losses and LAE in the consolidated statement of financial position is as follows: Carried Reserves Reconciliation amounts in thousands As of December 31, 2017 Total Net Reserves Reinsurance Recoverables on Unpaid Claims Total Gross Reserves U.S. Marine $ 101,091 $ 98,373 $ 199,464 U.S. P&C 849,009 371,651 1,220,660 U.S. Professional Liability 108,302 49,650 157,952 Int'l Marine 221,659 90,792 312,451 Int'l P&C 95,680 126,365 222,045 Int'l Professional Liability 116,783 55,393 172,176 GlobalRe 148,350 17,541 165,891 Liabilities for Unpaid Claims and Claim Adjustment Expenses $ 1,640,874 $ 809,765 $ 2,450,639 Unallocated Claims Adjustment Expenses 64,269 Other 237 Total Gross Liability for Unpaid Claims and Claim Adjustment Expense $ 2,515,145 F-1

Ceded Reinsurance

Ceded Reinsurance	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Ceded Reinsurance	NOTE 6. CEDED REINSURANCE We utilize reinsurance principally to reduce our exposure on individual risks, to protect against catastrophic losses and to stabilize loss ratios and net underwriting results. Reinsurers are liable to us to the extent the risk is transferred or ceded to them. However, ceded reinsurance arrangements do not eliminate our obligation to pay claims to our policyholders. Accordingly, we bear credit risk with respect to our reinsurers. We are protected by various treaty and facultative reinsurance agreements. Our exposure to credit risk from any one reinsurer is managed through diversification of reinsurers, principally in the U.S. and European reinsurance markets. A reinsurer generally must have a rating from A.M. Best and/or S&P of “A” or better, or an equivalent financial strength if not rated, plus at least $500 million in policyholders’ surplus to meet our standards of acceptability. Our Reinsurance Security Committee, which is part of our Enterprise Risk Management Finance and Credit Sub-Committee of our Board of Directors, monitors the financial strength of our reinsurers and the related reinsurance recoverables and periodically reviews the list of acceptable reinsurers. The following table lists our Company’s 10 largest reinsurers measured by the amount of total reinsurance recoverables as of December 31, 2017, and the reinsurers’ ratings from A.M. Best and S&P: Reinsurance Recoverables Unearned Paid/Unpaid Collateral amounts in thousands Premium Losses (1) Total Held A.M. Best S&P Everest Reinsurance Company 29,333 90,005 119,338 9,564 A+ A+ Swiss Reinsurance America Corporation 18,987 71,062 90,049 8,069 A+ AA- National Indemnity Company 5,770 61,413 67,183 5,423 A++ AA+ Transatlantic Reinsurance Company 8,587 50,487 59,074 2,588 A+ A+ Munich Reinsurance America Inc. 10,991 46,751 57,742 5,171 A+ AA- Allied World Reinsurance 6,511 32,422 38,933 1,637 A A- Aspen Insurance UK Ltd. 8,921 29,093 38,014 8,775 A A Employers Mutual Casualty Company 12,096 22,509 34,605 11,172 A NR Scor Global P&C SE 5,938 25,228 31,166 6,393 A+ AA- Partner Reinsurance Europe 5,257 25,472 30,729 11,408 A A+ Top 10 Reinsurers 112,391 454,442 566,833 70,200 Others 116,178 427,817 543,995 110,464 Total 228,569 882,259 1,110,828 180,664 (1) - Net of reserve for uncollectible reinsurance of approximately $12.6 million. Our Company holds reserves for uncollectible reinsurance in the amounts of $12.6 million and $12.1 million as of December 31, 2017 and 2016, respectively. This reserve is determined by reinsurer specific default risk as indicated by their financial strength ratings as well as additional default risk for asbestos and environmental related recoverables. Actual uncollectible reinsurance could exceed or be less than our reserve balance. The increase in our reserves for uncollectible reinsurance as compared to 2016 is driven primarily by one of our reinsurers having been placed in liquidation by the State of California. We continue to monitor the liquidation process and assess our potential exposure. Our Company holds collateral of $180.7 million, which consists of $136.4 million in ceded balances payable, $41.3 million in letters of credit and $3.0 million of funds held as trust account balances. NIC and NSIC are required to collateralize reinsurance obligations due to us from reinsurers not authorized by their respective states of domicile. The following table summarizes the components of Net Written Premium: Years Ended December 31, amounts in thousands 2017 2016 2015 Direct $ 1,489,422 $ 1,403,865 $ 1,277,728 Assumed 223,843 165,046 175,774 Ceded (441,935 ) (382,687 ) (409,642 ) Net Written Premiums $ 1,271,330 $ 1,186,224 $ 1,043,860 The following table summarizes the components of Net Earned Premium: Years Ended December 31, amounts in thousands 2017 2016 2015 Direct $ 1,425,582 $ 1,330,265 $ 1,223,840 Assumed 187,609 171,978 175,153 Ceded (426,771 ) (401,898 ) (414,906 ) Net Earned Premiums $ 1,186,420 $ 1,100,345 $ 984,087 The following table summarizes the components of Net Losses and LAE Incurred: Years Ended December 31, amounts in thousands 2017 2016 2015 Direct $ 938,640 $ 795,414 $ 703,361 Assumed 148,606 87,995 97,947 Ceded (280,981 ) (217,961 ) (228,710 ) Net Losses and LAE $ 806,265 $ 665,448 $ 572,598

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt	NOTE 7. DEBT Credit Facilities On November 4, 2016, NUAL entered into an Australian Dollar credit facility, which was subsequently amended on October 30, 2017, with Barclays Bank PLC. Letter of Credit commissions are payable under this facility at a rate of 1.55% per annum. The facility may be cancelled by either party after providing written notice. This credit facility contains customary covenants for facilities of this type, including a restriction on future encumbrances that are outside the ordinary course of business, and a requirement to maintain at least £75.0 million of Funds at Lloyd’s. As of December 31, 2017, letters of credit with an aggregate face amount of 24.0 million Australian Dollars were outstanding under the credit facility, and our Company was in compliance with all covenants. On November 7, 2016, we entered into a credit facility agreement with ING Bank N.V., London Branch, individually and as Administrative Agent for a syndicate of lenders (the “Club Facility”), which is secured by all the common stock of NIC and requires us to maintain at least forty percent of the outstanding amounts under such facility as Funds at Lloyd’s. The Club Facility has two tranches with one tranche extending a $140.0 million commitment and the other tranche extending a £60.0 million commitment. In addition, in order to support the increased underwriting capacity of the Syndicate for the 2017 UWY, we amended the $25.0 million credit facility with ING Bank N.V., London Branch, dated November 20, 2015, on November 7, 2016, to extend the term for an additional two years (the “Bilateral Facility”). Both of these facilities, as well as the November 4, 2016 facility, are used to fund underwriting obligations at Lloyd’s for the 2017 UWY, as well as open prior UWYs. The Bilateral Facility is a non-committed facility which has an applicable fee rate ranging from 0.85% to 1.20% per annum based upon our Company’s S&P rating. For the Club Facility the applicable fee rate payable ranges from 0.95% to 1.60% per annum based on a tiered schedule that is based on our then-current financial strength ratings issued by S&P and A.M. Best and the amount of our own collateral utilized to fund our participation in the Syndicate. If any letters of credit remain outstanding under these facilities after December 31, 2018, we would be required to post additional collateral to secure the remaining letters of credit. As of December 31, 2017, letters of credit with an aggregate face amount of $125.0 million and £60.0 million were outstanding under the Club Facility and we had an aggregate of $1.1 million of cash collateral posted. As of December 31, 2017 there were letters of credit with a face amount of $25.0 million outstanding under the Bilateral Facility. The Bilateral and Club Facilities contain customary covenants for facilities of this type, including restrictions on indebtedness and liens, limitations on mergers, dividends and the sale of assets, and requirements as to maintaining certain consolidated tangible net worth, statutory surplus and other financial ratios. These credit facilities also provide for customary events of default, including failure to pay principal, interest or fees when due, failure to comply with covenants, any representation or warranty made by our Company being false in any material respect, default under certain other indebtedness, certain insolvency or receivership events affecting our Company and our subsidiaries, the occurrence of certain material judgments, or a change in control of our Company. As of December 31, 2017, our Company was in compliance with all covenants. Senior notes On October 4, 2013, our Company completed a public debt offering of $265.0 million principal amount of the 5.75% Senior notes due on October 15, 2023 and received net proceeds of $263.3 million. Our Company used a portion of the proceeds for the redemption of the 7.0% Senior notes due May 1, 2016 (“7.0% Senior notes”), as well as a $17.9 million call premium in connection with the redemption of the 7.0% Senior notes. The unamortized discount as of December 31, 2017 and 2016 was $1.1 million and $1.3 million, respectively. The interest rate payable on the 5.75% Senior notes is subject to a tiered adjustment based on defined changes in our Company’s debt ratings. Our Company may redeem the 5.75% Senior notes in whole at any time or in part from time to time at a make-whole redemption price. The 5.75% Senior notes are our Company’s only senior unsecured obligation and will rank equally with future senior unsecured indebtedness. The terms of the 5.75% Senior notes contain various restrictive business and financial covenants, including a restriction on indebtedness, and other restrictions typical for debt obligations of this type, including limitations on mergers, liens and dispositions of the Common stock of certain subsidiaries. As of December 31, 2017, our Company was in compliance with all such covenants.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	NOTE 8. COMMITMENT AND CONTINGENCIES Rental Commitments Future minimum annual rental commitments as of December 31, 2017 under various non-cancellable operating leases for our office facilities, which expire at various dates through 2031, are as follows: Rental Commitments for Years Ended December 31, amounts in thousands 2018 $ 12,075 2019 11,469 2020 11,395 2021 10,315 2022 9,654 2023-2031 36,296 Total Minimum Operating Lease Payments $ 91,204 We are also liable for additional payments to the landlords for certain annual cost increases. Rent expense for the years ended December 31, 2017, 2016 and 2015 was $13.2 million, $13.2 million and $12.9 million, respectively. Legal Contingencies In the ordinary course of conducting business, our Parent Company’s subsidiaries are involved in various legal proceedings. Most of these proceedings consist of claims litigation involving our Parent Company’s subsidiaries as either: (a) liability insurers defending or providing indemnity for third party claims brought against insureds or (b) insurers defending first party coverage claims brought against them. In general, our Company believes we have valid defenses to these cases. Our Company’s management believes that the ultimate liability, if any, with respect to these legal proceedings, after consideration of provisions made for potential losses and cost of defense, will not be material to our Company’s Consolidated Balance Sheets, Statements of Income and Statements of Cash Flows Share Purchase Agreement On December 15, 2017, our Company entered into a Share Purchase Agreement with Ackermans & van Haaren NV, a Belgian limited liability company (“AvH”), SIPEF NV, a Belgian limited liability company (“SIPEF”), Mr. Jozef Gielen, an individual (“Gielen”), and Kapimar Comm.V, a Belgian limited liability company (collectively the “Sellers”), pursuant to which our Company agreed to purchase all of the shares of Bracht, Deckers & Mackelbert NV, an insurance underwriting agency organized under the laws of Belgium (“BDM”), and Assurances Continentales – Continentale Verzekeringen NV, an insurance company licensed under the laws of Belgium (“ASCO”). In the proposed transaction, our Company will also acquire Canal Re S.A., a reinsurance company licensed under the laws of the Grand Duchy of Luxembourg and a wholly-owned subsidiary of ASCO (“Canal Re”). As aggregate consideration for the purchase of the shares of BDM and ASCO, our Company will pay EUR 35 million (which is approximately USD $42 million, based on the exchange rate as of December 31, 2017) in cash at the closing of the transaction, which is subject to the satisfaction or waiver of customary closing conditions, including among other things, regulatory approvals from the National Bank of Belgium with respect to the transfer of ASCO shares and from the Luxembourg Insurance Commission with respect to the indirect transfer of the shares of Canal Re. The Purchase Agreement may be terminated under certain circumstances, including (i) by either our Company or Sellers due to a failure to obtain the necessary regulatory approvals noted above on or before May 31, 2018; or (ii) by either our Company or Sellers if the other party has failed to fulfill its specified conditions to closing the transaction on or before the scheduled date of the closing. In the event that the specified conditions to closing for the Sellers are otherwise satisfied and the Sellers terminate the Purchase Agreement due to a failure by our Company to fulfill its specified conditions to closing, our Company is required to pay the Sellers a termination fee equal to EUR 5 million, (which is approximately USD $6 million, based on the exchange rate as of December 31, 2017). In the event that the specified conditions to closing for our Company are otherwise satisfied and our Company terminates the Purchase Agreement due to a failure by a Seller to fulfill its specified conditions to closing, such Seller is required to pay our Company a termination fee equal to EUR 10 million, (which is approximately USD $12 million, based on the exchange rate as of December 31, 2017). Additionally, the Sellers have agreed to reimburse the Purchaser up to EUR 5 million, (which is approximately USD $6 million, based on the exchange rate as of December 31, 2017) in the event of adverse development of claims incurred prior to December 31, 2016 as measured on December 31, 2019. Guarantees On February 16, 2017, our Company entered into a Guarantee, pursuant to which it guaranteed all of the liabilities and obligations of NIIC (the “Guarantee”), to facilitate the issuance of a financial strength rating to NIIC by A.M. Best. The Guarantee would remain effective until all of such liabilities and obligations are discharged, and in the event that our Company does not meet its obligations under the Guarantee, any person who is covered by an insurance policy, certificate of coverage or reinsurance contract issued by NIIC would be a third party beneficiary under the Guarantee. Our Company’s obligations under the Guarantee may be terminated by providing twelve months prior written notice to NIIC. However the obligations of our Company under the Guarantee terminate immediately in the event that (i) the majority of the outstanding voting capital stock in NIIC is sold to any non-affiliated entity; (ii) A.M. Best confirms that NIIC would receive the same financial strength rating as NIC or NSIC, without the benefit of the Guarantee; or (iii) NIIC withdraws its request to be rated by A.M. Best, provided that NIIC has not been downgraded within the prior twelve months. State Loan and Grant Contingencies In 2013, the State of Connecticut (“the State”) awarded our Company up to $11.5 million ($8.0 million in loans and $3.5 million in grants) to move our corporate headquarters to Stamford, Connecticut. The loan is non-interest bearing has a term of 10 years and is subject to forgiveness based on our compliance with certain conditions set forth in the agreement with the State. The amount of the loan to be received is dependent on our Company reaching certain milestones for creation of new jobs over a five-year period, and the funds are to be used to offset certain equipment purchases, facility costs, training of employees and other eligible project-related costs. As of December 31, 2017, our Company received $9.9 million of the award ($7.5 million in loans and $2.4 million of the grant) and earned a loan forgiveness credit of $7.0 million with the State . .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	NOTE 9. INCOME TAXES Our Company is subject to the tax laws and regulations of the U.S. and the foreign countries in which it operates. Our Company files a consolidated U.S. Federal tax return, which includes all domestic subsidiaries and the U.K. Branch. The income from the foreign operations is designated as either U.S. connected income or non-U.S. connected income. Lloyd’s is required to pay U.S. income tax on U.S. connected income written by Lloyd’s syndicates. Lloyd’s and the Internal Revenue Service (“IRS”) have entered into an agreement whereby the amount of tax due on U.S. connected income is calculated by Lloyd’s and remitted directly to the IRS. These amounts are then charged to the corporate member in proportion to its participation in the relevant syndicates. Our Company’s corporate member is subject to this agreement and receives U.K. tax credits in the U.K. for any U.S. income tax incurred up to the U.K. income tax charged on the U.S. connected income. The non-U.S. connected insurance income would generally constitute taxable income under the Subpart F income section of the U.S. Internal Revenue Code (“Subpart F”) since less than 50% of the Syndicate’s premiums are derived within the U.K. and would therefore be subject to U.S. taxation when the Lloyd’s year of account closes. Taxes are accrued at a 35% rate on our Company’s foreign source insurance income and foreign tax credits, where available, are utilized to offset U.S. tax as permitted. Our Company’s effective tax rate for the Syndicate taxable income could substantially exceed 35% to the extent our Company is unable to offset U.S. taxes paid under Subpart F tax regulations with U.K. tax credits on future underwriting year distributions. Our Company’s undistributed foreign earnings are intended to be permanently reinvested. As a result of the Tax Act’s one-time Transition Tax on the earnings of foreign subsidiaries, as of December 31, 2017 our Company expects the unrecognized deferred tax liability attributable to indefinitely reinvested earnings to be immaterial. On December 22, 2017, the President signed the Tax Act. The Tax Act, among other things, lowered the U.S. corporate income tax rate from 35% to 21% effective January 1, 2018. Consequently, we revalued our net deferred tax assets as of December 31, 2017 by $17.1 million to reflect the estimated impact of the Tax Act. As a result of the Tax Act we are subject to the Transition Tax and we anticipate incurring a one-time adjustment of approximately $2.6 million due to the tax on previously untaxed accumulated and current earnings and profits on certain foreign subsidiaries. In computing taxable income, P&C insurers reduce underwriting income by loss and loss adjustment expenses incurred. The deduction for losses incurred is discounted for tax at the interest rates and for the loss payment patterns prescribed by the U.S. Treasury. The Tax Act changes the prescribed interest rates to rates based on corporate bond yield curves and extends the applicable time periods for the loss payment pattern. These changes are effective for tax years beginning after 2017 and are subject to a transition rule that spreads the additional tax payment from the amount determined by applying these changes over the subsequent eight years beginning in 2018. This item is a taxable temporary difference and has no direct impact on total tax expense for 2017 and future years. While we have substantially completed our provisional analysis of the income tax effects of the Tax Act and recorded a reasonable estimate of such effects, including but not limited to, the re-measurement of deferred tax assets (liabilities), future discounting for loss reserves and the Transition Tax, the net one-time charge related to the Tax Act may differ, possibly materially, due to further refinement of our calculations, changes in interpretations and assumptions that we have made, additional guidance that may be issued by the U.S. Government, and actions and related accounting policy decisions we may take as a result of the Tax Act. We will complete our analysis over a one-year measurement period, as described in SAB 118, and any adjustments during this measurement period will be included in net earnings from continuing operations as an adjustment to income tax expense in the reporting period when such adjustments are determined. The components of current and deferred income tax expense (benefit) are as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 Current Income Tax Expense (Benefit): Federal and Foreign $ 42,427 $ 39,900 $ 27,937 State and Local 764 638 1,476 Subtotal $ 43,191 $ 40,538 $ 29,413 Deferred Income Tax Expense (Benefit): Federal and Foreign $ (8,879 ) $ (5,755 ) $ 7,004 State and local — — — Subtotal $ (8,879 ) $ (5,755 ) $ 7,004 Total Income Tax Expense (Benefit) $ 34,312 $ 34,783 $ 36,417 A reconciliation of total income taxes applicable to pre‑tax operating income and the amounts computed by applying the federal statutory income tax rate to the pre‑tax operating income were as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 Computed Expected Tax Expense $ 26,182 35.0 % $ 41,128 35.0 % $ 41,116 35.0 % Tax-Exempt Interest (5,394 ) (7.2 %) (4,559 ) (3.9 %) (4,987 ) (4.2 %) Dividends Received Deduction (4,689 ) (6.3 %) (3,812 ) (3.2 %) (2,086 ) (1.8 %) Proration of DRD and Tax-Exempt Interest 1,512 2.0 % 1,256 1.1 % 1,061 0.9 % Current State and Local Income Taxes, Net of Federal Income Tax Deduction 497 0.7 % 415 0.4 % 959 0.8 % Stock Compensation Fair Market Value (5,027 ) (6.7 %) N/A N/A N/A N/A Change in Valuation Allowance 1,182 1.6 % — — — — Deferred Tax Expense due to Tax Law Changes 17,107 22.9 % N/A N/A N/A N/A Transition Tax 2,587 3.5 % N/A N/A N/A N/A Other 355 0.4 % 355 0.2 % 354 0.3 % Actual Tax Expense and Rate $ 34,312 45.9 % $ 34,783 29.6 % $ 36,417 31.0 % N/A - Not applicable The tax effects of cumulative temporary differences that give rise to federal, foreign, state and local deferred tax assets and deferred tax liabilities were as follows: December 31, amounts in thousands 2017 2016 Deferred Tax Assets: Loss Reserve Discount $ 20,425 $ 25,420 Unearned Premiums 24,352 33,886 Compensation Related 8,462 15,288 Lloyd's Year of Account Deferral 9,400 — State and Local Net Deferred Tax Assets 616 699 Net Currency Translation Adjustments 2,084 5,363 Other 152 3,618 Total Gross Deferred Tax Assets $ 65,491 $ 84,274 Less: Valuation Allowance (1,798 ) (699 ) Total Deferred Tax Assets $ 63,693 $ 83,575 Deferred Tax Liabilities: Loss Reserve Discount - (Transition Rule) $ (5,066 ) $ — Net Unrealized Gains/Losses on Securities (8,708 ) (10,142 ) Deferred Acquisition Costs (20,670 ) (29,401 ) Lloyd's Year of Account Deferral — (15,471 ) Net Unrealized Foreign Exchange (3,782 ) (3,575 ) Other (3,196 ) (4,048 ) Total Deferred Tax Liabilities $ (41,422 ) $ (62,637 ) Net Deferred Income Tax Asset (Liability) $ 22,271 $ 20,938 Unrecognized tax benefits are differences between tax positions taken in the tax returns and benefits recognized in the financial statements. Our Company has no unrecognized tax benefits as of December 31, 2017, 2016 and 2015. Our Company did not incur any interest or penalties related to unrecognized tax benefits for the years ended December 31, 2017, 2016 and 2015. Our Company is currently under examination by the IRS for tax years 2014 and 2015. In addition, our Company is generally subject to state or local or foreign tax examinations by tax authorities for open tax years in the respective locations. Our Company had state and local deferred tax assets amounting to potential future tax benefits of $0.6 and $0.7 million as of December 31, 2017 and 2016, respectively. Included in the deferred tax assets are minimal state and local net operating loss carry-forwards. A valuation allowance was established for the full amount of these potential future tax benefits due to the uncertainty associated with their realization. Our Company’s state and local tax carry-forwards as of December 31, 2017 expire from 2027 to 2035. Additionally, our Company has established a valuation allowance of $1.2 million for our deferred tax asset on our foreign operations in the U.K. Net operating losses were incurred as a result of the catastrophe losses incurred during the third quarter of 2017 and their significance to the 2017 financial results of our Company’s international operations. U.K. net operating losses have the ability to be carried forward without expiration. However, upon evaluation, management determined that a valuation allowance was necessary for a portion of the deferred tax assets where it was more likely than not that the deferred tax assets would not be utilized in future periods. In assessing the realization of deferred tax assets, management considers whether it is more likely than not that, the deferred tax assets will be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considers the scheduled reversal of deferred tax liabilities, tax planning strategies and anticipated future taxable income in making this assessment and believes it is more likely than not that our Company will realize the benefits of its deductible differences as of December 31, 2017, net of any valuation allowance.

Stockholders Equity

Stockholders Equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders' Equity	NOTE 10. STOCKHOLDERS’ EQUITY Our authorized share capital consists of 50 million common shares with a par value of $0.10 per share and 1 million preferred shares with a par value of $0.10 per share. Our Company has not issued any preferred shares as of December 31, 2017. The following table represents changes in our Company’s issued and outstanding common shares for the periods indicated. We completed a two-for-one stock split on January 20, 2017. All share and per share data prior to January 20, 2017 has been retroactively restated on a post-split basis. Years Ended December 31, amounts in thousands 2017 2016 2015 Beginning Balance 29,124 28,862 28,562 Net Vested Stock Grants 343 220 254 Employee Stock Purchase Plan 40 42 42 Stock Options Exercised — — 4 Ending Balance 29,507 29,124 28,862 On December 6, 2016, our Board of Directors declared a two-for-one stock split of The Navigators Group, Inc. Common stock, to be effected in the form of a stock dividend. Stockholders of record at the close of business on December 30, 2016 received one additional share of Common stock for every share of Common stock held. All disclosures of shares and per share data have been retroactively adjusted to reflect the stock split for all periods presented. For the years ended December 31, 2017 and 2016, our Company paid total dividends of $0.225 and $0.135 per share, respectively, to stockholders of record of our Company’s Common Stock. For the year ended December 31, 2015, our Company did not pay any dividends. The declaration and amount of any future dividend will be at the discretion of the Board of Directors, and will depend upon our Company’s financial condition, results of operations, business requirements, regulatory and legal constraints and any other factors the Board of Directors deems relevant. Refer to Footnote 16 – Subsequent Events. NIC may pay dividends to our Parent Company out of its statutory earned surplus pursuant to statutory restrictions imposed under the New York insurance law. As of December 31, 2017, the maximum amount available for the payment of dividends by NIC without prior regulatory approval is $105.7 million. NIC paid a dividend to our Parent Company of $19.0 million and $5.0 million in 2017 and 2016, respectively. NCUL may pay dividends to our Parent Company up to the extent of available profits that have been distributed from the Syndicate. The Syndicate’s capital and surplus as filed with Lloyd’s consists of undistributed profits on closed and open UWYs. In connection with the business plan approved in November 2017, NCUL posted all of the available undistributed profits on closed years of $218.3 million to support a portion of the FAL requirement and therefore that amount is not available for distribution to NCUL, which ultimately is not available to our Parent Company in the form of a dividend. As of December 31, 2017, NCUL does not have the ability to pay dividends to the Parent Company. NIIC may pay dividends to our Parent Company out of its statutory profits subject to the restrictions imposed under UK Company law and Insurance regulation. As of December 31, 2017, the maximum amount available for the payment of dividends by NIIC without prior regulatory approval is $8.4 million. The amount and nature of net assets that are restricted from payment of dividends as of December 31, 2017 and 2016 are presented in the following table: As of December 31, amounts in thousands 2017 2016 Restricted Net Assets: NIC and NSIC: Fixed Maturities at Fair Value (Amortized Cost: 2017, $9,856; 2016, $9,820) $ 10,655 $ 10,690 Short Term Investments, at Fair Value 290 290 Cash 1,242 7,168 Total NIC and NSIC (1) $ 12,187 $ 18,148 NHUK: Fixed Maturities at Fair Value (Amortized Cost: 2017, $472,701; 2016, $457,225) $ 471,831 $ 452,053 Short Term Investments, at Fair Value 83,778 63,729 Total NHUK (2) $ 555,609 $ 515,782 Total Restricted Net Assets $ 567,796 $ 533,930 (1) - The restricted net assets for NIC and NSIC primarily consist of fixed maturities on deposit with various state insurance departments. The cash as of December 31, 2017 and 2016, as presented in the table above, was on deposit with a U.K. bank to comply with the regulatory requirements of the Prudential Regulation Authority for the underwriting activities of the U.K. Branch. (2) - The restricted net assets for NHUK consists of fixed maturities and cash held in trust for the benefit of syndicate policyholders and short term investments primarily consisting of overseas deposits in various countries with Lloyd's to support underwriting activities in those countries.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Share	NOTE 11. EARNINGS PER SHARE The following is a reconciliation of the basic and diluted EPS computations for the years ended December 31, 2017, 2016 and 2015: Years Ended December 31, amounts in thousands, except per share amounts 2017 2016 2015 Net Income $ 40,494 $ 82,726 $ 81,057 Basic Weighted Average Shares 29,441 29,074 28,785 Effect of Common Stock Equivalents: Assumed Exercise of Stock Options and Vesting of Stock 630 958 866 Diluted Weighted Average Shares 30,071 30,032 29,651 Net Income per Common Share: Basic $ 1.38 $ 2.85 $ 2.82 Diluted $ 1.35 $ 2.75 $ 2.73

Statutory Financial Information

Statutory Financial Information	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Statutory Financial Information	NOTE 12. STATUTORY FINANCIAL INFORMATION The following table presents statutory Net Income and capital and surplus in accordance with statutory accounting practices: Years Ended December 31, amounts in millions 2017 2016 2015 NIC & NSIC: Statutory Net Income $ 79.3 $ 84.2 $ 60.8 Statutory Capital and Surplus $ 1,056.6 $ 1,027.3 $ 949.1 The Syndicate: Syndicate's Net Income $ (23.4 ) $ 46.3 $ 41.0 Syndicate's Capital and Surplus $ 168.7 $ 180.7 $ 177.5 NIIC: Statutory Net Income $ (8.4 ) $ 0.4 $ — Statutory Capital and Surplus $ 62.9 $ 70.7 $ — Our insurance subsidiaries file financial statements prepared in accordance with statutory accounting practices prescribed or permitted by domestic and foreign insurance regulatory authorities. The differences between statutory financial statements and financial statements prepared in accordance with U.S. GAAP vary between domestic and foreign jurisdictions. For NIC and NSIC, the National Association of Insurance Commissioners (“NAIC”) has codified Statutory Accounting Practices and Procedures (“SAP”) for insurance enterprises. We prepare our statutory basis financial statements in accordance with the most recently updated NAIC SAP manual subject to any deviations prescribed or permitted by the New York Insurance Commissioner. The following table represents some of the significant differences between SAP and U.S. GAAP as they relate to our operations: Differences SAP U.S. GAAP Acquisition and Commission Costs Expensed when incurred Costs are generally deferred Bonds Generally stated at amortized cost Stated at fair value Deferred tax assets Certain temporary differences are not recognized All temporary differences recognized Receivables over 90 days outstanding and other intangible assets Not recognized Generally recognized (subject to valuation allowances) Provision for Reinsurance Recorded through a charge to surplus Recorded through income when deemed uncollectible Unearned premiums and loss reserves Net of ceded amounts Gross of ceded amounts The Syndicate is subject to oversight by the Council of Lloyd’s. Lloyd’s as a whole is authorized and regulated by the PRA. Our other international businesses are also regulated by the PRA. The following table represents some of the significant differences between U.K. GAAP and U.S. GAAP as they relate to our operations: Differences U.K. GAAP U.S. GAAP Unrealized gains/losses Recognized in income Recognized in AOCI Foreign exchange gains/losses on translation Recognized in income Recognized in AOCI Lloyd’s membership costs Expensed when incurred Amortized over each UWY Refer to Note 1, Organization and Summary of Significant Accounting Policies For NIC and NSIC, aggregate minimum required statutory capital and surplus is based on the greater of the risk-based capital level that would trigger regulatory action or minimum requirements per state insurance regulation. Capital and surplus requirements of our foreign subsidiaries differ from those prescribed in the U.S. and vary by jurisdiction. The capital requirement of the Syndicate, known as FAL, is currently calculated using the internal Lloyd’s risk-based capital model. The FAL may be comprised of cash, investments and undrawn letters of credit provided by various banks. Lloyd’s sets the corporate member’s required capital annually based on the Syndicate business plans, rating environment, reserving environment and input arising from Lloyd’s discussions with regulatory and rating agencies. NIIC is subject to oversight and regulation by the PRA and the FCA. The following table represents some of the significant differences between U.K. GAAP and U.S. GAAP as they relate to our NIIC operations: Differences U.K. GAAP U.S. GAAP Unrealized gains/losses Recognized in income Recognized in AOCI Foreign exchange gains/losses on translation Recognized in income Recognized in AOCI As of December 31, 2017 and 2016, all insurance subsidiaries individually exceed the minimum required statutory capital and surplus requirements and all U.S. domestic insurance subsidiaries individually exceeded risk-based capital minimum requirements.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	NOTE 13. STOCK-BASED COMPENSATION In May 2005 our shareholders approved the 2005 Stock Incentive Plan authorizing the issuance in aggregate of 1,000,000 incentive stock options, non-incentive stock options, restricted shares and stock appreciation rights for our common stock. In April 2009, the stockholders approved an amendment to the 2005 Stock Incentive Plan increasing the available number of awards from 1,000,000 to 1,500,000. In April 2013, the stockholders further amended and restated the 2005 Stock Incentive Plan increasing the available number of awards from 1,500,000 to 2,000,000 in the Second Amended and Restated 2005 Stock Incentive Plan. The stock split that occurred on January 20, 2017 doubled the number of awards from 2,000,000 to 4,000,000. As of December 31, 2017, 3,156,664 of such awards were issued leaving 843,336 awards available to be issued in subsequent periods. Stock-based compensation granted under our Company’s stock plans is expensed in tranches over the vesting period. Options and non-performance based grants generally vest equally over a three or four year period and the options had a maximum term of ten years. Our Company’s performance based share grants generally consist of three types of awards. The performance units issued in 2017, 2016 and 2015 will cliff vest on the third anniversary of the date of the grant in an amount as determined by the rate of cumulative annual growth in tangible book value for the three years immediately prior to the vesting date, with actual shares that vest ranging between 150% to 50% of that portion of the original award. During 2017, 2016 and 2015, the total compensation cost for share-based payment arrangements recognized in the income statement was $13.8 million, $17.6 million and $15.0 million, and the recorded tax benefits related to share-based awards were $2.9 million, $6.2 million and $5.2 million, respectively. As of December 31, 2017 and 2016, the total unrecognized compensation expense related to nonvested stock awards was $18.0 million and $22.3 million, respectively, which is expected to be recognized as expense over weighted average periods of 1.9 years and 1.8 years, respectively. The aggregate fair value of all unvested restricted stock units as of December 31, 2017 and 2016 was $49.9 million and $87.2 million, respectively. The aggregate fair value did not change after the stock split, as shares were doubled and the price per share halved. Restricted Stock Units The activity related to our Company’s restricted stock unit awards was as follows: Year Ended December 31, 2017 Number of Awards Weighted Average Grant Date Fair Value (1) Nonvested at the beginning of the year 197,422 $ 37.24 Granted 35,297 $ 56.05 Vested (59,930 ) $ 36.21 Forfeited (13,250 ) $ 39.41 Nonvested at the end of the year 159,539 $ 41.60 ( 1) Fair value is based on the closing price of our common shares on the NASDAQ on the grant date . During 2017, 2016 and 2015, we granted 35,297, 84,000 and 64,316 restricted stock units, respectively, with weighted average grant-date fair values per share of $56.05, $44.52 and $38.72, respectively. During 2017, 2016 and 2015, the total fair value of restricted stock units vested was $3.3 million, $2.5 million and $2.6 million, respectively. Performance-based Equity Awards The activity related to our Company’s performance-based equity awards was as follows: Year Ended December 31, 2017 Number of Awards Weighted Average Grant Date Fair Value (1) Nonvested at the beginning of the year 1,176,742 $ 35.85 Granted 253,333 $ 55.70 Performance Adjustment 83,071 $ 30.49 Vested (520,285 ) $ 30.38 Forfeited (26,500 ) $ 41.60 Nonvested at the end of the year 966,361 $ 43.39 ( 1) Fair value is based on the closing price of our common shares on the NASDAQ on the grant date . During 2017, 2016 and 2015, we granted 253,333, 404,946 and 370,100 performance-based equity awards, respectively, with weighted average grant-date fair values per share of $55.70, $40.77 and $37.25, respectively. During 2017, 2016 and 2015, the total fair value of performance-based equity awards vested was $29.0 million, $11.5 million and $12.5 million, respectively. Stock Options During 2017, 2016 and 2015, our Company did not grant any stock options. There are currently no stock options outstanding with the last exercise of stock options occurring during 2015. Employee Stock Purchase Plan We offer an Employee Stock Purchase Plan (the “ESPP”) to all of our eligible employees. Employees are offered the opportunity to purchase our Company’s Common stock at 90% of fair market value at the lower of the price at the beginning or the end of each six month offering period. Each employee can invest up to 10% of their base compensation subject to the lesser of 1,000 common stock shares or total market value of $25,000. There were 40,488 shares purchased in 2017 from funds withheld during the July 1, 2016 to December 31, 2016 and January 1, 2017 to June 30, 2017 offering periods. There were 41,224 shares purchased in 2016 in the aggregate from funds withheld during the offering periods of July 1, 2015 to December 31, 2015 and January 1, 2016 to June 30, 2016. We expense both the value of the 10% discount and the “look-back” option, which provides for the more favorable price at either the beginning or end of the offering period.

Retirement Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Retirement Plans	NOTE 14. RETIREMENT PLANS We have a 401(k) Plan for all U.S. eligible employees. Each eligible employee can contribute a portion of their salary, limited by certain Federal regulations. The expense recorded for all contributions to the 401(k) Plan for the year ended December 31, 2017 was $7.5 million, consisting of $4.9 million for the retirement savings contributions and $2.6 million for the non-discretionary matching contributions. The expense recorded for all contributions to the 401(k) Plan for the year ended December 31, 2016 was $7.4 million, consisting of $4.2 million for the retirement savings contributions and $3.2 million for the non-discretionary matching contributions. The expense recorded for all contributions to the 401(k) Plan for the year ended December 31, 2015 was $6.3 million, consisting of $3.8 million for the retirement savings contributions and $2.5 million for the non-discretionary matching contributions. Our Company sponsors a defined contribution plan for all of our Company’s U.K. employees under U.K. regulations. Contributions, which are fully vested when made, are equal to 15.0% of each eligible employee’s gross base salary for all U.K. employees hired prior to November 2014 and 12.0% for all employees hired after November 2014. The expense recorded for the U.K. defined contribution plan was $2.6 million, $2.9 million and $2.9 million for the years ended December 31, 2017, 2016 and 2015, respectively. Our Company sponsors defined contribution plans for employees in several of our other European offices, outside of the U.K., under each countries’ regulations. Contributions, which are fully vested when made, range by European office between 2.1% and 26.0% of each eligible employee’s gross base salary. The expense recorded for the other European offices defined contribution plans was $0.3 million, $0.5 million and $0.5 million for the years ended December 31, 2017, 2016 and 2015, respectively. Such expenses are included in Other operating expenses.

Condensed Quarterly Financial D

Condensed Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Condensed Quarterly Financial Data (Unaudited)	NOTE 15. CONDENSED QUARTERLY FINANCIAL DATA (Unaudited) The following is a summary of quarterly financial data for the periods indicated: March 31, June 30, September December 31, amounts in thousands, except per share amounts 2017 2017 2017 2017 Gross Written Premiums $ 450,305 $ 452,179 $ 402,038 $ 408,743 Revenues: Net Written Premiums $ 337,163 $ 333,282 $ 296,016 $ 304,869 Change in Unearned Premiums (51,032 ) (39,447 ) 5,339 230 Net Earned Premiums $ 286,131 $ 293,835 $ 301,355 $ 305,099 Net Investment Income 21,448 22,265 22,598 22,982 Total Other-Than-Temporary Impairment Losses (1,077 ) 29 (957 ) 3 Portion of Loss Recognized in Other Comprehensive Income (Before Tax) (16 ) (29 ) (15 ) (3 ) Net Other-Than-Temporary Impairment Losses Recognized in Earnings (1,093 ) — (972 ) — Net Realized Gains 1,049 1,694 5,190 37,141 Other Income (Loss) 1,068 (411 ) (1,699 ) (3,201 ) Total Revenues $ 308,603 $ 317,383 $ 326,472 $ 362,021 Expenses: Net Losses and LAE $ 169,600 $ 177,110 $ 276,171 $ 183,384 Commission Expenses 47,844 48,173 45,509 43,205 Other Operating Expenses 58,538 60,766 45,773 68,153 Interest Expense 3,861 3,861 3,862 3,863 Total Expenses $ 279,843 $ 289,910 $ 371,315 $ 298,605 Income (Loss) Before Income Taxes 28,760 27,473 (44,843 ) 63,416 Income Tax Expense (Benefit) $ 7,650 $ 6,971 $ (16,864 ) $ 36,555 Net Income (Loss) $ 21,110 $ 20,502 $ (27,979 ) $ 26,861 Comprehensive Income (Loss) $ 35,911 $ 35,699 $ (18,311 ) $ (1,812 ) Combined Ratio 96.4 % 97.3 % 121.9 % 96.6 % Net Income (Loss) per Share: Basic $ 0.72 $ 0.70 $ (0.95 ) $ 0.91 Diluted $ 0.70 $ 0.69 $ (0.95 ) $ 0.89 March 31, June 30, September 30, December 31, amounts in thousands, except per share amounts 2016 2016 2016 2016 Gross Written Premiums $ 413,877 $ 412,565 $ 374,930 $ 367,539 Revenues: Net Written Premiums $ 319,820 $ 306,535 $ 277,001 $ 282,868 Change in Unearned Premiums (55,462 ) (38,543 ) 7,009 1,117 Net Earned Premiums $ 264,358 $ 267,992 $ 284,010 $ 283,985 Net Investment Income 19,594 19,875 19,875 20,107 Total Other-Than-Temporary Impairment Losses (109 ) (162 ) 23 21 Portion of Loss Recognized in Other Comprehensive Income (Before Tax) 109 12 (23 ) (21 ) Net Other-Than-Temporary Impairment Losses Recognized in Earnings — (150 ) — — Net Realized Gains 1,597 1,960 1,586 4,043 Other Income (Loss) 2,549 4,430 (183 ) 1,905 Total Revenues $ 288,098 $ 294,107 $ 305,288 $ 310,040 Expenses: Net Losses and LAE $ 152,956 $ 167,206 $ 172,793 $ 172,493 Commission Expenses 37,554 40,726 42,611 44,154 Other Operating Expenses 60,809 59,074 56,137 58,076 Interest Expense 3,858 3,858 3,859 3,860 Total Expenses $ 255,177 $ 270,864 $ 275,400 $ 278,583 Income Before Income Taxes 32,921 23,243 29,888 31,457 Income Tax Expense $ 9,989 $ 7,053 $ 7,875 $ 9,866 Net Income $ 22,932 $ 16,190 $ 22,013 $ 21,591 Comprehensive Income (Loss) $ 45,721 $ 35,228 $ 18,760 $ (31,712 ) Combined Ratio 95.1 % 99.6 % 95.6 % 96.7 % Net Income per Share: Basic $ 0.79 $ 0.56 $ 0.76 $ 0.74 Diluted $ 0.77 $ 0.54 $ 0.73 $ 0.71

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	NOTE 16. SUBSEQUENT EVENTS On February 15, 2018, our Board of Directors declared a cash dividend on our Company’s Common Stock of $0.07 per share, payable on March 23, 2018 to stockholders of record on March 2, 2018. On February 7, 2018, NMUK entered into a Renewal Rights Agreement with Thomas Miller Specialty Underwriting Agency Limited (“Thomas Miller”), pursuant to which Thomas Miller agreed to acquire the renewal rights to our Company’s fixed-premium protection and indemnity business. Our Company will remain responsible for all losses occurring prior to February 8, 2018 on such business. We do not expect this transaction to have a material effect on our financial statements.

Summary of Consolidated Investm

Summary of Consolidated Investments-Other Than Investments in Related Parties	12 Months Ended
	Dec. 31, 2017
Summary Of Investments Other Than Investments In Related Parties [Abstract]
Summary of Consolidated Investments-Other Than Investments in Related Parties	THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Summary of Consolidated Investments - Other than Investments in Related Parties December 31, 2017 Gross Gross Unrealized Unrealized Amortized amounts in thousands Fair Value (1) Gains Losses Cost Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 393,563 $ 2,081 $ (2,014 ) $ 393,496 States, Municipalities and Political Subdivisions 814,632 20,136 (1,423 ) 795,919 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities 407,619 2,352 (5,414 ) 410,681 Residential Mortgage Obligations 54,104 606 (79 ) 53,577 Asset-Backed Securities 328,753 2,138 (663 ) 327,278 Commercial Mortgage-Backed Securities 160,904 2,354 (1,182 ) 159,732 Subtotal $ 951,380 $ 7,450 $ (7,338 ) $ 951,268 Corporate Exposures (2) 897,479 14,491 (3,737 ) 886,725 Total Fixed Maturities $ 3,057,054 $ 44,158 $ (14,512 ) $ 3,027,408 Equity Securities: Common Stocks 52,439 7,423 (112 ) 45,128 Preferred Stocks 183,542 6,071 (1,560 ) 179,031 Total Equity Securities $ 235,981 $ 13,494 $ (1,672 ) $ 224,159 Other Invested Assets 1,720 — — 1,720 Short-Term Investments 127,128 3 — 127,125 Total Investments $ 3,421,883 $ 57,655 $ (16,184 ) $ 3,380,412 (1) Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value. (2) Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.

Condensed Financial Information

Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]
Condensed Financial Information of Registrant	THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Condensed Financial Information of Registrant The Navigators Group, Inc. Balance Sheets (Parent Company) December 31, amounts in thousands 2017 2016 ASSETS Investments $ 1,668 $ 1,660 Cash 7,567 11,003 Investments in Subsidiaries 1,423,843 1,381,652 Goodwill and Other Intangible Assets 2,534 2,534 Current Income Tax Receivable, Net 30,573 13,160 Other Assets 28,063 36,428 Total Assets $ 1,494,248 $ 1,446,437 LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities: Senior Notes $ 263,885 $ 263,728 Accounts Payable and Other Liabilities 1,224 1,347 Accrued Interest Payable 3,174 3,174 Total Liabilities $ 268,283 $ 268,249 Stockholders' Equity: Preferred Stock, $.10 par value, authorized 1,000 shares, none issued $ — $ — Common Stock, $.10 par value, authorized 50,000 shares, issued 36,530 shares for 2017 and 36,147 shares for 2016 3,650 3,612 Additional Paid-In Capital 376,868 373,983 Treasury Stock, at cost (7,023 shares for 2017 and 2016) (155,801 ) (155,801 ) Retained Earnings 981,380 947,519 Accumulated Other Comprehensive Income: Net Unrealized Gains (Losses) on Securities Available-for-Sale, Net of Tax 19,874 8,882 Foreign Currency Translation Adjustment, Net of Tax (6 ) (7 ) Total Stockholders' Equity $ 1,225,965 $ 1,178,188 Total Liabilities and Stockholders' Equity $ 1,494,248 $ 1,446,437 SCHEDULE II THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Condensed Financial Information of Registrant (Continued) The Navigators Group, Inc. Statements of Income (Parent Company) Years Ended December 31, amounts in thousands 2017 2016 2015 Revenues: Net Investment Income $ 7 $ 53 $ 147 Total Revenues $ 7 $ 53 $ 147 Expenses: Interest Expense 15,447 15,435 15,424 Other (Income) Expense (28 ) 22 — Total Expenses $ 15,419 $ 15,457 $ 15,424 Income (Loss) Before Income Tax Benefit $ (15,412 ) $ (15,404 ) $ (15,277 ) Income Tax Benefit (5,709 ) (8,009 ) (5,472 ) Income (Loss) Before Equity in Undistributed Net Income of Wholly Owned Subsidiaries $ (9,703 ) $ (7,395 ) $ (9,805 ) Equity in Undistributed Net Income of Wholly-Owned Subsidiaries 50,197 90,121 90,862 Net Income $ 40,494 $ 82,726 $ 81,057 SCHEDULE II THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Condensed Financial Information of Registrant (Continued) The Navigators Group, Inc. Statements of Cash Flows (Parent Company) Years Ended December 31, amounts in thousands 2017 2016 2015 Operating Activities: Net Income $ 40,494 $ 82,726 $ 81,057 Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations: Equity in Undistributed Net Income of Wholly-Owned Subsidiaries (50,197 ) (90,121 ) (90,862 ) Dividends Received from Subsidiaries 19,000 5,000 — Other 5,841 5,737 9,542 Net Cash Provided By (Used in) Operating Activities $ 15,138 $ 3,342 $ (263 ) Investing Activities: Fixed Maturities, Available-for-Sale Sales $ — $ — $ 1,250 Purchases — — — Equity Securities Sales — — — Purchases — — — Net (Increase) Decrease in Short-Term Investments (8 ) 87,942 3,918 Net Cash Provided By (Used in) Investing Activities $ (8 ) $ 87,942 $ 5,168 Financing Activities: Capital Contribution to Subsidiary $ (1 ) $ (79,250 ) $ — Proceeds of Stock Issued from Employee Stock Purchase Plan 1,846 1,840 1,352 Proceeds of Stock Issued from Exercise of Stock Options — — 29 Dividends Paid (6,633 ) (3,930 ) — Payment of Employee Tax Withholding on Stock Compensation (13,778 ) (5,358 ) (6,169 ) Net cash Provided By (Used in) Financing Activities $ (18,566 ) $ (86,698 ) $ (4,788 ) Increase (Decrease) in Cash $ (3,436 ) $ 4,586 $ 117 Cash at Beginning of Year 11,003 6,417 6,300 Cash at End of Year $ 7,567 $ 11,003 $ 6,417

Supplementary Insurance Informa

Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
Supplementary Insurance Information	SCHEDULE III THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Supplementary Insurance Information Amortization Deferred Other Policy of Deferred Policy Reserve Claims and Net Net Losses Policy Other Net Acquisition for Losses Unearned Benefits Earned Investment and LAE Acquisition Operating Written amounts in thousands Costs and LAE Premiums Payable Premiums Income (1) Incurred Costs Expenses Premiums Year ended December 31, 2017 U.S. Insurance $ 54,674 $ 1,629,015 $ 575,068 $ — $ 674,665 $ — $ 443,353 $ 77,729 $ 128,905 $ 721,300 Int'l Insurance 38,988 716,352 260,982 — 333,792 — 229,601 68,824 83,464 335,852 GlobalRe 41,587 169,778 151,631 — 177,963 — 133,311 39,136 20,861 214,178 Corporate — — — — — 89,293 — (958 ) — — $ 135,249 $ 2,515,145 $ 987,681 $ — $ 1,186,420 $ 89,293 $ 806,265 $ 184,731 $ 233,230 $ 1,271,330 Year ended December 31, 2016 U.S. Insurance $ 50,028 $ 1,564,524 $ 519,505 $ — $ 629,308 $ — $ 397,860 $ 70,812 $ 128,108 $ 683,568 Int'l Insurance 37,385 605,105 252,442 — 307,416 — 178,284 61,703 86,395 345,967 GlobalRe 32,247 120,098 115,397 — 163,621 — 89,304 34,008 19,593 156,689 Corporate — — — — — 79,451 — (1,478 ) — — $ 119,660 $ 2,289,727 $ 887,344 $ — $ 1,100,345 $ 79,451 $ 665,448 $ 165,045 $ 234,096 $ 1,186,224 Year ended December 31, 2015 U.S. Insurance $ 37,372 $ 1,478,391 $ 476,803 $ — $ 555,836 $ — $ 343,497 $ 56,319 $ 131,407 $ 597,006 Int'l Insurance 25,451 608,783 221,544 — 259,960 — 134,702 43,676 75,867 277,942 GlobalRe 29,160 115,470 122,329 — 168,291 — 94,399 32,240 16,242 168,912 Corporate — — — — — 68,718 — (2,258 ) — — $ 91,983 $ 2,202,644 $ 820,676 $ — $ 984,087 $ 68,718 $ 572,598 $ 129,977 $ 223,516 $ 1,043,860 (1) As we evaluate the underwriting results of each of our reportable segments separately from the results of our investment portfolio, we do not allocate net investment income to our reportable segments.

Reinsurance - Written Premium

Reinsurance - Written Premium	12 Months Ended
	Dec. 31, 2017
Supplemental Schedule Of Reinsurance Premiums For Insurance Companies [Abstract]
Reinsurance - Written Premium	SCHEDULE IV THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Reinsurance - Written Premium Ceded to Assumed Percentage Direct Other from Other Net of Amount amounts in thousands Amount Companies Companies Amount Assumed Year ended December 31, 2017 Accident & Health $ — $ — $ 80,605 $ 80,605 100 % Property & Liability 1,489,422 441,935 143,238 1,190,725 12 % Total $ 1,489,422 $ 441,935 $ 223,843 $ 1,271,330 18 % Year ended December 31, 2016 Accident & Health $ — $ — $ 53,814 $ 53,814 100 % Property & Liability 1,403,865 382,687 111,232 1,132,410 10 % Total $ 1,403,865 $ 382,687 $ 165,046 $ 1,186,224 14 % Year ended December 31, 2015 Accident & Health $ — $ — $ 87,088 $ 87,088 100 % Property & Liability 1,277,728 409,642 88,686 956,772 9 % Total $ 1,277,728 $ 409,642 $ 175,774 $ 1,043,860 17 %

Valuation and Qualifying Accoun

Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation And Qualifying Accounts [Abstract]
Valuation and Qualifying Accounts	SCHEDULE V THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Valuation and Qualifying Accounts Balance at Charged (Credited) to Charged to Deductions Balance at amounts in thousands January 1 Costs and Expenses Other Accounts (Describe) December 31 Year ended December 31, 2017 Allowance for Uncollectable Reinsurance $ 12,078 $ 519 $ — $ — $ 12,597 Valuation Allowance in Deferred Taxes $ 699 $ 1,099 $ — $ — $ 1,798 Year ended December 31, 2016 Allowance for Uncollectable Reinsurance $ 6,921 $ 5,157 $ — $ — $ 12,078 Valuation Allowance in Deferred Taxes $ 721 $ (22 ) $ — $ — $ 699 Year ended December 31, 2015 Allowance for Uncollectable Reinsurance $ 11,332 $ (4,411 ) $ — $ — $ 6,921 Valuation Allowance in Deferred Taxes $ 776 $ (55 ) $ — $ — $ 721

Supplementary Information Conce

Supplementary Information Concerning Property-Casualty Insurance Operations	12 Months Ended
	Dec. 31, 2017
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
Supplementary Information Concerning Property-Casualty Insurance Operations	SCHEDULE VI THE NAVIGATORS GROUP, INC. AND SUBSIDIARIES Supplementary Information Concerning P&C Insurance Operations Losses and LAE Amortization Deferred Policy Reserve Net Net Expenses Incurred Related to of Deferred Policy Other Net amounts in thousands Acquisition for Losses Unearned Earned Investment Current Prior Acquisition Operating Written Affiliation with Registrant Costs and LAE Premiums Premiums Income Year Years Costs Expenses Premiums Consolidated Subsidiaries: Year ended December 31, $ 135,249 $ 2,515,145 $ 987,681 $ 1,186,420 $ 89,293 $ 771,955 $ 34,310 $ 184,731 $ 233,230 $ 1,271,330 Year ended December 31, 2016 $ 119,660 $ 2,289,727 $ 887,344 $ 1,100,345 $ 79,451 $ 693,976 $ (28,528 ) $ 165,045 $ 234,096 $ 1,186,224 Year ended December 31, 2015 $ 91,983 $ 2,202,644 $ 820,676 $ 984,087 $ 68,718 $ 637,267 $ (64,669 ) $ 129,977 $ 223,516 $ 1,043,860

Organization and Summary of S32

Organization and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Unless the context requires otherwise, the terms “we,” “us,” “our,” or “our Company” are used to mean The Navigators Group, Inc., a Delaware holding company established in 1982, and its subsidiaries. The term “Parent Company” is used to mean The Navigators Group, Inc. without its subsidiaries. The accompanying consolidated financial statements of our Company have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP” or “U.S. GAAP”) and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”). All significant intercompany transactions and balances have been eliminated in consolidation. The preparation of these financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported revenues and expenses during the reporting periods. Certain amounts for the prior year have been reclassified to conform with the current period presentation.
Organization	Organization We are an international insurance company with a long-standing area of specialization in Marine insurance. We also offer Property and Casualty (“P&C”) insurance, which consists primarily of general liability coverage and umbrella & excess liability coverage to commercial enterprises through our Primary and Excess Casualty divisions. We have also developed niches in Professional Liability insurance, through our Directors & Officers (“D&O”) and Errors & Omissions (“E&O”) divisions, as well as assumed reinsurance products. We operate through various wholly-owned insurance and service companies. Our subsidiaries domiciled in the United States (“U.S.”) include two insurance companies, Navigators Insurance Company (“NIC”) and Navigators Specialty Insurance Company (“NSIC”), as well as our U.S. underwriting agency, Navigators Management Company, Inc. (“NMC”). NIC includes a branch in the United Kingdom (“U.K.”). We also have operations domiciled in the U.K., Hong Kong and Europe. Navigators International Insurance Company Ltd. (“NIIC”), Navigators Management (U.K.) Ltd. (“NMUK”) and Navigators Underwriting Ltd. (“NUL”) are domiciled in the U.K. and NUL includes European branches. Navigators Underwriting Agency Ltd. (“NUAL”), a Lloyd’s of London (“Lloyd’s”) underwriting agency, manages and provides the capital, through Navigators Corporate Underwriters Ltd. (“NCUL”), for our Lloyd’s Syndicate 1221 (“the Syndicate”), and is also domiciled in the U.K. We control 100% of the Syndicate’s stamp capacity. Effective January 1, 2017, we sold our underwriting agency operations in Sweden and Denmark. The transaction represented a 100% disposition of our Sweden and Denmark corporations, NUAL AB and Navigators A/S, respectively. This transaction did not have a material impact on our financial statements.
Cash	Cash Cash includes cash on hand and demand deposits with banks, excluding such amounts held by the Syndicate included as Funds at Lloyd’s (“FAL”), which are classified as Short-Term Investments.
Investments	Investments As of December 31, 2017 and 2016, all Fixed Maturity and Equity Securities held by our Company were carried at fair value and classified as available-for-sale. Available-for-sale securities are debt and equity securities not classified as either held-to-maturity securities or trading securities and are reported at fair value, with unrealized gains and losses excluded from earnings and reported in Accumulated Other Comprehensive Income (“AOCI”) as a separate component of Stockholders’ Equity. Fixed Maturity securities include bonds, mortgage-backed and asset-backed securities, and redeemable preferred stocks. Equity Securities consist of common stock, exchange traded funds and preferred stock. Other Invested Assets consist of investments our Company made in certain strategic companies which are accounted for using the equity method of accounting. In applying the equity method, these investments are initially recorded at cost and are subsequently adjusted based on our Company’s proportionate share of the net income or loss of the companies. Changes in the carrying value of such investments are recorded in Other Income. In applying the equity method, we use the most recently available financial information provided by the companies which is generally three months prior to the end of the reporting period. Short-Term Investments are carried at fair value. Short-Term Investments mature within one year from the purchase date. All prices for our Fixed Maturities, Equity Securities and Short-Term Investments are classified as Level 1, Level 2 or Level 3 under the fair value hierarchy, as defined in the Financial Accounting Standards Board (“FASB”) Accounting Standards Codification 820 (“ASC 820”). Premiums and discounts on Fixed Maturity securities are amortized into interest income over the life of the security using the interest method. For Mortgage-Backed and Asset-Backed Securities, anticipated prepayments and expected maturities are utilized in applying the interest rate method. An effective yield is calculated based on projected principal cash flows at the time of original purchase. The effective yield is used to amortize the purchase price of the security over the security’s expected life. Book values are adjusted to reflect the amortization of premium or accretion of discount on a monthly basis. The projected principal cash flows are based on certain prepayment assumptions, which are generated using a prepayment model. The prepayment model uses a number of factors to estimate prepayment activity including the current levels of interest rates (refinancing incentive), time of year (seasonality), economic activity (including housing turnover) and term and age of the underlying collateral (burnout, seasoning). Prepayment assumptions associated with the Mortgage-Backed and Asset-Backed Securities are reviewed on a periodic basis. When changes in prepayment assumptions are deemed necessary as the result of actual prepayments differing from anticipated prepayments, securities are revalued based upon the new prepayment assumptions utilizing the retrospective adjustment method, whereby the effective yield is recalculated to reflect actual payments to date and anticipated future payments. The investment in such securities is adjusted to the amount that would have existed had the new effective yield been applied since the acquisition of the security. Such adjustments, if any, are included in Net Investment Income for the current period. Realized Gains and Losses on sales of investments are recognized when the related trades are executed and are determined on the basis of the specific identification method.
Impairment of Invested Assets	Impairment of Invested Assets Management regularly reviews our Fixed Maturity and Equity Securities portfolios to evaluate the necessity of recording impairment losses for other-than-temporary declines in the fair value of securities. Our Company reviews the magnitude of a security’s unrealized loss compared to its cost/amortized cost and the length of time that the security has been impaired to determine if an unrealized loss is other-than-temporary. If warranted as a result of conditions relating to a particular security, our Company will also review securities with declines in fair value resulting from a headline news event involving the issuer, a headline news event involving the asset class, the advice of our external asset managers, or economic events that may impact the issuer to determine if an unrealized loss is other-than-temporary. The depth of analysis performed is dependent upon the nature and magnitude of the indicators of other-than-temporary impairment present in regards to each impaired security. For Equity Securities, our Company performs a fundamental analysis of the issuer, including an evaluation of the mean analysts’ target price, to assess the likelihood of recovery of our cost basis in the security. Management also assesses the likelihood of future cash flows, dividends and increases to dividends, all of which affect the securities eligible for our equity strategy and therefore our intent to hold the security. If an equity security is deemed to be other-than temporarily-impaired, the cost is written down to fair value with the loss recognized in earnings. For Fixed Maturities, our Company assesses the underlying fundamentals of each issuer to determine if there is a change in the amount or timing of expected cash flows. Management compares the amortized cost basis to the present value of the revised cash flows using the historical book yield to determine the credit loss portion of impairment which is recognized in earnings. All non-credit losses where we have the intent and ability to hold the security until recovery are recognized as changes in Other than Temporary Impairment (“ OTTI”) losses within AOCI. Specifically for structured Fixed Maturities, our Company analyzes projections provided by our investment managers with respect to an expected principal loss under a range of scenarios and utilizes the most likely outcomes. The analysis relies on actual collateral performance measures such as default rate, prepayment rate and loss severity. These assumptions are applied throughout the remaining term of the deal, incorporating the transaction structure and priority of payments, to generate loss adjusted cash flows. Results of the analysis will indicate whether the security is expected ultimately to incur a loss or whether there is a material impact on yield due to either a projected loss or a change in cash flow timing. A break-even default rate is also calculated. A comparison of the break-even default rate to the actual default rate provides an indication of the level of cushion or coverage to the first dollar principal loss. For securities in which a tranche loss is present and the net present value of loss adjusted cash flows is less than book value, credit impairment is recognized in earnings. The output data also includes a number of additional metrics such as average life remaining, original and current credit support, over 60 day delinquency and security rating. The significant inputs used to measure the amount of credit loss recognized in earnings are actual delinquency rates, default probability, severity and prepayment assumptions. Projected losses are a function of both loss severity and probability of default, which differ based on property type, vintage and the stress of the collateral .
Foreign Currency Remeasurement and Translation	Foreign Currency Remeasurement and Translation The functional currency of each of our operations is generally the currency of the local operating environment, except for our Lloyd’s business which is United States Dollar (“USD”). Transactions in currencies other than an operation’s functional currency are remeasured into the functional currency and the resulting foreign exchange gains and losses are reflected in net Other Income (Expense) in the Consolidated Statements of Income, with the exception of those related to foreign-denominated available-for-sale investments. For these investments, exchange rate fluctuations represent an unrealized appreciation/depreciation in the value of the securities and are included in the related component of AOCI. Functional currency assets and liabilities of foreign operations are translated into USD using period end rates of exchange and the related translation adjustments are recorded as a separate component of AOCI. Consolidated Statements of Income amounts expressed in functional currencies are translated using average exchange rates.
Premium Revenues	Premium Revenues Written premium is based on the insurance policies that have been reported to us and the policies that have been written by agents but not yet reported to us. We estimate the amount of written premium not yet reported based on judgments relative to current and historical trends of the business being written. Such estimates are regularly reviewed and updated and any resulting adjustments are included in the current year’s results. An unearned premium reserve is established to reflect the unexpired portion of each policy at the financial reporting date. Substantially all of our business is placed through agents and brokers. We record estimates for both unreported direct and assumed premiums. We also record the ceded portion of the estimated Gross Written Premiums and related acquisition costs. These estimates are mostly for our Marine and Energy & Engineering products written by our International Insurance (“Int’l Insurance”) reporting segment as well as our GlobalRe reporting segment. Such premium estimates are generally based on submission data received from brokers and agents and recorded when the insurance policy or reinsurance contract is written or bound. The earned gross, ceded and net premiums are calculated based on our earning methodology, which is generally pro-rata over the policy period or over the period of risk if the period of risk differs significantly from the contract period.
Reinsurance Ceded	Reinsurance Ceded In the normal course of business, we purchase reinsurance from insurers or reinsurers to reduce the amount of loss arising from claims. Management analyzes the reinsurance agreements to determine whether the reinsurance should be classified as prospective or retroactive based upon the terms of the reinsurance agreement and whether the reinsurer has assumed significant insurance risk to the extent that the reinsurer may realize a significant loss from the transaction. Prospective reinsurance is reinsurance in which an assuming company agrees to reimburse the ceding company for losses that may be incurred as a result of future insurable events covered under contracts subject to the reinsurance. Retroactive reinsurance is reinsurance in which an assuming company agrees to reimburse a ceding company for liabilities incurred as a result of past insurable events covered under contracts subject to the reinsurance. Ceded reinsurance premiums net of ceding commissions and ceded losses are reflected as reductions of the respective income or expense accounts over the terms of the reinsurance contracts. Prepaid Reinsurance Premiums represent the portion of premiums ceded to reinsurers applicable to the unexpired terms of the reinsurance contracts in force. Reinsurance reinstatement premiums (“RRPs”) are recognized in the same period as the loss event that gave rise to the reinstatement premiums. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policy. Ceded Unearned Premiums and estimates of amounts recoverable from reinsurers on paid and unpaid losses are reflected as assets. Provisions are made for estimated unrecoverable reinsurance.
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs Costs of acquiring business are deferred and amortized over the period that the related premiums are recognized as revenue. Such costs (e.g., Commission Expenses, Other Underwriting Expenses and premium taxes) are limited to the incremental direct costs related to the successful acquisition of new or renewal business. The method of computing Deferred Policy Acquisition Costs limits the deferral to their estimated net realizable value based on the related Unearned Premiums and takes into account anticipated Losses, Loss Adjustment Expense (“LAE”), Commission Expenses and Operating Expenses based on historical and current experience, as well as anticipated Investment Income.
Reserves for Losses and Loss Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses Unpaid losses and LAE are determined on: (a) individual claims reported on direct business for insureds, (b) from reports received from ceding insurers for assumed business and (c) on estimates based on Company and industry experience for incurred but not reported (“IBNR”) claims and LAE. Indicated IBNR reserves for losses and LAE are calculated by our actuaries using several standard actuarial methodologies, including the paid and incurred loss development and the paid and incurred Bornhuetter-Ferguson loss methods. Frequency/severity analyses are performed for certain books of business. The Reserve for Losses and LAE has been established to cover the estimated unpaid cost of claims incurred. Such estimates are regularly compared to indicated reserves and updated and any resulting adjustments are included in the current year’s results. Management believes that the liability recognized for unpaid losses and LAE is a reasonable estimate of the ultimate unpaid claims incurred, however, no representation is made that the ultimate liability will not differ materially from the amounts recorded in the accompanying Consolidated Financial Statements. Losses and LAE are recorded on an undiscounted basis.
Earnings per Share	Earnings per Share Basic earnings per share (“EPS”) is computed by dividing Net income by the weighted average number of common shares outstanding for the period. Diluted EPS reflects the basic EPS adjusted for the potential dilution that would occur if all issued stock options were exercised and all stock grants were fully vested.
Share-Based Compensation	Share-Based Compensation Our Company applies a fair value based measurement method in accounting for its share-based payment arrangements with eligible employees and directors. The associated expense is estimated based on the fair value of the award at the grant date and is recognized in Net Income over the requisite service period with a corresponding increase in Shareholder’s Equity. Forfeitures of share-based payment awards are recognized as they occur.
Depreciation and Amortization	Depreciation and Amortization Depreciation of furniture and fixtures, electronic data processing equipment and computer software is provided over the estimated useful lives of the respective assets, ranging from three to seven years, using the straight-line method. Amortization of leasehold improvements are provided over the shorter of the useful lives of those improvements or the contractual terms of the leases, ranging from five to ten years, using the straight-line method.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill represents the excess of the cost of acquiring a business enterprise over the fair value of the net assets acquired. Our Company’s recorded indefinite lived intangible assets represent acquired stamp capacity in the Syndicate. Goodwill and indefinite lived intangible assets are reported at carrying value and are tested for impairment at least annually, and when permitted by the applicable accounting guidance, qualitative factors are assessed to determine whether it is necessary to calculate an asset’s fair value when testing an asset with an indefinite life for impairment. Goodwill and indefinite lived intangible assets are considered impaired if the estimated fair value is less than its carrying value and any impairment loss is measured as the difference between the implied fair value and the carrying value. Our Company did not recognize an impairment of goodwill or the indefinite lived intangible assets for any of the years ended December 31, 2017, 2016 and 2015. As of December 31, 2017, the carrying value of goodwill and indefinite lived intangible assets was $6.6 million, which is $0.1 million more than the carrying value as of December 31, 2016, $6.5 million. Changes in the carrying value of the goodwill and indefinite lived intangible assets are due to fluctuations in currency exchange rates between the USD and the GBP.
Income Taxes	Income Taxes We recognize deferred tax assets and liabilities based on the differences between the financial statement carrying amounts and the tax bases of assets and liabilities. Deferred tax assets are reduced by a valuation allowance if, based on the weight of available evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. All available evidence, both positive and negative, is considered to determine whether, based on the weight of that evidence, a valuation allowance is needed. Future realization of the tax benefit of an existing deferred tax asset ultimately depends on the existence of sufficient taxable income of the appropriate character within the carryback/carryforward period available under the tax law. In determining whether a valuation allowance is needed, management considers the timing of the reversal of each deferred tax asset as well as expected future levels of taxable income, amounts of taxable income in carryback years, and tax planning strategies. On December 22, 2017, the 2017 Tax Cuts and Jobs Act (the “Tax Act”) was enacted into law and the new legislation contains several key tax provisions that affected us, including a one-time mandatory Deemed Repatriation Transition Tax (“Transition Tax”) on accumulated foreign earnings and a reduction of the corporate income tax rate to 21% effective January 1, 2018, among others. We are required to recognize the effect of the tax law changes in the period of enactment, such as determining the Transition Tax, re-measuring our U.S. deferred tax assets and liabilities as well as reassessing the net realizability of our deferred tax assets and liabilities. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), Income Tax Accounting Implications of the Tax Act, which allows us to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. Since the Tax Act was passed late in the fourth quarter of 2017, and ongoing guidance and accounting interpretation is expected over the next 12 months, as of December 31, 2017, we have made a reasonable estimate of the effects on our deferred tax balances and in relation to the Transition Tax; however we will continue to gather additional information to more precisely compute these income tax impacts. We expect to complete our analysis within the measurement period in accordance with SAB 118. Additional information can be found in Note 9, Income Taxes
New Accounting Standards Adopted in 2017	New Accounting Standards Adopted in 2017 Share-Based Compensation Accounting Effective January 1, 2017, our Company adopted ASU 2016-09, “Improvements to Employee Share-Based Payment Accounting” issued by the FASB. This guidance requires all income tax effects of awards to be recognized in the income statement when the awards vest or are settled, eliminates the requirement that excess tax benefits be realized before companies can recognize them, requires the presentation of excess tax benefits as an operating activity on the statement of cash flows, allows employers to increase the amounts withheld to cover income taxes on share-based compensation awards without requiring liability classification, requires the presentation of employee taxes paid as a financing activity on the statement of cash flows, and requires companies to elect whether they will account for award forfeitures by recognizing forfeitures only as they occur or by estimating the number of awards expected to be forfeited. The requirement to recognize all income tax effects of awards in the income statement when the awards vest or are settled was adopted on a prospective basis. Previously, these amounts were recorded in Additional Paid-In Capital. This change also prospectively impacts the calculation of potential common shares used to determine Diluted Net Income per Common Share under 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 previously would have been recognized in Additional Paid-In Capital. We elected to account for forfeitures of share-based payment awards by recognizing forfeitures of awards as they occur, and upon adoption of this guidance, the payment of employee taxes was retrospectively adjusted on the Consolidated Statements of Cash Flows to be classified as a financing activity. All other changes in the ASU did not result in cumulative-effect or retrospective adjustments. Transition to Equity Method of Accounting Effective January 1, 2017, our Company adopted ASU 2016-07, “Simplifying the Transition to the Equity Method of Accounting” issued by the FASB. This guidance eliminates the requirement to retrospectively apply equity method accounting when an investment that had been accounted for by another method initially qualifies for the equity method. Our Company did not have any investments transitioning to the equity method of accounting during 2017. The adoption of this guidance did not impact our results of operations, financial condition or liquidity. Share-Based Compensation Modification Accounting During the second quarter of 2017, our Company early adopted ASU 2017-09, “Scope of Modification Accounting” issued by the FASB. This guidance clarifies when changes to the terms or conditions of a share-based award must be accounted for as modifications. Under the new guidance, modification accounting is required if the value, vesting conditions or classification of the award changes. This guidance will be applied prospectively to awards modified on or after the adoption date. The adoption of this guidance did not impact our results of operations, financial condition or liquidity.
Recently Issued Accounting Standards Not Yet Adopted	Recently Issued Accounting Standards Not Yet Adopted Revenue From Contracts With Customers In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers (Topic 606)”. This guidance affects any entity that either enters into contracts with customers to transfer goods or services or enters into contracts for the transfer of nonfinancial assets unless those contracts are within the scope of other standards (for example, insurance contracts are not in scope of the new guidance). The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In August 2015, the FASB delayed the effective date by one year through the issuance of ASU 2015-14, “Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date”. This guidance is effective for interim and annual reporting periods beginning after December 15, 2017. Early adoption was permitted for interim and annual reporting periods beginning after December 15, 2016. Substantially all of our revenue is not included in the scope of this guidance. We do generate an insignificant amount of fee income that is in the scope of this guidance, but will not be materially impacted by the adoption of this guidance. The adoption of this guidance will not have a material impact on our results of operations, financial condition or liquidity. Classification and Measurement of Financial Instruments In January 2016, the FASB issued ASU 2016-01 “Financial Instruments – Overall (Subtopic 825-10) – Recognition and Measurement of Financial Assets and Financial Liabilities” which requires equity investments (except those accounted for under the equity method of accounting, investments that are consolidated or those that meet a practicability exception) to be measured at fair value with changes in fair value recognized in net income, simplifies the impairment assessment of equity investments without readily determinable values by requiring a qualitative assessment to identify impairment, eliminates the requirement to disclose the methods and significant assumptions used to estimate the fair value for financial instruments measured at amortized cost, requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes, requires separate presentation in other comprehensive income of the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the organization has elected to measure the liabilities in accordance with the fair value option, requires the separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or the accompanying notes to the financial statements and clarifies that the reporting organization should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the organization’s other deferred tax assets. This guidance is effective for interim and annual reporting periods beginning after December 15, 2017 with early adoption permitted for certain of the amendments. The adoption of this guidance will result in a cumulative-effect adjustment of approximately $11.8 million relating to net unrealized gains in accumulated other comprehensive income for available-for-sale Equity Securities that will be measured at fair value with changes in fair value recognized in net income upon adoption of this guidance. The other aspects of this guidance impacting our Company relate to disclosures and will not impact our results of operations, financial condition or liquidity. Leases In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842)” which provides a new comprehensive model for lease accounting. The guidance will require a lessee to recognize a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. This guidance is effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. Our Company’s future minimum lease payments, which represent minimum annual rental commitments and will be subject to this new guidance, totaled $91.2 million at December 31, 2017. The calculation of the lease liability and right-of-use asset requires further analysis of the underlying leases. Adoption of this guidance will impact our Company’s consolidated balance sheets but is not expected to have a material impact on our Company’s results of operations, financial condition and liquidity. Our Company is still evaluating the impact of adopting this standard on our consolidated financial statements. Credit Losses In June 2016, the FASB issued ASU 2016-13, “Financial Instruments – Credit Losses (Topic 326) – Measurement of Credit Losses on Financial Instruments” which replaces the “incurred loss” impairment methodology with an approach based on “expected losses” to estimate credit losses on certain types of financial instruments and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The guidance requires financial assets measured at amortized cost to be presented at the net amount expected to be collected. The allowance for credit losses is a valuation account that is deducted from the amortized cost of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. Credit losses relating to available-for-sale debt securities will be recorded through an allowance for credit losses. The guidance also provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. This guidance is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted for interim and annual periods beginning after December 15, 2018. The application of this guidance is not expected to significantly impact our Company’s available-for-sale debt investment portfolio, but will impact our Company’s other financial assets, including our reinsurance recoverables and premium receivable. Upon adoption of this guidance, any impairment losses on our available-for-sale debt securities will be recorded as an allowance, subject to reversal, rather than as a reduction in amortized cost. Our Company is currently evaluating the impact of this guidance on our results of operations, financial condition and liquidity. Cash Flows In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230) – Classification of Certain Cash Receipts and Cash Payments” which addresses diversity in practice in how eight specific cash receipts and cash payments should be presented and classified on the statement of cash flows. This guidance is effective for interim and annual periods beginning after December 15, 2017, with early adoptions permitted. The adoption of this guidance is not expected to impact our results of operations, financial condition or liquidity. In November 2016, the FASB issued ASU 2016-18, “Statement of Cash Flows (Topic 230) – Restricted Cash” which addresses diversity in practice in the classification and presentation of changes in restricted cash on the statement of cash flows. This guidance will require a statement of cash flows to explain the change during the period in the total of cash, cash equivalents, restricted cash and restricted cash equivalents. Transfers between cash and cash equivalents and restricted cash and restricted cash equivalents will no longer be presented on the statement of cash flows. This guidance is effective for interim and annual periods beginning after December 15, 2017, with early adoption permitted. The guidance will be adopted on a retrospective basis. The adoption of this guidance is not expected to impact our results of operations, financial condition or liquidity. Income Taxes In October 2016, the FASB issued ASU 2016-16, “Income Taxes (Topic 740) – Intra-Entity Transfers of Assets Other than Inventory” that will require companies to account for the income tax effects of intercompany transfers of assets other than inventory when the transfer occurs. This guidance is effective for interim and annual periods beginning after December 15, 2017, with early adoption permitted as of the beginning of an annual period. The adoption of this guidance is not expected to impact our results of operations, financial condition or liquidity. Definition of a Business In January 2017, the FASB issued ASU 2017-01, “Clarifying the Definition of a Business” with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. This guidance is effective for annual periods beginning after December 15, 2017, and interim periods within those fiscal years with early adoption permitted. This guidance will be applied prospectively to any transactions occurring within the period of adoption. Goodwill Impairment In January 2017, the FASB issued ASU 2017-04, “Simplifying the Test for Goodwill Impairment” that eliminates the requirement to calculate the implied fair value of goodwill (i.e., Step 2 of the current goodwill impairment test) to measure a goodwill impairment charge. Instead, an impairment charge will be based on the excess of a reporting unit’s carrying amount over its fair value (i.e., measure the charge based on Step 1 of the current goodwill impairment test). This guidance is effective for annual and interim impairment tests performed in periods beginning after December 15, 2019, with early adoption permitted for annual and interim goodwill impairment testing dates after January 1, 2017. This guidance will be adopted on a prospective basis. Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, “Premium Amortization on Purchased Callable Debt Securities” that shortens the amortization period for the premium on certain purchased callable debt securities to the earliest call date. This guidance is effective for annual periods beginning after December 15, 2018, and interim periods within those fiscal years with early adoption permitted. This guidance will be adopted using a modified retrospective transition approach. The adoption of this guidance is not expected to materially impact our results of operations, financial condition or liquidity. Tax Reform Reclassification from Other Comprehensive Income In February 2018, the FASB issued ASU 2018-02, “Reclassification of Certain Tax Effects form Accumulated Other Comprehensive Income” that will permit entities to reclassify tax effects stranded in accumulated other comprehensive income as a result of tax reform to retained earnings. This guidance is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years with early adoption in any period permitted. Our Company is currently evaluating the impact of this guidance on our results of operations, financial condition and liquidity.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Financial Data by Segment	The following tables set forth the financial data by segment for the years ended December 31, 2017, 2016 and 2015: Year Ended December 31, 2017 U.S. Int'l amounts in thousands Insurance Insurance GlobalRe Corporate (1) Total Net Earned Premiums $ 674,665 $ 333,792 $ 177,963 $ — $ 1,186,420 Net Losses and LAE (443,353 ) (229,601 ) (133,311 ) — (806,265 ) Commission Expenses (77,729 ) (68,824 ) (39,136 ) 958 (184,731 ) Other Operating Expenses (128,905 ) (83,464 ) (20,861 ) — (233,230 ) Other Underwriting Income (Expense) 461 — 565 (958 ) 68 Underwriting Profit (Loss) $ 25,139 $ (48,097 ) $ (14,780 ) $ — $ (37,738 ) Net Investment Income 89,293 89,293 Net Realized Gains 43,009 43,009 Interest Expense (15,447 ) (15,447 ) Other Loss (4,311 ) (4,311 ) Income (Loss) Before Income Taxes $ 25,139 $ (48,097 ) $ (14,780 ) $ 112,544 $ 74,806 Income Tax Expense (34,312 ) (34,312 ) Net Income $ 40,494 Losses and LAE ratio 65.7 % 68.8 % 74.9 % 68.0 % Commission Expense Ratio 11.5 % 20.6 % 22.0 % 15.6 % Other Operating Expense Ratio (2) 19.1 % 25.0 % 11.4 % 19.6 % Combined Ratio 96.3 % 114.4 % 108.3 % 103.2 % ( 1) - Includes Corporate segment intercompany eliminations. (2) - Includes Other Operating Expenses and Other Underwriting Income (Expense). Year Ended December 31, 2016 U.S. Int'l amounts in thousands Insurance Insurance GlobalRe Corporate (1) Total Net Earned Premiums $ 629,308 $ 307,416 $ 163,621 $ — $ 1,100,345 Net Losses and LAE (397,860 ) (178,284 ) (89,304 ) — (665,448 ) Commission Expenses (70,812 ) (61,703 ) (34,008 ) 1,478 (165,045 ) Other Operating Expenses (128,108 ) (86,395 ) (19,593 ) — (234,096 ) Other Underwriting Income (Expense) 1,092 — 522 (1,478 ) 136 Underwriting Profit (Loss) $ 33,620 $ (18,966 ) $ 21,238 $ — $ 35,892 Net Investment Income 79,451 79,451 Net Realized Gains 9,036 9,036 Interest Expense (15,435 ) (15,435 ) Other Income 8,565 8,565 Income (Loss) Before Income Taxes $ 33,620 $ (18,966 ) $ 21,238 $ 81,617 $ 117,509 Income Tax Expense (34,783 ) (34,783 ) Net Income $ 82,726 Losses and LAE ratio 63.2 % 58.0 % 54.6 % 60.5 % Commission Expense Ratio 11.3 % 20.1 % 20.8 % 15.0 % Other Operating Expense Ratio (2) 20.2 % 28.1 % 11.6 % 21.2 % Combined Ratio 94.7 % 106.2 % 87.0 % 96.7 % (1) - Includes Corporate segment intercompany eliminations. (2) - Includes Other Operating Expenses and Other Underwriting Income (Expense ). Year Ended December 31, 2015 U.S. Int'l amounts in thousands Insurance Insurance GlobalRe Corporate (1) Total Net Earned Premiums $ 555,836 $ 259,960 $ 168,291 $ — $ 984,087 Net Losses and LAE (343,497 ) (134,702 ) (94,399 ) — (572,598 ) Commission Expenses (56,319 ) (43,676 ) (32,240 ) 2,258 (129,977 ) Other Operating Expenses (131,407 ) (75,867 ) (16,242 ) — (223,516 ) Other Underwriting Income (Expense) 1,690 — 690 (2,258 ) 122 Underwriting Profit $ 26,303 $ 5,715 $ 26,100 $ — $ 58,118 Net Investment Income 68,718 68,718 Net Realized Gains 6,675 6,675 Interest Expense (15,424 ) (15,424 ) Other Loss (613 ) (613 ) Income Before Income Taxes $ 26,303 $ 5,715 $ 26,100 $ 59,356 $ 117,474 Income Tax Expense (36,417 ) (36,417 ) Net Income $ 81,057 Losses and LAE ratio 61.8 % 51.8 % 56.1 % 58.2 % Commission Expense Ratio 10.1 % 16.8 % 19.2 % 13.2 % Other Operating Expense Ratio (2) 23.4 % 29.2 % 9.2 % 22.7 % Combined Ratio 95.3 % 97.8 % 84.5 % 94.1 % (1) - Includes Corporate segment intercompany eliminations. (2) - Includes Other Operating Expenses and Other Underwriting Income (Expense ).
Additional Financial Data by Segment	The following tables provide additional financial data by operating segment for the years ended December 31, 2017, 2016 and 2015: Year Ended December 31, 2017 amounts in thousands Gross written premiums Ceded premiums Net written premiums Net earned premiums U.S. Insurance Marine $ 156,171 $ (72,431 ) $ 83,740 $ 86,605 P&C 713,539 (173,501 ) 540,038 495,260 Professional Liability 118,583 (21,061 ) 97,522 92,800 Total $ 988,293 $ (266,993 ) $ 721,300 $ 674,665 Int'l Insurance Marine $ 198,241 $ (42,018 ) $ 156,223 $ 152,396 P&C 159,123 (85,298 ) 73,825 88,517 Professional Liability 143,766 (37,962 ) 105,804 92,879 Total $ 501,130 $ (165,278 ) $ 335,852 $ 333,792 GlobalRe $ 223,842 $ (9,664 ) $ 214,178 $ 177,963 Total $ 1,713,265 $ (441,935 ) $ 1,271,330 $ 1,186,420 Year Ended December 31, 2016 amounts in thousands Gross written premiums Ceded written premiums Net written premiums Net earned premiums U.S. Insurance Marine $ 169,405 $ (70,858 ) $ 98,547 $ 100,132 P&C 631,562 (135,888 ) 495,674 453,673 Professional Liability 118,428 (29,081 ) 89,347 75,503 Total $ 919,395 $ (235,827 ) $ 683,568 $ 629,308 Int'l Insurance Marine $ 183,228 $ (40,092 ) $ 143,136 $ 141,593 P&C 181,094 (69,606 ) 111,488 89,455 Professional Liability 120,149 (28,806 ) 91,343 76,368 Total $ 484,471 $ (138,504 ) $ 345,967 $ 307,416 GlobalRe $ 165,045 $ (8,356 ) $ 156,689 $ 163,621 Total $ 1,568,911 $ (382,687 ) $ 1,186,224 $ 1,100,345 Year Ended December 31, 2015 amounts in thousands Gross written premiums Ceded written premiums Net written premiums Net earned premiums U.S. Insurance Marine $ 158,124 $ (61,916 ) $ 96,208 $ 96,082 P&C 596,673 (152,168 ) 444,505 401,408 Professional Liability 110,984 (54,691 ) 56,293 58,346 Total $ 865,781 $ (268,775 ) $ 597,006 $ 555,836 Int'l Insurance Marine $ 183,707 $ (36,515 ) $ 147,192 $ 149,256 P&C 130,729 (67,722 ) 63,007 55,320 Professional Liability 97,511 (29,768 ) 67,743 55,384 Total $ 411,947 $ (134,005 ) $ 277,942 $ 259,960 GlobalRe $ 175,774 $ (6,862 ) $ 168,912 $ 168,291 Total $ 1,453,502 $ (409,642 ) $ 1,043,860 $ 984,087
Summary of Gross Written Premiums Allocated Based on Location of Insured's Head Office	The following is a summary of our Company's Gross Written Premiums allocated based on the location of the insured's head office: Years Ended December 31, amounts in thousands 2017 2016 2015 United States $ 1,203,695 $ 1,101,337 $ 1,057,895 Worldwide (Excluding the United States) 509,570 467,574 395,607 Total $ 1,713,265 $ 1,568,911 $ 1,453,502

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income	The following tables set forth our Company’s available-for-sale investments as of December 31, 2017 and 2016 and include OTTI securities recognized within AOCI: December 31, 2017 Gross Gross Unrealized Unrealized Amortized amounts in thousands Fair Value Gains Losses Cost Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 393,563 $ 2,081 $ (2,014 ) $ 393,496 States, Municipalities and Political Subdivisions 814,632 20,136 (1,423 ) 795,919 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities 407,619 2,352 (5,414 ) 410,681 Residential Mortgage Obligations 54,104 606 (79 ) 53,577 Asset-Backed Securities 328,753 2,138 (663 ) 327,278 Commercial Mortgage-Backed Securities 160,904 2,354 (1,182 ) 159,732 Subtotal $ 951,380 $ 7,450 $ (7,338 ) $ 951,268 Corporate Exposures (1) 897,479 14,491 (3,737 ) 886,725 Total Fixed Maturities $ 3,057,054 $ 44,158 $ (14,512 ) $ 3,027,408 Equity Securities: Common Stocks $ 52,439 $ 7,423 $ (112 ) $ 45,128 Preferred Stocks 183,542 6,071 (1,560 ) 179,031 Total Equity Securities $ 235,981 $ 13,494 $ (1,672 ) $ 224,159 Short-Term Investments 127,128 3 — 127,125 Total Investments $ 3,420,163 $ 57,655 $ (16,184 ) $ 3,378,692 (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks. December 31, 2016 Gross Gross Unrealized Unrealized Amortized amounts in thousands Fair Value Gains Losses Cost Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 273,776 $ 2,192 $ (5,128 ) $ 276,712 States, Municipalities and Political Subdivisions 547,415 11,542 (4,036 ) 539,909 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities 487,364 4,016 (6,585 ) 489,933 Residential Mortgage Obligations 20,530 453 (55 ) 20,132 Asset-Backed Securities 314,601 824 (1,178 ) 314,955 Commercial Mortgage-Backed Securities 154,139 2,859 (1,904 ) 153,184 Subtotal $ 976,634 $ 8,152 $ (9,722 ) $ 978,204 Corporate Exposures (1) 838,057 10,185 (5,528 ) 833,400 Total Fixed Maturities $ 2,635,882 $ 32,071 $ (24,414 ) $ 2,628,225 Equity Securities: Common Stocks $ 164,087 $ 24,677 $ (964 ) $ 140,374 Preferred Stocks 185,055 2,339 (4,821 ) 187,537 Total Equity Securities $ 349,142 $ 27,016 $ (5,785 ) $ 327,911 Short-Term Investments 143,539 88 — 143,451 Total Investments $ 3,128,563 $ 59,175 $ (30,199 ) $ 3,099,587 (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.
Scheduled Maturity Dates for Fixed Maturity Securities by Number of Years Until Maturity	The contractual maturity dates for Fixed Maturities categorized by the number of years until maturity as of December 31, 2017 are shown in the following table: December 31, 2017 Amortized amounts in thousands Fair Value Cost Due in One Year or Less $ 228,479 $ 228,255 Due after One Year Through Five Years 809,718 809,195 Due After Five Years Through Ten Years 359,745 352,524 Due After Ten Years 707,732 686,166 Mortgage-Backed and Asset-Backed Securities 951,380 951,268 Total $ 3,057,054 $ 3,027,408
Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position	The following tables summarize all securities in a gross unrealized loss position as of December 31, 2017 and 2016, showing the aggregate fair value and gross unrealized loss by the length of time those securities have continuously been in a gross unrealized loss position: December 31, 2017 Less than 12 months Greater than 12 months Total Gross Gross Gross Fair Unrealized Fair Unrealized Fair Unrealized amounts in thousands Value (Losses) Value (Losses) Value (Losses) Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 273,672 $ (1,502 ) $ 54,484 $ (512 ) $ 328,156 $ (2,014 ) States, Municipalities and Political Subdivisions 74,097 (503 ) 45,085 (920 ) 119,182 (1,423 ) Mortgage-Backed and Asset-Backed Securities: Agency Residential Mortgage-Backed Securities 87,496 (346 ) 236,745 (5,068 ) 324,241 (5,414 ) Residential Mortgage Obligations 12,418 (62 ) 546 (17 ) 12,964 (79 ) Asset-Backed Securities 85,877 (468 ) 24,733 (195 ) 110,610 (663 ) Commercial Mortgage-Backed Securities 20,482 (95 ) 22,903 (1,087 ) 43,385 (1,182 ) Subtotal $ 206,273 $ (971 ) $ 284,927 $ (6,367 ) $ 491,200 $ (7,338 ) Corporate Exposures (1) 295,433 (1,690 ) 121,410 (2,047 ) 416,843 (3,737 ) Total Fixed Maturities $ 849,475 $ (4,666 ) $ 505,906 $ (9,846 ) $ 1,355,381 $ (14,512 ) Equity Securities: Common Stocks $ 11,245 $ (81 ) $ 1,770 $ (31 ) $ 13,015 $ (112 ) Preferred Stocks 50,861 (1,524 ) 662 (36 ) 51,523 (1,560 ) Total Equity Securities $ 62,106 $ (1,605 ) $ 2,432 $ (67 ) $ 64,538 $ (1,672 ) Total Fixed Maturities and Equity Securities $ 911,581 $ (6,271 ) $ 508,338 $ (9,913 ) $ 1,419,919 $ (16,184 ) (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks. December 31, 2016 Less than 12 months Greater than 12 months Total Gross Gross Gross Fair Unrealized Fair Unrealized Fair Unrealized amounts in thousands Value (Losses) Value (Losses) Value (Losses) Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 150,891 $ (2,570 ) $ 16,819 $ (2,558 ) $ 167,710 $ (5,128 ) States, Municipalities and Political Subdivisions 137,731 (3,111 ) 13,255 (925 ) 150,986 (4,036 ) Mortgage-Backed and Asset-Backed Securities: Agency Residential Mortgage-Backed Securities 349,119 (6,155 ) 12,401 (430 ) 361,520 (6,585 ) Residential Mortgage Obligations 953 (18 ) 926 (37 ) 1,879 (55 ) Asset-Backed Securities 95,514 (970 ) 48,093 (208 ) 143,607 (1,178 ) Commercial Mortgage-Backed Securities 51,932 (1,164 ) 7,910 (740 ) 59,842 (1,904 ) Subtotal $ 497,518 $ (8,307 ) $ 69,330 $ (1,415 ) $ 566,848 $ (9,722 ) Corporate Exposures (1) 325,733 (5,086 ) 26,005 (442 ) 351,738 (5,528 ) Total Fixed Maturities $ 1,111,873 $ (19,074 ) $ 125,409 $ (5,340 ) $ 1,237,282 $ (24,414 ) Equity Securities: Common Stocks $ 31,272 $ (634 ) $ 1,471 $ (330 ) $ 32,743 $ (964 ) Preferred Stocks 113,742 (4,785 ) 523 (36 ) 114,265 (4,821 ) Total Equity Securities $ 145,014 $ (5,419 ) $ 1,994 $ (366 ) $ 147,008 $ (5,785 ) Total Fixed Maturities and Equity Securities $ 1,256,887 $ (24,493 ) $ 127,403 $ (5,706 ) $ 1,384,290 $ (30,199 ) (1) - Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.
Summary of Cumulative Amounts Related to Credit Loss Portion of Other-Than-Temporary Impairment Losses on Debt Securities	The following table summarizes the cumulative amounts related to our Company’s credit loss portion of the OTTI losses on Fixed Maturities for the years ended December 31, 2017, 2016 and 2015. Years Ended December 31, amounts in thousands 2017 2016 2015 Beginning Balance $ 2,361 $ 2,361 $ 2,361 Additions for Credit Loss Impairments recognized in the current period on securities not previously impaired — 150 — Additions for Credit Loss Impairments recognized in the current period on securities previously impaired — — — Reductions for Credit Loss Impairments previously recognized on securities sold during the period — (150 ) — Ending Balance $ 2,361 $ 2,361 $ 2,361
Net Investment Income	Our Company’s Net Investment Income was derived from the following sources: Years Ended December 31, amounts in thousands 2017 2016 2015 Fixed Maturities $ 76,784 $ 67,772 $ 61,572 Equity Securities 15,108 14,271 9,813 Short-Term Investments 1,064 727 683 Total Investment Income $ 92,956 $ 82,770 $ 72,068 Investment Expenses (3,663 ) (3,319 ) (3,350 ) Net Investment Income $ 89,293 $ 79,451 $ 68,718
Realized Gains and Losses, Excluding Net Other-Than-Temporary Impairment Losses Recognized in Earnings	Realized Gains and Losses, excluding net OTTI losses recognized in earnings, for the periods indicated, were as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 Fixed Maturities: Gains $ 4,041 $ 5,681 $ 4,756 Losses (3,249 ) (4,271 ) (5,926 ) Fixed Maturities, net $ 792 $ 1,410 $ (1,170 ) Short-Term Investments: Gains $ 1,631 $ 890 $ 130 Losses (612 ) (1,552 ) (383 ) Short-Term Investments, Net $ 1,019 $ (662 ) $ (253 ) Equity Securities: Gains $ 44,783 $ 9,096 $ 14,331 Losses (1,521 ) (658 ) (4,535 ) Equity Securities, Net $ 43,262 $ 8,438 $ 9,796 Net Realized Gains (Losses) $ 45,073 $ 9,186 $ 8,373

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value	The following tables present, for each of the fair value hierarchy levels as defined by the accounting guidance for fair value measurements and described below, our Company’s Fixed Maturities and Equity Securities by asset class that are measured at fair value on a recurring basis, as well as the fair value of the 5.75% Senior notes due October 15, 2023 (the “5.75% Senior notes”) carried at amortized cost as of December 31, 2017 and 2016: December 31, 2017 amounts in thousands Level 1 Level 2 Level 3 Total Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 178,251 $ 215,312 $ — $ 393,563 States, Municipalities and Political Subdivisions — 814,632 — 814,632 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities — 407,619 — 407,619 Residential Mortgage Obligations — 54,104 — 54,104 Asset-Backed Securities — 328,753 — 328,753 Commercial Mortgage-Backed Securities — 160,904 — 160,904 Subtotal $ — $ 951,380 $ — $ 951,380 Corporate Exposures — 897,479 — 897,479 Total Fixed Maturities $ 178,251 $ 2,878,803 $ — $ 3,057,054 Equity Securities: Common Stocks 52,439 — — 52,439 Preferred Stocks — 183,542 — 183,542 Total Equity Securities $ 52,439 $ 183,542 $ — $ 235,981 Short-Term Investments 127,128 — — 127,128 Total Assets Measured at Fair Value $ 357,818 $ 3,062,345 $ — $ 3,420,163 Senior Notes $ — $ 277,951 $ — $ 277,951 Total Liabilities Measured at Fair Value $ — $ 277,951 $ — $ 277,951 December 31, 2016 amounts in thousands Level 1 Level 2 Level 3 Total Fixed Maturities: U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds $ 47,704 $ 226,072 $ — $ 273,776 States, Municipalities and Political Subdivisions — 547,415 — 547,415 Mortgage-Backed and Asset-Backed Securities: Agency Mortgage-Backed Securities — 487,364 — 487,364 Residential Mortgage Obligations — 20,530 — 20,530 Asset-Backed Securities — 314,601 — 314,601 Commercial Mortgage-Backed Securities — 154,139 — 154,139 Subtotal $ — $ 976,634 $ — $ 976,634 Corporate Exposures — 838,057 — 838,057 Total Fixed Maturities $ 47,704 $ 2,588,178 $ — $ 2,635,882 Equity Securities: Common Stocks 164,087 — — 164,087 Preferred Stocks — 185,055 — 185,055 Total Equity Securities $ 164,087 $ 185,055 $ — $ 349,142 Short-Term Investments 143,539 — — 143,539 Total Assets Measured at Fair Value $ 355,330 $ 2,773,233 $ — $ 3,128,563 Senior Notes $ — $ 280,316 $ — $ 280,316 Total Liabilities Measured at Fair Value $ — $ 280,316 $ — $ 280,316

Reserves for Losses and LAE (Ta

Reserves for Losses and LAE (Tables)	12 Months Ended
	Dec. 31, 2017
Summary of Activity in Reserve for Losses and LAE	The following table summarizes activity for our Company’s Reserves for Losses and LAE for the years ended December 31, 2017, 2016 and 2015: Years Ended December 31, amounts in thousands 2017 2016 2015 Net Reserves for Losses and LAE at Beginning of Year $ 1,510,451 $ 1,393,126 $ 1,308,136 Provision for Losses and LAE for Claims Occurring in the Current Year 771,955 693,976 637,267 Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years 34,310 (28,528 ) (64,669 ) Incurred Losses and LAE $ 806,265 $ 665,448 $ 572,598 Losses and LAE Paid for Claims Occurring During: Current Year (144,095 ) (119,485 ) (159,802 ) Prior Years (478,979 ) (412,544 ) (320,863 ) Losses and LAE Payments $ (623,074 ) $ (532,029 ) $ (480,665 ) Foreign Currency Adjustment 11,738 (16,094 ) (6,943 ) Net Reserves for Losses and LAE at End of Period 1,705,380 1,510,451 1,393,126 Reinsurance Recoverables on Unpaid Losses and LAE 809,765 779,276 809,518 Gross Reserves for Losses and LAE at End of Period $ 2,515,145 $ 2,289,727 $ 2,202,644
Reporting and Operating Segments Break Down of Prior Years' Net Reserve Deficiency (Redundancy)	The reporting and operating segments breakdowns of prior period net reserve strengthening (releases) for the years ended December 31, 2017, 2016 and 2015 are as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 U.S. Insurance Marine $ 1,696 $ (10,122 ) $ (24,825 ) P&C 1,613 2,790 (706 ) Professional Liability 13,405 5,984 (3,788 ) Total $ 16,714 $ (1,348 ) $ (29,319 ) Int'l Insurance Marine $ 6,518 $ (22,068 ) $ (21,914 ) P&C 3,118 (7,051 ) (8,458 ) Professional Liability 5,198 4,100 4,156 Total $ 14,834 $ (25,019 ) $ (26,216 ) GlobalRe $ 2,762 $ (2,161 ) $ (9,134 ) Total Strengthening (Releases) $ 34,310 $ (28,528 ) $ (64,669 )
Summary of Reconciliation of Net Incurred and Paid Claims Development to the Reserve for Losses and LAE	The reconciliation of the net incurred and paid claims development tables to the Reserve for losses and LAE in the consolidated statement of financial position is as follows: Carried Reserves Reconciliation amounts in thousands As of December 31, 2017 Total Net Reserves Reinsurance Recoverables on Unpaid Claims Total Gross Reserves U.S. Marine $ 101,091 $ 98,373 $ 199,464 U.S. P&C 849,009 371,651 1,220,660 U.S. Professional Liability 108,302 49,650 157,952 Int'l Marine 221,659 90,792 312,451 Int'l P&C 95,680 126,365 222,045 Int'l Professional Liability 116,783 55,393 172,176 GlobalRe 148,350 17,541 165,891 Liabilities for Unpaid Claims and Claim Adjustment Expenses $ 1,640,874 $ 809,765 $ 2,450,639 Unallocated Claims Adjustment Expenses 64,269 Other 237 Total Gross Liability for Unpaid Claims and Claim Adjustment Expense $ 2,515,145
U.S.Insurance | Marine
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	U.S. Marine Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 56,878 $ 67,421 $ 72,259 $ 73,734 $ 73,494 $ 68,393 $ 68,557 $ 66,717 $ 66,937 $ 66,849 $ 282 3,194 2009 62,556 59,278 56,052 50,501 48,366 49,571 51,783 50,874 51,537 205 2,323 2010 64,753 63,812 60,035 54,813 52,847 57,008 56,458 56,281 (224 ) 2,007 2011 70,219 69,293 67,640 63,099 61,912 60,329 59,750 630 1,910 2012 85,275 87,606 72,219 68,841 70,337 70,887 (619 ) 1,856 2013 61,356 57,865 51,748 54,381 56,319 144 1,814 2014 59,270 42,684 41,006 45,762 (377 ) 1,927 2015 61,106 51,864 50,331 5,148 2,187 2016 53,011 48,791 12,748 2,352 2017 54,061 23,327 1,986 Total $ 560,568 U.S. Marine Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 7,554 $ 36,397 $ 54,175 $ 56,460 $ 62,398 $ 62,700 $ 63,027 $ 63,140 $ 64,029 $ 64,433 2009 12,947 29,266 35,449 41,650 43,450 46,033 46,894 49,573 50,191 2010 16,582 32,323 39,825 42,896 48,114 51,887 55,096 55,249 2011 14,955 39,821 48,850 52,408 56,955 57,929 58,558 2012 27,254 46,308 54,673 60,734 67,220 69,059 2013 17,831 35,471 41,488 47,162 51,466 2014 10,717 23,415 34,899 41,021 2015 16,945 31,010 40,586 2016 11,610 25,154 2017 18,947 Total $ 474,664 Net reserves < 2008 15,187 Total Net reserves $ 101,091 U.S. Marine Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 U.S. Marine 27.7 % 31.8 % 16.7 % 8.4 % 7.7 % 3.3 % 2.2 % 1.9 % 1.3 % 0.6 %
U.S.Insurance | Property Casualty
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	U.S. P&C Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 151,401 $ 152,659 $ 138,860 $ 135,255 $ 136,498 $ 138,499 $ 142,641 $ 147,763 $ 152,814 $ 154,169 $ 3,706 10,937 2009 127,237 129,613 130,860 135,984 141,186 146,321 147,036 150,294 149,622 6,143 8,515 2010 96,885 96,446 94,317 100,006 103,038 104,955 107,914 110,170 7,776 7,874 2011 79,832 78,093 86,557 89,666 90,613 98,238 100,461 12,051 6,450 2012 103,089 89,758 91,502 99,207 107,245 111,146 18,194 4,579 2013 139,517 125,952 129,100 137,298 149,409 20,569 3,898 2014 191,515 159,477 148,541 155,248 51,231 3,880 2015 244,311 218,973 214,547 76,700 3,672 2016 271,351 243,754 190,052 3,136 2017 295,386 263,648 2,527 Total $ 1,683,912 U.S. P&C Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 10,266 $ 32,307 $ 54,663 $ 78,024 $ 99,825 $ 111,420 $ 119,766 $ 128,886 $ 142,284 $ 146,295 2009 11,245 33,049 55,773 86,736 110,648 123,981 129,461 136,636 138,829 2010 11,985 26,482 46,478 65,094 77,302 85,740 90,625 97,125 2011 3,630 14,579 30,359 46,638 58,489 70,486 79,682 2012 4,761 17,311 27,513 47,005 67,502 82,906 2013 7,146 20,914 48,716 84,494 118,506 2014 5,229 13,264 44,468 74,979 2015 3,575 24,057 78,856 2016 5,831 29,131 2017 12,860 Total $ 859,169 Net reserves < 2008 24,266 Total Net reserves $ 849,009 U.S. P&C Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 U.S. P&C 5.0 % 10.9 % 17.1 % 18.6 % 15.7 % 10.0 % 5.7 % 5.5 % 5.1 % 2.6 %
U.S.Insurance | Professional Liability
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	U.S. Professional Liability Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 34,346 $ 38,449 $ 39,130 $ 35,844 $ 37,603 $ 37,939 $ 37,497 $ 37,620 $ 41,766 $ 43,003 $ 875 770 2009 47,473 53,273 61,023 58,600 60,789 58,487 58,660 58,143 61,083 788 1,370 2010 52,302 64,020 68,375 75,619 74,535 86,787 85,348 86,164 932 1,431 2011 58,116 62,087 70,240 73,028 79,181 79,136 82,558 699 1,726 2012 61,196 54,480 57,097 55,917 58,344 58,563 2,254 2,175 2013 60,899 54,982 49,104 48,298 54,228 3,915 1,982 2014 50,020 35,870 39,826 42,060 5,255 2,017 2015 31,659 30,764 31,088 8,721 1,674 2016 41,673 36,650 12,608 1,806 2017 51,708 44,396 1,902 Total $ 547,105 U.S. Professional Liability Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 1,880 $ 15,288 $ 25,042 $ 30,581 $ 34,902 $ 35,206 $ 35,598 $ 37,175 $ 42,056 $ 42,046 2009 3,837 21,742 41,675 51,642 55,378 53,859 57,744 58,095 58,150 2010 2,981 23,147 46,868 54,019 59,765 82,013 84,511 84,529 2011 3,204 23,878 43,551 59,576 65,710 75,641 78,258 2012 3,855 22,635 35,681 44,028 49,675 54,728 2013 4,677 22,167 32,904 38,876 47,062 2014 2,304 14,371 29,392 35,114 2015 2,257 11,249 18,428 2016 2,382 18,687 2017 2,682 Total $ 439,684 Net reserves < 2008 881 Total Net reserves $ 108,302 U.S. Professional Liability Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 U.S. Professional Liability 5.8 % 30.6 % 25.9 % 13.7 % 9.2 % 8.9 % 3.3 % 1.4 % 5.7 % 0.0 %
International Insurance | Marine
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	Int'l Marine Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 85,148 $ 80,793 $ 80,589 $ 84,122 $ 84,449 $ 83,962 (135 ) 5,733 2009 96,465 91,705 90,527 83,477 82,074 81,729 (265 ) 5,045 2010 132,080 130,102 119,156 114,940 112,259 108,523 (428 ) 5,129 2011 100,162 85,720 82,441 77,848 73,970 74,309 (500 ) 4,643 2012 110,316 132,245 107,636 100,029 93,052 92,983 (1,224 ) 4,908 2013 87,987 94,614 82,641 81,989 84,100 3,168 4,716 2014 104,397 117,974 116,899 119,767 (1,483 ) 4,986 2015 99,067 95,001 96,614 (1,913 ) 7,400 2016 88,388 95,584 8,335 9,114 2017 107,915 32,198 7,385 Total $ 945,486 Int'l Marine Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 71,355 $ 73,799 $ 75,255 $ 79,024 $ 80,185 $ 80,183 2009 66,388 70,553 73,321 76,783 77,487 78,772 2010 78,509 90,871 96,920 100,897 103,188 103,646 2011 45,896 59,341 63,778 68,702 69,672 70,022 2012 23,382 54,954 70,657 78,789 81,411 83,381 2013 23,731 46,552 58,687 64,859 67,730 2014 29,977 57,629 81,941 90,619 2015 22,862 54,550 76,044 2016 23,665 55,420 2017 29,082 Total $ 734,899 Net reserves < 2008 11,072 Total Net reserves $ 221,659 Int'l Marine Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 Int'l Marine 25.1 % 29.7 % 19.0 % 7.8 % 4.3 % 2.4 % 2.3 % 1.9 % 1.5 % 0.0 %
International Insurance | Property Casualty
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	Int'l P&C Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 37,256 $ 34,428 $ 34,495 $ 33,877 $ 41,536 $ 41,472 $ 67 913 2009 38,680 40,896 40,984 40,692 34,186 34,461 206 885 2010 43,096 42,527 41,922 42,196 42,822 42,613 309 1,089 2011 71,531 61,564 59,405 59,529 59,516 55,127 221 1,216 2012 45,693 37,520 36,444 32,472 32,622 33,075 152 1,358 2013 36,044 37,520 36,599 34,499 35,443 1,027 1,592 2014 27,829 22,787 20,997 16,449 (1,396 ) 1,300 2015 34,889 36,689 33,969 10 1,986 2016 79,393 92,108 18,665 3,556 2017 56,953 10,229 2,213 Total $ 441,670 Int'l P&C Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 38,275 $ 39,489 $ 39,869 $ 40,398 $ 40,249 $ 40,333 2009 28,919 33,223 33,685 33,317 33,405 33,494 2010 35,192 40,880 43,429 41,102 41,204 41,691 2011 38,548 47,851 50,753 53,318 54,239 52,225 2012 13,238 21,578 27,084 30,135 31,332 31,596 2013 7,160 18,441 25,840 28,682 30,279 2014 3,902 9,015 13,232 16,805 2015 6,845 18,436 29,624 2016 26,190 55,749 2017 15,019 Total $ 346,815 Net reserves < 2008 825 Total Net reserves $ 95,680 Int'l P&C Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 Int'l P&C 25.0 % 32.4 % 26.5 % 13.0 % 4.3 % -1.0 % -1.5 % 0.9 % -0.1 % 0.2 %
International Insurance | Professional Liability
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	Int'l Professional Liability Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 11,749 $ 13,524 $ 13,798 $ 13,829 $ 14,082 $ 14,463 $ 258 439 2009 16,661 17,225 17,365 17,016 16,020 15,988 758 624 2010 14,423 14,023 14,225 14,221 15,864 14,639 247 745 2011 15,390 18,125 19,429 8,923 6,989 12,823 1,769 665 2012 6,835 11,506 12,789 13,457 17,168 18,068 2,365 1,102 2013 10,218 4,099 14,532 15,748 16,842 1,607 1,174 2014 20,662 24,519 23,861 21,758 4,216 1,747 2015 28,319 30,539 35,586 9,550 2,370 2016 36,167 34,218 22,567 2,744 2017 47,697 41,954 1,522 Total $ 232,082 Int'l Professional Liability Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 12,198 $ 13,360 $ 13,754 $ 13,784 $ 14,071 $ 14,105 2009 8,954 10,901 12,983 14,008 14,331 14,918 2010 5,909 7,320 10,659 12,840 13,597 13,643 2011 1,221 2,038 2,994 4,087 4,677 16,642 2012 2,141 3,236 4,293 5,600 7,526 6,933 2013 733 2,285 3,078 9,478 13,906 2014 1,387 2,493 5,228 8,945 2015 3,457 5,939 17,502 2016 2,076 6,955 2017 2,024 Total $ 115,573 Net reserves < 2008 274 Total Net reserves $ 116,783 Int'l Professional Liability Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 Int'l Professional 6.7 % 8.8 % 16.6 % 20.8 % 15.2 % 5.4 % 35.0 % 0.8 % 2.8 % 0.2 %
Global Reinsurance
Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance	GlobalRe Loss and ALAE, Net of Reinsurance amounts in thousands except # of reported claims For the Years Ended December 31, As of December 31, 2017 AY 2012 2013 2014 2015 2016 2017 Total IBNR # of Reported Claims 2008 $ 14,467 $ 14,846 $ 14,198 $ 13,792 $ 13,487 $ 13,123 $ 529 306 2009 11,247 11,381 11,267 10,516 10,567 10,406 53 200 2010 13,150 13,110 14,142 11,908 12,631 13,159 (54 ) 201 2011 53,868 76,362 76,164 73,610 72,877 73,530 (438 ) 769 2012 107,189 99,116 92,730 88,367 115,058 119,771 (815 ) 1,456 2013 114,873 118,779 102,632 105,172 102,433 1,067 1,285 2014 119,379 142,058 121,716 118,255 5,062 1,070 2015 101,764 91,767 94,428 6,897 896 2016 88,543 90,755 15,972 749 2017 129,150 60,576 428 Total $ 765,010 GlobalRe Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance amounts in thousands For the Years Ended December 31, AY 2012 2013 2014 2015 2016 2017 2008 $ 11,775 $ 11,799 $ 11,661 $ 11,640 $ 11,702 $ 11,797 2009 8,694 8,930 9,060 10,018 10,133 10,133 2010 10,005 10,349 10,529 11,154 11,207 11,272 2011 43,450 71,138 71,816 72,102 72,246 72,414 2012 37,746 77,227 83,408 84,801 111,962 117,782 2013 53,476 82,701 90,721 98,484 100,059 2014 66,286 119,249 105,648 108,794 2015 41,660 64,527 76,973 2016 36,352 66,277 2017 43,663 Total $ 619,164 Net reserves < 2008 2,504 Total Net reserves $ 148,350 GlobalRe Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 7 8 9 10 GlobalRe 39.3 % 34.0 % 3.2 % 3.8 % 8.2 % 3.3 % 3.3 % 0.5 % 0.2 % 0.7 %

Ceded Reinsurance (Tables)

Ceded Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable	The following table lists our Company’s 10 largest reinsurers measured by the amount of total reinsurance recoverables as of December 31, 2017, and the reinsurers’ ratings from A.M. Best and S&P: Reinsurance Recoverables Unearned Paid/Unpaid Collateral amounts in thousands Premium Losses (1) Total Held A.M. Best S&P Everest Reinsurance Company 29,333 90,005 119,338 9,564 A+ A+ Swiss Reinsurance America Corporation 18,987 71,062 90,049 8,069 A+ AA- National Indemnity Company 5,770 61,413 67,183 5,423 A++ AA+ Transatlantic Reinsurance Company 8,587 50,487 59,074 2,588 A+ A+ Munich Reinsurance America Inc. 10,991 46,751 57,742 5,171 A+ AA- Allied World Reinsurance 6,511 32,422 38,933 1,637 A A- Aspen Insurance UK Ltd. 8,921 29,093 38,014 8,775 A A Employers Mutual Casualty Company 12,096 22,509 34,605 11,172 A NR Scor Global P&C SE 5,938 25,228 31,166 6,393 A+ AA- Partner Reinsurance Europe 5,257 25,472 30,729 11,408 A A+ Top 10 Reinsurers 112,391 454,442 566,833 70,200 Others 116,178 427,817 543,995 110,464 Total 228,569 882,259 1,110,828 180,664 (1) - Net of reserve for uncollectible reinsurance of approximately $12.6 million.
Summary of Components of Net Written Premium	The following table summarizes the components of Net Written Premium: Years Ended December 31, amounts in thousands 2017 2016 2015 Direct $ 1,489,422 $ 1,403,865 $ 1,277,728 Assumed 223,843 165,046 175,774 Ceded (441,935 ) (382,687 ) (409,642 ) Net Written Premiums $ 1,271,330 $ 1,186,224 $ 1,043,860
Summary of Components of Net Earned Premium	The following table summarizes the components of Net Earned Premium: Years Ended December 31, amounts in thousands 2017 2016 2015 Direct $ 1,425,582 $ 1,330,265 $ 1,223,840 Assumed 187,609 171,978 175,153 Ceded (426,771 ) (401,898 ) (414,906 ) Net Earned Premiums $ 1,186,420 $ 1,100,345 $ 984,087
Summary of Components of Net Losses and Loss Adjustment Expenses Incurred	The following table summarizes the components of Net Losses and LAE Incurred: Years Ended December 31, amounts in thousands 2017 2016 2015 Direct $ 938,640 $ 795,414 $ 703,361 Assumed 148,606 87,995 97,947 Ceded (280,981 ) (217,961 ) (228,710 ) Net Losses and LAE $ 806,265 $ 665,448 $ 572,598

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Annual Rental Commitments under Various Non-cancellable Operating Leases	Rental Commitments Future minimum annual rental commitments as of December 31, 2017 under various non-cancellable operating leases for our office facilities, which expire at various dates through 2031, are as follows: Rental Commitments for Years Ended December 31, amounts in thousands 2018 $ 12,075 2019 11,469 2020 11,395 2021 10,315 2022 9,654 2023-2031 36,296 Total Minimum Operating Lease Payments $ 91,204

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Components of Current and Deferred Income Tax Expense (Benefit)	The components of current and deferred income tax expense (benefit) are as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 Current Income Tax Expense (Benefit): Federal and Foreign $ 42,427 $ 39,900 $ 27,937 State and Local 764 638 1,476 Subtotal $ 43,191 $ 40,538 $ 29,413 Deferred Income Tax Expense (Benefit): Federal and Foreign $ (8,879 ) $ (5,755 ) $ 7,004 State and local — — — Subtotal $ (8,879 ) $ (5,755 ) $ 7,004 Total Income Tax Expense (Benefit) $ 34,312 $ 34,783 $ 36,417
Reconciliation of Total Income Taxes Applicable to Pre-Tax Operating Income and Amounts Computed by Applying Federal Statutory Income Tax Rate to Pre-Tax Operating Income	A reconciliation of total income taxes applicable to pre‑tax operating income and the amounts computed by applying the federal statutory income tax rate to the pre‑tax operating income were as follows: Years Ended December 31, amounts in thousands 2017 2016 2015 Computed Expected Tax Expense $ 26,182 35.0 % $ 41,128 35.0 % $ 41,116 35.0 % Tax-Exempt Interest (5,394 ) (7.2 %) (4,559 ) (3.9 %) (4,987 ) (4.2 %) Dividends Received Deduction (4,689 ) (6.3 %) (3,812 ) (3.2 %) (2,086 ) (1.8 %) Proration of DRD and Tax-Exempt Interest 1,512 2.0 % 1,256 1.1 % 1,061 0.9 % Current State and Local Income Taxes, Net of Federal Income Tax Deduction 497 0.7 % 415 0.4 % 959 0.8 % Stock Compensation Fair Market Value (5,027 ) (6.7 %) N/A N/A N/A N/A Change in Valuation Allowance 1,182 1.6 % — — — — Deferred Tax Expense due to Tax Law Changes 17,107 22.9 % N/A N/A N/A N/A Transition Tax 2,587 3.5 % N/A N/A N/A N/A Other 355 0.4 % 355 0.2 % 354 0.3 % Actual Tax Expense and Rate $ 34,312 45.9 % $ 34,783 29.6 % $ 36,417 31.0 % N/A - Not applicable
Tax Effects of Temporary Differences That Give Rise to Federal, Foreign, State and Local Deferred Tax Assets and Deferred Tax Liabilities	The tax effects of cumulative temporary differences that give rise to federal, foreign, state and local deferred tax assets and deferred tax liabilities were as follows: December 31, amounts in thousands 2017 2016 Deferred Tax Assets: Loss Reserve Discount $ 20,425 $ 25,420 Unearned Premiums 24,352 33,886 Compensation Related 8,462 15,288 Lloyd's Year of Account Deferral 9,400 — State and Local Net Deferred Tax Assets 616 699 Net Currency Translation Adjustments 2,084 5,363 Other 152 3,618 Total Gross Deferred Tax Assets $ 65,491 $ 84,274 Less: Valuation Allowance (1,798 ) (699 ) Total Deferred Tax Assets $ 63,693 $ 83,575 Deferred Tax Liabilities: Loss Reserve Discount - (Transition Rule) $ (5,066 ) $ — Net Unrealized Gains/Losses on Securities (8,708 ) (10,142 ) Deferred Acquisition Costs (20,670 ) (29,401 ) Lloyd's Year of Account Deferral — (15,471 ) Net Unrealized Foreign Exchange (3,782 ) (3,575 ) Other (3,196 ) (4,048 ) Total Deferred Tax Liabilities $ (41,422 ) $ (62,637 ) Net Deferred Income Tax Asset (Liability) $ 22,271 $ 20,938

Stockholders Equity (Tables)

Stockholders Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Changes in Company's Issued and Outstanding Common Shares	The following table represents changes in our Company’s issued and outstanding common shares for the periods indicated. We completed a two-for-one stock split on January 20, 2017. All share and per share data prior to January 20, 2017 has been retroactively restated on a post-split basis. Years Ended December 31, amounts in thousands 2017 2016 2015 Beginning Balance 29,124 28,862 28,562 Net Vested Stock Grants 343 220 254 Employee Stock Purchase Plan 40 42 42 Stock Options Exercised — — 4 Ending Balance 29,507 29,124 28,862
Amount and Nature of Net Assets Restricted from Payment of Dividends	The amount and nature of net assets that are restricted from payment of dividends as of December 31, 2017 and 2016 are presented in the following table: As of December 31, amounts in thousands 2017 2016 Restricted Net Assets: NIC and NSIC: Fixed Maturities at Fair Value (Amortized Cost: 2017, $9,856; 2016, $9,820) $ 10,655 $ 10,690 Short Term Investments, at Fair Value 290 290 Cash 1,242 7,168 Total NIC and NSIC (1) $ 12,187 $ 18,148 NHUK: Fixed Maturities at Fair Value (Amortized Cost: 2017, $472,701; 2016, $457,225) $ 471,831 $ 452,053 Short Term Investments, at Fair Value 83,778 63,729 Total NHUK (2) $ 555,609 $ 515,782 Total Restricted Net Assets $ 567,796 $ 533,930 (1) - The restricted net assets for NIC and NSIC primarily consist of fixed maturities on deposit with various state insurance departments. The cash as of December 31, 2017 and 2016, as presented in the table above, was on deposit with a U.K. bank to comply with the regulatory requirements of the Prudential Regulation Authority for the underwriting activities of the U.K. Branch. (2) - The restricted net assets for NHUK consists of fixed maturities and cash held in trust for the benefit of syndicate policyholders and short term investments primarily consisting of overseas deposits in various countries with Lloyd's to support underwriting activities in those countries.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Reconciliation of Basic and Diluted Earnings per Share Computations	The following is a reconciliation of the basic and diluted EPS computations for the years ended December 31, 2017, 2016 and 2015: Years Ended December 31, amounts in thousands, except per share amounts 2017 2016 2015 Net Income $ 40,494 $ 82,726 $ 81,057 Basic Weighted Average Shares 29,441 29,074 28,785 Effect of Common Stock Equivalents: Assumed Exercise of Stock Options and Vesting of Stock 630 958 866 Diluted Weighted Average Shares 30,071 30,032 29,651 Net Income per Common Share: Basic $ 1.38 $ 2.85 $ 2.82 Diluted $ 1.35 $ 2.75 $ 2.73

Statutory Financial Informati42

Statutory Financial Information (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Summary of Statutory Net Income (Loss) and Capital and Surplus	The following table presents statutory Net Income and capital and surplus in accordance with statutory accounting practices: Years Ended December 31, amounts in millions 2017 2016 2015 NIC & NSIC: Statutory Net Income $ 79.3 $ 84.2 $ 60.8 Statutory Capital and Surplus $ 1,056.6 $ 1,027.3 $ 949.1 The Syndicate: Syndicate's Net Income $ (23.4 ) $ 46.3 $ 41.0 Syndicate's Capital and Surplus $ 168.7 $ 180.7 $ 177.5 NIIC: Statutory Net Income $ (8.4 ) $ 0.4 $ — Statutory Capital and Surplus $ 62.9 $ 70.7 $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Restricted Stock Unit Awards	The activity related to our Company’s restricted stock unit awards was as follows: Year Ended December 31, 2017 Number of Awards Weighted Average Grant Date Fair Value (1) Nonvested at the beginning of the year 197,422 $ 37.24 Granted 35,297 $ 56.05 Vested (59,930 ) $ 36.21 Forfeited (13,250 ) $ 39.41 Nonvested at the end of the year 159,539 $ 41.60 ( 1) Fair value is based on the closing price of our common shares on the NASDAQ on the grant date .
Performance-based Equity Awards	The activity related to our Company’s performance-based equity awards was as follows: Year Ended December 31, 2017 Number of Awards Weighted Average Grant Date Fair Value (1) Nonvested at the beginning of the year 1,176,742 $ 35.85 Granted 253,333 $ 55.70 Performance Adjustment 83,071 $ 30.49 Vested (520,285 ) $ 30.38 Forfeited (26,500 ) $ 41.60 Nonvested at the end of the year 966,361 $ 43.39 ( 1) Fair value is based on the closing price of our common shares on the NASDAQ on the grant date .

Condensed Quarterly Financial44

Condensed Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Summary of Condensed Quarterly Financial Data	The following is a summary of quarterly financial data for the periods indicated: March 31, June 30, September December 31, amounts in thousands, except per share amounts 2017 2017 2017 2017 Gross Written Premiums $ 450,305 $ 452,179 $ 402,038 $ 408,743 Revenues: Net Written Premiums $ 337,163 $ 333,282 $ 296,016 $ 304,869 Change in Unearned Premiums (51,032 ) (39,447 ) 5,339 230 Net Earned Premiums $ 286,131 $ 293,835 $ 301,355 $ 305,099 Net Investment Income 21,448 22,265 22,598 22,982 Total Other-Than-Temporary Impairment Losses (1,077 ) 29 (957 ) 3 Portion of Loss Recognized in Other Comprehensive Income (Before Tax) (16 ) (29 ) (15 ) (3 ) Net Other-Than-Temporary Impairment Losses Recognized in Earnings (1,093 ) — (972 ) — Net Realized Gains 1,049 1,694 5,190 37,141 Other Income (Loss) 1,068 (411 ) (1,699 ) (3,201 ) Total Revenues $ 308,603 $ 317,383 $ 326,472 $ 362,021 Expenses: Net Losses and LAE $ 169,600 $ 177,110 $ 276,171 $ 183,384 Commission Expenses 47,844 48,173 45,509 43,205 Other Operating Expenses 58,538 60,766 45,773 68,153 Interest Expense 3,861 3,861 3,862 3,863 Total Expenses $ 279,843 $ 289,910 $ 371,315 $ 298,605 Income (Loss) Before Income Taxes 28,760 27,473 (44,843 ) 63,416 Income Tax Expense (Benefit) $ 7,650 $ 6,971 $ (16,864 ) $ 36,555 Net Income (Loss) $ 21,110 $ 20,502 $ (27,979 ) $ 26,861 Comprehensive Income (Loss) $ 35,911 $ 35,699 $ (18,311 ) $ (1,812 ) Combined Ratio 96.4 % 97.3 % 121.9 % 96.6 % Net Income (Loss) per Share: Basic $ 0.72 $ 0.70 $ (0.95 ) $ 0.91 Diluted $ 0.70 $ 0.69 $ (0.95 ) $ 0.89 March 31, June 30, September 30, December 31, amounts in thousands, except per share amounts 2016 2016 2016 2016 Gross Written Premiums $ 413,877 $ 412,565 $ 374,930 $ 367,539 Revenues: Net Written Premiums $ 319,820 $ 306,535 $ 277,001 $ 282,868 Change in Unearned Premiums (55,462 ) (38,543 ) 7,009 1,117 Net Earned Premiums $ 264,358 $ 267,992 $ 284,010 $ 283,985 Net Investment Income 19,594 19,875 19,875 20,107 Total Other-Than-Temporary Impairment Losses (109 ) (162 ) 23 21 Portion of Loss Recognized in Other Comprehensive Income (Before Tax) 109 12 (23 ) (21 ) Net Other-Than-Temporary Impairment Losses Recognized in Earnings — (150 ) — — Net Realized Gains 1,597 1,960 1,586 4,043 Other Income (Loss) 2,549 4,430 (183 ) 1,905 Total Revenues $ 288,098 $ 294,107 $ 305,288 $ 310,040 Expenses: Net Losses and LAE $ 152,956 $ 167,206 $ 172,793 $ 172,493 Commission Expenses 37,554 40,726 42,611 44,154 Other Operating Expenses 60,809 59,074 56,137 58,076 Interest Expense 3,858 3,858 3,859 3,860 Total Expenses $ 255,177 $ 270,864 $ 275,400 $ 278,583 Income Before Income Taxes 32,921 23,243 29,888 31,457 Income Tax Expense $ 9,989 $ 7,053 $ 7,875 $ 9,866 Net Income $ 22,932 $ 16,190 $ 22,013 $ 21,591 Comprehensive Income (Loss) $ 45,721 $ 35,228 $ 18,760 $ (31,712 ) Combined Ratio 95.1 % 99.6 % 95.6 % 96.7 % Net Income per Share: Basic $ 0.79 $ 0.56 $ 0.76 $ 0.74 Diluted $ 0.77 $ 0.54 $ 0.73 $ 0.71

Organization and Summary of S45

Organization and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage of ownership interest sold					100.00%
Impairment of goodwill and indefinite lived intangible assets		$ 0	$ 0	$ 0
Goodwill and other intangible assets		6,596,000	$ 6,451,000
Changes in the carrying value of the goodwill and indefinite lived intangible assets		$ 100,000
Corporate income tax rate		35.00%	35.00%	35.00%
Total future minimum lease payments		$ 91,204,000
Scenario Forecast
Organization And Summary Of Significant Accounting Policies [Line Items]
Corporate income tax rate	21.00%
Pro Forma
Organization And Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to net unrealized gains in accumulated other comprehensive income for available-for-sale equity securities		$ 11,800,000
Minimum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life		3 years
Maximum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life		7 years
Leasehold Improvements | Minimum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Leasehold Improvements | Maximum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life		10 years
Syndicate 1,221
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage control of stamp capacity		100.00%

Segment Information - Additiona

Segment Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017Segment
Segment Reporting Information [Line Items]
Number of underwriting Segments	3
U.S.Insurance
Segment Reporting Information [Line Items]
Number of Operating Segments	3
International Insurance
Segment Reporting Information [Line Items]
Number of Operating Segments	3

Financial Data by Segment (Deta

Financial Data by Segment (Detail) - 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 Information [Line Items]
Net Earned Premiums		$ 305,099	$ 301,355	$ 293,835	$ 286,131	$ 283,985	$ 284,010	$ 267,992	$ 264,358	$ 1,186,420	$ 1,100,345	$ 984,087
Net Losses and LAE		(183,384)	(276,171)	(177,110)	(169,600)	(172,493)	(172,793)	(167,206)	(152,956)	(806,265)	(665,448)	(572,598)
Commission Expenses		(43,205)	(45,509)	(48,173)	(47,844)	(44,154)	(42,611)	(40,726)	(37,554)	(184,731)	(165,045)	(129,977)
Other Operating Expenses		(68,153)	(45,773)	(60,766)	(58,538)	(58,076)	(56,137)	(59,074)	(60,809)	(233,230)	(234,096)	(223,516)
Other Underwriting Income (Expense)										68	136	122
Underwriting Profit (Loss)										(37,738)	35,892	58,118
Net Investment Income		22,982	22,598	22,265	21,448	20,107	19,875	19,875	19,594	89,293	79,451	68,718
Net Realized Gains										43,009	9,036	6,675
Interest Expense		(3,863)	(3,862)	(3,861)	(3,861)	(3,860)	(3,859)	(3,858)	(3,858)	(15,447)	(15,435)	(15,424)
Other Income (Loss)										(4,311)	8,565	(613)
Income (Loss) Before Income Taxes		63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income Tax Expense		(36,555)	16,864	(6,971)	(7,650)	(9,866)	(7,875)	(7,053)	(9,989)	(34,312)	(34,783)	(36,417)
Net Income (Loss)		$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Losses and LAE ratio										68.00%	60.50%	58.20%
Commission Expense Ratio										15.60%	15.00%	13.20%
Other Operating Expense Ratio	[1]									19.60%	21.20%	22.70%
Combined Ratio		96.60%	121.90%	97.30%	96.40%	96.70%	95.60%	99.60%	95.10%	103.20%	96.70%	94.10%
U.S.Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums										$ 674,665	$ 629,308	$ 555,836
Net Losses and LAE										(443,353)	(397,860)	(343,497)
Commission Expenses										(77,729)	(70,812)	(56,319)
Other Operating Expenses										(128,905)	(128,108)	(131,407)
Other Underwriting Income (Expense)										461	1,092	1,690
Underwriting Profit (Loss)										25,139	33,620	26,303
Income (Loss) Before Income Taxes										$ 25,139	$ 33,620	$ 26,303
Losses and LAE ratio										65.70%	63.20%	61.80%
Commission Expense Ratio										11.50%	11.30%	10.10%
Other Operating Expense Ratio	[1]									19.10%	20.20%	23.40%
Combined Ratio										96.30%	94.70%	95.30%
International Insurance
Segment Reporting Information [Line Items]
Net Earned Premiums										$ 333,792	$ 307,416	$ 259,960
Net Losses and LAE										(229,601)	(178,284)	(134,702)
Commission Expenses										(68,824)	(61,703)	(43,676)
Other Operating Expenses										(83,464)	(86,395)	(75,867)
Underwriting Profit (Loss)										(48,097)	(18,966)	5,715
Income (Loss) Before Income Taxes										$ (48,097)	$ (18,966)	$ 5,715
Losses and LAE ratio										68.80%	58.00%	51.80%
Commission Expense Ratio										20.60%	20.10%	16.80%
Other Operating Expense Ratio	[1]									25.00%	28.10%	29.20%
Combined Ratio										114.40%	106.20%	97.80%
Global Reinsurance
Segment Reporting Information [Line Items]
Net Earned Premiums										$ 177,963	$ 163,621	$ 168,291
Net Losses and LAE										(133,311)	(89,304)	(94,399)
Commission Expenses										(39,136)	(34,008)	(32,240)
Other Operating Expenses										(20,861)	(19,593)	(16,242)
Other Underwriting Income (Expense)										565	522	690
Underwriting Profit (Loss)										(14,780)	21,238	26,100
Income (Loss) Before Income Taxes										$ (14,780)	$ 21,238	$ 26,100
Losses and LAE ratio										74.90%	54.60%	56.10%
Commission Expense Ratio										22.00%	20.80%	19.20%
Other Operating Expense Ratio	[1]									11.40%	11.60%	9.20%
Combined Ratio										108.30%	87.00%	84.50%
Corporate
Segment Reporting Information [Line Items]
Net Investment Income	[2]									$ 89,293	$ 79,451	$ 68,718
Net Realized Gains	[2]									43,009	9,036	6,675
Interest Expense	[2]									(15,447)	(15,435)	(15,424)
Other Income (Loss)	[2]									(4,311)	8,565	(613)
Income (Loss) Before Income Taxes	[2]									112,544	81,617	59,356
Income Tax Expense	[2]									(34,312)	(34,783)	(36,417)
Intersegment Elimination
Segment Reporting Information [Line Items]
Commission Expenses	[2]									958	1,478	2,258
Other Underwriting Income (Expense)	[2]									$ (958)	$ (1,478)	$ (2,258)
[1]	Includes Other Operating Expenses and Other Underwriting Income (Expense).
[2]	Includes Corporate segment intercompany eliminations.

Additional Financial Data by Se

Additional Financial Data by Segment (Detail) - 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 Information [Line Items]
Gross written premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
Ceded written premiums									(441,935)	(382,687)	(409,642)
Net written premiums	304,869	296,016	333,282	337,163	282,868	277,001	306,535	319,820	1,271,330	1,186,224	1,043,860
Net earned premiums	$ 305,099	$ 301,355	$ 293,835	$ 286,131	$ 283,985	$ 284,010	$ 267,992	$ 264,358	1,186,420	1,100,345	984,087
Property Casualty
Segment Reporting Information [Line Items]
Ceded written premiums									(441,935)	(382,687)	(409,642)
Net written premiums									1,190,725	1,132,410	956,772
U.S.Insurance
Segment Reporting Information [Line Items]
Gross written premiums									988,293	919,395	865,781
Ceded written premiums									(266,993)	(235,827)	(268,775)
Net written premiums									721,300	683,568	597,006
Net earned premiums									674,665	629,308	555,836
U.S.Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									156,171	169,405	158,124
Ceded written premiums									(72,431)	(70,858)	(61,916)
Net written premiums									83,740	98,547	96,208
Net earned premiums									86,605	100,132	96,082
U.S.Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									713,539	631,562	596,673
Ceded written premiums									(173,501)	(135,888)	(152,168)
Net written premiums									540,038	495,674	444,505
Net earned premiums									495,260	453,673	401,408
U.S.Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									118,583	118,428	110,984
Ceded written premiums									(21,061)	(29,081)	(54,691)
Net written premiums									97,522	89,347	56,293
Net earned premiums									92,800	75,503	58,346
International Insurance
Segment Reporting Information [Line Items]
Gross written premiums									501,130	484,471	411,947
Ceded written premiums									(165,278)	(138,504)	(134,005)
Net written premiums									335,852	345,967	277,942
Net earned premiums									333,792	307,416	259,960
International Insurance | Marine
Segment Reporting Information [Line Items]
Gross written premiums									198,241	183,228	183,707
Ceded written premiums									(42,018)	(40,092)	(36,515)
Net written premiums									156,223	143,136	147,192
Net earned premiums									152,396	141,593	149,256
International Insurance | Property Casualty
Segment Reporting Information [Line Items]
Gross written premiums									159,123	181,094	130,729
Ceded written premiums									(85,298)	(69,606)	(67,722)
Net written premiums									73,825	111,488	63,007
Net earned premiums									88,517	89,455	55,320
International Insurance | Professional Liability
Segment Reporting Information [Line Items]
Gross written premiums									143,766	120,149	97,511
Ceded written premiums									(37,962)	(28,806)	(29,768)
Net written premiums									105,804	91,343	67,743
Net earned premiums									92,879	76,368	55,384
Global Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									223,842	165,045	175,774
Ceded written premiums									(9,664)	(8,356)	(6,862)
Net written premiums									214,178	156,689	168,912
Net earned premiums									$ 177,963	$ 163,621	$ 168,291

Summary of Gross Written Premiu

Summary of Gross Written Premiums Allocated Based on Location of Insured's Head Office (Detail) - 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 Information [Line Items]
Gross Written Premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
United States
Segment Reporting Information [Line Items]
Gross Written Premiums									1,203,695	1,101,337	1,057,895
Worldwide (Excluding the United States)
Segment Reporting Information [Line Items]
Gross Written Premiums									$ 509,570	$ 467,574	$ 395,607

Available-for-Sale Investments

Available-for-Sale Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Total investments		$ 3,420,163		$ 3,128,563
Gross Unrealized Gains		57,655		59,175
Gross Unrealized Losses		(16,184)		(30,199)
Amortized Cost		3,378,692		3,099,587
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Total investments		3,057,054	[1]	2,635,882
Gross Unrealized Gains		44,158		32,071
Gross Unrealized Losses		(14,512)		(24,414)
Amortized Cost		3,027,408		2,628,225
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Total investments		393,563	[1]	273,776
Gross Unrealized Gains		2,081		2,192
Gross Unrealized Losses		(2,014)		(5,128)
Amortized Cost		393,496		276,712
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Total investments		814,632	[1]	547,415
Gross Unrealized Gains		20,136		11,542
Gross Unrealized Losses		(1,423)		(4,036)
Amortized Cost		795,919		539,909
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		407,619	[1]	487,364
Gross Unrealized Gains		2,352		4,016
Gross Unrealized Losses		(5,414)		(6,585)
Amortized Cost		410,681		489,933
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		54,104	[1]	20,530
Gross Unrealized Gains		606		453
Gross Unrealized Losses		(79)		(55)
Amortized Cost		53,577		20,132
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		328,753	[1]	314,601
Gross Unrealized Gains		2,138		824
Gross Unrealized Losses		(663)		(1,178)
Amortized Cost		327,278		314,955
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		160,904	[1]	154,139
Gross Unrealized Gains		2,354		2,859
Gross Unrealized Losses		(1,182)		(1,904)
Amortized Cost		159,732		153,184
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments		951,380	[1]	976,634
Gross Unrealized Gains		7,450		8,152
Gross Unrealized Losses		(7,338)		(9,722)
Amortized Cost		951,268		978,204
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Total investments	[2]	897,479	[1]	838,057
Gross Unrealized Gains	[2]	14,491		10,185
Gross Unrealized Losses	[2]	(3,737)		(5,528)
Amortized Cost	[2]	886,725		833,400
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments		235,981	[1]	349,142
Gross Unrealized Gains		13,494		27,016
Gross Unrealized Losses		(1,672)		(5,785)
Amortized Cost		224,159		327,911
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Total investments		52,439	[1]	164,087
Gross Unrealized Gains		7,423		24,677
Gross Unrealized Losses		(112)		(964)
Amortized Cost		45,128		140,374
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Total investments		183,542	[1]	185,055
Gross Unrealized Gains		6,071		2,339
Gross Unrealized Losses		(1,560)		(4,821)
Amortized Cost		179,031		187,537
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Total investments		127,128	[1]	143,539
Gross Unrealized Gains		3		88
Amortized Cost		$ 127,125		$ 143,451
[1]	Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value.
[2]	Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.

Investments - Additional Inform

Investments - Additional Information (Detail) - 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 Investments [Line Items]
Other Invested Assets	$ 1,720				$ 1,960				$ 1,720	$ 1,960
Non-credit OTTI on fixed maturities in a net unrealized net gain position									500	400
Gross Unrealized Loss	16,184				30,199				16,184	30,199
OTTI losses									2,064	150	$ 1,698
Net Realized Gains	37,141	$ 5,190	$ 1,694	$ 1,049	4,043	$ 1,586	$ 1,960	$ 1,597	45,073	9,186	8,373
Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	14,512				24,414				14,512	24,414
OTTI losses										200
Net Realized Gains									792	1,410	(1,170)
Equity Securities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 1,672				$ 5,785				1,672	5,785
OTTI losses									2,100		1,700
Net Realized Gains									$ 43,262	$ 8,438	$ 9,796
Non-government-backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%				5.00%				5.00%	5.00%
Gross Unrealized Loss	$ 700				$ 1,000				$ 700	$ 1,000
Mortgage-Backed and Asset-Backed Securities
Schedule of Investments [Line Items]
Securities, effective maturities									5 years
Mortgage-Backed and Asset-Backed Securities | Fixed Maturities
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 7,338				$ 9,722				$ 7,338	$ 9,722

Scheduled Maturity Dates for Fi

Scheduled Maturity Dates for Fixed Maturity Securities by Number of Years Until Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value
Due in One Year or Less	$ 228,479
Due after One Year Through Five Years	809,718
Due After Five Years Through Ten Years	359,745
Due After Ten Years	707,732
Mortgage-Backed and Asset-Backed Securities	951,380
Total	3,057,054	$ 2,635,882
Amortized Cost
Due in One Year or Less	228,255
Due after One Year Through Five Years	809,195
Due After Five Years Through Ten Years	352,524
Due After Ten Years	686,166
Mortgage-Backed and Asset-Backed Securities	951,268
Total	$ 3,027,408	$ 2,628,225

Summary of Aggregate Fair Value

Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		$ 911,581	$ 1,256,887
Less than 12 months Gross Unrealized Losses		(6,271)	(24,493)
Greater than 12 months Fair Value		508,338	127,403
Greater than 12 months Gross Unrealized Losses		(9,913)	(5,706)
Fair Value		1,419,919	1,384,290
Gross Unrealized Losses		(16,184)	(30,199)
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		849,475	1,111,873
Less than 12 months Gross Unrealized Losses		(4,666)	(19,074)
Greater than 12 months Fair Value		505,906	125,409
Greater than 12 months Gross Unrealized Losses		(9,846)	(5,340)
Fair Value		1,355,381	1,237,282
Gross Unrealized Losses		(14,512)	(24,414)
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		273,672	150,891
Less than 12 months Gross Unrealized Losses		(1,502)	(2,570)
Greater than 12 months Fair Value		54,484	16,819
Greater than 12 months Gross Unrealized Losses		(512)	(2,558)
Fair Value		328,156	167,710
Gross Unrealized Losses		(2,014)	(5,128)
Fixed Maturities | States, Municipalities and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		74,097	137,731
Less than 12 months Gross Unrealized Losses		(503)	(3,111)
Greater than 12 months Fair Value		45,085	13,255
Greater than 12 months Gross Unrealized Losses		(920)	(925)
Fair Value		119,182	150,986
Gross Unrealized Losses		(1,423)	(4,036)
Fixed Maturities | Agency Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		87,496	349,119
Less than 12 months Gross Unrealized Losses		(346)	(6,155)
Greater than 12 months Fair Value		236,745	12,401
Greater than 12 months Gross Unrealized Losses		(5,068)	(430)
Fair Value		324,241	361,520
Gross Unrealized Losses		(5,414)	(6,585)
Fixed Maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		12,418	953
Less than 12 months Gross Unrealized Losses		(62)	(18)
Greater than 12 months Fair Value		546	926
Greater than 12 months Gross Unrealized Losses		(17)	(37)
Fair Value		12,964	1,879
Gross Unrealized Losses		(79)	(55)
Fixed Maturities | Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		85,877	95,514
Less than 12 months Gross Unrealized Losses		(468)	(970)
Greater than 12 months Fair Value		24,733	48,093
Greater than 12 months Gross Unrealized Losses		(195)	(208)
Fair Value		110,610	143,607
Gross Unrealized Losses		(663)	(1,178)
Fixed Maturities | Commercial Mortgage-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		20,482	51,932
Less than 12 months Gross Unrealized Losses		(95)	(1,164)
Greater than 12 months Fair Value		22,903	7,910
Greater than 12 months Gross Unrealized Losses		(1,087)	(740)
Fair Value		43,385	59,842
Gross Unrealized Losses		(1,182)	(1,904)
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		206,273	497,518
Less than 12 months Gross Unrealized Losses		(971)	(8,307)
Greater than 12 months Fair Value		284,927	69,330
Greater than 12 months Gross Unrealized Losses		(6,367)	(1,415)
Fair Value		491,200	566,848
Gross Unrealized Losses		(7,338)	(9,722)
Fixed Maturities | Corporate Exposures
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	[1]	295,433	325,733
Less than 12 months Gross Unrealized Losses	[1]	(1,690)	(5,086)
Greater than 12 months Fair Value	[1]	121,410	26,005
Greater than 12 months Gross Unrealized Losses	[1]	(2,047)	(442)
Fair Value	[1]	416,843	351,738
Gross Unrealized Losses	[1]	(3,737)	(5,528)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		62,106	145,014
Less than 12 months Gross Unrealized Losses		(1,605)	(5,419)
Greater than 12 months Fair Value		2,432	1,994
Greater than 12 months Gross Unrealized Losses		(67)	(366)
Fair Value		64,538	147,008
Gross Unrealized Losses		(1,672)	(5,785)
Equity Securities | Common Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		11,245	31,272
Less than 12 months Gross Unrealized Losses		(81)	(634)
Greater than 12 months Fair Value		1,770	1,471
Greater than 12 months Gross Unrealized Losses		(31)	(330)
Fair Value		13,015	32,743
Gross Unrealized Losses		(112)	(964)
Equity Securities | Preferred Stocks
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value		50,861	113,742
Less than 12 months Gross Unrealized Losses		(1,524)	(4,785)
Greater than 12 months Fair Value		662	523
Greater than 12 months Gross Unrealized Losses		(36)	(36)
Fair Value		51,523	114,265
Gross Unrealized Losses		$ (1,560)	$ (4,821)
[1]	Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.

Summary of Cumulative Amounts R

Summary of Cumulative Amounts Related to Credit Loss Portion of Other-Than-Temporary Impairment Losses on Debt Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Beginning Balance	$ 2,361	$ 2,361	$ 2,361
Additions for Credit Loss Impairments recognized in the current period on securities not previously impaired	0	150	0
Additions for Credit Loss Impairments recognized in the current period on securities previously impaired	0	0	0
Reductions for Credit Loss Impairments previously recognized on securities sold during the period	0	(150)	0
Ending Balance	$ 2,361	$ 2,361	$ 2,361

Net Investment Income (Detail)

Net Investment Income (Detail) - 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 [Line Items]
Investment Income									$ 92,956	$ 82,770	$ 72,068
Investment Expenses									(3,663)	(3,319)	(3,350)
Net Investment Income	$ 22,982	$ 22,598	$ 22,265	$ 21,448	$ 20,107	$ 19,875	$ 19,875	$ 19,594	89,293	79,451	68,718
Fixed Maturities
Net Investment Income [Line Items]
Investment Income									76,784	67,772	61,572
Equity Securities
Net Investment Income [Line Items]
Investment Income									15,108	14,271	9,813
Short-Term Investments
Net Investment Income [Line Items]
Investment Income									$ 1,064	$ 727	$ 683

Realized Gains and Losses, Excl

Realized Gains and Losses, Excluding Net Other-Than-Temporary Impairment Losses Recognized in Earnings (Detail) - 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
Gain (Loss) on Investments [Line Items]
Net Realized Gains (Losses)	$ 37,141	$ 5,190	$ 1,694	$ 1,049	$ 4,043	$ 1,586	$ 1,960	$ 1,597	$ 45,073	$ 9,186	$ 8,373
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gains									4,041	5,681	4,756
Losses									(3,249)	(4,271)	(5,926)
Net Realized Gains (Losses)									792	1,410	(1,170)
Short-Term Investments
Gain (Loss) on Investments [Line Items]
Gains									1,631	890	130
Losses									(612)	(1,552)	(383)
Net Realized Gains (Losses)									1,019	(662)	(253)
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains									44,783	9,096	14,331
Losses									(1,521)	(658)	(4,535)
Net Realized Gains (Losses)									$ 43,262	$ 8,438	$ 9,796

Fair Value Measurement - Additi

Fair Value Measurement - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Significant transfers between Level 1 and Level 2	$ 0	$ 0
Level 3 assets	$ 0	$ 0
5.75% Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate stated on senior notes	5.75%
Senior notes due date	Oct. 15, 2023

Fixed Maturities and Equity Sec

Fixed Maturities and Equity Securities by Asset Class that are Measured at Fair Value (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		$ 3,420,163		$ 3,128,563
Total Liabilities Measured at Fair Value		277,951		280,316
Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		3,057,054	[1]	2,635,882
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		393,563	[1]	273,776
Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		814,632	[1]	547,415
Fixed Maturities | Agency Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		407,619	[1]	487,364
Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		54,104	[1]	20,530
Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		328,753	[1]	314,601
Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		160,904	[1]	154,139
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		951,380	[1]	976,634
Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value	[2]	897,479	[1]	838,057
Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		235,981	[1]	349,142
Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		52,439	[1]	164,087
Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		183,542	[1]	185,055
Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		127,128	[1]	143,539
Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value		277,951		280,316
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		357,818		355,330
Total Liabilities Measured at Fair Value		0		0
Level 1 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		178,251		47,704
Level 1 | Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		178,251		47,704
Level 1 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Fixed Maturities | Agency Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		52,439		164,087
Level 1 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		52,439		164,087
Level 1 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 1 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		127,128		143,539
Level 1 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value		0		0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		3,062,345		2,773,233
Total Liabilities Measured at Fair Value		277,951		280,316
Level 2 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		2,878,803		2,588,178
Level 2 | Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		215,312		226,072
Level 2 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		814,632		547,415
Level 2 | Fixed Maturities | Agency Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		407,619		487,364
Level 2 | Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		54,104		20,530
Level 2 | Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		328,753		314,601
Level 2 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		160,904		154,139
Level 2 | Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		951,380		976,634
Level 2 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		897,479		838,057
Level 2 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		183,542		185,055
Level 2 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 2 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		183,542		185,055
Level 2 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 2 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value		277,951		280,316
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Total Liabilities Measured at Fair Value		0		0
Level 3 | Fixed Maturities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | Agency Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | Residential Mortgage Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Fixed Maturities | Corporate Exposures
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Equity Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Equity Securities | Common Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Equity Securities | Preferred Stocks
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Short-Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed Maturities and Equity Securities, at Fair Value		0		0
Level 3 | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes, at Fair Value		$ 0		$ 0
[1]	Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value.
[2]	Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.

Summary of Activity in Reserve

Summary of Activity in Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,510,451	$ 1,393,126	$ 1,308,136
Provision for Losses and LAE for Claims Occurring in the Current Year	771,955	693,976	637,267
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	34,310	(28,528)	(64,669)
Incurred Losses and LAE	806,265	665,448	572,598
Losses and LAE Paid for Claims Occurring During:
Current Year	(144,095)	(119,485)	(159,802)
Prior Years	(478,979)	(412,544)	(320,863)
Losses and LAE Payments	(623,074)	(532,029)	(480,665)
Foreign Currency Adjustment	11,738	(16,094)	(6,943)
Net Reserves for Losses and LAE at End of Period	1,705,380	1,510,451	1,393,126
Reinsurance Recoverables on Unpaid Losses and LAE	809,765	779,276	809,518
Gross Reserves for Losses and LAE at End of Period	$ 2,515,145	$ 2,289,727	$ 2,202,644

Reporting and Operating Segment

Reporting and Operating Segments Breakdowns of Prior Period Net Reserve Deficiency (Redundancy) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 34,310	$ (28,528)	$ (64,669)
U.S.Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	16,714	(1,348)	(29,319)
U.S.Insurance | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,696	(10,122)	(24,825)
U.S.Insurance | Property Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,613	2,790	(706)
U.S.Insurance | Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	13,405	5,984	(3,788)
International Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,834	(25,019)	(26,216)
International Insurance | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	6,518	(22,068)	(21,914)
International Insurance | Property Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,118	(7,051)	(8,458)
International Insurance | Professional Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	5,198	4,100	4,156
Global Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	$ 2,762	$ (2,161)	$ (9,134)

Reserves for Losses and LAE - A

Reserves for Losses and LAE - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Increase in incurred losses and LAE	$ 140,800	$ 92,900
Provision for Losses and LAE for Claims Occurring in the Current Year	771,955	693,976	$ 637,267
Net reserve deficiencies (redundancies) in prior years	34,310	(28,528)	(64,669)
U.S.Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	16,714	(1,348)	(29,319)
U.S.Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,613	2,790	(706)
U.S.Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	1,696	(10,122)	(24,825)
U.S.Insurance | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	13,405	5,984	(3,788)
International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	14,834	(25,019)	(26,216)
International Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	3,118	(7,051)	(8,458)
International Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	6,518	(22,068)	(21,914)
International Insurance | Professional Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	5,198	4,100	4,156
Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve deficiencies (redundancies) in prior years	2,762	(2,161)	$ (9,134)
Irma
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	28,300
Maria
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	23,400
Harvey
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	15,800
Typhoon Hato
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	4,200
Puebla, Mexico Earthquake
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	3,500
Other CAT Events
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	9,600
Alberta Wildfires
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		11,700
Hurricane Matthew
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		7,100
Ecuador Earthquake
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		3,800
Taiwan Earthquake
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		3,300
Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	84,800	26,800
Net incurred but not reported (“IBNR”) claims	18,500
Catastrophe | U.S.Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	8,200
Catastrophe | U.S.Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	6,600
Catastrophe | U.S.Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	1,600
Catastrophe | International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	32,900
Catastrophe | International Insurance | Property Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	16,900
Catastrophe | International Insurance | Marine
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	16,000
Catastrophe | Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year	$ 43,700
Hurricane Matthew And Alberta Wildfires | International Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		17,400
Alberta Wildfires And Ecuador Earthquake | Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Provision for Losses and LAE for Claims Occurring in the Current Year		$ 9,400

Summary of Information about In

Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance (Detail) $ in Thousands	Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	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]
Total Net reserves	$ 1,640,874
U.S.Insurance | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	560,568
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	474,664
Net reserves lesser than 2008	15,187
Total Net reserves	$ 101,091
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	27.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	31.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	16.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	8.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	7.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	3.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	2.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	1.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	1.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	0.60%
U.S.Insurance | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 1,683,912
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	859,169
Net reserves lesser than 2008	24,266
Total Net reserves	$ 849,009
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	5.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	10.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	17.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	18.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	15.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	10.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	5.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	5.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	5.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	2.60%
U.S.Insurance | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 547,105
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	439,684
Net reserves lesser than 2008	881
Total Net reserves	$ 108,302
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	5.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	30.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	25.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	13.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	9.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	8.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	3.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	1.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	5.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	0.00%
U.S.Insurance | 2008 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 66,849	$ 66,937	$ 66,717	$ 68,557	$ 68,393	$ 73,494	$ 73,734	$ 72,259	$ 67,421	$ 56,878
Total IBNR	$ 282
Number of Reported Claims | Claim	3,194
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 64,433	64,029	63,140	63,027	62,700	62,398	56,460	54,175	36,397	7,554
U.S.Insurance | 2008 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	154,169	152,814	147,763	142,641	138,499	136,498	135,255	138,860	152,659	151,401
Total IBNR	$ 3,706
Number of Reported Claims | Claim	10,937
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 146,295	142,284	128,886	119,766	111,420	99,825	78,024	54,663	32,307	10,266
U.S.Insurance | 2008 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	43,003	41,766	37,620	37,497	37,939	37,603	35,844	39,130	38,449	34,346
Total IBNR	$ 875
Number of Reported Claims | Claim	770
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 42,046	42,056	37,175	35,598	35,206	34,902	30,581	25,042	15,288	$ 1,880
U.S.Insurance | 2009 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	51,537	50,874	51,783	49,571	48,366	50,501	56,052	59,278	62,556
Total IBNR	$ 205
Number of Reported Claims | Claim	2,323
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 50,191	49,573	46,894	46,033	43,450	41,650	35,449	29,266	12,947
U.S.Insurance | 2009 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	149,622	150,294	147,036	146,321	141,186	135,984	130,860	129,613	127,237
Total IBNR	$ 6,143
Number of Reported Claims | Claim	8,515
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 138,829	136,636	129,461	123,981	110,648	86,736	55,773	33,049	11,245
U.S.Insurance | 2009 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	61,083	58,143	58,660	58,487	60,789	58,600	61,023	53,273	47,473
Total IBNR	$ 788
Number of Reported Claims | Claim	1,370
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 58,150	58,095	57,744	53,859	55,378	51,642	41,675	21,742	$ 3,837
U.S.Insurance | 2010 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	56,281	56,458	57,008	52,847	54,813	60,035	63,812	64,753
Total IBNR	$ (224)
Number of Reported Claims | Claim	2,007
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 55,249	55,096	51,887	48,114	42,896	39,825	32,323	16,582
U.S.Insurance | 2010 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	110,170	107,914	104,955	103,038	100,006	94,317	96,446	96,885
Total IBNR	$ 7,776
Number of Reported Claims | Claim	7,874
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 97,125	90,625	85,740	77,302	65,094	46,478	26,482	11,985
U.S.Insurance | 2010 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	86,164	85,348	86,787	74,535	75,619	68,375	64,020	52,302
Total IBNR	$ 932
Number of Reported Claims | Claim	1,431
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 84,529	84,511	82,013	59,765	54,019	46,868	23,147	$ 2,981
U.S.Insurance | 2011 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	59,750	60,329	61,912	63,099	67,640	69,293	70,219
Total IBNR	$ 630
Number of Reported Claims | Claim	1,910
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 58,558	57,929	56,955	52,408	48,850	39,821	14,955
U.S.Insurance | 2011 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	100,461	98,238	90,613	89,666	86,557	78,093	79,832
Total IBNR	$ 12,051
Number of Reported Claims | Claim	6,450
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 79,682	70,486	58,489	46,638	30,359	14,579	3,630
U.S.Insurance | 2011 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	82,558	79,136	79,181	73,028	70,240	62,087	58,116
Total IBNR	$ 699
Number of Reported Claims | Claim	1,726
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 78,258	75,641	65,710	59,576	43,551	23,878	$ 3,204
U.S.Insurance | 2012 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	70,887	70,337	68,841	72,219	87,606	85,275
Total IBNR	$ (619)
Number of Reported Claims | Claim	1,856
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 69,059	67,220	60,734	54,673	46,308	27,254
U.S.Insurance | 2012 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	111,146	107,245	99,207	91,502	89,758	103,089
Total IBNR	$ 18,194
Number of Reported Claims | Claim	4,579
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 82,906	67,502	47,005	27,513	17,311	4,761
U.S.Insurance | 2012 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	58,563	58,344	55,917	57,097	54,480	61,196
Total IBNR	$ 2,254
Number of Reported Claims | Claim	2,175
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 54,728	49,675	44,028	35,681	22,635	3,855
U.S.Insurance | 2013 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	56,319	54,381	51,748	57,865	61,356
Total IBNR	$ 144
Number of Reported Claims | Claim	1,814
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 51,466	47,162	41,488	35,471	17,831
U.S.Insurance | 2013 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	149,409	137,298	129,100	125,952	139,517
Total IBNR	$ 20,569
Number of Reported Claims | Claim	3,898
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 118,506	84,494	48,716	20,914	7,146
U.S.Insurance | 2013 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	54,228	48,298	49,104	54,982	60,899
Total IBNR	$ 3,915
Number of Reported Claims | Claim	1,982
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 47,062	38,876	32,904	22,167	4,677
U.S.Insurance | 2014 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	45,762	41,006	42,684	59,270
Total IBNR	$ (377)
Number of Reported Claims | Claim	1,927
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 41,021	34,899	23,415	10,717
U.S.Insurance | 2014 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	155,248	148,541	159,477	191,515
Total IBNR	$ 51,231
Number of Reported Claims | Claim	3,880
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 74,979	44,468	13,264	5,229
U.S.Insurance | 2014 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	42,060	39,826	35,870	50,020
Total IBNR	$ 5,255
Number of Reported Claims | Claim	2,017
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 35,114	29,392	14,371	2,304
U.S.Insurance | 2015 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	50,331	51,864	61,106
Total IBNR	$ 5,148
Number of Reported Claims | Claim	2,187
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 40,586	31,010	16,945
U.S.Insurance | 2015 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	214,547	218,973	244,311
Total IBNR	$ 76,700
Number of Reported Claims | Claim	3,672
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 78,856	24,057	3,575
U.S.Insurance | 2015 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	31,088	30,764	31,659
Total IBNR	$ 8,721
Number of Reported Claims | Claim	1,674
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 18,428	11,249	2,257
U.S.Insurance | 2016 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	48,791	53,011
Total IBNR	$ 12,748
Number of Reported Claims | Claim	2,352
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 25,154	11,610
U.S.Insurance | 2016 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	243,754	271,351
Total IBNR	$ 190,052
Number of Reported Claims | Claim	3,136
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 29,131	5,831
U.S.Insurance | 2016 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	36,650	41,673
Total IBNR	$ 12,608
Number of Reported Claims | Claim	1,806
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 18,687	2,382
U.S.Insurance | 2017 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	54,061
Total IBNR	$ 23,327
Number of Reported Claims | Claim	1,986
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 18,947
U.S.Insurance | 2017 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	295,386
Total IBNR	$ 263,648
Number of Reported Claims | Claim	2,527
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 12,860
U.S.Insurance | 2017 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	51,708
Total IBNR	$ 44,396
Number of Reported Claims | Claim	1,902
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,682
International Insurance | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	945,486
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	734,899
Net reserves lesser than 2008	11,072
Total Net reserves	$ 221,659
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	25.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	29.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	19.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	7.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	4.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	2.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	2.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	1.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	1.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	0.00%
International Insurance | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 441,670
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	346,815
Net reserves lesser than 2008	825
Total Net reserves	$ 95,680
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	25.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	32.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	26.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	13.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	4.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	(1.00%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	(1.50%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	0.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	(0.10%)
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	0.20%
International Insurance | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 232,082
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	115,573
Net reserves lesser than 2008	274
Total Net reserves	$ 116,783
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	6.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	8.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	16.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	20.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	15.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	5.40%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	35.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	0.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	2.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	0.20%
International Insurance | 2008 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 83,962	84,449	84,122	80,589	80,793	85,148
Total IBNR	$ (135)
Number of Reported Claims | Claim	5,733
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 80,183	80,185	79,024	75,255	73,799	71,355
International Insurance | 2008 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	41,472	41,536	33,877	34,495	34,428	37,256
Total IBNR	$ 67
Number of Reported Claims | Claim	913
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 40,333	40,249	40,398	39,869	39,489	38,275
International Insurance | 2008 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	14,463	14,082	13,829	13,798	13,524	11,749
Total IBNR	$ 258
Number of Reported Claims | Claim	439
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 14,105	14,071	13,784	13,754	13,360	12,198
International Insurance | 2009 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	81,729	82,074	83,477	90,527	91,705	96,465
Total IBNR	$ (265)
Number of Reported Claims | Claim	5,045
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 78,772	77,487	76,783	73,321	70,553	66,388
International Insurance | 2009 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	34,461	34,186	40,692	40,984	40,896	38,680
Total IBNR	$ 206
Number of Reported Claims | Claim	885
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 33,494	33,405	33,317	33,685	33,223	28,919
International Insurance | 2009 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	15,988	16,020	17,016	17,365	17,225	16,661
Total IBNR	$ 758
Number of Reported Claims | Claim	624
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 14,918	14,331	14,008	12,983	10,901	8,954
International Insurance | 2010 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	108,523	112,259	114,940	119,156	130,102	132,080
Total IBNR	$ (428)
Number of Reported Claims | Claim	5,129
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 103,646	103,188	100,897	96,920	90,871	78,509
International Insurance | 2010 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	42,613	42,822	42,196	41,922	42,527	43,096
Total IBNR	$ 309
Number of Reported Claims | Claim	1,089
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 41,691	41,204	41,102	43,429	40,880	35,192
International Insurance | 2010 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	14,639	15,864	14,221	14,225	14,023	14,423
Total IBNR	$ 247
Number of Reported Claims | Claim	745
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 13,643	13,597	12,840	10,659	7,320	5,909
International Insurance | 2011 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	74,309	73,970	77,848	82,441	85,720	100,162
Total IBNR	$ (500)
Number of Reported Claims | Claim	4,643
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 70,022	69,672	68,702	63,778	59,341	45,896
International Insurance | 2011 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	55,127	59,516	59,529	59,405	61,564	71,531
Total IBNR	$ 221
Number of Reported Claims | Claim	1,216
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 52,225	54,239	53,318	50,753	47,851	38,548
International Insurance | 2011 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	12,823	6,989	8,923	19,429	18,125	15,390
Total IBNR	$ 1,769
Number of Reported Claims | Claim	665
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 16,642	4,677	4,087	2,994	2,038	1,221
International Insurance | 2012 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	92,983	93,052	100,029	107,636	132,245	110,316
Total IBNR	$ (1,224)
Number of Reported Claims | Claim	4,908
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 83,381	81,411	78,789	70,657	54,954	23,382
International Insurance | 2012 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	33,075	32,622	32,472	36,444	37,520	45,693
Total IBNR	$ 152
Number of Reported Claims | Claim	1,358
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 31,596	31,332	30,135	27,084	21,578	13,238
International Insurance | 2012 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	18,068	17,168	13,457	12,789	11,506	6,835
Total IBNR	$ 2,365
Number of Reported Claims | Claim	1,102
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 6,933	7,526	5,600	4,293	3,236	2,141
International Insurance | 2013 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	84,100	81,989	82,641	94,614	87,987
Total IBNR	$ 3,168
Number of Reported Claims | Claim	4,716
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 67,730	64,859	58,687	46,552	23,731
International Insurance | 2013 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	35,443	34,499	36,599	37,520	36,044
Total IBNR	$ 1,027
Number of Reported Claims | Claim	1,592
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 30,279	28,682	25,840	18,441	7,160
International Insurance | 2013 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	16,842	15,748	14,532	4,099	10,218
Total IBNR	$ 1,607
Number of Reported Claims | Claim	1,174
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 13,906	9,478	3,078	2,285	733
International Insurance | 2014 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	119,767	116,899	117,974	104,397
Total IBNR	$ (1,483)
Number of Reported Claims | Claim	4,986
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 90,619	81,941	57,629	29,977
International Insurance | 2014 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	16,449	20,997	22,787	27,829
Total IBNR	$ (1,396)
Number of Reported Claims | Claim	1,300
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 16,805	13,232	9,015	3,902
International Insurance | 2014 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	21,758	23,861	24,519	20,662
Total IBNR	$ 4,216
Number of Reported Claims | Claim	1,747
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 8,945	5,228	2,493	1,387
International Insurance | 2015 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	96,614	95,001	99,067
Total IBNR	$ (1,913)
Number of Reported Claims | Claim	7,400
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 76,044	54,550	22,862
International Insurance | 2015 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	33,969	36,689	34,889
Total IBNR	$ 10
Number of Reported Claims | Claim	1,986
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 29,624	18,436	6,845
International Insurance | 2015 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	35,586	30,539	28,319
Total IBNR	$ 9,550
Number of Reported Claims | Claim	2,370
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 17,502	5,939	3,457
International Insurance | 2016 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	95,584	88,388
Total IBNR	$ 8,335
Number of Reported Claims | Claim	9,114
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 55,420	23,665
International Insurance | 2016 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	92,108	79,393
Total IBNR	$ 18,665
Number of Reported Claims | Claim	3,556
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 55,749	26,190
International Insurance | 2016 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	34,218	36,167
Total IBNR	$ 22,567
Number of Reported Claims | Claim	2,744
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 6,955	2,076
International Insurance | 2017 | Marine
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	107,915
Total IBNR	$ 32,198
Number of Reported Claims | Claim	7,385
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 29,082
International Insurance | 2017 | Property Casualty
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	56,953
Total IBNR	$ 10,229
Number of Reported Claims | Claim	2,213
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 15,019
International Insurance | 2017 | Professional Liability
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	47,697
Total IBNR	$ 41,954
Number of Reported Claims | Claim	1,522
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,024
Global Reinsurance
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	765,010
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	619,164
Net reserves lesser than 2008	2,504
Total Net reserves	$ 148,350
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1	39.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2	34.00%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3	3.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4	3.80%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5	8.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6	3.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7	3.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8	0.50%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9	0.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10	0.70%
Global Reinsurance | 2008
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	$ 13,123	13,487	13,792	14,198	14,846	14,467
Total IBNR	$ 529
Number of Reported Claims | Claim	306
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 11,797	11,702	11,640	11,661	11,799	11,775
Global Reinsurance | 2009
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	10,406	10,567	10,516	11,267	11,381	11,247
Total IBNR	$ 53
Number of Reported Claims | Claim	200
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 10,133	10,133	10,018	9,060	8,930	8,694
Global Reinsurance | 2010
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	13,159	12,631	11,908	14,142	13,110	13,150
Total IBNR	$ (54)
Number of Reported Claims | Claim	201
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 11,272	11,207	11,154	10,529	10,349	10,005
Global Reinsurance | 2011
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	73,530	72,877	73,610	76,164	76,362	53,868
Total IBNR	$ (438)
Number of Reported Claims | Claim	769
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 72,414	72,246	72,102	71,816	71,138	43,450
Global Reinsurance | 2012
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	119,771	115,058	88,367	92,730	99,116	107,189
Total IBNR	$ (815)
Number of Reported Claims | Claim	1,456
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 117,782	111,962	84,801	83,408	77,227	$ 37,746
Global Reinsurance | 2013
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	102,433	105,172	102,632	118,779	114,873
Total IBNR	$ 1,067
Number of Reported Claims | Claim	1,285
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 100,059	98,484	90,721	82,701	$ 53,476
Global Reinsurance | 2014
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	118,255	121,716	142,058	119,379
Total IBNR	$ 5,062
Number of Reported Claims | Claim	1,070
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 108,794	105,648	119,249	$ 66,286
Global Reinsurance | 2015
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	94,428	91,767	101,764
Total IBNR	$ 6,897
Number of Reported Claims | Claim	896
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 76,973	64,527	$ 41,660
Global Reinsurance | 2016
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	90,755	88,543
Total IBNR	$ 15,972
Number of Reported Claims | Claim	749
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 66,277	$ 36,352
Global Reinsurance | 2017
Claims Development [Line Items]
Loss and ALAE, Net of Reinsurance	129,150
Total IBNR	$ 60,576
Number of Reported Claims | Claim	428
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 43,663

Summary of Reconciliation of Ne

Summary of Reconciliation of Net Incurred and Paid Claims Development to the Reserve for Losses and LAE (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	$ 1,640,874
Reinsurance Recoverables on Unpaid Losses and LAE	809,765	$ 779,276	$ 809,518
Total Gross Reserves	2,450,639
Unallocated Claims Adjustment Expenses	64,269
Other	237
Total Gross Liability for Unpaid Claims and Claim Adjustment Expense	2,515,145	$ 2,289,727	$ 2,202,644
U.S.Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	101,091
Reinsurance Recoverables on Unpaid Losses and LAE	98,373
Total Gross Reserves	199,464
U.S.Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	849,009
Reinsurance Recoverables on Unpaid Losses and LAE	371,651
Total Gross Reserves	1,220,660
U.S.Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	108,302
Reinsurance Recoverables on Unpaid Losses and LAE	49,650
Total Gross Reserves	157,952
International Insurance | Marine
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	221,659
Reinsurance Recoverables on Unpaid Losses and LAE	90,792
Total Gross Reserves	312,451
International Insurance | Property Casualty
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	95,680
Reinsurance Recoverables on Unpaid Losses and LAE	126,365
Total Gross Reserves	222,045
International Insurance | Professional Liability
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	116,783
Reinsurance Recoverables on Unpaid Losses and LAE	55,393
Total Gross Reserves	172,176
Global Reinsurance
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Total Net reserves	148,350
Reinsurance Recoverables on Unpaid Losses and LAE	17,541
Total Gross Reserves	$ 165,891

Ceded Reinsurance - Additional

Ceded Reinsurance - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Minimum amount of policyholders' surplus for reinsurance	$ 500,000
Reinsurance rating description	A reinsurer generally must have a rating from A.M. Best and/or S&P of “A” or better, or an equivalent financial strength if not rated, plus at least $500 million in policyholders’ surplus to meet our standards of acceptability.
Reserve for uncollectible reinsurance amount	$ 12,600	$ 12,100
Collateral held	180,664
Ceded Balances Payable
Effects of Reinsurance [Line Items]
Collateral held	136,400
Letter of Credit
Effects of Reinsurance [Line Items]
Collateral held	41,300
Trust Account Balances
Effects of Reinsurance [Line Items]
Collateral held	$ 3,000

Lists of 10 Largest Reinsurers

Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 228,569	$ 213,377
Reinsurance Recoverables Paid/Unpaid Losses	[1]	882,259
Reinsurance Recoverables Total		1,110,828
Collateral Held		180,664
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		5,770
Reinsurance Recoverables Paid/Unpaid Losses	[1]	61,413
Reinsurance Recoverables Total		67,183
Collateral Held		$ 5,423
National Indemnity Company | AM Best, A++ Rating | AM Best | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating		A++
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating		AA+
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 29,333
Reinsurance Recoverables Paid/Unpaid Losses	[1]	90,005
Reinsurance Recoverables Total		119,338
Collateral Held		$ 9,564
Everest Reinsurance Company | AM Best, A+ Rating | AM Best | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 18,987
Reinsurance Recoverables Paid/Unpaid Losses	[1]	71,062
Reinsurance Recoverables Total		90,049
Collateral Held		$ 8,069
Swiss Reinsurance America Corporation | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		AA-
Transatlantic Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 8,587
Reinsurance Recoverables Paid/Unpaid Losses	[1]	50,487
Reinsurance Recoverables Total		59,074
Collateral Held		$ 2,588
Transatlantic Reinsurance Company | AM Best, A+ Rating | AM Best | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Transatlantic Reinsurance Company | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Munich Reinsurance America Inc | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 10,991
Reinsurance Recoverables Paid/Unpaid Losses	[1]	46,751
Reinsurance Recoverables Total		57,742
Collateral Held		$ 5,171
Munich Reinsurance America Inc | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
Munich Reinsurance America Inc | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		AA-
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 6,511
Reinsurance Recoverables Paid/Unpaid Losses	[1]	32,422
Reinsurance Recoverables Total		38,933
Collateral Held		$ 1,637
Allied World Reinsurance | AM Best, A Rating | AM Best | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating		A
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating		A-
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 12,096
Reinsurance Recoverables Paid/Unpaid Losses	[1]	22,509
Reinsurance Recoverables Total		34,605
Collateral Held		$ 11,172
Employers Mutual Casualty Company | AM Best, A Rating | AM Best | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating		A
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating		NR
Aspen Insurance UK Ltd. | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 8,921
Reinsurance Recoverables Paid/Unpaid Losses	[1]	29,093
Reinsurance Recoverables Total		38,014
Collateral Held		$ 8,775
Aspen Insurance UK Ltd. | AM Best, A Rating | AM Best | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating		A
Aspen Insurance UK Ltd. | AM Best, A Rating | Standard & Poor's | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating		A
Scor Global P&C SE | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 5,938
Reinsurance Recoverables Paid/Unpaid Losses	[1]	25,228
Reinsurance Recoverables Total		31,166
Collateral Held		$ 6,393
Scor Global P&C SE | AM Best, A+ Rating | AM Best | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		A+
Scor Global P&C SE | AM Best, A+ Rating | Standard & Poor's | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating		AA-
Partner Reinsurance Europe | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 5,257
Reinsurance Recoverables Paid/Unpaid Losses	[1]	25,472
Reinsurance Recoverables Total		30,729
Collateral Held		$ 11,408
Partner Reinsurance Europe | AM Best, A Rating | AM Best | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A
Partner Reinsurance Europe | AM Best, A Rating | Standard & Poor's | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating		A+
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		$ 112,391
Reinsurance Recoverables Paid/Unpaid Losses	[1]	454,442
Reinsurance Recoverables Total		566,833
Collateral Held		70,200
All Others
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium		116,178
Reinsurance Recoverables Paid/Unpaid Losses	[1]	427,817
Reinsurance Recoverables Total		543,995
Collateral Held		$ 110,464
[1]	Net of reserve for uncollectible reinsurance of approximately $12.6 million.

Lists of 10 Largest Reinsurer66

Lists of 10 Largest Reinsurers Measured by Reinsurance Recoverable (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Reserve for uncollectible reinsurance amount	$ 12.6	$ 12.1

Summary of Components of Net Wr

Summary of Components of Net Written Premium (Detail) - 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
Insurance [Abstract]
Direct									$ 1,489,422	$ 1,403,865	$ 1,277,728
Assumed									223,843	165,046	175,774
Ceded written premiums									(441,935)	(382,687)	(409,642)
Net written premiums	$ 304,869	$ 296,016	$ 333,282	$ 337,163	$ 282,868	$ 277,001	$ 306,535	$ 319,820	$ 1,271,330	$ 1,186,224	$ 1,043,860

Summary of Components of Net Ea

Summary of Components of Net Earned Premium (Detail) - 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
Insurance [Abstract]
Direct									$ 1,425,582	$ 1,330,265	$ 1,223,840
Assumed									187,609	171,978	175,153
Ceded									(426,771)	(401,898)	(414,906)
Net Earned Premiums	$ 305,099	$ 301,355	$ 293,835	$ 286,131	$ 283,985	$ 284,010	$ 267,992	$ 264,358	$ 1,186,420	$ 1,100,345	$ 984,087

Summary of Components of Net Lo

Summary of Components of Net Losses and Loss Adjustment Expenses Incurred (Detail) - 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
Insurance [Abstract]
Direct									$ 938,640	$ 795,414	$ 703,361
Assumed									148,606	87,995	97,947
Ceded									(280,981)	(217,961)	(228,710)
Net Losses and LAE	$ 183,384	$ 276,171	$ 177,110	$ 169,600	$ 172,493	$ 172,793	$ 167,206	$ 152,956	$ 806,265	$ 665,448	$ 572,598

Debt - Additional Information (

Debt - Additional Information (Detail) £ in Millions, AUD in Millions	Nov. 07, 2016USD ($)Tranche	Nov. 04, 2016GBP (£)	Oct. 04, 2013USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2017AUD	Dec. 31, 2016USD ($)	Nov. 07, 2016GBP (£)Tranche
Line of Credit Facility [Line Items]
Issuance of new letters of credit, expiry date				Dec. 31, 2018
Line of credit facility, covenant terms				The Bilateral and Club Facilities contain customary covenants for facilities of this type, including restrictions on indebtedness and liens, limitations on mergers, dividends and the sale of assets, and requirements as to maintaining certain consolidated tangible net worth, statutory surplus and other financial ratios. These credit facilities also provide for customary events of default, including failure to pay principal, interest or fees when due, failure to comply with covenants, any representation or warranty made by our Company being false in any material respect, default under certain other indebtedness, certain insolvency or receivership events affecting our Company and our subsidiaries, the occurrence of certain material judgments, or a change in control of our Company. As of December 31, 2017, our Company was in compliance with all covenants.
Senior Notes
Line of Credit Facility [Line Items]
Unamortized discount				$ 1,100,000			$ 1,300,000
5.75% Senior Notes
Line of Credit Facility [Line Items]
Interest rate stated on senior notes				5.75%	5.75%	5.75%
Senior notes due date				Oct. 15, 2023
5.75% Senior Notes | Senior Notes
Line of Credit Facility [Line Items]
Principal amount of senior notes			$ 265,000,000
Interest rate stated on senior notes			5.75%
Net proceeds from issuance of senior notes			$ 263,300,000
Debt instrument, covenant compliance				As of December 31, 2017, our Company was in compliance with all such covenants
7.0% Senior Notes | Senior Notes
Line of Credit Facility [Line Items]
Interest rate stated on senior notes				7.00%	7.00%	7.00%
Senior notes due date				May 1, 2016
Call premium on redemption of Senior notes				$ 17,900,000
Club Facility
Line of Credit Facility [Line Items]
Number of tranches | Tranche	2							2
Club Facility | Minimum
Line of Credit Facility [Line Items]
Credit facility fee rate	0.95%
Club Facility | Maximum
Line of Credit Facility [Line Items]
Credit facility fee rate	1.60%
Club Facility | Tranche One Extending
Line of Credit Facility [Line Items]
Line of credit, commitment	$ 140,000,000
Club Facility | Tranche Two Extending
Line of Credit Facility [Line Items]
Line of credit, commitment | £								£ 60
Bilateral Facility
Line of Credit Facility [Line Items]
Line of credit facility	$ 25,000,000
Additional extended term	2 years
Bilateral Facility | Minimum
Line of Credit Facility [Line Items]
Credit facility fee rate	0.85%
Bilateral Facility | Maximum
Line of Credit Facility [Line Items]
Credit facility fee rate	1.20%
Syndicate 1,221
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding | AUD						AUD 24
Line of credit facility, covenant compliance				As of December 31, 2017, our Company was in compliance with all covenants.
Syndicate 1221 | Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility, interest rate		1.55%
Lloyd's | Syndicate 1221
Line of Credit Facility [Line Items]
Line of credit facility, underwriting reserve required | £		£ 75
Lloyd's | Syndicate 1221 | Club Facility
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding				$ 125,000,000	£ 60
Cash collateral posted				1,100,000
Lloyd's | Syndicate 1221 | Bilateral Facility
Line of Credit Facility [Line Items]
Aggregate face amount of letters of credit outstanding				$ 25,000,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands, € in Millions	Dec. 31, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2017EUR (€)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2017EUR (€)
Loss Contingencies [Line Items]
Rent expense		$ 13,200		$ 13,200	$ 12,900
Incentive awarded to the company by the State of Connecticut, total	$ 9,900					$ 11,500
Incentive awarded to the company by the State of Connecticut, non-interest bearing loans	7,500					8,000
Incentive awarded to the company by the State of Connecticut, grants	2,400					$ 3,500
Non-interest bearing incentive loan term		10 years	10 years
Forgiveness awarded to the company by the state of Connecticut	7,000
Incentive awarded to the company by the State of Connecticut, revenue recognized		$ 1,300		1,400	1,400
Incentive awarded to the company by the State of Connecticut, deferred revenue	4,400	4,400		4,800
Aggregate consideration payment for purchase of shares		62,887		$ 92,329	$ 215,405
Ackermans & van Haaren NV, Mr. Jozef Gielen and Kapimar Comm.V | Share Purchase Agreement
Loss Contingencies [Line Items]
Aggregate consideration payment for purchase of shares		42,000	€ 35
Termination fee payable	6,000	6,000					€ 5
Termination fee receivable	12,000	12,000					10
Additional reimbursement amount receivable	$ 6,000	$ 6,000					€ 5
Maximum
Loss Contingencies [Line Items]
Non-cancellable operating leases expiration year		2,031	2,031

Future Minimum Annual Rental Co

Future Minimum Annual Rental Commitments Under Various Non-cancellable Operating Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Leases [Abstract]
2,018	$ 12,075
2,019	11,469
2,020	11,395
2,021	10,315
2,022	9,654
2023-2031	36,296
Total Minimum Operating Lease Payments	$ 91,204

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Corporate income tax rate		35.00%	35.00%	35.00%
Deferred tax expense due to tax law changes		$ 17,100,000
Transition tax		$ 2,600,000
Tax Cuts and Jobs Act of 2017, accounting complete		false
Unrecognized tax benefits		$ 0	$ 0	$ 0
Deferred Tax Assets: State and Local		616,000	$ 699,000
Valuation allowance for deferred tax assets		$ 1,182,000
Scenario Forecast
Income Taxes [Line Items]
Corporate income tax rate	21.00%
United Kingdom
Income Taxes [Line Items]
Foreign effective income tax rate		35.00%
United Kingdom | Syndicate 1221 | Maximum
Income Taxes [Line Items]
Percentage of premiums		50.00%
United Kingdom | Syndicate 1221 | Minimum
Income Taxes [Line Items]
Foreign effective income tax rate		35.00%
State and Local Jurisdiction | Maximum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year		2,035
State and Local Jurisdiction | Minimum
Income Taxes [Line Items]
State and local tax carry-forwards expiration year		2,027

Components of Current and Defer

Components of Current and Deferred Income Tax Expense (Benefit) (Detail) - 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
Current Income Tax Expense (Benefit):
Federal and Foreign									$ 42,427	$ 39,900	$ 27,937
State and Local									764	638	1,476
Subtotal									43,191	40,538	29,413
Deferred Income Tax Expense (Benefit):
Federal and Foreign									(8,879)	(5,755)	7,004
State and local									0	0	0
Subtotal									(8,879)	(5,755)	7,004
Actual Tax Expense and Rate	$ 36,555	$ (16,864)	$ 6,971	$ 7,650	$ 9,866	$ 7,875	$ 7,053	$ 9,989	$ 34,312	$ 34,783	$ 36,417

Reconciliation of Total Income

Reconciliation of Total Income Taxes Applicable to Pre-Tax Operating Income and Amounts Computed by Applying Federal Statutory Income Tax Rate to Pre-Tax Operating Income (Detail) - 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 Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed Expected Tax Expense									$ 26,182	$ 41,128	$ 41,116
Tax-Exempt Interest									(5,394)	(4,559)	(4,987)
Dividends Received Deduction									(4,689)	(3,812)	(2,086)
Proration of DRD and Tax-Exempt Interest									1,512	1,256	1,061
Current State and Local Income Taxes, Net of Federal Income Tax Deduction									497	415	959
Stock Compensation Fair Market Value									(5,027)
Change in Valuation Allowance									1,182
Deferred Tax Expense due to Tax Law Changes									17,107
Transition Tax									2,587
Other									355	355	354
Actual Tax Expense and Rate	$ 36,555	$ (16,864)	$ 6,971	$ 7,650	$ 9,866	$ 7,875	$ 7,053	$ 9,989	$ 34,312	$ 34,783	$ 36,417
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed Expected Tax Expense									35.00%	35.00%	35.00%
Tax-Exempt Interest									(7.20%)	(3.90%)	(4.20%)
Dividends Received Deduction									(6.30%)	(3.20%)	(1.80%)
Proration of DRD and Tax-Exempt Interest									2.00%	1.10%	0.90%
Current State and Local Income Taxes, Net of Federal Income Tax Deduction									0.70%	0.40%	0.80%
Stock Compensation Fair Market Value									(6.70%)
Change in Valuation Allowance									1.60%
Deferred Tax Expense due to Tax Law Changes									22.90%
Transition Tax									3.50%
Other									0.40%	0.20%	0.30%
Actual Tax Expense and Rate									45.90%	29.60%	31.00%

Tax Effects of Temporary Differ

Tax Effects of Temporary Differences That Give Rise to Federal, Foreign, State and Local Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Loss Reserve Discount	$ 20,425	$ 25,420
Unearned Premiums	24,352	33,886
Compensation Related	8,462	15,288
State and Local Net Deferred Tax Assets	616	699
Net Currency Translation Adjustments	2,084	5,363
Other	152	3,618
Total Gross Deferred Tax Assets	65,491	84,274
Less: Valuation Allowance	(1,798)	(699)
Total Deferred Tax Assets	63,693	83,575
Deferred Tax Liabilities:
Loss Reserve Discount - (Transition Rule)	(5,066)
Net Unrealized Gains/Losses on Securities	(8,708)	(10,142)
Deferred Acquisition Costs	(20,670)	(29,401)
Net Unrealized Foreign Exchange	(3,782)	(3,575)
Other	(3,196)	(4,048)
Total Deferred Tax Liabilities	(41,422)	(62,637)
Net Deferred Income Tax Asset (Liability)	22,271	20,938
Lloyd's
Deferred Tax Assets:
Lloyd's Year of Account Deferral	$ 9,400
Deferred Tax Liabilities:
Lloyd's Year of Account Deferral		$ (15,471)

Stockholders Equity - Additiona

Stockholders Equity - Additional Information (Detail)	Jan. 20, 2017	Dec. 30, 2016shares	Dec. 06, 2016	Nov. 30, 2017USD ($)	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)
Stockholders Equity [Line Items]
Common stock, authorized shares | shares					50,000,000	50,000,000
Common stock, par value | $ / shares					$ 0.10	$ 0.10
Preferred stock, authorized shares | shares					1,000,000	1,000,000
Preferred stock, par value | $ / shares					$ 0.10	$ 0.10
Stock split ratio	2		2
Stockholders equity, stock split	We completed a two-for-one stock split on January 20, 2017.		On December 6, 2016, our Board of Directors declared a two-for-one stock split of The Navigators Group, Inc. Common stock, to be effected in the form of a stock dividend.
Number of additional shares for every share held | shares		1
Common stock dividend paid per share | $ / shares					$ 0.225	$ 0.135
Dividend paid							$ 0
NCUL
Stockholders Equity [Line Items]
Undistributed Profit				$ 218,300,000
NIIC
Stockholders Equity [Line Items]
Maximum amount available for payment of dividends without prior regulatory approval					$ 8,400,000
Navigators Insurance Company
Stockholders Equity [Line Items]
Dividend paid					19,000,000	$ 5,000,000
Maximum amount available for payment of dividends without prior regulatory approval					$ 105,700,000

Changes in Issued and Outstandi

Changes in Issued and Outstanding Common Shares (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Beginning Balance	29,124,000	28,862,000	28,562,000
Net Vested Stock Grants	343,000	220,000	254,000
Employee Stock Purchase Plan	40,488	41,224	42,000
Stock Options Exercised			4,000
Ending Balance	29,507,000	29,124,000	28,862,000

Amount and Nature of Net Assets

Amount and Nature of Net Assets Restricted from Payment of Dividends (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 567,796	$ 533,930
NIC and NSIC
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[1]	12,187	18,148
NIC and NSIC | Fixed Maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		10,655	10,690
NIC and NSIC | Short-Term Investments
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		290	290
NIC and NSIC | Cash [Member]
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		1,242	7,168
NHUK
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets	[2]	555,609	515,782
NHUK | Fixed Maturities
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		471,831	452,053
NHUK | Short-Term Investments
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Total Restricted Net Assets		$ 83,778	$ 63,729
[1]	The restricted net assets for NIC and NSIC primarily consist of fixed maturities on deposit with various state insurance departments. The cash as of December 31, 2017 and 2016, as presented in the table above, was on deposit with a U.K. bank to comply with the regulatory requirements of the Prudential Regulation Authority for the underwriting activities of the U.K. Branch.
[2]	The restricted net assets for NHUK consists of fixed maturities and cash held in trust for the benefit of syndicate policyholders and short term investments primarily consisting of overseas deposits in various countries with Lloyd's to support underwriting activities in those countries.

Amount and Nature of Net Asse80

Amount and Nature of Net Assets Restricted from Payment of Dividends (Parenthetical) (Detail) - Fixed Maturities - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
NIC and NSIC
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Restricted net assets, amortized cost	$ 9,856	$ 9,820
NHUK
Restricted Assets And Liabilities Relating To Securitization [Line Items]
Restricted net assets, amortized cost	$ 472,701	$ 457,225

Reconciliation of Basic and Dil

Reconciliation of Basic and Diluted Earnings per Share Computations (Detail) - 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
Earning Per Share Basic And Diluted [Abstract]
Net Income	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Basic weighted average shares									29,441	29,074	28,785
Effect of Common Stock Equivalents:
Assumed Exercise of Stock Options and Vesting of Stock Grants									630	958	866
Diluted Weighted Average Shares									30,071	30,032	29,651
Net Income Per Common Share:
Basic	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 0.74	$ 0.76	$ 0.56	$ 0.79	$ 1.38	$ 2.85	$ 2.82
Diluted	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 0.71	$ 0.73	$ 0.54	$ 0.77	$ 1.35	$ 2.75	$ 2.73

Summary of Statutory Net Income

Summary of Statutory Net Income (Loss) and Capital and Surplus (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NIC and NSIC
Statutory Accounting Practices [Line Items]
Net Income	$ 79.3	$ 84.2	$ 60.8
Capital and Surplus	1,056.6	1,027.3	949.1
Syndicate
Statutory Accounting Practices [Line Items]
Net Income	(23.4)	46.3	41
Capital and Surplus	168.7	180.7	$ 177.5
NIIC
Statutory Accounting Practices [Line Items]
Net Income	(8.4)	0.4
Capital and Surplus	$ 62.9	$ 70.7

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Jan. 20, 2017	Dec. 06, 2016	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	Apr. 30, 2013	Apr. 30, 2009	May 31, 2005
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock split effective date											Jan. 20, 2017
Stockholders equity, stock split	We completed a two-for-one stock split on January 20, 2017.	On December 6, 2016, our Board of Directors declared a two-for-one stock split of The Navigators Group, Inc. Common stock, to be effected in the form of a stock dividend.
Total compensation cost for share-based payment arrangements recognized in income statement											$ 13,800,000	$ 17,600,000	$ 15,000,000
Income tax benefits			$ 36,555,000	$ (16,864,000)	$ 6,971,000	$ 7,650,000	$ 9,866,000	$ 7,875,000	$ 7,053,000	$ 9,989,000	34,312,000	34,783,000	$ 36,417,000
Unrecognized compensation expense			18,000,000				22,300,000				18,000,000	$ 22,300,000
Weighted Average Periods												1 year 10 months 24 days	1 year 9 months 18 days
Aggregate fair value of all unvested RSU			$ 49,900,000				$ 87,200,000				$ 49,900,000	$ 87,200,000
Number of stock options granted											0	0	0
Number of stock options outstanding			0								0
Employee Stock Purchase Plan, shares purchased by employees											40,488	41,224	42,000
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price as percentage of fair market value											90.00%
Percentage of employees base compensation that can be invested for the purchase of common stock, maximum			10.00%								10.00%
Number of shares which can be acquired by employee, maximum											1,000
Total market value of shares that can be acquired by employee											$ 25,000
Employee Stock Purchase Plan, shares purchased by employees											40,488	41,224
Discount on employee stock purchase plan											10.00%
Share-based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Income tax benefits											$ 2,900,000	$ 6,200,000	$ 5,200,000
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards, granted											35,297	84,000	64,316
Weighted average grant-date fair values per share											$ 56.05	$ 44.52	$ 38.72
Total fair value of shares vested											$ 3,300,000	$ 2,500,000	$ 2,600,000
Performance-based Equity Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards, granted											253,333	404,946	370,100
Weighted average grant-date fair values per share											$ 55.70	$ 40.77	$ 37.25
Total fair value of shares vested											$ 29,000,000	$ 11,500,000	$ 12,500,000
Minimum | Options and Non Performance Based Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period											3 years
Minimum | Performance Based Shares Issued in 2017, 2016 and 2015 | Third Anniversary of Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vesting over the period											50.00%
Maximum | Options and Non Performance Based Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award, award vesting period											4 years
Options expiration term (in years)											10 years
Maximum | Performance Based Shares Issued in 2017, 2016 and 2015 | Third Anniversary of Grant Date
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares vesting over the period											150.00%
2005 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock authorized for issuance																1,000,000
Second Amended and Restated 2005 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock authorized for issuance	4,000,000													2,000,000
Stockholders equity, stock split											The stock split that occurred on January 20, 2017 doubled the number of awards from 2,000,000 to 4,000,000.
Stock issued during period under incentive plan											3,156,664
Number of stock available to be issued			843,336								843,336
First Amended and Restated 2005 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock authorized for issuance															1,500,000

Restricted Stock Unit Awards (D

Restricted Stock Unit Awards (Detail) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Awards
Nonvested at the beginning of the year	197,422
Granted	35,297	84,000	64,316
Vested	(59,930)
Forfeited	(13,250)
Nonvested at the end of the year	159,539	197,422
Weighted Average Grant Date Fair Value
Nonvested at the beginning of the year	$ 37.24
Granted	56.05	$ 44.52	$ 38.72
Vested	36.21
Forfeited	39.41
Nonvested at the end of the year	$ 41.60	$ 37.24

Performance-based Equity Awards

Performance-based Equity Awards (Detail) - Performance-based Equity Awards - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Awards
Nonvested at the beginning of the year	1,176,742
Granted	253,333	404,946	370,100
Performance Adjustment	83,071
Vested	(520,285)
Forfeited	(26,500)
Nonvested at the end of the year	966,361	1,176,742
Weighted Average Grant Date Fair Value
Nonvested at the beginning of the year	$ 35.85
Granted	55.70	$ 40.77	$ 37.25
Performance Adjustment	30.49
Vested	30.38
Forfeited	41.60
Nonvested at the end of the year	$ 43.39	$ 35.85

Retirement Plans - Additional I

Retirement Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United States
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	$ 7.5	$ 7.4	$ 6.3
United States | Retirement Savings Contributions
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	4.9	4.2	3.8
United States | Non Discretionary Matching Contributions
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	2.6	3.2	2.5
United Kingdom
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	$ 2.6	2.9	2.9
United Kingdom | Employees hired prior to November 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	15.00%
United Kingdom | Employees hired after November 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	12.00%
European Offices
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan	$ 0.3	$ 0.5	$ 0.5
European Offices | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	2.10%
European Offices | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	26.00%

Summary of Condensed Quarterly

Summary of Condensed Quarterly Financial Data (Detail) - 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
Income Statement [Abstract]
Gross Written Premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
Revenues:
Net Written Premiums	304,869	296,016	333,282	337,163	282,868	277,001	306,535	319,820	1,271,330	1,186,224	1,043,860
Change in Net Unearned Premiums	230	5,339	(39,447)	(51,032)	1,117	7,009	(38,543)	(55,462)	(84,910)	(85,879)	(59,773)
Net Earned Premiums	305,099	301,355	293,835	286,131	283,985	284,010	267,992	264,358	1,186,420	1,100,345	984,087
Net Investment Income	22,982	22,598	22,265	21,448	20,107	19,875	19,875	19,594	89,293	79,451	68,718
Total Other-Than-Temporary Impairment Losses	3	(957)	29	(1,077)	21	23	(162)	(109)	(2,002)	(227)	(1,870)
Portion of Loss Recognized in Other Comprehensive Income (Before Tax)	(3)	(15)	(29)	(16)	(21)	(23)	12	109	(62)	77	172
Net Other-Than-Temporary Impairment Losses Recognized in Earnings		(972)		(1,093)			(150)		(2,064)	(150)	(1,698)
Net Realized Gains	37,141	5,190	1,694	1,049	4,043	1,586	1,960	1,597	45,073	9,186	8,373
Other Income (Loss)	(3,201)	(1,699)	(411)	1,068	1,905	(183)	4,430	2,549	(4,243)	8,701	(491)
Total Revenues	362,021	326,472	317,383	308,603	310,040	305,288	294,107	288,098	1,314,479	1,197,533	1,058,989
Expenses:
Net Losses and LAE	183,384	276,171	177,110	169,600	172,493	172,793	167,206	152,956	806,265	665,448	572,598
Commission Expenses	43,205	45,509	48,173	47,844	44,154	42,611	40,726	37,554	184,731	165,045	129,977
Other Operating Expenses	68,153	45,773	60,766	58,538	58,076	56,137	59,074	60,809	233,230	234,096	223,516
Interest Expense	3,863	3,862	3,861	3,861	3,860	3,859	3,858	3,858	15,447	15,435	15,424
Total Expenses	298,605	371,315	289,910	279,843	278,583	275,400	270,864	255,177	1,239,673	1,080,024	941,515
Income (Loss) Before Income Taxes	63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income tax benefits	36,555	(16,864)	6,971	7,650	9,866	7,875	7,053	9,989	34,312	34,783	36,417
Net Income (Loss)	26,861	(27,979)	20,502	21,110	21,591	22,013	16,190	22,932	40,494	82,726	81,057
Comprehensive Income (Loss)	$ (1,812)	$ (18,311)	$ 35,699	$ 35,911	$ (31,712)	$ 18,760	$ 35,228	$ 45,721	$ 51,487	$ 67,997	$ 58,102
Combined Ratio	96.60%	121.90%	97.30%	96.40%	96.70%	95.60%	99.60%	95.10%	103.20%	96.70%	94.10%
Net Income (Loss) per Share:
Basic	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 0.74	$ 0.76	$ 0.56	$ 0.79	$ 1.38	$ 2.85	$ 2.82
Diluted	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 0.71	$ 0.73	$ 0.54	$ 0.77	$ 1.35	$ 2.75	$ 2.73

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - $ / shares	Feb. 15, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Cash dividends declared per common share		$ 0.225	$ 0.135	$ 0
Subsequent Event
Subsequent Event [Line Items]
Dividend declaration date	Feb. 15, 2018
Cash dividends declared per common share	$ 0.07
Dividend payable date	Mar. 23, 2018
Dividend payable record date	Mar. 2, 2018

Summary of Consolidated Inves89

Summary of Consolidated Investments-Other Than Investments in Related Parties (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		$ 3,420,163		$ 3,128,563
Gross Unrealized Gains		57,655		59,175
Gross Unrealized Losses		(16,184)		(30,199)
Amortized Cost		3,378,692		3,099,587
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		3,057,054	[1]	2,635,882
Gross Unrealized Gains		44,158		32,071
Gross Unrealized Losses		(14,512)		(24,414)
Amortized Cost		3,027,408		2,628,225
Fixed Maturities | U.S. Treasury Bonds, Agency Bonds and Foreign Government Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		393,563	[1]	273,776
Gross Unrealized Gains		2,081		2,192
Gross Unrealized Losses		(2,014)		(5,128)
Amortized Cost		393,496		276,712
Fixed Maturities | States, Municipalities and Political Subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		814,632	[1]	547,415
Gross Unrealized Gains		20,136		11,542
Gross Unrealized Losses		(1,423)		(4,036)
Amortized Cost		795,919		539,909
Fixed Maturities | Agency Mortgage-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		407,619	[1]	487,364
Gross Unrealized Gains		2,352		4,016
Gross Unrealized Losses		(5,414)		(6,585)
Amortized Cost		410,681		489,933
Fixed Maturities | Residential Mortgage Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		54,104	[1]	20,530
Gross Unrealized Gains		606		453
Gross Unrealized Losses		(79)		(55)
Amortized Cost		53,577		20,132
Fixed Maturities | Asset-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		328,753	[1]	314,601
Gross Unrealized Gains		2,138		824
Gross Unrealized Losses		(663)		(1,178)
Amortized Cost		327,278		314,955
Fixed Maturities | Commercial Mortgage-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		160,904	[1]	154,139
Gross Unrealized Gains		2,354		2,859
Gross Unrealized Losses		(1,182)		(1,904)
Amortized Cost		159,732		153,184
Fixed Maturities | Mortgage-Backed and Asset-Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		951,380	[1]	976,634
Gross Unrealized Gains		7,450		8,152
Gross Unrealized Losses		(7,338)		(9,722)
Amortized Cost		951,268		978,204
Fixed Maturities | Corporate Exposures
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments	[2]	897,479	[1]	838,057
Gross Unrealized Gains	[2]	14,491		10,185
Gross Unrealized Losses	[2]	(3,737)		(5,528)
Amortized Cost	[2]	886,725		833,400
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		235,981	[1]	349,142
Gross Unrealized Gains		13,494		27,016
Gross Unrealized Losses		(1,672)		(5,785)
Amortized Cost		224,159		327,911
Equity Securities | Common Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		52,439	[1]	164,087
Gross Unrealized Gains		7,423		24,677
Gross Unrealized Losses		(112)		(964)
Amortized Cost		45,128		140,374
Equity Securities | Preferred Stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		183,542	[1]	185,055
Gross Unrealized Gains		6,071		2,339
Gross Unrealized Losses		(1,560)		(4,821)
Amortized Cost		179,031		187,537
Short-Term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments		127,128	[1]	143,539
Gross Unrealized Gains		3		88
Amortized Cost		127,125		$ 143,451
Investment Including Other Invested Assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments	[1]	3,421,883
Gross Unrealized Gains		57,655
Gross Unrealized Losses		(16,184)
Amortized Cost		3,380,412
Other Invested Assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments	[1]	1,720
Amortized Cost		$ 1,720
[1]	Other invested assets are accounted for using the equity method of accounting. All other investments are shown at fair value.
[2]	Corporate Exposures consist of investments in corporate bonds, hybrid bonds and redeemable preferred stocks.

Condensed Financial Informati90

Condensed Financial Information - Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Investments	$ 3,421,883	$ 3,130,523
Cash	67,084	64,643	$ 69,901	$ 90,751
Investments in Subsidiaries	1,720	1,960
Goodwill and Other Intangible Assets	6,596	6,451
Current Income Tax Receivable, Net	16,667	20,556
Other Assets	73,171	52,030
Total Assets	5,224,622	4,814,037
Liabilities:
Reserves for Losses and Loss Adjustment Expenses	2,515,145	2,289,727	2,202,644
Unearned Premiums	987,681	887,344
Reinsurance Balances Payable	136,192	108,980
Senior Notes	263,885	263,728
Accounts Payable and Other Liabilities	95,754	86,070
Total Liabilities	3,998,657	3,635,849
Stockholders' Equity:
Preferred Stock, ($.10 par value, authorized 1,000 shares, none issued)	0	0
Common Stock, ($.10 par value, authorized 50,000 shares, issued 36,530 shares for 2017 and 36,147 shares for 2016)	3,650	3,612
Additional Paid-In Capital	376,868	373,983
Retained Earnings	981,380	947,519
Accumulated Other Comprehensive Income:
Total Stockholders' Equity	1,225,965	1,178,188	1,096,148	1,027,224
Total Liabilities and Stockholders' Equity	5,224,622	4,814,037
Parent Company
ASSETS
Investments	1,668	1,660
Cash	7,567	11,003	$ 6,417	$ 6,300
Investments in Subsidiaries	1,423,843	1,381,652
Goodwill and Other Intangible Assets	2,534	2,534
Current Income Tax Receivable, Net	30,573	13,160
Other Assets	28,063	36,428
Total Assets	1,494,248	1,446,437
Liabilities:
Senior Notes	263,885	263,728
Accounts Payable and Other Liabilities	1,224	1,347
Accrued Interest Payable	3,174	3,174
Total Liabilities	268,283	268,249
Stockholders' Equity:
Preferred Stock, ($.10 par value, authorized 1,000 shares, none issued)
Common Stock, ($.10 par value, authorized 50,000 shares, issued 36,530 shares for 2017 and 36,147 shares for 2016)	3,650	3,612
Additional Paid-In Capital	376,868	373,983
Treasury Stock, at cost (7,023 shares for 2017 and 2016)	(155,801)	(155,801)
Retained Earnings	981,380	947,519
Accumulated Other Comprehensive Income:
Net Unrealized Gains (Losses) on Securities Available-for-Sale, Net of Tax	19,874	8,882
Foreign Currency Translation Adjustment, Net of Tax	(6)	(7)
Total Stockholders' Equity	1,225,965	1,178,188
Total Liabilities and Stockholders' Equity	$ 1,494,248	$ 1,446,437

Condensed Financial Informati91

Condensed Financial Information - Balance Sheets (Parenthetical) (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements Captions [Line Items]
Preferred Stock, par value	$ 0.10	$ 0.10
Preferred Stock, authorized shares	1,000,000	1,000,000
Preferred Stock, issued shares	0	0
Common Stock, par value	$ 0.10	$ 0.10
Common Stock, authorized shares	50,000,000	50,000,000
Common Stock, issued shares	36,530,000	36,147,000
Parent Company
Condensed Financial Statements Captions [Line Items]
Preferred Stock, par value	$ 0.10	$ 0.10
Preferred Stock, authorized shares	1,000,000	1,000,000
Preferred Stock, issued shares	0	0
Common Stock, par value	$ 0.10	$ 0.10
Common Stock, authorized shares	50,000,000	50,000,000
Common Stock, issued shares	36,530,000	36,147,000
Treasury Stock, shares	7,023,000	7,023,000

Condensed Financial Informati92

Condensed Financial Information - Statement of Income (Detail) - 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:
Net Investment Income	$ 22,982	$ 22,598	$ 22,265	$ 21,448	$ 20,107	$ 19,875	$ 19,875	$ 19,594	$ 89,293	$ 79,451	$ 68,718
Total Revenues	362,021	326,472	317,383	308,603	310,040	305,288	294,107	288,098	1,314,479	1,197,533	1,058,989
Expenses:
Interest Expense	3,863	3,862	3,861	3,861	3,860	3,859	3,858	3,858	15,447	15,435	15,424
Other (Income) Expense	3,201	1,699	411	(1,068)	(1,905)	183	(4,430)	(2,549)	4,243	(8,701)	491
Total Expenses	298,605	371,315	289,910	279,843	278,583	275,400	270,864	255,177	1,239,673	1,080,024	941,515
Income (Loss) Before Income Taxes	63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income Tax Expense	36,555	(16,864)	6,971	7,650	9,866	7,875	7,053	9,989	34,312	34,783	36,417
Net Income (Loss)	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	40,494	82,726	81,057
Parent Company
Revenues:
Net Investment Income									7	53	147
Total Revenues									7	53	147
Expenses:
Interest Expense									15,447	15,435	15,424
Other (Income) Expense									(28)	22
Total Expenses									15,419	15,457	15,424
Income (Loss) Before Income Taxes									(15,412)	(15,404)	(15,277)
Income Tax Expense									(5,709)	(8,009)	(5,472)
Income (Loss) Before Equity in Undistributed Net Income of Wholly Owned Subsidiaries									(9,703)	(7,395)	(9,805)
Equity in Undistributed Net Income of Wholly-Owned Subsidiaries									$ 50,197	$ 90,121	$ 90,862

Condensed Financial Informati93

Condensed Financial Information - Statement of Cash Flows (Detail) - 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	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations:
Other									936	27,791	(13,654)
Net Cash Provided by Operating Activities									265,343	229,425	233,646
Fixed Maturities, Available-for-Sale
Sales									225,962	446,666	376,522
Purchases									(1,008,537)	(962,419)	(648,059)
Equity Securities
Sales									207,838	54,798	96,405
Purchases									(62,887)	(92,329)	(215,405)
Net (Increase) Decrease in Short-Term Investments									17,877	73,541	(38,468)
Net Cash Used in Investing Activities									(247,706)	(211,432)	(249,855)
Financing Activities:
Proceeds from Employee Stock Purchase Plan									2,076	1,840	1,352
Proceeds of Stock Issued from Exercise of Stock Options									0	0	29
Dividends Paid									(6,633)	(3,930)	0
Payment of Employee Tax Withholding on Stock Compensation									(13,887)	(5,198)	(6,022)
Net Cash Used in Financing Activities									(18,444)	(7,288)	(4,641)
Change in Cash									2,441	(5,258)	(20,850)
Cash at Beginning of Year				64,643				69,901	64,643	69,901	90,751
Cash at End of Period	67,084				64,643				67,084	64,643	69,901
Parent Company
Adjustments to Reconcile Net Income to Net Cash Provided By (Used in) Operations:
Equity in Undistributed Net Income of Wholly-Owned Subsidiaries									(50,197)	(90,121)	(90,862)
Dividends Received from Subsidiaries									19,000	5,000
Other									5,841	5,737	9,542
Net Cash Provided by Operating Activities									15,138	3,342	(263)
Fixed Maturities, Available-for-Sale
Sales											1,250
Equity Securities
Sales									0	0	0
Purchases									0	0	0
Net (Increase) Decrease in Short-Term Investments									(8)	87,942	3,918
Net Cash Used in Investing Activities									(8)	87,942	5,168
Financing Activities:
Capital Contribution to Subsidiary									(1)	(79,250)
Proceeds from Employee Stock Purchase Plan									1,846	1,840	1,352
Proceeds of Stock Issued from Exercise of Stock Options											29
Dividends Paid									(6,633)	(3,930)
Payment of Employee Tax Withholding on Stock Compensation									(13,778)	(5,358)	(6,169)
Net Cash Used in Financing Activities									(18,566)	(86,698)	(4,788)
Change in Cash									(3,436)	4,586	117
Cash at Beginning of Year				$ 11,003				$ 6,417	11,003	6,417	6,300
Cash at End of Period	$ 7,567				$ 11,003				$ 7,567	$ 11,003	$ 6,417

Supplementary Insurance Infor94

Supplementary Insurance Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		$ 135,249	$ 119,660	$ 91,983
Reserve for Losses and LAE		2,515,145	2,289,727	2,202,644
Unearned Premiums		987,681	887,344	820,676
Net Earned Premiums		1,186,420	1,100,345	984,087
Net Investment Income	[1]	89,293	79,451	68,718
Losses and LAE Incurred		806,265	665,448	572,598
Amortization of Deferred Policy Acquisition Costs		184,731	165,045	129,977
Other Operating Expenses		233,230	234,096	223,516
Net Written Premiums		1,271,330	1,186,224	1,043,860
U.S.Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		54,674	50,028	37,372
Reserve for Losses and LAE		1,629,015	1,564,524	1,478,391
Unearned Premiums		575,068	519,505	476,803
Net Earned Premiums		674,665	629,308	555,836
Losses and LAE Incurred		443,353	397,860	343,497
Amortization of Deferred Policy Acquisition Costs		77,729	70,812	56,319
Other Operating Expenses		128,905	128,108	131,407
Net Written Premiums		721,300	683,568	597,006
International Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		38,988	37,385	25,451
Reserve for Losses and LAE		716,352	605,105	608,783
Unearned Premiums		260,982	252,442	221,544
Net Earned Premiums		333,792	307,416	259,960
Losses and LAE Incurred		229,601	178,284	134,702
Amortization of Deferred Policy Acquisition Costs		68,824	61,703	43,676
Other Operating Expenses		83,464	86,395	75,867
Net Written Premiums		335,852	345,967	277,942
Global Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs		41,587	32,247	29,160
Reserve for Losses and LAE		169,778	120,098	115,470
Unearned Premiums		151,631	115,397	122,329
Net Earned Premiums		177,963	163,621	168,291
Losses and LAE Incurred		133,311	89,304	94,399
Amortization of Deferred Policy Acquisition Costs		39,136	34,008	32,240
Other Operating Expenses		20,861	19,593	16,242
Net Written Premiums		214,178	156,689	168,912
Corporate
Supplementary Insurance Information, by Segment [Line Items]
Net Investment Income	[1]	89,293	79,451	68,718
Amortization of Deferred Policy Acquisition Costs		$ (958)	$ (1,478)	$ (2,258)
[1]	As we evaluate the underwriting results of each of our reportable segments separately from the results of our investment portfolio, we do not allocate net investment income to our reportable segments.

Reinsurance Written Premium (De

Reinsurance Written Premium (Detail) - 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
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,489,422	$ 1,403,865	$ 1,277,728
Ceded to Other Companies									441,935	382,687	409,642
Assumed from Other Companies									223,843	165,046	175,774
Net written premiums	$ 304,869	$ 296,016	$ 333,282	$ 337,163	$ 282,868	$ 277,001	$ 306,535	$ 319,820	$ 1,271,330	$ 1,186,224	$ 1,043,860
Percentage of Amount Assumed to Net									18.00%	14.00%	17.00%
Accident & Health
Supplementary Insurance Information, by Segment [Line Items]
Assumed from Other Companies									$ 80,605	$ 53,814	$ 87,088
Net written premiums									$ 80,605	$ 53,814	$ 87,088
Percentage of Amount Assumed to Net									100.00%	100.00%	100.00%
Property Casualty
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,489,422	$ 1,403,865	$ 1,277,728
Ceded to Other Companies									441,935	382,687	409,642
Assumed from Other Companies									143,238	111,232	88,686
Net written premiums									$ 1,190,725	$ 1,132,410	$ 956,772
Percentage of Amount Assumed to Net									12.00%	10.00%	9.00%

Valuation and Qualifying Acco96

Valuation and Qualifying Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Uncollectable Reinsurance
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 12,078	$ 6,921	$ 11,332
Charged (Credited) to Costs and Expenses	519	5,157	(4,411)
Charged to Other Accounts	0	0	0
Deductions (Describe)	0	0	0
Ending Balance	12,597	12,078	6,921
Valuation Allowance in Deferred Taxes
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	699	721	776
Charged (Credited) to Costs and Expenses	1,099	(22)	(55)
Charged to Other Accounts	0	0	0
Deductions (Describe)	0	0	0
Ending Balance	$ 1,798	$ 699	$ 721

Supplementary Information Con97

Supplementary Information Concerning P&C Insurance Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
Deferred Policy Acquisition Costs	$ 135,249	$ 119,660	$ 91,983
Reserve for Losses and LAE	2,515,145	2,289,727	2,202,644
Unearned Premiums	987,681	887,344	820,676
Net Earned Premiums	1,186,420	1,100,345	984,087
Net Investment Income	89,293	79,451	68,718
Losses and LAE Expenses Incurred Related to Current Year	771,955	693,976	637,267
Losses and LAE Expenses Incurred Related to Prior Years	34,310	(28,528)	(64,669)
Amortization of Deferred Policy Acquisition Costs	184,731	165,045	129,977
Other Operating Expenses	233,230	234,096	223,516
Net Written Premiums	$ 1,271,330	$ 1,186,224	$ 1,043,860