James River (JRVR) 10-K 1 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 26, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	James River Group Holdings, Ltd.
Entity Central Index Key	1,620,459
Trading Symbol	jrvr
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	No
Entity Common Stock, Shares Outstanding		29,739,509
Entity Public Float			$ 879,232,250
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2,017

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fixed maturity securities:
Available-for-sale, at fair value (amortized cost: 2017 – $1,008,662; 2016 – $940,212)	$ 1,016,098	$ 941,077
Trading, at fair value (amortized cost: 2017 – $3,801; 2016 – $5,052)	3,808	5,063
Equity securities available-for-sale, at fair value (cost: 2017 – $75,318; 2016 – $74,553)	82,522	76,401
Bank loan participations held-for-investment, at amortized cost, net of allowance	238,214	203,526
Short-term investments	36,804	50,844
Other invested assets	70,208	55,419
Total invested assets	1,447,654	1,332,330
Cash and cash equivalents	163,495	109,784
Accrued investment income	8,381	7,246
Premiums receivable and agents’ balances, net	352,436	265,315
Reinsurance recoverable on unpaid losses	302,524	182,737
Reinsurance recoverable on paid losses	11,292	2,877
Prepaid reinsurance premiums	91,979	90,147
Deferred policy acquisition costs	72,365	64,789
Intangible assets, net	38,334	38,931
Goodwill	181,831	181,831
Income tax receivable	2,806	5,566
Other assets	83,598	64,980
Total assets	2,756,695	2,346,533
Liabilities:
Reserve for losses and loss adjustment expenses	1,292,349	943,865
Unearned premiums	418,114	390,563
Payables to reinsurers	56,268	39,899
Senior debt	98,300	88,300
Junior subordinated debt	104,055	104,055
Accrued expenses	39,295	36,884
Deferred tax liabilities, net	5,247	2,902
Other liabilities	48,368	46,844
Total liabilities	2,061,996	1,653,312
Commitments and contingent liabilities	0	0
Shareholders’ equity:
Common Shares – $0.0002 par value; 200,000,000 shares authorized. 2017 and 2016: 29,696,682 and 29,257,566 shares issued and outstanding, respectively	6	6
Preferred Shares – 2017 and 2016: $0.00125 par value; 20,000,000 shares authorized; no shares issued and outstanding	0	0
Additional paid-in capital	636,149	636,856
Retained earnings	48,198	55,232
Accumulated other comprehensive income	10,346	1,127
Total shareholders’ equity	694,699	693,221
Total liabilities and shareholders’ equity	$ 2,756,695	$ 2,346,533

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Available-for-sale debt securities, amortized cost (in dollars)	$ 1,008,662	$ 940,212
Trading securities, debt, amortized cost (in dollars)	3,801	5,052
Available-for-sale equity securities, amortized cost (in dollars)	$ 75,318	$ 74,553
Common stock, par value (in dollars per share)	$ 0.0002	$ 0.0002
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	29,696,682	29,257,566
Common stock, shares outstanding	29,696,682	29,257,566
Preferred stock par value (in dollars per share)	$ 0.00125	$ 0.00125
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Consolidated Statements of Inco

Consolidated Statements of Income and Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Gross written premiums	$ 1,081,905,000	$ 737,398,000	$ 572,194,000
Ceded written premiums	(315,279,000)	(179,690,000)	(101,162,000)
Net written premiums	766,626,000	557,708,000	471,032,000
Change in net unearned premiums	(25,517,000)	(42,045,000)	(9,827,000)
Net earned premiums	741,109,000	515,663,000	461,205,000
Net investment income	61,119,000	52,638,000	44,835,000
Net realized investment (losses) gains	(1,989,000)	7,565,000	(4,547,000)
Other income	17,386,000	10,361,000	3,428,000
Total revenues	817,625,000	586,227,000	504,921,000
Expenses:
Losses and loss adjustment expenses	555,377,000	325,421,000	279,016,000
Other operating expenses	196,993,000	170,828,000	157,803,000
Other expenses	539,000	1,590,000	730,000
Interest expense	8,974,000	8,448,000	6,999,000
Amortization of intangible assets	597,000	597,000	597,000
Total expenses	762,480,000	506,884,000	445,145,000
Income before income taxes	55,145,000	79,343,000	59,776,000
Income tax expense (benefit):
Current	11,943,000	(221,000)	5,357,000
Deferred	(364,000)	5,093,000	922,000
Total income tax expense	11,579,000	4,872,000	6,279,000
Net income	43,566,000	74,471,000	53,497,000
Other comprehensive income:
Net unrealized gains (losses), net of taxes of $2,707 in 2017, $(1,723) in 2016 and $(939) in 2015	9,219,000	(2,059,000)	(15,170,000)
Total comprehensive income	$ 52,785,000	$ 72,412,000	$ 38,327,000
Per share data:
Basic earnings per share (in dollars per share)	$ 1.48	$ 2.56	$ 1.87
Diluted earnings per share (in dollars per share)	1.44	2.49	1.82
Dividend declared per share (in dollars per share)	$ 1.70	$ 2.25	$ 1.64
Weighted-average common shares outstanding:
Basic (in shares)	29,461,717	29,063,075	28,662,051
Diluted (in shares)	30,273,149	29,894,378	29,334,918

Consolidated Statements of Inc5

Consolidated Statements of Income and Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Tax on net unrealized gains (losses)	$ 2,707	$ (1,723)	$ (939)

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity - USD ($)	Total	Common Shares	Preferred stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income
Balances at Dec. 31, 2014	$ 687,921,000	$ 6,000	$ 0	$ 628,236,000	$ 41,323,000	$ 18,356,000
Balances (in shares) at Dec. 31, 2014		28,540,350
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	53,497,000				53,497,000
Other comprehensive loss	(15,170,000)					(15,170,000)
Dividends	(47,794,000)				(47,794,000)
Exercise of stock options and related excess tax benefits	15,000			15,000
Exercise of stock options and related excess tax benefits (in shares)		341,264
Vesting of RSUs and related excess tax benefits	1,136,000			1,136,000
Vesting of RSUs and related excess tax benefits (in shares)		59,933
Compensation expense under share incentive plans	3,735,000			3,735,000
Balances at Dec. 31, 2015	$ 681,038,000	$ 6,000	0	630,820,000	47,026,000	3,186,000
Balances (in shares) at Dec. 31, 2015	28,941,547	28,941,547
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 74,471,000				74,471,000
Other comprehensive loss	(2,059,000)					(2,059,000)
Dividends	(66,265,000)				(66,265,000)
Exercise of stock options and related excess tax benefits	(1,536,000)			(1,536,000)
Exercise of stock options and related excess tax benefits (in shares)		260,672
Vesting of RSUs and related excess tax benefits	992,000			992,000
Vesting of RSUs and related excess tax benefits (in shares)		55,347
Compensation expense under share incentive plans	5,492,000			5,492,000
Balances at Dec. 31, 2016	$ 693,221,000	$ 6,000	0	636,856,000	55,232,000	1,127,000
Balances (in shares) at Dec. 31, 2016	29,257,566	29,257,566
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 43,566,000				43,566,000
Other comprehensive loss	9,219,000					9,219,000
Dividends	(50,600,000)				(50,600,000)
Exercise of stock options and related excess tax benefits	6,213,000			6,213,000
Exercise of stock options and related excess tax benefits (in shares)		358,967
Vesting of RSUs and related excess tax benefits	2,182,000			2,182,000
Vesting of RSUs and related excess tax benefits (in shares)		80,149
Compensation expense under share incentive plans	7,688,000			7,688,000
Balances at Dec. 31, 2017	$ 694,699,000	$ 6,000	$ 0	$ 636,149,000	$ 48,198,000	$ 10,346,000
Balances (in shares) at Dec. 31, 2017	29,696,682	29,696,682

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	$ 43,566	$ 74,471	$ 53,497
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred policy acquisition costs	(123,577)	(105,659)	(98,302)
Amortization of policy acquisition costs	116,001	101,624	97,750
Net realized investment losses (gains)	1,989	(7,565)	4,547
Distributions from equity method investments	7,333	3,467	2,885
Income from equity method investments	(13,209)	(8,743)	(2,468)
Trading securities purchases, sales, and maturities, net	1,250	0	2,319
Deferred U.S. federal income tax (benefit) expense	(364)	5,093	922
Provision for depreciation and amortization	2,123	2,414	2,004
Share based compensation expense	7,688	5,492	3,735
Excess tax benefits from equity incentive plan transactions	(2,115)	3,191	3,580
Change in operating assets and liabilities:
Reserve for losses and loss adjustment expenses	348,484	158,543	69,026
Unearned premiums	27,551	89,459	23,525
Premiums receivable and agents’ balances	(87,121)	(88,630)	(14,158)
Reinsurance balances	(113,665)	(68,497)	(28,213)
Payable to insurance companies	445	(950)	4,170
Other	(8,563)	(9,361)	(4,848)
Net cash provided by operating activities	207,816	154,349	119,971
Securities available-for-sale:
Purchases – fixed maturity securities	(270,014)	(300,135)	(393,168)
Sales – fixed maturity securities	77,781	110,124	110,122
Maturities and calls – fixed maturity securities	121,890	135,472	122,791
Purchases – equity securities	(5,540)	(3,680)	(18,519)
Sales – equity securities	3,522	14,850	14,068
Bank loan participations:
Purchases	(240,799)	(156,638)	(109,107)
Sales	138,214	51,077	113,391
Maturities	69,740	97,097	37,388
Other invested assets:
Purchases	(8,913)	(2,365)	(52,663)
Return of capital	0	226	237
Disposals	0	0	1,374
Maturities and repayments	0	6,500	30,753
Securities receivable or payable, net	(2,525)	1,018	(2,104)
Short-term investments, net	14,040	(31,574)	112,586
Other	(2,137)	(2,736)	(1,312)
Net cash used in investing activities	(104,741)	(80,764)	(34,163)
Financing activities
Senior debt issuances	10,000	0	10,000
Senior debt repayments	0	0	(10,000)
Dividends paid	(50,832)	(65,988)	(47,405)
Issuances of common shares under equity incentive plans	1,708	2,260	1,730
Common share repurchases	(9,448)	(4,907)	(6,461)
Other financing activities	(792)	(1,572)	(649)
Net cash used in financing activities	(49,364)	(70,207)	(52,785)
Change in cash and cash equivalents	53,711	3,378	33,023
Cash and cash equivalents at beginning of year	109,784	106,406	73,383
Cash and cash equivalents at end of year	163,495	109,784	106,406
Supplemental information
Income taxes paid (refunded), net	9,848	(59)	(2,827)
Interest paid	$ 8,909	$ 8,121	$ 7,342

Accounting Policies

Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Accounting Policies	Accounting Policies ​ Organization James River Group Holdings, Ltd. (referred to as “JRG Holdings” or, with its subsidiaries, the “Company”) is an exempted holding company registered in Bermuda, organized for the purpose of acquiring and managing insurance and reinsurance entities. The Company owns six insurance companies based in the United States (“U.S.”) focused on specialty insurance niches and a Bermuda-based reinsurance company as described below: • James River Group Holdings UK Limited (“James River UK”) is an insurance holding company formed in 2015 in the United Kingdom (“U.K.”). The Company contributed James River Group, Inc. (“James River Group”), a U.S. insurance holding company, to James River UK in 2015. ​ • James River Group is a Delaware domiciled insurance holding company formed in 2002, which owns all of the Company’s U.S.-based subsidiaries, either directly or indirectly through one of its wholly-owned U.S. subsidiaries. James River Group oversees the Company’s U.S. insurance operations and maintains all of the outstanding debt in the U.S. ​ • James River Insurance Company (“James River Insurance”) is an Ohio domiciled excess and surplus lines insurance company that, with its wholly-owned insurance subsidiary, James River Casualty Company, is authorized to write business in every state and the District of Columbia. ​ • Falls Lake National Insurance Company (“Falls Lake National”) is an Ohio domiciled insurance company which wholly owns Stonewood Insurance Company (“Stonewood Insurance”), a North Carolina domiciled company, Falls Lake General Insurance Company, an Ohio domiciled company, and Falls Lake Fire and Casualty Company, a California domiciled company. Falls Lake National began writing specialty admitted fronting and program business in late 2013. Falls Lake Fire and Casualty Company began operations in 2016. ​​ • JRG Reinsurance Company, Ltd. (“JRG Re”) was formed in 2007 and commenced operations in 2008. JRG Re, a Bermuda domiciled reinsurer, provides non-catastrophe casualty reinsurance to U.S. third parties and to the Company’s U.S.-based insurance subsidiaries. ​ Basis of Presentation and Principles of Consolidation The consolidated financial statements are prepared in accordance with U.S. generally accepted accounting principles (“GAAP”), which vary in some respects from statutory accounting practices (“SAP”) which are prescribed or permitted by the various state insurance departments in the U.S. or by insurance regulators in Bermuda. The accompanying consolidated financial statements include the accounts and operations of the Company and its subsidiaries. Intercompany transactions and balances have been eliminated. Estimates and Assumptions Preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying disclosures. Those estimates are inherently subject to change, and actual results may ultimately differ from those estimates. Fixed Maturity and Equity Securities Fixed maturity and equity securities classified as “available-for-sale” are carried at fair value, and unrealized gains and losses on such securities, net of any deferred taxes, are reported as a separate component of accumulated other comprehensive income. Fixed maturity securities purchased for short-term resale are classified as “trading” and are carried at fair value with unrealized gains and losses included in earnings as a component of net investment income. The Company does not have any securities classified as “held-to-maturity”. Fair value generally represents quoted market value prices for securities traded in the public marketplace or prices analytically determined using bid or closing prices for securities not traded in the public marketplace. 1. Accounting Policies (continued) Premiums and discounts on mortgage-backed securities and asset-backed securities are amortized or accrued using the constant yield method which considers anticipated prepayments at the date of purchase. To the extent that the estimated lives of such securities change as a result of changes in estimated prepayment rates, the adjustments are included in net investment income using the retrospective method. Realized investment gains or losses are determined on a specific identification basis. Interest income is recognized as earned, and dividend income is recognized on the ex-dividend date. The Company evaluates its available-for-sale investments regularly to determine whether there are declines in value that are other-than-temporary. The Company’s outside investment managers assist the Company in this evaluation. When the Company determines that a security has experienced an other-than-temporary impairment, the impairment loss is recognized as a realized investment loss. The factors that the Company considers in evaluating whether such an other-than-temporary impairment has occurred include the amount and percentage that fair value is below amortized cost or cost and the length of time that fair value has been below amortized cost or cost. For fixed maturity securities, the Company considers the credit quality rating of the security, with a special emphasis on securities downgraded below investment grade. Management does not intend to sell available-for-sale fixed maturity securities in an unrealized loss position, and it is not “more likely than not” that the Company will be required to sell these securities before a recovery in fair value to their amortized cost basis occurs. For equity securities, management evaluates the near-term prospects of these investments in relation to the severity and duration of the impairment, and the Company’s ability and intent to hold these investments until a recovery of fair value occurs. Bank Loan Participations Held-for-Investment and Allowance for Credit Losses Bank loan participations held-for-investment are managed by a specialized outside investment manager and are generally stated at their outstanding unpaid principal balances net of unamortized premiums or discounts and net of any allowance for credit losses. Interest income is accrued on the unpaid principal balance. Discounts and premiums are amortized to income using the interest method. Generally, the accrual of interest on a bank loan participation is discontinued when the contractual payment of principal or interest has become 90 days past due or management has serious doubts about further collectability of principal or interest. A bank loan participation may remain on accrual status if it is in the process of collection and is either guaranteed or well secured. Interest received on nonaccrual loans generally is reported as investment income. There were no bank loans on nonaccrual status at December 31, 2017 or 2016 . Generally, bank loan participations are restored to accrual status when the obligation is brought current, has performed in accordance with the contractual terms for a reasonable period of time, and the ultimate collectability of the total contractual principal and interest is no longer in doubt. ​ The allowance for credit losses is maintained at a level believed adequate by management to absorb estimated probable credit losses. Management’s periodic evaluation of the adequacy of the allowance is based on consultations and advice of the Company’s specialized investment manager, known and inherent risks in the portfolio, adverse situations that may affect the borrower’s ability to repay, the estimated value of any underlying collateral, current economic conditions, and other relevant factors. When an observable market price for a loan is available, the Company has recorded an allowance equal to the difference between the fair value and the amortized cost of bank loans that it has determined to be impaired as a practical expedient for an estimate of probable future cash flows to be collected on those bank loans. If an observable market price for a loan is not available, the Company records an allowance equal to the difference between the present value of expected future cash flows discounted at the loan’s effective interest rate and the amortized cost of the loan. Bank loans are charged off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote. Other Invested Assets Other invested assets at December 31, 2017 and 2016 include the Company’s interests in private debt and equity investments. The investments are primarily focused in renewable energy, limited partnerships, and bank holding companies. Equity interests in various limited liability companies (“LLCs”) and limited partnerships are accounted for under the equity method, as the Company has determined that the equity method best reflects its economic interest in the underlying equity investment. For certain note agreements, original discounts and commitment fees received are recognized over the terms of the notes under the effective interest method. 1. Accounting Policies (continued) Short-Term Investments Short-term investments are carried at cost, which approximates fair value. Short-term investments have maturities greater than three months but less than one year at the date of purchase. Cash Equivalents The Company considers highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents. Direct Written Premiums Direct written premiums are earned on a pro rata basis over the terms of the policies, generally 12 months. The portion of premiums written applicable to the unexpired terms of the policies in force is recorded as unearned premiums. Policies are accounted for on an individual basis, with no aggregation by counterparty. Assumed Reinsurance Premiums Assumed reinsurance written premiums include amounts reported by brokers and ceding companies, supplemented by the Company’s own estimates of premiums when reports have not been received. Premiums on the Company’s excess of loss and pro rata reinsurance contracts are estimated when the business is underwritten. For excess of loss contracts, the deposit premium, as defined in the contract, is generally recorded as an estimate of premiums written at the inception date of the treaty. Estimates of premiums written under pro rata contracts are recorded in the period in which the underlying risks are expected to begin and are based on information provided by the brokers and the ceding companies. Reinsurance premium estimates are reviewed by management periodically. Any adjustment to these estimates is recorded in the period in which it becomes known. ​ Reinsurance premiums assumed are earned over the terms of the underlying policies or reinsurance contracts. Contracts and policies written on a “losses occurring” basis cover claims that may occur during the term of the contract or policy, which is typically 12 months. Accordingly, the premium is earned evenly over the term. Contracts which are written on a “risks attaching” basis cover claims which attach to the underlying insurance policies written during the terms of such contracts. Premiums earned on such contracts usually extend beyond the original term of the reinsurance contract, typically resulting in recognition of premiums earned over a 24-month period in proportion to the level of underlying exposure. Contracts are accounted for on an individual basis, with no aggregation by counterparty. Premiums Receivable and Agents’ Balances, Net Premiums receivable and agents’ balances are carried at face value net of any allowance for doubtful accounts. The allowance for doubtful accounts represents an estimate of amounts considered uncollectible based on the Company’s assessment of the collectability of receivables that are past due. Receivables greater than 90 days past due were $2.6 million and $2.0 million at December 31, 2017 and 2016 , respectively. The allowance for doubtful accounts was $2.8 million and $2.1 million at December 31, 2017 and 2016 , respectively. Bad debt expense was $1.0 million for the year ended December 31, 2017 , $813,000 for the year ended December 31, 2016 , and $1.1 million for the year ended December 31, 2015 . Receivables written off against the allowance for doubtful accounts totaled $408,000 for the year ended December 31, 2017 , $1.5 million for the year ended December 31, 2016 , and $268,000 for the year ended December 31, 2015 . Account balances are charged off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote. Deferred Policy Acquisition Costs Costs which are incrementally or directly related to the successful acquisition of new or renewal insurance business are deferred. These deferred costs are primarily commissions to agents, ceding commissions paid on reinsurance assumed, premium taxes, and the portion of underwriting fixed compensation and payroll related fringe benefits directly related to an insurance contract that has been acquired, net of ceding commissions related to reinsurance ceded. Amortization of such policy acquisition costs is charged to expense in proportion to premium earned over the estimated policy life. To the extent that unearned premiums on existing policies are not adequate to cover projected related costs and expenses, deferred policy acquisition costs are charged to earnings. The Company considers anticipated investment income in determining whether a premium deficiency exists. 1. Accounting Policies (continued) Reinsurance and Adjustable Features of Insurance and Reinsurance Contracts Certain premiums and losses are ceded to other insurance companies or assumed from other insurance companies under various excess of loss and quota-share reinsurance contracts. The Company enters into ceded reinsurance contracts to limit its exposure to large losses, to limit exposure on new lines of insurance written by the Company, and to provide additional capacity for growth. Premiums, commissions, and losses and loss adjustment expenses on reinsured business are accounted for on a basis consistent with that used in accounting for the original policies issued and the terms of the reinsurance contracts. Reinsurance recoverables and prepaid reinsurance premiums are reported as assets. Other amounts payable to insurance companies and reinsurers or receivable from insurance companies and reinsurers are netted where the right of offset exists. The Company receives ceding commissions in connection with certain ceded reinsurance. The ceding commissions are recorded as a reduction of other operating expenses. Certain reinsurance contracts of the Casualty Reinsurance segment include provisions that adjust premiums or acquisition expenses based upon the experience under the contracts. Premiums written and earned, as well as related acquisition expenses are recorded based upon the projected experience under the contracts. The Company’s Specialty Admitted Insurance segment writes insurance under specialty admitted fronting and program arrangements. The fronting and program arrangements may contain contractual provisions that adjust acquisition expenses based upon loss experience under the contracts. The specialty admitted fronting and program arrangements are significantly reinsured. These reinsurance contracts may also contain provisions that adjust premiums or acquisition expenses based upon the loss experience under the contracts. Premiums written and earned, as well as related acquisition expenses, are recorded based upon the projected experience under the contracts. Other Income Other income is principally comprised of fee income earned on policies for which the Company has no exposure to underwriting risk. Fee income of $17.0 million , $10.1 million , and $3.2 million is included in other income for the years ended December 31, 2017 , 2016 , and 2015 , respectively. Fees are earned on a pro rata basis over the service period of the underlying business. Policies are accounted for on an individual basis, with no aggregation by counterparty. Income Taxes Deferred tax assets and deferred tax liabilities are provided for the future tax consequences attributable to temporary differences between the financial statement carrying amounts of assets and liabilities and their respective U.S. tax bases. Deferred tax assets and liabilities are measured using enacted U.S. corporate tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. Deferred tax assets are reduced by a valuation allowance only when management believes it is more likely than not that some, or all, of the deferred tax assets will not be realized. Goodwill Goodwill is tested annually for impairment in the fourth quarter of each calendar year, or more frequently if events or changes in circumstances indicate that the carrying amount of the Company’s reporting units, including goodwill, may exceed their fair values. The Company first assesses qualitative factors in determining whether it is necessary to perform the quantitative goodwill impairment test. If management determines that it is more likely than not that the fair value of a reporting unit is less than the carrying value based on qualitative factors then they will perform the quantitative goodwill impairment test. For the quantitative goodwill impairment testing, the fair value of the reporting units is determined using a combination of a market approach and an income approach which projects the future cash flows produced by the reporting units and discounts those cash flows to their present value. The projection of future cash flows is necessarily dependent upon assumptions on the future levels of income as well as business trends, prospects, market, and economic conditions. The results of the two approaches are weighted to determine the fair value of each reporting unit. When the fair value is less than the carrying value of the net assets of the reporting unit, including goodwill, an impairment loss is charged to operations. To determine the amount of any goodwill impairment, the implied fair value of reporting unit goodwill is compared to the carrying amount of that goodwill. The implied fair value of goodwill is determined in the same manner as the amount of goodwill recognized in a business combination is determined. That is, the fair value of a reporting unit is assigned to all of the assets and liabilities of that unit (including any unrecognized intangible assets) as if the reporting unit had been acquired in a business combination. The excess of the fair value of a reporting unit over the amounts assigned to its assets and liabilities is the implied fair value of goodwill. 1. Accounting Policies (continued) Intangible Assets, Net Intangible assets are initially recognized and measured at fair value. Specifically identified intangible assets with indefinite lives include trademarks and state insurance licenses and authorities. Other specifically identified intangible assets with lives ranging from 7.0 to 27.5 years represent relationships with brokers. These intangible assets are amortized on a straight-line basis over their estimated useful lives. Intangible assets with indefinite useful lives are reviewed for impairment at least annually. In evaluating whether there has been impairment to the intangible asset, management determines the fair value of the intangible asset and compares the resulting fair value to the carrying value of the intangible asset. If the carrying value exceeds the fair value, the intangible asset is written down to fair value, and the impairment is reported through earnings. The Company evaluates intangible assets with definite lives for impairment when impairment indicators are noted. Impairment of Long-Lived Assets Long-lived assets with finite lives are tested for impairment whenever recognized events or changes in circumstances indicate the carrying value of these assets may not be recoverable. If indicators of impairment are present, fair value is calculated using estimated future cash flows expected to be generated from the use of those assets. An impairment loss is recognized only if the carrying amount of a long-lived asset or asset group is not recoverable and exceeds its fair value. The carrying amount of a long-lived asset or asset group is not recoverable if it exceeds the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the asset or asset group. That assessment is based on the carrying amount of the asset or asset group at the date it is tested for recoverability. An impairment loss is measured as the amount by which the carrying amount of a long-lived asset or asset group exceeds its fair value. Property and Equipment, Net Property and equipment, which is included in “other assets” in the accompanying consolidated balance sheets, is reported at cost less accumulated depreciation and is depreciated principally on a straight-line basis over the estimated useful lives of the depreciable assets, generally three to ten years . In the event the Company has been deemed the owner for accounting purposes of construction projects in lease arrangements, the estimated construction costs incurred to date are recorded as assets in property and equipment, net and included in “other assets” in the accompanying consolidated balance sheets. Upon occupancy of facilities under lease, the Company assesses whether arrangements qualify for sales recognition under the sale-leaseback accounting guidance. If the Company continues to be the deemed owner for accounting purposes, the cost of the building is depreciated over its estimated useful life. Reserve for Losses and Loss Adjustment Expenses The reserve for losses and loss adjustment expenses represents the estimated ultimate cost of all reported and unreported losses and loss adjustment expenses incurred and unpaid at the balance sheet date. The Company does not discount this reserve. The process of estimating the reserve for losses and loss adjustment expenses requires a high degree of judgment and is subject to a number of variables. The reserve for losses and loss adjustment expenses is estimated using individual case-basis valuations and statistical analyses. Those estimates are subject to the effects of trends in loss severity and frequency. The Company utilizes various actuarially-accepted reserving methodologies in determining the continuum of expected outcomes for its reserves. These methodologies utilize various inputs, including management’s initial expected loss ratio (the ratio of losses and loss adjustment expenses incurred to net earned premiums), expected reporting patterns and payment patterns for losses and loss adjustment expenses (based on insurance industry data and the Company’s own experience), and the Company’s actual paid and reported losses and loss adjustment expenses. An internal actuary reviews these results and (after applying appropriate professional judgment and other actuarial techniques that are considered necessary) presents recommendations to the Company’s management. Management uses this information and its judgment to make decisions on the final recorded reserve for losses and loss adjustment expenses. Management believes that the use of judgment is necessary to arrive at a best estimate for the reserve for losses and loss adjustment expenses given the long-tailed nature of the business generally written by the Company and the limited operating experience of the Casualty Reinsurance segment, the fronting and program business in the Specialty Admitted Insurance segment, and the commercial auto business in the Excess and Surplus Lines segment. 1. Accounting Policies (continued) Catastrophes of significant magnitude, including hurricanes and earthquakes, involve complex coverage issues. In estimating the reserve for losses and loss adjustment expenses for these catastrophes, management uses case reserve estimates based on information obtained from site inspections by the Company’s adjustors and the terms of coverage provided in the policies. Management estimates reserves for incurred but not reported claims for these catastrophes using judgment based on an assessment of the Company’s property insurance exposures where the catastrophes occur and the Company’s progress in settling claims. Although management believes that the reserve for losses and loss adjustment expenses is reasonable, it is possible that the Company’s actual incurred losses and loss adjustment expenses will not develop in a manner consistent with the assumptions inherent in the determination of these reserves. Specifically, the Company’s actual ultimate loss ratio could differ from management’s initial expected loss ratio and/or the Company’s actual reporting patterns for losses could differ from the expected reporting patterns. Accordingly, the ultimate settlement of losses and the related loss adjustment expenses may vary significantly from the estimates included in the Company’s consolidated financial statements. These estimates are reviewed continually by management and are adjusted as necessary as experience develops or new information becomes known; such adjustments are included in current operations. Share Based Compensation The Company expenses the fair value of share equity awards over the vesting period of the award on a straight-line basis. The Black-Scholes-Merton option pricing model is used to value the options granted (see Note 11). As the share based compensation expense is incurred, a corresponding increase to additional paid-in capital in shareholders’ equity is recognized. Share based compensation expense is reflected in “other operating expenses” in the accompanying consolidated statements of income and comprehensive income. Financing Obligations In a lease arrangement where the Company made a minority investment in a partnership that was involved in the construction of a building, the Company was deemed the owner for accounting purposes during the construction period. The Company recorded an asset for the amount of the total project costs and the related financing obligation is included in “other liabilities” in the accompanying consolidated balance sheets. Once construction was completed, the Company determined the arrangement did not qualify for sale-lease back treatment. Accordingly, the Company continues to reduce the obligation over the lease term as payments are made and depreciates the asset over its useful life. The Company does not report rent expense for the portion of the rent payment determined to be related to the assets which are owned for accounting purposes. Rather, this portion of the rent payment under the lease is recognized as a reduction of the financing obligation and as interest expense. ​ Variable Interest Entities Entities that do not have sufficient equity at risk to allow the entity to finance its activities without additional financial support or in which the equity investors, as a group, do not have the characteristic of a controlling financial interest are referred to as variable interest entities (“VIE”). A VIE is consolidated by the variable interest holder that is determined to have the controlling financial interest (primary beneficiary) as a result of having both the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance and the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. The Company determines whether it is the primary beneficiary of an entity subject to consolidation based on a qualitative assessment of the VIE’s capital structure, contractual terms, nature of the VIE’s operations and purpose, and the Company’s relative exposure to the related risks of the VIE on the date it becomes initially involved in the VIE. The Company reassesses its VIE determination with respect to an entity on an ongoing basis. The Company holds interests in VIEs through certain equity method investments included in “other invested assets” in the accompanying consolidated balance sheets. The Company has determined that it should not consolidate any of the VIEs as it is not the primary beneficiary in any of the relationships. Although the investments resulted in the Company holding variable interests in the entities, they did not empower the Company to direct the activities that most significantly impact the economic performance of the entities. The Company’s investments related to these VIEs totaled $32.1 million and $27.1 million as of December 31, 2017 and 2016 , respectively, representing the Company’s maximum exposure to loss. Earnings Per Share Basic earnings per share excludes dilution and is computed by dividing net income by the weighted-average number of common shares outstanding for the period. Diluted earnings per share reflects the dilution that could occur if securities or other contracts to issue common shares or common share equivalents were exercised or converted into common shares as calculated 1. Accounting Policies (continued) using the treasury stock method. When inclusion of common share equivalents increases the earnings per share or reduces the loss per share, the effect on earnings is anti-dilutive, and the diluted net earnings or net loss per share is computed excluding these common share equivalents. The following represents a reconciliation of the numerator and denominator of the basic and diluted earnings per share computations contained in the consolidated financial statements. ​ Net Income Weighted-Average Earnings ​ (in thousands, except per share data) Year ended December 31, 2017 ​ ​ ​ Basic $ 43,566 29,461,717 $ 1.48 Common share equivalents — 811,432 (0.04 ) Diluted $ 43,566 30,273,149 $ 1.44 Year ended December 31, 2016 ​ ​ ​ Basic $ 74,471 29,063,075 $ 2.56 Common share equivalents — 831,303 (0.07 ) Diluted $ 74,471 29,894,378 $ 2.49 Year ended December 31, 2015 ​ ​ ​ Basic $ 53,497 28,662,051 $ 1.87 Common share equivalents — 672,867 (0.05 ) Diluted $ 53,497 29,334,918 $ 1.82 Common share equivalents relate to our stock options and restricted stock units (“RSUs”). For the year ended December 31, 2017 , common share equivalents of 174,380 shares are excluded from the calculations of diluted earnings per share as their effects are anti-dilutive. For the years ended December 31, 2016 and 2015 , all common share equivalents are dilutive. Adopted Accounting Standards Effective January 1, 2017, the Company adopted Accounting Standards Update (“ASU”) 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting using the prospective method. The guidance requires that all tax effects related to share-based payments be made through the income statement at the time of settlement as opposed to excess tax benefits being recognized in additional paid-in capital under the previous guidance . The ASU also removes the requirement to delay recognition of a tax benefit until it reduces current taxes payable. Additionally, all tax related cash flows resulting from share-based payments are now reported as operating activities on the statement of cash flows, a change from the previous requirement to present tax benefits as an inflow from financing activities and an outflow from operating activities. For the years ended December 31, 2016 and 2015 , net cash provided by operating activities increased by $3.2 million and $3.6 million , respectively, and net cash used in financing activities increased by $3.2 million and $3.6 million , respectively, for the adoption of ASU 2016-09. The Company has elected to recognize forfeitures as they occur in accordance with ASU 2016-09. The adoption of ASU 2016-09 did not materially impact the Company’s consolidated income statements. Prospective Accounting Standards In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) . The guidance applies to all companies that either enter into contracts with customers to transfer goods or services or enter into contracts for the transfer of nonfinancial assets, unless those contracts are within the scope of other standards, such as insurance contracts. Under this guidance, a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. Judgments required in adopting this u

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The Company's available-for-sale investments are summarized as follows: ​ Cost or Gross Gross Fair ​ (in thousands) December 31, 2017 Fixed maturity securities: State and municipal $ 139,382 $ 5,587 $ (603 ) $ 144,366 Residential mortgage-backed 160,379 723 (2,441 ) 158,661 Corporate 408,857 7,503 (2,639 ) 413,721 Commercial mortgage and asset-backed 182,595 714 (698 ) 182,611 Obligations of U.S. government corporations and agencies 35,948 — (101 ) 35,847 U.S. Treasury securities and obligations guaranteed by the U.S. government 79,476 37 (639 ) 78,874 Redeemable preferred stock 2,025 — (7 ) 2,018 Total fixed maturity securities 1,008,662 14,564 (7,128 ) 1,016,098 Equity securities 75,318 7,830 (626 ) 82,522 Total investments available-for-sale $ 1,083,980 $ 22,394 $ (7,754 ) $ 1,098,620 December 31, 2016 Fixed maturity securities: State and municipal $ 101,793 $ 5,032 $ (984 ) $ 105,841 Residential mortgage-backed 152,703 1,070 (2,975 ) 150,798 Corporate 379,727 4,382 (5,661 ) 378,448 Commercial mortgage and asset-backed 167,967 906 (826 ) 168,047 Obligations of U.S. government corporations and agencies 64,823 276 (85 ) 65,014 U.S. Treasury securities and obligations guaranteed by the U.S. government 71,174 131 (185 ) 71,120 Redeemable preferred stock 2,025 — (216 ) 1,809 Total fixed maturity securities 940,212 11,797 (10,932 ) 941,077 Equity securities 74,553 4,503 (2,655 ) 76,401 Total investments available-for-sale $ 1,014,765 $ 16,300 $ (13,587 ) $ 1,017,478 The amortized cost and fair value of available-for-sale investments in fixed maturity securities at December 31, 2017 are summarized, by contractual maturity, as follows: ​ Amortized Fair ​ (in thousands) One year or less $ 66,852 $ 66,770 After one year through five years 274,095 274,421 After five years through ten years 195,333 195,995 After ten years 127,383 135,622 Residential mortgage-backed 160,379 158,661 Commercial mortgage and asset-backed 182,595 182,611 Redeemable preferred stock 2,025 2,018 Total $ 1,008,662 $ 1,016,098 2. Investments (continued) ​ Actual maturities may differ for some securities because borrowers have the right to call or prepay obligations with or without penalties. The following table shows the Company’s gross unrealized losses and fair value for available-for-sale securities aggregated by investment category and the length of time that individual securities have been in a continuous unrealized loss position: ​ Less Than 12 Months 12 Months or More Total ​ Fair Gross Fair Gross Fair Gross ​ (in thousands) December 31, 2017 Fixed maturity securities: State and municipal $ 40,117 $ (318 ) $ 10,662 $ (285 ) $ 50,779 $ (603 ) Residential mortgage-backed 50,447 (261 ) 84,193 (2,180 ) 134,640 (2,441 ) Corporate 113,464 (846 ) 66,954 (1,793 ) 180,418 (2,639 ) Commercial mortgage and asset-backed 53,965 (244 ) 25,299 (454 ) 79,264 (698 ) Obligations of U.S. government corporations and agencies 3,024 (1 ) 32,154 (100 ) 35,178 (101 ) U.S. Treasury securities and obligations guaranteed by the U.S. government 50,760 (430 ) 26,707 (209 ) 77,467 (639 ) Redeemable preferred stock 2,018 (7 ) — — 2,018 (7 ) Total fixed maturity securities 313,795 (2,107 ) 245,969 (5,021 ) 559,764 (7,128 ) Equity securities 5,859 (65 ) 5,665 (561 ) 11,524 (626 ) Total investments available-for-sale $ 319,654 $ (2,172 ) $ 251,634 $ (5,582 ) $ 571,288 $ (7,754 ) December 31, 2016 Fixed maturity securities: State and municipal $ 28,398 $ (984 ) $ — $ — $ 28,398 $ (984 ) Residential mortgage-backed 93,242 (1,548 ) 32,330 (1,427 ) 125,572 (2,975 ) Corporate 199,841 (4,212 ) 8,477 (1,449 ) 208,318 (5,661 ) Commercial mortgage and asset-backed 47,990 (799 ) 7,195 (27 ) 55,185 (826 ) Obligations of U.S. government corporations and agencies 50,573 (85 ) — — 50,573 (85 ) U.S. Treasury securities and obligations guaranteed by the U.S. government 48,989 (185 ) — — 48,989 (185 ) Redeemable preferred stock 1,809 (216 ) — — 1,809 (216 ) Total fixed maturity securities 470,842 (8,029 ) 48,002 (2,903 ) 518,844 (10,932 ) Equity securities 21,345 (1,071 ) 6,558 (1,584 ) 27,903 (2,655 ) Total investments available-for-sale $ 492,187 $ (9,100 ) $ 54,560 $ (4,487 ) $ 546,747 $ (13,587 ) The Company held securities of 166 issuers that were in an unrealized loss position at December 31, 2017 with a total fair value of $571.3 million and gross unrealized losses of $7.8 million . None of the fixed maturity securities with unrealized losses has ever missed, or been delinquent on, a scheduled principal or interest payment. 2. Investments (continued) ​ At December 31, 2017 , 99.4% of the Company’s fixed maturity security portfolio was rated “BBB-” or better (“investment grade”) by Standard & Poor’s or received an equivalent rating from another nationally recognized rating agency. Fixed maturity securities with ratings below investment grade by Standard & Poor’s or another nationally recognized rating agency at December 31, 2017 had an aggregate fair value of $6.5 million and an aggregate net unrealized gain of $305,000 . The Company previously held two municipal bonds issued by the Commonwealth of Puerto Rico. Puerto Rico’s weak economic conditions and heavy debt burden heightened the risk of default on the bonds and management concluded that the bonds, which had been downgraded to below investment grade, were other-than-temporarily impaired. The Company recognized impairment losses of $660,000 on these bonds for the year ended December 31, 2015 . The bonds were sold during the second quarter of 2015 and a net realized gain of $22,000 was recognized on the sales. Management concluded that none of the other fixed maturity securities with an unrealized loss at December 31, 2017 , 2016 , and 2015 experienced an other-than-temporary impairment. Management does not intend to sell available-for-sale securities in an unrealized loss position, and it is not “more likely than not” that the Company will be required to sell these securities before a recovery in their value to their amortized cost basis occurs. Management concluded that based on the severity and duration of the impairment associated with an equity security, the security had experienced an other-than-temporary impairment at December 31, 2017 . Accordingly, the Company recorded an impairment loss of $1.5 million in 2017. Management concluded that none of the other equity securities with an unrealized loss at December 31, 2017 , 2016 , and 2015 experienced an other-than-temporary impairment. Management has evaluated the near-term prospects of these other equity securities in relation to the severity and duration of the impairment, and management has the ability and intent to hold these securities until a recovery of their fair value. Bank loan participations generally have a credit rating that is below investment grade (i.e. below “BBB-” for Standard & Poor’s) at the date of purchase. These bank loans are primarily senior, secured floating-rate debt rated “BB”, “B”, or “CCC” by Standard & Poor’s or an equivalent rating from another nationally recognized rating agency. These bank loans include assignments of, and participations in, performing and non-performing senior corporate debt generally acquired through primary bank syndications and in secondary markets. Bank loans consist of, but are not limited to, term loans, the funded and unfunded portions of revolving credit loans, and other similar loans and investments. Management believed that it was probable at the time that these loans were acquired that the Company would be able to collect all contractually required payments receivable. At December 31, 2015 , the Company held participations in two loans issued by companies that produce and supply power to Puerto Rico through power purchase agreements with Puerto Rico Electric Power Authority (“PREPA”), a public corporation and governmental agency of the Commonwealth of Puerto Rico. PREPA’s credit strength and ability to make timely payments was impacted by the economic conditions in Puerto Rico, thus raising doubt about the companies’ ability to meet the debt obligations held by the Company. Management concluded that the loans were impaired at December 31, 2015 and established an allowance for credit losses on the loans of $414,000 . After recording this impairment, the loans had a carrying value of $3.9 million at December 31, 2015 and unpaid principal of $4.6 million . In June 2016, one of the loans was repaid in full at its scheduled maturity. Management concluded that the remaining loan was still impaired at December 31, 2016 . The loan had a carrying value of $1.7 million , unpaid principal of $2.0 million , and an allowance for credit losses of $177,000 at December 31, 2016 . In July 2017, PREPA filed a petition for relief in U.S. District Court under Title III of the Puerto Rico Oversight, Management, and Economic Stability Act. Also in 2017, the devastation caused by Hurricane Maria on the island of Puerto Rico raised additional doubt about PREPA's ability to make payments to the issuer. The maturity of the remaining loan, originally scheduled for November 2017, was deferred until March 2018. Management established an allowance for credit losses of $759,000 to reduce the loan's carrying value to zero at both September 30, 2017 and December 31, 2017. The unpaid principal on the loan was $807,000 at December 31, 2017 . The Company’s bank loan portfolio includes loans to oil and gas companies in the energy sector. The market values of these loans have been impacted by declining energy prices. At December 31, 2017 , the Company’s oil and gas exposure in the bank loan portfolio was in four loans with a carrying value of $7.9 million and unrealized losses of $180,000 . Management concluded that three of these loans were impaired as of December 31, 2017 , and accordingly, an allowance for credit losses of $2.3 million was established on the loans. After recording this impairment, the loans had a carrying value of $4.6 million at December 31, 2017 and unpaid principal of $6.9 million . All of the other loans are current at December 31, 2017 . At December 31, 2016, one loan in the energy sector was impaired with a carrying value of $1.6 million , unpaid principal of $2.2 million and an allowance for credit losses of $545,000 . At December 31, 2015, two loans in the energy sector were impaired with a carrying value of $1.7 million , unpaid principal of $5.8 million and an allowance for credit losses of $3.9 million . 2. Investments (continued) ​ Management also concluded that one non-energy sector loan was impaired at December 31, 2017 . At December 31, 2017 , the impaired loan had a carrying value of $561,000 , unpaid principal of $706,000 , and an allowance for credit losses of $145,000 . At December 31, 2016 , three non-energy sector loans were impaired with a total carrying value of $3.2 million , unpaid principal of $3.5 million , and an allowance for credit losses of $221,000 . At December 31, 2015 , one non-energy sector loan was impaired with a carrying value of $689,000 , unpaid principal of $722,000 , and an allowance for credit losses of $34,000 . The aggregate allowance for credit losses was $3.2 million at December 31, 2017 on five impaired loans with a total carrying value of $5.1 million and unpaid principal of $8.4 million . At December 31, 2016 , the aggregate allowance for credit losses was $943,000 on five impaired loans with a total carrying value of $6.5 million and unpaid principal of $7.6 million . At December 31, 2015 , the aggregate allowance for credit losses was $4.3 million on five impaired loans with a total carrying value of $6.3 million and unpaid principal of $11.1 million . The average recorded investment in impaired bank loans was $5.8 million , $6.4 million , and $6.7 million , during the years ended December 31, 2017 , 2016 , and 2015 , respectively, and investment income of $300,000 , $297,000 , and $229,000 , was recognized during the time that the loans were impaired. The Company recorded realized losses of $2.4 million , realized gains of $415,000 , and realized losses of $3.4 million during the years ended December 31, 2017 , 2016 , and 2015 , respectively, for changes in the fair value of impaired bank loans. At December 31, 2017 , unamortized discounts on bank loan participations were $1.1 million , and unamortized premiums were $3,000 . At December 31, 2016 , unamortized discounts on bank loan participations were $2.9 million , and unamortized premiums were $14,000 . Major categories of the Company’s net investment income are summarized as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Fixed maturity securities $ 26,833 $ 25,917 $ 24,178 Bank loan participations 17,388 14,486 13,432 Equity securities 5,045 5,617 4,444 Other invested assets 14,079 9,536 5,947 Cash, cash equivalents, short-term investments, and other 1,708 824 672 Trading gains (losses) (4 ) 18 (9 ) Gross investment income 65,049 56,398 48,664 Investment expense (3,930 ) (3,760 ) (3,829 ) Net investment income $ 61,119 $ 52,638 $ 44,835   Changes in unrealized gains or losses on securities held for trading are recorded as trading gains or losses within net investment income. Net investment income for the year ended December 31, 2017 included $4,000 of net trading losses, all of which related to securities still held at December 31, 2017 . Net investment income for the year ended December 31, 2016 included $18,000 of net trading gains, all of which related to securities still held at December 31, 2016 . Net investment income for the year ended December 31, 2015 included $9,000 of net trading losses of which $7,000 of net trading losses related to securities still held at December 31, 2015 . 2. Investments (continued) ​ The Company’s realized gains and losses on investments are summarized as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Fixed maturity securities: Gross realized gains $ 840 $ 1,916 $ 2,197 Gross realized losses (512 ) (106 ) (826 ) ​ 328 1,810 1,371 Equity securities: Gross realized gains 429 4,761 1,041 Gross realized losses (1,591 ) — (10 ) ​ (1,162 ) 4,761 1,031 Bank loan participations: Gross realized gains 2,407 2,827 1,269 Gross realized losses (3,557 ) (1,832 ) (8,258 ) ​ (1,150 ) 995 (6,989 ) Short-term investments and other: Gross realized gains 1 3 54 Gross realized losses (6 ) (4 ) (14 ) ​ (5 ) (1 ) 40 Total $ (1,989 ) $ 7,565 $ (4,547 ) The following table summarizes the change in the Company’s available-for-sale gross unrealized gains or losses by investment type: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Change in gross unrealized gains (losses): Fixed maturity securities $ 6,571 $ (1,350 ) $ (16,832 ) Equity securities 5,356 (2,433 ) 724 Total $ 11,927 $ (3,783 ) $ (16,108 )   2. Investments (continued) ​ The Company invests selectively in private debt and equity opportunities. These investments, which together comprise the Company’s other invested assets, are primarily focused in renewable energy, limited partnerships, and bank holding companies. ​ Carrying Value Investment Income ​ Year Ended ​ December 31, ​ 2017 2016 2017 2016 2015 ​ (in thousands) (in thousands) Category: Renewable energy LLCs (a) $ 32,063 $ 27,067 $ 10,578 $ 3,480 $ 3,936 Renewable energy notes receivable (b) 7,278 — 526 450 3,136 Limited partnerships (c) 26,367 23,852 2,632 5,263 (1,468 ) Bank holding companies (d) 4,500 4,500 343 343 343 Total other invested assets $ 70,208 $ 55,419 $ 14,079 $ 9,536 $ 5,947 (a) The Company’s Corporate and Other segment owns equity interests ranging from 2.6% to 32.8% in various LLCs whose principal objective is capital appreciation and income generation from owning and operating renewable energy production facilities (wind and solar). The LLCs are managed by an affiliate of the Company’s largest shareholder and the Company’s Chairman and Chief Executive Officer has invested in certain of these LLCs. The equity method is used to account for the Company’s LLC investments. Income for the LLCs primarily reflects adjustments to the carrying values of investments in renewable energy projects to their determined fair values. The fair value adjustments are included in revenues for the LLCs. Expenses for the LLCs are not significant and are comprised of administrative and interest expenses. The Company received cash distributions from these investments totaling $5.6 million and $2.4 million for the years ended December 31, 2017 and 2016 , respectively. ​ (b) The Company has invested in notes receivable for renewable energy projects. In 2017 , the Company invested $7.3 million in a note with an affiliate of the Company's largest shareholder. Interest on the note, which matures in 2021, is fixed at 15% . The Company is committed to investing another $1.5 million in the note and commitment fees of 2.5% are earned on the unfunded commitment. Interest income on the note was $526,000 for the year ended December 31, 2017 . In 2016, the outstanding balance of a $6.5 million note was fully repaid. Income prior to repayment of the note was $450,000 for the year ended December 31, 2016 . During the year ended December 31, 2015, the Company invested a total of $36.3 million in these notes and received maturities and repayments totaling $30.8 million . ​ (c) The Company owns investments in limited partnerships that invest in concentrated portfolios including publicly-traded small cap equities, loans of middle market private equity sponsored companies, equity tranches of collateralized loan obligations (CLOs), and tranches of distressed home loans. Income from the partnerships is recognized under the equity method of accounting. The Company’s Corporate and Other segment held investments in limited partnerships of $3.0 million and $2.6 million at December 31, 2017 and 2016 , and recognized investment income of $394,000 and $455,000 and investment losses of $510,000 for the years ended December 31, 2017 , 2016 and 2015 , respectively. The Chairman and Chief Executive Officer of the Company is an investor in one limited partnership held by the Corporate and Other segment. The Company’s Excess and Surplus Lines segment holds investments in limited partnerships of $23.4 million and $21.2 million at December 31, 2017 and 2016 , respectively. Investment income of $2.2 million and $4.8 million and investment losses of $958,000 were recognized on the investments for the years ended December 31, 2017 , 2016 , and 2015 , respectively. At December 31, 2017 , the Company’s Excess and Surplus Lines segment has outstanding commitments to invest another $625,000 in a limited partnership that invests in loans of middle market private equity sponsored companies and another $5.0 million in a limited partnership that invests in tranches of distressed home loans. The Chairman and former Chief Executive Officer of the Company and one of the Company's directors are investors in one limited partnership held by the Excess and Surplus Lines segment. 2. Investments (continued) ​ (d) The Company holds $4.5 million of subordinated notes issued by a bank holding company. Interest on the notes, which mature in 2023, is fixed at 7.6% per annum. Interest income on the notes was $343,000 in each of the years ended December 31, 2017 , 2016 and 2015 . The Company's Chairman and former Chief Executive Officer was previously the Lead Independent Director of the bank holding company and an investor in the bank holding company. Additionally, one of the Company’s directors is a former investor in the bank holding company and is currently a lender to the bank holding company. The Company previously held common shares issued by the bank holding company. The shares were carried as available for-sale equity securities. Dividend income of $299,000 and $66,000 was recorded on the shares for the years ended December 31, 2016 and 2015 , respectively. Realized investment gains of $409,000 and $3.6 million were recognized on the sale of the common shares for the years ended December 31, 2017 and 2016 , respectively. At December 31, 2017 and 2016 , the Company held an investment in a collateralized loan obligation (CLO) where one of the underlying loans was issued by the bank holding company. The investment, with a carrying value of $4.7 million at December 31, 2017 , is classified as an available-for-sale fixed maturity. The Company holds a $1.0 million certificate of deposit issued by the bank holding company. The certificate of deposit, which matures on December 19, 2018, is carried as a short-term investment. Interest income of $3,000 , $4,000 and $5,000 was recognized on this investment for the years ended December 31, 2017 , 2016 , 2015 , respectively. Two of the Company’s directors were previously directors of First Wind Holdings, LLC (“First Wind”), which is an affiliate of the Company’s largest shareholder. At December 31, 2014, the Company held fixed maturity securities with a fair value of $12.6 million issued by a subsidiary of First Wind. These securities were called in March 2015, resulting in a realized gain of $845,000 . Also at December 31, 2014, the Company held a bank loan participation with a carrying value of $4.6 million from another subsidiary of First Wind. The loan was repaid in full in January 2015. The Company maintains fixed maturity securities, short-term investments, and cash and cash equivalents amounting to $472.0 million at December 31, 2017 in trust accounts or on deposit as collateral for outstanding letters of credit issued as security to third-party reinsureds on reinsurance assumed by JRG Re. At December 31, 2017 and 2016 , investments with a fair value of $16.5 million and $16.4 million , respectively, were on deposit with state insurance departments to satisfy regulatory requirements. At December 31, 2017 , the Company held no investments in securitizations of alternative-A mortgages or sub-prime mortgages.

Deferred Policy Acquisition Cos

Deferred Policy Acquisition Costs	12 Months Ended
	Dec. 31, 2017
Deferred Policy Acquisition Costs Disclosures [Abstract]
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs ​ An analysis of deferred policy acquisition costs is as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Balance at beginning of period $ 64,789 $ 60,754 $ 60,202 Policy acquisition costs deferred: ​ ​ ​ Commissions 107,387 92,736 90,342 Underwriting and other issue expenses 16,190 12,923 7,960 ​ 123,577 105,659 98,302 Amortization of policy acquisition costs (116,001 ) (101,624 ) (97,750 ) Net change 7,576 4,035 552 Balance at end of period $ 72,365 $ 64,789 $ 60,754

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets ​ On December 11, 2007, the Company completed an acquisition of James River Group by acquiring 100% of the outstanding shares of James River Group common stock, referred to herein as the “Merger”. The transaction was accounted for under the purchase method of accounting, and goodwill and intangible assets were recognized by the Company as a result of the transaction. All of the Company’s goodwill is an asset of the Excess and Surplus Lines segment. The Company’s annual testing performed in the fourth quarter of 2017 , 2016 and 2015 indicated that no impairment of goodwill had occurred. The carrying amount of goodwill at December 31, 2017 and 2016 was $181.8 million . Accumulated goodwill impairment losses were $99.6 million at December 31, 2017 and 2016 . The most recent goodwill impairment losses occurred in 2010. Specifically identifiable intangible assets were acquired in the Merger. During the fourth quarters of 2017 , 2016 and 2015 , the indefinite-lived intangible assets for trademarks and insurance licenses and authorities were tested for impairment. Intangible assets for customer and broker relationships that have specific lives and are subject to amortization were also reviewed for impairment. There were no impairments recognized in 2017 , 2016 , or 2015 . The gross carrying amounts and accumulated amortization for each major specifically identifiable intangible asset class were as follows: ​ December 31, ​ 2017 2016 Weighted- Average Life (Years) Gross Accumulated Gross Accumulated ​ ​ (in thousands) Trademarks Indefinite $ 22,200 $ — $ 22,200 $ — Insurance licenses and authorities Indefinite 9,164 — 9,164 — Identifiable intangibles not subject to amortization ​ 31,364 — 31,364 — Broker relationships 24.6 11,611 4,641 11,611 4,044 Identifiable intangible assets subject to amortization ​ 11,611 4,641 11,611 4,044 ​ ​ $ 42,975 $ 4,641 $ 42,975 $ 4,044 4. Goodwill and Intangible Assets (continued) Future estimated amortization of specifically identifiable intangible assets as of December 31, 2017 is as follows ( in thousands) : 2018 $ 597 2019 597 2020 538 2021 363 2022 363 Thereafter 4,512 Total $ 6,970 The table below summarizes the changes in the net carrying values of intangible assets by segment for 2017: December 31, 2016 December 31, 2017 ​ Net Carrying Amortization Impairment Net Carrying ​ (in thousands) Excess and Surplus Lines Trademarks $ 19,700 $ — $ — $ 19,700 Insurance licenses and authorities 4,900 — — 4,900 Broker relationships 6,689 (362 ) — 6,327 ​ 31,289 (362 ) — 30,927 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — — 4,265 Broker relationships 877 (235 ) — 642 ​ 7,642 (235 ) — 7,407 Total identifiable intangible assets $ 38,931 $ (597 ) $ — $ 38,334   ​ The table below summarizes the changes in the net carrying values of intangible assets by segment for 2016: December 31, 2015 December 31, 2016 ​ Net Carrying Amortization Impairment Net Carrying ​ (in thousands) Excess and Surplus Lines Trademarks $ 19,700 $ — $ — $ 19,700 Insurance licenses and authorities 4,900 — — 4,900 Broker relationships 7,051 (362 ) — 6,689 ​ 31,651 (362 ) — 31,289 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — — 4,265 Broker relationships 1,112 (235 ) — 877 ​ 7,877 (235 ) — 7,642 Total identifiable intangible assets $ 39,528 $ (597 ) $ — $ 38,931 4. Goodwill and Intangible Assets (continued) Amortization of intangible assets was $362,000 for the Excess and Surplus Lines segment and $235,000 for the Specialty Admitted Insurance segment for the year ended December 31, 2015 .

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net ​ Property and equipment, net of accumulated depreciation, consists of the following: ​ December 31, ​ 2017 2016 ​ (in thousands) Building, leased for which the Company has been deemed the owner for accounting purposes (Note 21) $ 30,902 $ 30,902 Electronic data processing hardware and software 6,156 4,460 Furniture and equipment 2,659 2,706 Property and equipment, cost basis 39,717 38,068 Accumulated depreciation (14,716 ) (13,388 ) Property and equipment, net $ 25,001 $ 24,680

Reserve for Losses and Loss Adj

Reserve for Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Abstract]
Reserve for Losses and Loss Adjustment Expenses	Reserve for Losses and Loss Adjustment Expenses ​ In establishing the reserve for losses and loss adjustment expenses, the Company’s internal actuaries estimate an initial expected ultimate loss ratio for each of our lines of business by accident year (or for our Casualty Reinsurance segment, on a contract by contract basis). Input from the Company’s underwriting and claims departments, including premium pricing assumptions and historical experience, are considered by the Company’s internal actuaries in estimating the initial expected loss ratios. The Company’s internal actuaries generally utilize five actuarial methods in their estimation process for the reserve for losses and loss adjustment expenses. These five methods utilize, to varying degrees, the initial expected loss ratio, detailed statistical analysis of past claims reporting and payment patterns, claims frequency and severity, paid loss experience, industry loss experience, and changes in market conditions, policy forms, exclusions, and exposures. In applying these methods to develop an estimate of the reserve for losses and loss adjustment expenses, our internal actuaries use judgment to determine three key parameters for each accident year and line of business: the initial expected loss ratios, the incurred and paid loss development factors and the weighting of the five actuarial methods to be used for each accident year and line of business. For the Excess and Surplus Lines and Specialty Admitted Insurance segments, the internal actuary performs a study on each of these parameters annually in the third quarter and makes recommendations for the initial expected loss ratios, the incurred and paid loss development factors and the weighting of the five actuarial methods by accident year and line of business. Members of management’s Reserve Committee review and approve the parameter review actuarial recommendations, and these approved parameters are used in the reserve estimation process for the next four quarters at which time a new parameter study is performed. For the Casualty Reinsurance segment, periodic assessments are made on a contract by contract basis with the goal of keeping the initial expected loss ratios and the incurred and paid loss development factors as constant as possible until sufficient evidence presents itself to support adjustments. Method weights are generally less rigid for the Casualty Reinsurance segment given the heterogeneous nature of the various contracts, and the potential for significant changes in mix of business within individual treaties. Different reserving methods are appropriate in different situations, and the Company’s internal actuaries use their judgment and experience to determine the weighting of the methods to use for each accident year and each line of business and, for our Casualty Reinsurance segment, on a contract by contract basis. For example, the current accident year has very little incurred and paid loss development data on which to base reserve projections. As a result, the Company relies heavily on the initial expected loss ratio in estimating reserves for the current accident year. The Company generally sets the initial expected loss ratio for the current accident year consistent with the internal actuaries’ pricing assumptions. We believe that this is a reasonable and appropriate reserving assumption for the current accident year since our pricing assumptions are actuarially driven and since the Company expects to make an acceptable return on the new business written. If actual loss emergence is better than our initial expected loss ratio assumptions, we will experience favorable development and if it is worse than our initial expected loss ratio assumptions, we will experience adverse development. Conversely, sufficient incurred and paid loss 6. Reserve for Losses and Loss Adjustment Expenses (continued) development is available for the oldest accident years, so more weight is given to this development data and less weight is given to the initial expected loss ratio. Historically, the Company’s reserve selections for the Excess and Surplus Lines segment gave more weight to industry indications due to the Company’s limited operating history. When reviewing the Excess and Surplus Lines segment’s reserve parameters in 2013, our internal actuaries’ felt that there was enough Company history to give more weight to the Company’s own experience. Accordingly, the initial expected loss ratios, the paid loss development factors and the incurred loss development factors were adjusted to more closely resemble the Company’s own internal indications. Method weights were also changed as management, in consultation with our actuaries, deemed appropriate. These changes had the cumulative effect of reducing our then best estimate for the reserve for losses and loss adjustment expenses.   The following table provides a reconciliation of the beginning and ending reserve balances for losses and loss adjustment expenses, net of reinsurance, to the gross amounts reported in the consolidated balance sheets: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period $ 761,128 $ 653,534 $ 589,042 Add: Incurred losses and loss adjustment expenses net of reinsurance: ​ ​ ​ Current year 533,905 349,137 295,334 Prior years 21,472 (23,716 ) (16,318 ) Total incurred losses and loss and adjustment expenses 555,377 325,421 279,016 Deduct: Loss and loss adjustment expense payments net of reinsurance: ​ ​ ​ Current year 103,205 39,473 31,957 Prior years 223,475 178,354 182,567 Total loss and loss adjustment expense payments 326,680 217,827 214,524 Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period 989,825 761,128 653,534 Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period 302,524 182,737 131,788 Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period $ 1,292,349 $ 943,865 $ 785,322 The foregoing reconciliation shows that $21.5 million of adverse development was experienced in 2017 on the reserve for losses and loss adjustment expenses held at December 31, 2016 . This adverse reserve development included $20.0 million of adverse development in the Excess and Surplus Lines segment, including $38.7 million of adverse development in the commercial auto line of business that was primarily related to the 2016 contract year with one insured. The adverse development for commercial auto was partially offset by favorable development of $18.6 million in other Excess and Surplus Lines underwriting divisions primarily from the 2014 through 2016 accident years. This favorable development occurred because our actuarial studies at December 31, 2017 for the Excess and Surplus Lines segment indicated that our loss experience on our casualty business excluding commercial auto continues to be below our initial expected ultimate loss ratios. The Company also experienced $2.7 million of favorable development on prior accident years in the Specialty Admitted Insurance segment primarily from accident years 2010 through 2015, as losses on our workers’ compensation business written prior to 2016 continued to develop more favorably than we had anticipated. The Casualty Reinsurance segment experienced $4.2 million of adverse development on prior accident years primarily from two contracts from 2010 through 2013 that had higher than expected reported losses in 2017. The foregoing reconciliation shows that a $23.7 million redundancy developed in 2016 on the reserve for losses and loss adjustment expenses held at December 31, 2015 . This favorable reserve development included $24.1 million of favorable development in the Excess and Surplus Lines segment primarily from the 2013 , 2014 and 2015 accident years with favorable development of $4.5 million , $10.7 million and $10.0 million , respectively. This favorable development occurred because our actuarial studies at December 31, 2016 for the Excess and Surplus Lines segment indicated that our loss experience on our 6. Reserve for Losses and Loss Adjustment Expenses (continued) casualty business continues to be below our initial expected ultimate loss ratios. The Company also experienced $3.8 million of favorable development on prior accident years in the Specialty Admitted Insurance segment primarily from accident years 2010 through 2014, as losses on our workers’ compensation business written prior to 2015 continued to develop more favorably than we had anticipated. The Casualty Reinsurance segment experienced $4.2 million of adverse development on prior accident years primarily from two contracts from 2012 and 2013 that had higher than expected reported losses in 2016. The foregoing reconciliation shows that a $16.3 million redundancy developed in 2015 on the reserve for losses and loss adjustment expenses held at December 31, 2014 . This favorable reserve development included $25.4 million of favorable development in the Excess and Surplus Lines segment. The Excess and Surplus Lines segment favorable development included $17.3 million of favorable development from the 2014 accident year and $10.5 million of favorable development from the 2013 accident year. This favorable development occurred because our actuarial studies at December 31, 2015 for the Excess and Surplus Lines segment indicated that our loss experience for our our shorter-tailed general casualty division book for the 2014 accident year is below our initial expected ultimate loss ratios. The actuarial studies at December 31, 2015 also showed that the experience on our other casualty business continues to be below our initial expected ultimate loss ratios. Favorable reserve development written in the Specialty Admitted Insurance segment was $3.5 million and primarily came from accident years 2011 through 2013, as losses on our workers’ compensation business written prior to 2014 continued to develop more favorably than we had anticipated. In addition, $12.6 million of adverse development occurred in the Casualty Reinsurance segment, with a majority of this adverse development coming from three reinsurers principally in the 2011, 2012, and 2013 underwriting years that experienced higher loss development in 2015 than expected. The following tables present incurred and paid losses and loss adjustment expenses, net of reinsurance as of December 31, 2017 for: (1) the Excess and Surplus Lines segment split between all excess and surplus lines business excluding commercial auto and commercial auto, (2) the Specialty Admitted Insurance segment split between individual risk workers’ compensation and fronting and programs, and (3) the Casualty Reinsurance segment. The information provided herein about incurred and paid accident year claims development for the years ended December 31, 2016 and prior is presented as unaudited supplementary information. Excess and Surplus Lines — Excluding Commercial Auto Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 120,760 $ 119,736 $ 116,817 $ 112,227 $ 107,927 $ 102,122 $ 99,169 $ 98,765 $ 96,687 $ 95,930 2009 114,834 110,783 106,480 98,502 86,691 81,764 83,431 83,846 85,470 2010 78,424 80,569 78,117 73,035 69,080 69,964 70,294 70,913 2011 111,190 119,927 114,473 106,564 106,381 106,130 106,643 2012 97,908 98,672 97,829 96,497 97,306 99,619 2013 96,729 96,064 85,433 81,009 82,830 2014 114,942 104,092 90,267 82,232 2015 126,443 113,417 104,847 2016 138,507 125,093 2017 144,349 Total $ 997,926   6. Reserve for Losses and Loss Adjustment Expenses (continued) Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 16,922 $ 35,228 $ 53,566 $ 69,585 $ 80,385 $ 83,937 $ 87,630 $ 91,750 $ 91,768 $ 92,332 2009 29,860 41,687 51,731 61,548 67,293 71,245 76,091 79,014 81,496 2010 13,673 26,418 35,812 45,641 52,071 57,371 61,307 64,214 2011 27,684 53,109 72,732 81,696 90,884 94,998 98,684 2012 6,944 33,757 49,604 63,216 74,869 82,545 2013 3,867 14,509 30,382 44,421 59,641 2014 3,412 16,969 28,212 43,891 2015 4,048 17,164 34,801 2016 5,180 22,852 2017 5,290 Total $ 585,746 All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance $ 9,033 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 421,213 Excess and Surplus Lines — Commercial Auto Incurred losses and adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 1,255 $ 1,300 $ 1,451 $ 1,351 $ 1,301 2014 20,487 14,071 17,233 18,953 2015 30,109 33,113 35,149 2016 74,340 109,286 2017 207,355 Total $ 372,044 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 60 $ 1,182 $ 1,285 $ 1,291 $ 1,275 2014 6,166 8,645 12,679 16,359 2015 8,356 15,234 24,282 2016 18,295 54,054 2017 41,467 Total $ 137,437 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 234,607 6. Reserve for Losses and Loss Adjustment Expenses (continued) Specialty Admitted — Individual Risk Workers’ Compensation Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 37,602 $ 36,641 $ 38,654 $ 38,807 $ 40,330 $ 40,552 $ 39,321 $ 39,320 $ 39,131 $ 39,001 2009 28,691 28,526 27,535 28,116 27,795 26,171 26,169 26,232 26,194 2010 27,209 28,736 30,464 30,373 28,963 28,938 27,590 27,098 2011 37,834 41,421 40,154 38,999 38,311 37,455 36,594 2012 32,116 32,420 31,490 29,689 28,255 28,174 2013 12,525 13,668 12,786 11,578 10,907 2014 16,638 16,652 14,620 13,890 2015 20,938 21,274 19,741 2016 21,678 20,299 2017 24,869 Total $ 246,767 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 10,479 $ 22,664 $ 28,436 $ 31,432 $ 33,885 $ 36,778 $ 37,905 $ 37,933 $ 37,965 $ 37,976 2009 7,277 16,945 21,095 22,646 24,231 24,192 24,350 24,418 25,660 2010 7,157 16,245 21,805 23,898 25,210 25,477 26,345 26,352 2011 10,123 23,127 29,021 33,204 34,240 34,287 34,334 2012 9,222 20,308 24,755 26,435 26,897 26,932 2013 4,487 8,723 9,846 10,246 10,263 2014 4,633 10,648 12,041 12,236 2015 6,604 13,285 15,118 2016 4,664 10,227 2017 6,546 Total $ 205,644 All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance $ 357 Outstanding losses and loss adjustment expenses assumed from involuntary workers’ compensation pools $ 4,830 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 46,310 Specialty Admitted — Fronting and Programs Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 104 $ 80 $ 52 $ 52 $ 52 2014 3,460 3,468 3,818 3,425 2015 7,136 9,632 9,358 2016 11,542 15,670 2017 21,229 Total $ 49,734 6. Reserve for Losses and Loss Adjustment Expenses (continued) Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 28 $ 52 $ 52 $ 52 $ 52 2014 883 1,687 2,369 2,728 2015 2,058 4,666 6,165 2016 1,894 5,123 2017 1,223 Total $ 15,291 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 34,443 Casualty Reinsurance Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 1,418 $ 3,215 $ 2,897 $ 2,859 $ 3,199 $ 3,473 $ 3,462 $ 3,467 $ 3,470 $ 3,488 2009 34,587 28,244 24,125 26,458 27,078 27,116 26,989 26,931 26,980 2010 64,413 60,476 61,068 62,714 61,344 60,949 60,978 61,619 2011 114,908 103,123 97,366 97,812 98,993 99,282 101,276 2012 148,251 132,388 131,281 135,594 136,813 139,978 2013 133,230 130,361 131,352 134,446 137,801 2014 118,881 115,927 114,636 116,981 2015 119,157 108,870 108,699 2016 112,759 105,533 2017 134,628 Total $ 936,983 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 320 $ 616 $ 999 $ 1,595 $ 2,087 $ 2,478 $ 2,706 $ 2,929 $ 3,029 $ 3,074 2009 6,487 9,926 12,956 16,466 19,672 21,646 23,024 23,796 24,649 2010 21,918 31,500 38,430 44,921 49,263 52,761 54,659 57,013 2011 48,688 61,922 68,616 78,164 87,267 90,287 94,627 2012 73,124 81,859 97,215 113,943 121,026 128,567 2013 59,756 75,094 93,902 108,396 119,256 2014 41,421 58,601 76,302 89,899 2015 40,021 53,986 68,002 2016 36,268 50,905 2017 47,739 Total $ 683,731 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 253,252 6. Reserve for Losses and Loss Adjustment Expenses (continued) The reconciliation of the net incurred and paid claims development tables to the reserve for losses and loss adjustment expenses in the consolidated balance sheet at December 31, 2017 is as follows (in thousands): E&S – excluding commercial auto $ 421,213 E&S – commercial auto 234,607 Specialty Admitted – individual risk workers’ compensation 46,310 Specialty Admitted – fronting and programs 34,443 Casualty Reinsurance 253,252 Net reserve for losses and loss adjustment expenses 989,825 Reinsurance recoverables on unpaid losses 302,524 Gross reserve for losses and loss adjustment expenses $ 1,292,349 The following is unaudited supplementary information about average annual percentage payouts of incurred claims by age, net of reinsurance, for the Excess and Surplus Lines segment and the Specialty Admitted Insurance segments as of December 31, 2017 . The Specialty Admitted Insurance segment’s first full year of writing fronting and programs business was 2014, so the average annual percentage payouts for fronting and programs only shows four years of payout information. ​ Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 E&S – excluding commercial auto 10.3 % 16.6 % 17.1 % 17.5 % 14.4 % 7.7 % 5.5 % 3.3 % 2.5 % 1.1 % E&S – commercial auto 21.6 % 40.4 % 15.1 % 13.9 % 7.1 % Specialty Admitted – individual risk workers’ compensation 28.0 % 32.7 % 13.6 % 6.1 % 3.4 % 1.4 % 1.0 % 0.1 % 0.5 % 0.0 % Specialty Admitted – fronting and programs 12.2 % 13.4 % 4.4 % 0.7 % Casualty Reinsurance 28.8 % 21.1 % 14.8 % 10.3 % 7.3 % 5.0 % 3.5 % 2.4 % 1.7 % 1.3 % In determining the cumulative number of reported claims, the Company measures claim counts by individual claimant for individual risk workers’ compensation policies in the Specialty Admitted Insurance segment. In the Excess and Surplus Lines insurance segment and for fronting and programs in the Specialty Admitted Insurance segment, the Company measures claim counts by claim event. The claim counts include all claims reported, even if the Company does not establish a liability for the claim (i.e. reserve for loss and loss adjustment expenses). The Casualty Reinsurance segment typically assumes written premium under quota share arrangements. The Company typically does not have direct access to claim frequency information underlying its assumed quota arrangements given the nature of that business. In addition, multiple claims are often aggregated by the ceding company before being reported to the Company. We do not use claim frequency information in the Casualty Reinsurance segment in the determination of loss reserves or for other internal purposes. Based on these considerations, the Company does not believe providing claims frequency information is practicable as it relates to the Casualty Reinsurance segment. The table below provides information on IBNR liabilities and claims frequency for: (1) the Excess and Surplus Lines segment split between commercial auto and all non commercial auto, and (2) the Specialty Admitted Insurance segment split between individual risk workers’ compensation and fronting and programs: 6. Reserve for Losses and Loss Adjustment Expenses (continued) Excess and Surplus Lines — Excluding Commercial Auto Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2008 $ 87,112 $ 3,169 2,283 2009 58,371 2,831 1,677 2010 60,883 3,563 1,367 2011 55,514 5,834 1,443 2012 82,142 8,692 1,725 2013 82,830 14,382 2,223 2014 82,232 23,241 2,024 2015 104,847 46,275 2,292 2016 125,093 77,940 2,468 2017 144,349 123,432 2,052 Excess and Surplus Lines — Commercial Auto Accident Year Incurred Losses and Loss Adj Expenses IBNR Cumulative # of Reported Claims ​ ($ in thousands) 2013 $ 1,301 $ 26 54 2014 18,953 791 7,748 2015 35,149 5,098 41,640 2016 109,286 20,321 87,920 2017 207,355 105,097 125,863 Specialty Admitted - Individual Risk Workers’ Compensation Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2008 $ 39,001 $ 663 2,012 2009 26,194 534 1,490 2010 27,098 700 1,604 2011 36,594 1,632 1,814 2012 28,174 809 1,323 2013 10,907 502 540 2014 13,890 1,506 850 2015 19,741 3,431 975 2016 20,299 6,409 831 2017 24,803 4,617 1,036   6. Reserve for Losses and Loss Adjustment Expenses (continued) Specialty Admitted — Fronting and Programs Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2013 $ 52 $ — 22 2014 3,041 384 857 2015 8,081 1,277 1,347 2016 12,050 3,620 2,730 2017 8,523 12,706 5,421 The Company has not provided insurance coverage that could reasonably be expected to produce material levels of asbestos claims activity. In addition, management does not believe that the Company is exposed to environmental liability claims other than those which it has specifically underwritten and priced as an environmental exposure.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance The Company remains liable to policyholders if its reinsurers are unable to meet their contractual obligations under applicable reinsurance agreements. To minimize exposure to significant losses from reinsurance insolvencies, the Company evaluates the financial condition of its reinsurers and monitors concentrations of credit risk. The Company’s reinsurance contracts generally require reinsurers that are not authorized as reinsurers under U.S. state insurance regulations or that experience rating downgrades from rating agencies below specified levels to fund their share of the Company’s ceded outstanding losses and loss adjustment expense reserves, typically through the use of irrevocable and unconditional letters of credit. In fronting arrangements, which the Company conducts through its Specialty Admitted Insurance segment, the Company is subject to credit risk with regard to insurance companies who act as reinsurers for the Company in such arrangements. The Company customarily requires a collateral trust arrangement to secure the obligations of the insurance entity for whom we are fronting. At December 31, 2017 , the Company had reinsurance recoverables on unpaid losses of $302.5 million and reinsurance recoverables on paid losses of $11.3 million . All material reinsurance recoverables are from companies with A.M. Best Company ratings of “A-” (Excellent) or better, or are collateralized with letters of credit or by a trust agreement. At December 31, 2017 , reinsurance recoverables on unpaid losses from the Company’s three largest reinsurers were $88.8 million , $37.8 million , and $23.8 million , representing 49.7% of the total balance. At December 31, 2017 , prepaid reinsurance premiums ceded to three reinsurers totaled $27.9 million , $11.2 million , and $10.9 million , representing 54.3% of the total balance. 7. Reinsurance (continued) Premiums written, premiums earned, and losses and loss adjustment expenses incurred are summarized as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Written premiums: ​ ​ ​ Direct $ 843,719 $ 549,700 $ 395,816 Assumed 238,186 187,698 176,378 Ceded (315,279 ) (179,690 ) (101,162 ) Net $ 766,626 $ 557,708 $ 471,032 Earned premiums: ​ ​ ​ Direct $ 842,182 $ 483,166 $ 367,340 Assumed 173,472 164,771 181,326 Ceded (274,545 ) (132,274 ) (87,461 ) Net $ 741,109 $ 515,663 $ 461,205 Losses and loss adjustment expenses: ​ ​ ​ Direct $ 626,318 $ 314,920 $ 201,755 Assumed 142,818 108,029 127,276 Ceded (213,759 ) (97,528 ) (50,015 ) Net $ 555,377 $ 325,421 $ 279,016

Senior Debt

Senior Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Senior Debt	Senior Debt ​ The Company’s $215.0 million senior revolving credit facility (the “Facility”) is comprised of the following: • A $102.5 million secured revolving facility utilized by JRG Re to issue letters of credit for the benefit of third-party reinsureds. At December 31, 2017 , the Company had $98.5 million of letters of credit issued under the secured facility. ​ • A $112.5 million unsecured revolving facility to meet the working capital needs of the Company. All unpaid principal on the revolver is due at maturity. Interest accrues quarterly and is payable in arrears at LIBOR plus a margin of 1.5% , which is subject to change according to terms in the credit agreement. At December 31, 2017 and 2016 , the Company had a drawn balance of $73.3 million outstanding on the unsecured revolver. ​ James River Group Holdings, Ltd. and JRG Re are borrowers on the Facility. On December 7, 2016, the Company entered into an Amended and Restated Credit Agreement for the Facility which, among other things, extended the maturity date of the Facility until December 7, 2021 and modified other terms including reducing the rate of interest and reducing the number of financial covenants. On June 8, 2017, the Company amended the credit agreement to, among other things, increase the amount of additional debt which the Company may incur under new financings, adjust the amount of capital that the Company may invest in merger and acquisition activity, and exclude hybrid securities from inclusion of consolidated debt up to a specified maximum. A subsidiary of a bank holding company is one of the lenders for the Facility, with a $25.0 million commitment allocation on the total $215.0 million Facility. The Company's Chairman and Chief Executive Officer was previously the Lead Independent Director of the bank holding company and an investor in the bank holding company. Additionally, one of the Company’s directors is a former investor in the bank holding company and is currently a lender to the bank holding company. The Company is in compliance with all financial and other covenants of the Facility, as contained in the Amended and Restated Credit Agreement, as amended, at December 31, 2017 . On August 2, 2017, the Company and its wholly-owned subsidiary, JRG Re (together with the Company, the “Borrowers”), entered into a new credit agreement. The credit agreement provides the Borrowers with a revolving line of credit of up to $100.0 million , which may be used for loans and letters of credit made or issued, at Borrowers’ option, on a secured or 8. Senior Debt (continued) ​ unsecured basis. The loans and letters of credit made or issued under the revolving line of credit may be used to finance the Borrowers’ general corporate purposes. The Borrowers’ obligations under the credit agreement will mature on the date which is 30 days after notice from the lender terminating the availability period. Interest accrues quarterly and is payable in arrears at variable rates which are subject to change according to terms in the credit agreement. At December 31, 2017 , unsecured loans of $10.0 million and secured letters of credit totaling $6.9 million were outstanding under the facility. The credit agreement contains certain financial and other covenants which the Company is in compliance with at December 31, 2017 . In order to secure borrowings and letters of credit made or issued under the secured portion of the revolving line of credit, JRG Re entered into a pledge and security agreement on August 2, 2017 with the lender, pursuant to which JRG Re will pledge certain investment securities. In the event the Company elects to pledge investment securities as collateral for the secured portion of the revolving credit facility, the Company will enter into a similar pledge and security agreement. The lender under the credit agreement and its affiliate is a joint lead arranger under the Company’s Amended and Restated Credit Agreement dated as of December 7, 2016, as amended, and its affiliate was also an underwriter in the December 2014 initial public offering of the Company’s common shares. On May 26, 2004, James River Group issued $15.0 million of unsecured, floating rate senior debentures (the “Senior Debt”), due April 29, 2034 unless accelerated earlier, through an indenture. The Senior Debt is not redeemable by the holder and is not subject to sinking fund requirements. Interest accrues quarterly and is payable in arrears at a per annum rate of the three-month LIBOR on the Determination Date (as defined in the indenture) plus 3.85% . The Senior Debt is redeemable prior to its stated maturity in whole or in part, at the option of James River Group. The terms of the indenture generally provide that so long as the Senior Debt is outstanding, neither James River Group nor any of its subsidiaries may: • assume or permit to exist any indebtedness that is secured by any encumbrance on the capital stock of James River Group or any of its subsidiaries which is senior to the Senior Debt; or ​ • issue, sell, transfer or otherwise dispose of any shares of, securities convertible into, or warrants, rights or options to subscribe for or purchase shares of, capital stock of any subsidiary. ​ James River Group is in compliance with all covenants of the indenture at December 31, 2017 . Interest payable is included in “accrued expenses” in the accompanying consolidated balance sheets.

Junior Subordinated Debt

Junior Subordinated Debt	12 Months Ended
	Dec. 31, 2017
Subordinated Borrowings [Abstract]
Junior Subordinated Debt	Junior Subordinated Debt ​ The Company issued trust preferred securities (“Trust Preferred Securities”) through James River Capital Trust I, James River Capital Trust II, James River Capital Trust III, James River Capital Trust IV, and Franklin Holdings II (Bermuda) Capital Trust I, (each, a “Trust”; collectively, the “Trusts”). These Delaware statutory trusts are sponsored and wholly-owned by the Company. Each Trust was created solely for the purpose of issuing the Trust Preferred Securities. Each Trust used proceeds from the sale of its Trust Preferred Securities to purchase the Company’s floating rate junior subordinated debentures (the “Junior Subordinated Debt”) issued to the Trust under an indenture (each, an “Indenture”; collectively, the “Indentures”). The Junior Subordinated Debt is the sole asset of each Trust, and the Trust Preferred Securities are the sole liabilities of each Trust. The Company purchased all of the outstanding common stock of the Trusts, and the investment in the Trusts is included in “other assets” in the accompanying consolidated balance sheets.   9. Junior Subordinated Debt​ (continued) The following table summarizes the nature and terms of the Junior Subordinated Debt and Trust Preferred Securities: ​ James River Capital Trust I James River James River Capital Trust III James River Franklin ​ ($ in thousands) Issue date May 26, December 15, June 15, December 11, January 10, Principal amount of Trust Preferred Securities $7,000 $15,000 $20,000 $54,000 $30,000 Principal amount of Junior Subordinated Debt $7,217 $15,464 $20,619 $55,670 $30,928 Carrying amount of Junior Subordinated Debt net of repurchases $7,217 $15,464 $20,619 $44,827 $15,928 Maturity date of Junior Subordinated Debt, unless accelerated earlier May 24, December 15, June 15, December 15, March 15, Trust common stock $217 $464 $619 $1,670 $928 Interest rate, per annum Three-Month LIBOR plus 4.0% Three-Month LIBOR plus 3.4% Three-Month LIBOR plus 3.0% Three-Month LIBOR plus 3.1% Three-Month LIBOR plus 4.0% All of the Junior Subordinated Debt is currently redeemable at 100.0% of the unpaid principal amount at the Company’s option. Interest on the Trust Preferred Securities and interest paid to the Trusts on the Junior Subordinated Debt is payable quarterly in arrears at a per annum rate as described in the table above. The Company has the right to defer interest payments on the Junior Subordinated Debt for up to five years without triggering an event of default. The Trust Preferred Securities are subject to mandatory redemption in a like amount (a) upon repayment of all of the Junior Subordinated Debt on the stated maturity date, (b) contemporaneously with the optional prepayment of all of the Junior Subordinated Debt in conjunction with a special event (as defined), and (c) five years or more after the issue date, contemporaneously with the optional prepayment, in whole or in part, of the Junior Subordinated Debt. The Indentures contain certain covenants which the Company is in compliance with as of December 31, 2017 . Interest payable is included in “accrued expenses” on the accompanying consolidated balance sheets.

Capital Stock

Capital Stock	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Capital Stock	Capital Stock ​ The Company’s authorized share capital consists of 200,000,000 common shares, par value $0.0002 per share ( 29,696,682 shares issued and outstanding at December 31, 2017 ) and 20,000,000 undesignated preferred shares, par value $0.00125 per share ( no shares issued or outstanding). Share activity in 2016 and 2017 includes issuances from stock option exercises and RSU vesting, increasing the number of common shares outstanding from 28,941,547 at December 31, 2015 to 29,257,566 at December 31, 2016 and 29,696,682 at December 31, 2017 . The Company has 4,385,213 common shares reserved for future issuance upon exercise or vesting of equity awards, as applicable. 10. Capital Stock​ (continued) The Board of Directors declared the following cash dividends in 2017 , 2016 , and 2015 : Date of Declaration Dividend per Common Share Payable to Shareholders of Record on Payment Date Total Amount 2017 February 14, 2017 $0.30 March 13, 2017 March 31, 2017 $8.9 May 2, 2017 $0.30 June 12, 2017 June 30, 2017 $8.9 August 1, 2017 $0.30 September 11, 2017 September 29, 2017 $8.9 November 1, 2017 $0.30 December 15, 2017 December 28, 2017 $9.0 November 1, 2017 $0.50 December 15, 2017 December 28, 2017 $15.0 Total $1.70 $50.7 2016 ​ ​ ​ ​ February 16, 2016 $0.20 March 14, 2016 March 28, 2016 $5.8 May 3, 2016 $0.20 June 13, 2016 June 30, 2016 $5.9 August 3, 2016 $0.20 September 12, 2016 September 30, 2016 $5.9 November 1, 2016 $0.30 December 16, 2016 December 29, 2016 $8.9 November 1, 2016 $1.35 December 16, 2016 December 29, 2016 $39.8 Total $2.25 ​ ​ $66.3 2015 February 17, 2015 $0.16 March 16, 2015 March 31, 2015 $4.6 May 5, 2015 $0.16 June 15, 2015 June 30, 2015 $4.6 August 5, 2015 $0.16 September 14, 2015 September 30, 2015 $4.7 November 3, 2015 $0.16 December 14, 2015 December 28, 2015 $4.7 November 3, 2015 $1.00 December 14, 2015 December 28, 2015 $29.2 Total $1.64 $47.8 Included in the dividends are $450,000 , $664,000 and $558,000 of dividend equivalents on RSUs, of which $434,000 , $666,000 and $385,000 were payable as of December 31, 2017 , 2016 , and 2015 , respectively.

Equity Awards

Equity Awards	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity Awards	Equity Awards ​ Equity Incentive Plans The Company’s shareholders have approved various equity incentive plans, including the Amended and Restated 2009 Equity Incentive Plan (the “Legacy Plan”), the 2014 Long Term Incentive Plan (“2014 LTIP”), and the 2014 Non-Employee Director Incentive Plan (“2014 Director Plan”) (collectively, the “Plans”). All awards issued under the Plans are issued at the discretion of the Board of Directors. Under the Legacy Plan, employees received non-qualified stock options. Options are outstanding under the Legacy Plan; however, no additional awards may be granted. Employees are eligible to receive non-qualified stock options, incentive stock options, share appreciation rights, performance shares, restricted shares, restricted share units, and other awards under the 2014 LTIP. The maximum number of shares available for issuance under the 2014 LTIP is 4,171,150 , and at December 31, 2017 , 1,862,291 shares are available for grant. Non-employee directors of the Company are eligible to receive non-qualified stock options, share appreciation rights, performance shares, restricted shares, restricted share units, and other awards under the 2014 Director Plan. The maximum number of shares available for issuance under the 2014 Director Plan is 50,000 , and at December 31, 2017 , 32,994 shares are available for grant. Generally, awards issued under the 2014 LTIP and 2014 Director Plan vest immediately in the event that an award recipient is terminated without Cause (as defined), and in the case of the 2014 LTIP for Good Reason (as defined), at any time following a Change in Control (as defined in the applicable plans). 11. Equity Awards (continued) Options The following table summarizes the option activity for the periods ended December 31, 2017 , 2016 , and 2015 : ​ Year Ended December 31, ​ 2017 2016 2015 ​ Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Outstanding: ​ ​ ​ ​ ​ ​ Beginning of year 2,234,699 $ 22.84 2,058,085 $ 18.11 3,104,768 $ 17.27 Granted 205,244 $ 42.24 706,203 $ 32.07 10,627 $ 24.32 Exercised (898,218 ) $ 18.53 (496,550 ) $ 16.02 (1,047,500 ) $ 15.66 Forfeited (62,489 ) $ 30.80 (33,039 ) $ 27.68 (9,810 ) $ 21.00 End of year 1,479,236 $ 27.81 2,234,699 $ 22.84 2,058,085 $ 18.11 Exercisable, end of year 846,371 $ 22.35 1,207,479 $ 18.14 1,217,903 $ 16.85 All of the outstanding options vest over three or four years and have a contractual life of seven years from the original date of grant. All of the outstanding options have an exercise price greater than or equal to the fair value of the underlying shares at the date of grant. For 2017 , 2016 and 2015 , the fair value of the underlying share was equal to the market price. The intrinsic value of each option is determined based on the difference between the fair value of the underlying share and the exercise price of the underlying option. The total intrinsic value of options exercised during 2017 , 2016 and 2015 was $20.4 million , $10.2 million and $12.8 million , respectively. The aggregate intrinsic value of options outstanding at December 31, 2017 , 2016 and 2015 was $18.5 million , $41.8 million and $31.8 million , respectively. The aggregate intrinsic value of options exercisable at December 31, 2017 , 2016 and 2015 was $14.9 million , $28.3 million and $20.3 million , respectively. The fair value used for calculating intrinsic values was $40.01 , $41.55 and $33.54 at December 31, 2017 , 2016 and 2015 , respectively. The weighted-average remaining contractual life of the options outstanding and options exercisable at December 31, 2017 is 4.5 years and 3.8 years , respectively. The weighted-average fair value of options granted during 2017 , 2016 and 2015 was $8.21 , $5.55 and $4.31 , respectively. The value of the options granted was estimated at the date of grant using the Black-Scholes-Merton option pricing model using the following assumptions: ​ Year Ended December 31, ​ 2017 2016 2015 Range of risk-free interest rates 1.97 % 1.23 % 1.54 % Dividend yield 2.85 % 2.50 % 4.00 % Expected share price volatility 27.39 % 25.00 % 25.00 % Expected life 5.0 years 5.0 years 5.0 years The risk-free interest rate assumption is based on the five-year U.S. Treasury rate at the date of the grant. The dividend yield assumption is based upon dividends expected to be declared over the life of the options at the date of grant. The share price volatility assumption is based upon the Company's 100-day realized volatility. In prior years, as a relatively new public company, the share price volatility assumption was based upon historical data for property/casualty companies which the Company deemed to be its peers. The expected life is determined using the simplified method, which factors in the average of the midpoint and the contractual term of each tranche in determining a single expected life. The simplified method is used as the Company does not have sufficient historical exercise data to estimate an expected term. The Black-Scholes-Merton option pricing model was developed for use in estimating the fair value of traded options that have no vesting restrictions and are fully transferable. In addition, option valuation models require the input of highly subjective assumptions including expected share price volatility. Because the Company’s share options have characteristics significantly different from those of traded options and because changes in the subjective input assumptions can materially 11. Equity Awards (continued) affect the fair value estimate, the existing models do not necessarily provide a reliable single measure of the fair value of such share options. RSUs The following table summarizes the RSU activity for the years ended December 31, 2017 , 2016 , and 2015 : ​ Year Ended December 31, ​ 2017 2016 2015 ​ Shares Weighted- Average Grant Date Fair Value Shares Weighted- Average Grant Date Fair Value Shares Weighted- Average Grant Date Fair Value Unvested, beginning of year 196,800 $ 24.38 234,922 $ 21.00 340,474 $ 21.00 Granted 137,034 $ 42.20 60,291 $ 32.03 — $ — Vested (132,764 ) $ 24.24 (98,413 ) $ 21.00 (105,552 ) $ 21.00 Forfeited (22,188 ) $ 26.06 — $ — — $ — Unvested, end of year 178,882 $ 37.93 196,800 $ 24.38 234,922 $ 21.00 The vesting period of RSUs granted to employees range from one to five years and vest ratably over the respective vesting period, and the majority vest in three years. All RSUs granted to date to non-employee directors had a one year vesting period. The total fair value of shares vested in 2017 , 2016 and 2015 was $5.5 million , $3.8 million and $3.5 million , respectively. The holders of RSUs are entitled to dividend equivalents. The dividend equivalents are settled in cash at the same time that the underlying RSUs vest and are subject to the same risk of forfeiture as the underlying shares. The fair value of the RSUs granted is based on the market price of the underlying shares. Compensation Expense Share based compensation expense is recognized on a straight line basis over the vesting period. The amount of expense and related tax benefit is summarized below: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Share based compensation expense $ 7,688 $ 5,492 $ 3,735 U.S. tax benefit on share based compensation expense $ 2,093 $ 1,532 $ 1,025 As of December 31, 2017 , the Company had $6.9 million of unrecognized share based compensation expense expected to be charged to earnings over a weighted-average period of 1.9 years .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes ​ Under current Bermuda law, James River Group Holdings, Ltd. and its Bermuda based subsidiary are not required to pay any Bermuda taxes on their income or capital gains. The Company has received an undertaking from the Minister of Finance in Bermuda that, in the event of any taxes being imposed, the Company will be exempt from taxation in Bermuda until March 2035. Distributions from the Company’s U.S. subsidiaries to its U.K. intermediate holding company, James River UK, are generally subject to a 5% dividend withholding tax. James River Group paid a $21.1 million dividend to James River UK during 2017 and remitted $1.1 million of dividend withholding taxes to the U.S. tax authorities for the year ending December 31, 2017 . No similar distributions occurred in 2016. James River Group paid a $50.0 million dividend to James River UK during 2015 and remitted $2.5 million of dividend withholding taxes to the U.S. tax authorities for the year ending December 31, 2015. The Company’s U.S. subsidiaries are subject to federal, state and local corporate income taxes, and other taxes applicable to U.S. corporations. The Company’s U.S.-domiciled subsidiaries file a consolidated U.S. federal income tax return. The 12. Income Taxes​ (continued) Company’s U.S.-based subsidiaries are generally no longer subject to income tax examination by U.S. income tax authorities for the tax years ending before January 1, 2014. Financial results for 2017 reflect provisional amounts related to the December 2017 enactment of the TCJA. These provisional estimates are based on the Company’s initial analysis and current interpretation of the legislation. Given the complexity of the legislation, anticipated guidance from the U.S. Treasury, and the potential for additional guidance from the Securities and Exchange Commission (SEC) or the Financial Accounting Standards Board, these estimates may be adjusted during 2018. The expected income tax provision computed from pre-tax income at the weighted-average tax rate has been calculated as the sum of the pre-tax income in each jurisdiction multiplied by that jurisdiction’s applicable Federal statutory tax rate. Federal statutory tax rates of 0% and 35% have been used for Bermuda and the U.S., respectively. U.S. deferred taxes have been remeasured at 21%, down from 35% in prior periods. U.S. income before Federal income taxes was $50.1 million , $18.3 million , and $15.2 million for the years ending December 31, 2017 , 2016 , and 2015 , respectively. A reconciliation of the difference between the Company’s Federal income tax provision on U.S. income and the expected Federal tax provision on U.S. income using the weighted-average tax rate as well as a reconciliation to total tax expense is as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Federal income tax expense at applicable statutory rates $ 17,541 $ 6,401 $ 5,317 Tax-exempt investment income (586 ) (643 ) (858 ) Dividends received deduction (792 ) (880 ) (867 ) Excess tax benefits on share based compensation (2,114 ) — — Effect of tax rate reduction on deferred tax liability (3,498 ) — — Other (90 ) (6 ) 379 Federal income tax expense $ 10,461 $ 4,872 $ 3,971 U.S. state income tax expense (benefit) 65 — (192 ) U.S. dividend withholding tax 1,053 — 2,500 Total income tax expense $ 11,579 $ 4,872 $ 6,279 12. Income Taxes (continued) The significant components of the net deferred tax liability at the corporate income tax rate of 21% for the year ended December 31, 2017 and 35% for December 31, 2016 are summarized as follows: ​ December 31, ​ 2017 2016 ​ (in thousands) Deferred tax assets: ​ ​ Accrued compensation expenses $ 1,712 $ 1,604 Reserve for losses and loss adjustment expenses 3,036 4,810 Unearned premiums 1,709 2,859 Share based compensation 1,143 2,281 Allowance for doubtful accounts 579 748 Deferred policy acquisition costs 1,509 1,983 Property and equipment 1,601 2,649 AMT credit — 464 Invested asset impairments 791 291 Other 1,564 2,946 Total deferred tax assets 13,644 20,635 Deferred tax liabilities: Intangible assets 7,521 12,554 Net unrealized gains 2,578 1,586 Deferred gain on extinguishment of debt 63 212 Equity method investments 8,348 8,668 Other 381 517 Total deferred tax liabilities 18,891 23,537 Net deferred tax liabilities $ (5,247 ) $ (2,902 ) Deferred income taxes have not been accrued with respect to certain undistributed earnings of foreign subsidiaries. If the earnings were to be distributed, as dividends or otherwise, such amounts may be subject to withholding taxation in the jurisdiction of the paying entity. The Company asserts that U.S. unremitted earnings as of December 31, 2017 will be permanently reinvested in the U.S. and, accordingly, no provision for withholding taxes arising in respect to U.S. unremitted earnings has been made. The Company had no reserve for future tax contingencies or liabilities (“unrecognized tax benefits”) at December 31, 2017 or 2016 . The U.S. imposes a 1% excise tax on reinsurance premiums paid to non-U.S. reinsurers with respect to risks located in the U.S. The rates of tax are established based on the nature of the risk, unless reduced by an applicable U.S. tax treaty. For the years ended December 31, 2017 , 2016 , and 2015 , the Company paid $3.4 million , $2.6 million , and $1.9 million , respectively, of federal excise taxes on its intercompany reinsurance transactions. The Company also paid excise taxes of $2.2 million , $1.6 million , and $1.7 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively, on written premiums assumed from third-party insurers with respect to risks located in the U.S. These excise taxes are reflected as “other operating expenses” in the Company’s consolidated income statements.

Other Operating Expenses and Ot

Other Operating Expenses and Other Expenses	12 Months Ended
	Dec. 31, 2017
Other Operating Expenses And Other Expenses [Abstract]
Other Operating Expenses and Other Expenses	Other Operating Expenses and Other Expenses ​ Other operating expenses consist of the following: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Amortization of policy acquisition costs $ 116,001 $ 101,624 $ 97,750 Other underwriting expenses of the insurance segments 55,662 48,771 41,499 Other operating expenses of the Corporate and Other segment 25,330 20,433 18,554 Total $ 196,993 $ 170,828 $ 157,803 Other expenses in the accompanying financial statements for the year ended December 31, 2017 were $539,000 and included $535,000 of legal and other professional services related to secondary share offerings in 2017. For the year ended December 31, 2016 , other expenses of $1.6 million included $1.5 million of employee severance expenses. For the year ended December 31, 2015 , other expenses of $730,000 included $276,000 of expenses associated with a related party leasing agreement (Note 21), $170,000 of employee severance expenses, and $284,000 of legal, tax, and other professional services related to the formation of James River UK, the December 2015 intercompany dividend, and a securities registration statement.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2017
Employee Benefits and Share-based Compensation [Abstract]
Employee Benefits	Employee Benefits ​ The Company and its subsidiaries offer savings plans (the “Savings Plans”) which qualify under Section 401(k) of the U.S. Internal Revenue Code. Participants may contribute certain percentages of their pre-tax salary to the Savings Plans subject to statutory limitations. The Company and its subsidiaries match employee contributions at various rates up to a maximum contribution of 6.0% of the participant’s earnings subject to certain statutory limits. For the years ended December 31, 2017 , 2016 , and 2015 , the expense associated with the Savings Plans totaled $2.5 million , $1.9 million , and $1.6 million , respectively.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingent Liabilities	Commitments and Contingent Liabilities ​ The Company is a party to various lawsuits arising in the ordinary course of its operations. The Company believes that the ultimate resolution of these matters will not materially impact its financial position, cash flows, or results of operations. The Company leases certain office space under operating leases that expire at various times and are subject to renewal options at market rates prevailing at the time of renewal. Rental expense for such leases was $4.2 million , $3.7 million , and $2.6 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. As of December 31, 2017 , future minimum payments under non-cancelable operating leases for office space are as follows (in thousands): 2018 $ 4,025 2019 3,644 2020 3,342 2021 3,154 2022 2,768 Thereafter 7,658 ​ $ 24,591 Included in the future minimum lease payments is $19.3 million related to the building constructed and owned by a partnership in which the Company has a minority investment (see Note 21). The Company’s Specialty Admitted Insurance segment entered into an agreement to lease certain policy management software. The five year lease began January 1, 2015. Lease expense for the year ended December 31, 2017 was $440,000 and total remaining payments of $880,000 are to be spread evenly at $440,000 per year in 2018 and 2019. 15. Commitments and Contingent Liabilities (continued) The Company’s reinsurance subsidiary, JRG Re, entered into three letter of credit facilities with banks as security to third-party reinsureds on reinsurance assumed by JRG Re. JRG Re has established custodial accounts to secure these letters of credit. Under a $100.0 million facility, $47.4 million of letters of credit were issued through December 31, 2017 which were secured by deposits of $93.9 million . Under a $102.5 million facility, $98.5 million of letters of credit were issued through December 31, 2017 which were secured by deposits of $102.9 million . Under a $100.0 million facility, $6.9 million of letters of credit were issued through December 31, 2017 which were secured by deposits of $10.2 million . JRG Re has also established trust accounts to secure its obligations to selected reinsureds. The total amount deposited in the trust accounts for the benefit of third-party reinsureds was $265.0 million at December 31, 2017 (see Note 2). The Company is also exposed to credit risk relating to a set of insurance contracts with an insured group of companies under which the Company pays losses and loss adjustment expenses on the contract. The Company has indemnity agreements with this group of insured parties (non-insurance entities) and is contractually entitled to receive reimbursement for a significant portion of the losses and loss adjustment expenses paid on behalf of the insured parties and other expenses incurred by the Company. The insured parties are required to collateralize all amounts currently due to the Company and to provide additional collateral sufficient to cover the amounts that may be recoverable under the indemnity agreement, including among other things case loss and loss adjustment expense reserves, IBNR loss and loss adjustment expense reserves, extra contractual obligations and excess of policy limits liabilities. This collateral is currently provided through a collateral trust arrangement established in favor of the Company by a captive insurance company affiliate of the insured group. At December 31, 2017 , the cash equivalent collateral held in the collateral trust arrangement was approximately $780.9 million , which exceeds the amount of claims receivable and unpaid reported losses and loss adjustment expenses outstanding. This is a rapidly growing relationship, and as such, there is ongoing exposure to estimated losses and expenses on these contracts growing at a faster pace than growth in our collateral balances. In addition, we have credit exposure if our estimates of future losses and loss adjustment expenses and other amounts recoverable, which are the basis for establishing collateral balances, are lower than actual amounts paid or payable. The amount of our credit exposure in any of these instances could be material. To mitigate these risks, we closely and frequently monitor our exposure compared to our collateral held, and we request additional collateral when our analysis indicates that we have uncollateralized exposure.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) The following table summarizes the components of other comprehensive (loss) income: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Unrealized gains (losses) arising during the period, before U.S. income taxes $ 11,091 $ 2,790 $ (13,707 ) U.S. income taxes (2,374 ) (414 ) 517 Unrealized gains (losses) arising during the period, net of U.S. income taxes 8,717 2,376 (13,190 ) Less reclassification adjustment: Net realized investment (losses) gains (835 ) 6,572 2,402 U.S. income taxes 333 (2,137 ) (422 ) Reclassification adjustment for investment (losses) gains realized in net income (502 ) 4,435 1,980 Other comprehensive income (loss) $ 9,219 $ (2,059 ) $ (15,170 ) In addition to the $835,000 of realized investment losses, and $6.6 million and $2.4 million of realized investment gains on available-for-sale securities for the years ended December 31, 2017 , 2016 , and 2015 , the Company recognized $1.2 million of realized investment losses, $995,000 of realized investment gains, and $7.0 million of realized investment losses in the respective years on its investments in bank loan participations.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment Information	Segment Information ​ The Company has four reportable segments, three of which are separately managed business units and the fourth (“Corporate and Other”) includes the Company’s remaining operations. The Excess and Surplus Lines segment primarily offers commercial excess and surplus lines liability and excess property insurance products. The Specialty Admitted Insurance segment offers workers’ compensation insurance coverage as well as specialty admitted fronting and program business. The Casualty Reinsurance segment offers commercial liability and non-catastrophe property reinsurance to U.S. insurance 17. Segment Information (continued) companies and to the Company’s U.S.-based insurance subsidiaries. The Corporate and Other segment consists of certain management and treasury activities of James River Group, James River UK, and JRG Holdings as well as interest expense associated with senior debt and Junior Subordinated Debt, and investment income from investments classified as trading or other invested assets. The accounting policies of the reportable segments are the same as those described in the summary of significant accounting policies. Segment revenues for each reportable segment consist of net earned premiums, net investment income, and realized investment gains (losses). Segment profit (loss) for each reportable segment is measured by underwriting profit (loss), which is generally defined as net earned premiums less losses and loss adjustment expenses and other operating expenses of the operating segments. Fee income and expenses of the Excess and Surplus Lines segment are included in that segment’s underwriting profit (loss). Fee income of $17.0 million , $10.1 million and $3.2 million was included in underwriting profit for the years ended December 31, 2017 , 2016 and 2015 , respectively. Segment results are reported prior to the effects of the intercompany reinsurance agreements between the Company’s insurance subsidiaries. All gross written premiums and net earned premiums for all periods presented were generated from policies issued to U.S. based insureds. ​ Excess and Surplus Lines Specialty Admitted Insurance Casualty Reinsurance Corporate and Other Total ​ (in thousands) As of and for the Year Ended December 31, 2017 ​ ​ ​ ​ ​ ​ Gross written premiums $ 530,120 $ 316,430 $ 235,355 $ — ​ $ 1,081,905 Net earned premiums 463,521 68,110 209,478 — ​ 741,109 Segment revenues 493,853 70,366 240,751 12,655 ​ 817,625 Net investment income 15,014 2,532 31,507 12,066 ​ 61,119 Interest expense — — — 8,974 ​ 8,974 Underwriting profit (loss) of operating segments 29,693 3,166 (1,765 ) — ​ 31,094 Segment goodwill 181,831 — — — ​ 181,831 Segment assets 843,486 439,416 1,379,866 93,927 ​ 2,756,695 17. Segment Information (continued) ​ ​ Excess and Surplus Lines Specialty Admitted Insurance Casualty Reinsurance Corporate and Other Total ​ As of and for the Year Ended December 31, 2016 ​ ​ ​ ​ ​ Gross written premiums $ 370,844 $ 182,221 $ 184,333 $ — $ 737,398 Net earned premiums 301,404 52,281 161,978 — 515,663 Segment revenues 331,090 55,412 190,064 9,661 586,227 Net investment income 18,051 2,542 27,257 4,788 52,638 Interest expense — — — 8,448 8,448 Underwriting profit (loss) of operating segments 47,235 2,872 (194 ) — 49,913 Segment goodwill 181,831 — — — 181,831 Segment assets 740,144 300,519 1,195,230 110,640 2,346,533 As of and for the Year Ended December 31, 2015 ​ ​ ​ ​ ​ Gross written premiums $ 308,717 $ 90,978 $ 172,499 $ — $ 572,194 Net earned premiums 240,878 42,206 178,121 — 461,205 Segment revenues 255,529 44,791 197,586 7,015 504,921 Net investment income 13,427 2,316 22,706 6,386 44,835 Interest expense — — — 6,999 6,999 Underwriting profit (loss) of operating segments 47,607 1,074 (2,558 ) — 46,123 Segment goodwill 181,831 — — — 181,831 Segment assets 671,143 173,094 1,108,278 102,982 2,055,497 17. Segment Information (continued) ​ The following table reconciles the underwriting profit (loss) of insurance segments by individual segment to income before taxes: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Underwriting profit (loss) of the operating segments: ​ ​ ​ ​ ​ Excess and Surplus Lines $ 29,693 $ 47,235 $ 47,607 Specialty Admitted Insurance 3,166 2,872 1,074 Casualty Reinsurance (1,765 ) (194 ) (2,558 ) Total underwriting profit of operating segments 31,094 49,913 46,123 Other operating expenses of the Corporate and Other segment (25,330 ) (20,433 ) (18,554 ) Underwriting profit 5,764 29,480 27,569 Net investment income 61,119 52,638 44,835 Net realized investment (losses) gains (1,989 ) 7,565 (4,547 ) Other income 361 295 245 Other expenses (539 ) (1,590 ) (730 ) Interest expense (8,974 ) (8,448 ) (6,999 ) Amortization of intangible assets (597 ) (597 ) (597 ) Income before income taxes $ 55,145 $ 79,343 $ 59,776 The Company currently has 15 underwriting divisions, including 13 in the Excess and Surplus Lines segment, one in the Specialty Admitted Insurance segment, and one in the Casualty Reinsurance segment. Each underwriting division focuses on a specific industry group or coverage. Gross written premiums by segment and underwriting division are presented below: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Commercial Auto $ 247,960 ​ $ 110,050 ​ $ 73,770 Manufacturers and Contractors 85,719 ​ 83,279 ​ 78,315 Excess Casualty 51,160 ​ 43,574 ​ 32,458 General Casualty 38,097 ​ 36,858 ​ 30,972 Energy 29,704 ​ 29,709 ​ 30,623 Allied Health 19,181 ​ 14,413 ​ 13,513 Excess Property 14,447 ​ 14,083 ​ 12,498 Life Sciences 12,981 ​ 11,132 ​ 8,917 Small Business 11,307 ​ 9,104 ​ 6,916 Professional Liability 6,326 ​ 8,361 ​ 10,046 Environmental 7,920 ​ 5,321 ​ 4,437 Medical Professionals 2,297 ​ 2,739 ​ 3,585 Sports and Entertainment 3,021 ​ 2,221 ​ 2,667 Total Excess and Surplus Lines segment 530,120 ​ 370,844 ​ 308,717 Specialty Admitted Insurance segment 316,430 ​ 182,221 ​ 90,978 Casualty Reinsurance segment 235,355 ​ 184,333 ​ 172,499 Total $ 1,081,905 ​ $ 737,398 ​ $ 572,194 17. Segment Information (continued) The Company does business with two brokers that generated $253.7 million and $70.3 million of gross written premiums for the Excess and Surplus Lines segment for the year ended December 31, 2017 , representing 23.5% (BB&T Insurance Services) and 6.5% of consolidated gross written premiums and 47.9% and 13.3% of the Excess and Surplus Lines segment’s gross written premiums, respectively. The Company has agency contracts with various branches within the aforementioned brokers. No other broker generated 10.0% or more of the gross written premiums for the Excess and Surplus Lines segment for the year ended December 31, 2017 . The Company does business with one insured that generated $216.1 million of gross written premiums and $16.9 million of fee income for the Excess and Surplus Lines segment for the year ended December 31, 2017 , representing 20.0% (Rasier LLC) of consolidated gross written premiums and 40.8% of the Excess and Surplus Lines segment’s gross written premiums. No other insured generated 10.0% or more of the gross written premiums for the Excess and Surplus Lines segment. The Specialty Admitted Insurance segment accepts applications for insurance from a variety of sources, including independent retail agents, program administrators and managing general agents (“MGAs”). The Company does business with one agency that generated $183.0 million of gross written premiums for the Specialty Admitted Insurance segment for the year ended December 31, 2017 , representing 16.9% (Atlas General Insurance Services) of the consolidated gross written premiums and 57.8% of the Specialty Admitted Insurance segment’s gross written premiums. No other agency generated 10.0% or more of the gross written premiums for the Specialty Admitted Insurance segment for the year ended December 31, 2017 . The Company does business with four brokers that generated $56.1 million , $55.0 million , $38.1 million , and $33.4 million of gross written premiums for the Casualty Reinsurance segment for the year ended December 31, 2017 , representing 5.2% , 5.1% , 3.5% , and 3.1% of consolidated gross written premiums and 23.8% , 23.4% , 16.2% , and 14.2% of the Casualty Reinsurance segment’s gross written premiums, respectively. No other broker generated 10.0% or more of the gross written premiums for the Casualty Reinsurance segment for the year ended December 31, 2017 . The Casualty Reinsurance segment assumed business from two unaffiliated ceding companies that generated $120.3 million and $48.4 million of gross written premiums for the year ended December 31, 2017 , representing 11.1% (State National Insurance Company) and 4.5% of consolidated gross written premiums and 51.1% and 20.6% of the Casualty Reinsurance segment’s gross written premiums, respectively.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements ​ Three levels of inputs are used to measure fair value of financial instruments: (1) Level 1: quoted price (unadjusted) in active markets for identical assets, (2) Level 2: inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the instrument, and (3) Level 3: inputs to the valuation methodology are unobservable for the asset or liability. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability in the principal or most advantageous market in an orderly transaction between market participants on the measurement date. To measure fair value, the Company obtains quoted market prices for its investment securities from its outside investment managers. If a quoted market price is not available, the Company uses prices of similar securities. Values for U.S. Treasury and publicly-traded equity securities are generally based on Level 1 inputs which use the market approach valuation technique. The values for all other fixed maturity securities (including state and municipal securities and obligations of U.S. government corporations and agencies) generally incorporate significant Level 2 inputs, and in some cases, Level 3 inputs, using the market approach and income approach valuation techniques. There have been no changes in the Company’s use of valuation techniques since December 31, 2015. The Company reviews fair value prices provided by its outside investment managers for reasonableness by comparing the fair values provided by the managers to those provided by our investment custodian. The Company also reviews and monitors changes in unrealized gains and losses. The Company has not historically adjusted security prices. The Company obtains an understanding of the methods, models and inputs used by the investment managers and independent pricing services, and controls are in place to validate that prices provided represent fair values. The Company’s control process includes, but is not limited to, initial and ongoing evaluation of the methodologies used, a review of specific securities and an assessment for proper classification within the fair value hierarchy, and obtaining and reviewing internal control reports for our investment manager that obtains fair values from independent pricing services. 18. Fair Value Measurements (continued) Assets measured at fair value on a recurring basis as of December 31, 2017 are summarized below: Fair Value Measurements Using ​ Quoted Prices in Active Markets for Identical Assets Level 1 Significant Other Observable Inputs Level 2 ​ Significant Unobservable Inputs Level 3 Total ​ (in thousands) Available-for-sale securities ​ ​ ​ ​ Fixed maturity securities: ​ ​ ​ ​ State and municipal $ — $ 144,366 $ — $ 144,366 Residential mortgage-backed — 158,661 — 158,661 Corporate — 413,721 — 413,721 Commercial mortgage and asset-backed — 177,931 4,680 182,611 Obligations of U.S. government corporations and agencies — 35,847 — 35,847 U.S. Treasury securities and obligations guaranteed by the U.S. government 78,265 609 — 78,874 Redeemable preferred stock — 2,018 — 2,018 Total fixed maturity securities 78,265 933,153 4,680 1,016,098 Equity securities: Preferred stock — 66,281 — 66,281 Common stock 15,507 734 — 16,241 Total equity securities 15,507 67,015 — 82,522 Total available-for-sale securities $ 93,772 $ 1,000,168 $ 4,680 $ 1,098,620 Trading securities: Fixed maturity securities $ — $ 3,808 $ — $ 3,808 Short-term investments $ 1,000 $ 35,804 $ — $ 36,804 18. Fair Value Measurements (continued) Assets measured at fair value on a recurring basis as of December 31, 2016 are summarized below: ​ Fair Value Measurements Using ​ Quoted Prices in Active Markets for Identical Assets Level 1 Significant Other Observable Inputs Level 2 ​ Significant Unobservable Inputs Level 3 Total ​ ​ (in thousands) Available-for-sale securities ​ ​ ​ ​ ​ ​ Fixed maturity securities: ​ ​ ​ ​ ​ ​ State and municipal $ — ​ $ 105,841 ​ $ — $ 105,841 Residential mortgage-backed — ​ 150,798 ​ — 150,798 Corporate — ​ 378,448 ​ — 378,448 Commercial mortgage and asset-backed — ​ 163,047 ​ 5,000 168,047 Obligations of U.S. government corporations and agencies — ​ 65,014 ​ — 65,014 U.S. Treasury securities and obligations guaranteed by the U.S. government 70,465 ​ 655 ​ — 71,120 Redeemable preferred stock — ​ 1,809 ​ — 1,809 Total fixed maturity securities 70,465 ​ 865,612 ​ 5,000 941,077 Equity securities: Preferred stock — ​ 64,827 ​ — 64,827 Common stock 10,840 ​ 734 ​ — 11,574 Total equity securities 10,840 ​ 65,561 ​ — 76,401 Total available-for-sale securities $ 81,305 ​ $ 931,173 ​ $ 5,000 $ 1,017,478 Trading securities: Fixed maturity securities $ 1,250 ​ $ 3,813 ​ $ — $ 5,063 Short-term investments $ 1,100 ​ $ 49,744 ​ $ — $ 50,844 A reconciliation of the beginning and ending balances of available-for-sale fixed maturity securities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) is shown below: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Beginning balance $ 5,000 $ 5,000 ​ $ — Transfers out of Level 3 — — ​ — Transfers in to Level 3 — — ​ — Purchases — — ​ 5,000 Sales — — ​ — Maturities and calls (320 ) — — Amortization of discount — — ​ — Total gains or losses (realized/unrealized): — — ​ — Included in earnings — — ​ — Included in other comprehensive income — — ​ — Ending balance $ 4,680 $ 5,000 ​ $ 5,000 18. Fair Value Measurements (continued) The Company held one available-for-sale fixed maturity security at December 31, 2017 and 2016 for which the fair value was determined using significant unobservable inputs (Level 3). A market approach using prices in trades of comparable securities was utilized to determine a fair value of $4.7 million for the security at December 31, 2017 and $5.0 million at December 31, 2016 . Transfers out of Level 3 occur when the Company is able to obtain reliable prices from pricing vendors for which the Company was previously unable to obtain reliable prices. Transfers in to Level 3 occur when the Company is unable to obtain reliable prices for securities from pricing vendors and instead must use broker price quotes. There were no transfers between Level 1 and Level 2 during 2017 , 2016 or 2015 . The Company recognizes transfers between levels at the beginning of the reporting period. There were no realized gains or losses included in earnings for the year ended December 31, 2017 attributable to the change in unrealized gains or losses relating to Level 3 assets valued at fair value on a recurring basis that are still held at December 31, 2017 . The Company measures certain bank loan participations at fair value on a non-recurring basis during the year as part of the Company’s impairment evaluation when loans are determined by management to be impaired. Assets measured at fair value on a nonrecurring basis are summarized below: ​ Fair Value Measurements Using ​ Quoted Prices in Active Markets for Identical Assets Level 1 Significant Other Observable Inputs Level 2 Significant Unobservable Inputs Level 3 ​ Total ​ (in thousands) December 31, 2017 ​ ​ ​ ​ ​ ​ ​ Bank loan participations held-for-investment $ — $ — ​ $ 5,111 ​ $ 5,111 December 31, 2016 Bank loan participations held-for-investment $ — $ — ​ $ 4,849 ​ $ 4,849 At December 31, 2017 and 2016 , bank loan participations held for investment that were determined to be impaired were written down to their fair value of $5.1 million and $4.8 million , respectively. In the determination of the fair value for bank loan participations and certain high yield bonds, the Company’s investment manager endeavors to obtain data from multiple external pricing sources. External pricing sources may include brokers, dealers and price data vendors that provide a composite price based on prices from multiple dealers. Such external pricing sources typically provide valuations for normal institutional size trading units of such securities using methods based on market transactions for comparable securities, and various relationships between securities, as generally recognized by institutional dealers. For investments in which the investment manager determines that only one external pricing source is appropriate or if only one external price is available, the relevant investment is generally recorded at fair value based on such price. Investments for which external sources are not available or are determined by the investment manager not to be representative of fair value are recorded at fair value as determined by the Company, with input from its investment managers and valuation specialists as considered necessary. In determining the fair value of such investments, the Company considers one or more of the following factors: type of security held, convertibility or exchangeability of the security, redeemability of the security (including the timing of redemptions), application of industry accepted valuation models, recent trading activity, liquidity, estimates of liquidation value, purchase cost, and prices received for securities with similar terms of the same issuer or similar issuers. At December 31, 2017 , there was one bank loan participation with an unpaid principal balance of $807,000 and a carrying value of zero for which external sources were unavailable to determine fair value. At December 31, 2016 , there were two bank loan participations with an unpaid principal balance of $2.3 million and a carrying value of $2.0 million for which external sources were unavailable to determine fair value. 18. Fair Value Measurements (continued) The carrying values and fair values of financial instruments are summarized below: ​ December 31, ​ 2017 2016 ​ Carrying Value Fair Value Carrying Value Fair Value ​ (in thousands) Assets ​ ​ ​ ​ ​ Available-for-sale: ​ ​ ​ ​ Fixed maturity securities $ 1,016,098 ​ $ 1,016,098 $ 941,077 $ 941,077 Equity securities 82,522 ​ 82,522 76,401 76,401 Trading: ​ ​ ​ ​ ​ Fixed maturity securities 3,808 ​ 3,808 5,063 5,063 Bank loan participations held-for-investment 238,214 ​ 236,532 203,526 203,123 Cash and cash equivalents 163,495 ​ 163,495 109,784 109,784 Short-term investments 36,804 ​ 36,804 50,844 50,844 Other invested assets – notes receivable 11,778 ​ 17,104 4,500 6,008 Liabilities ​ ​ ​ ​ ​ Senior debt 98,300 ​ 97,884 88,300 85,404 Junior subordinated debt 104,055 ​ 116,569 104,055 99,397 The fair values of fixed maturity securities and equity securities have been determined using quoted market prices for securities traded in the public market or prices using bid or closing prices for securities not traded in the public marketplace. The fair values of cash and cash equivalents and short-term investments approximate their carrying values due to their short-term maturity. The fair values of other invested assets-notes receivable, senior debt, and Junior Subordinated Debt at December 31, 2017 and 2016 were determined by calculating the present value of expected future cash flows under the terms of the note agreements or debt agreements, as applicable, discounted at an estimated market rate of interest at December 31, 2017 and 2016 , respectively. The fair values of bank loan participations held-for-investment, senior debt, and Junior Subordinated Debt at December 31, 2017 and 2016 were determined using inputs to the valuation methodology that are unobservable (Level 3).

Statutory Matters

Statutory Matters	12 Months Ended
	Dec. 31, 2017
Statutory Matters [Abstract]
Statutory Matters	Statutory Matters ​ U.S. U.S. state insurance laws and regulations prescribe accounting practices for determining statutory net income and capital and surplus for insurance companies. In addition, state regulators may permit statutory accounting practices that differ from prescribed practices. Statutory accounting practices prescribed or permitted by regulatory authorities for the Company’s insurance subsidiaries differ from U.S. GAAP. The principal differences between SAP and GAAP as they relate to the financial statements of the Company’s insurance subsidiaries are (a) policy acquisition costs are expensed as incurred under SAP, whereas they are deferred and amortized under GAAP, (b) certain assets are not admitted for purposes of determining surplus under SAP, (c) the classification and carrying amounts of investments in certain securities are different under SAP and GAAP, and (d) the criteria for providing asset valuation allowances and the methodologies used to determine the amount thereof are different under SAP and GAAP. Combined net income, statutory capital and surplus and minimum required statutory capital and surplus, as determined in accordance with statutory accounting practices, for the U.S. insurance subsidiaries as of December 31, 2017 , 2016 , and 2015 and for the years then ended are summarized as follows: 19. Statutory Matters (continued) ​ ​ 2017 2016 2015 ​ (in thousands) Statutory net income $ 31,881 $ 13,468 $ 14,747 Statutory capital and surplus 219,132 184,859 176,884 Minimum required statutory capital and surplus 25,000 25,000 25,000 Risk-Based Capital (“RBC”) requirements promulgated by the National Association of Insurance Commissioners require property/casualty insurers to maintain minimum capitalization levels determined based on formulas incorporating various business risks of the insurance subsidiaries. As of December 31, 2017 , the insurance subsidiaries’ adjusted capital and surplus exceeds their authorized control level RBC. Bermuda Under the Bermuda Insurance Act, 1978 and related regulations, JRG Re is required to maintain certain solvency and liquidity levels. The minimum statutory solvency margin required at December 31, 2017 was approximately $109.9 million . Actual statutory capital and surplus at December 31, 2017 was $409.2 million . JRG Re had statutory net income of $20.1 million for 2017 , $74.7 million for 2016 , and $54.3 million for 2015 . JRG Re had shareholders’ equity of $409.7 million on a GAAP basis at December 31, 2017 . JRG Re maintains a Class 3B license and thus must maintain a minimum liquidity ratio in which the value of its relevant assets is not less than 75.0% of the amount of its relevant liabilities for general business. Relevant assets include cash and cash equivalents, fixed maturities, alternative investments, accrued interest income, premiums receivable, losses recoverable from reinsurers, and funds withheld. The relevant liabilities include total general business insurance reserves and total other liabilities, less sundry liabilities. As of December 31, 2017 , the Company met the minimum liquidity ratio requirement.

Dividend Restrictions

Dividend Restrictions	12 Months Ended
	Dec. 31, 2017
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments [Abstract]
Dividend Restrictions	Dividend Restrictions ​ U.S. The insurance statutes of the U.S.-based insurance subsidiaries’ states of domicile limit the amount of dividends that they may pay annually without first obtaining regulatory approval. Generally, the limitations are based on the greater of statutory net income for the preceding year or 10.0% of statutory surplus at the end of the preceding year. The maximum amount of dividends available to James River Group from its U.S. insurance subsidiaries during 2018 without regulatory approval is $37.0 million . However, U.S. insurance regulators have broad powers to prevent the reduction of statutory surplus to inadequate levels and could refuse to permit the payment of dividends. Distributions from the Company’s U.S.-based subsidiaries to its U.K. intermediate holding company, James River UK, are generally subject to a 5% dividend withholding tax. The payment of any dividends by the Company’s U.S.-based subsidiaries directly to a Bermuda-based entity is subject to U.S. taxes at a 30.0% tax rate. JRG Holdings has determined that earnings of its U.S. subsidiaries have been and will be indefinitely reinvested in U.S. operations. Bermuda Bermuda regulations limit the amount of dividends and return of capital paid by a regulated entity. A class 3B insurer is prohibited from declaring or paying a dividend if it is in breach of its minimum solvency margin, its enhanced capital requirement, or its minimum liquidity ratio, or if the declaration or payment of such dividend would cause such a breach. Pursuant to Bermuda regulations, the maximum amount of dividends and return of capital available to be paid by a reinsurer is determined pursuant to a formula. Under this formula, the maximum amount of dividends and return of capital available to the Company from JRG Re during 2018 is calculated to be approximately $102.3 million . However, this dividend amount is subject to annual enhanced solvency requirement calculations. As of December 31, 2017 , JRG Holdings had consolidated retained earnings of $48.2 million , all of which was available for the payment of dividends to shareholders.

Other Related Party Transaction

Other Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Other Related Party Transactions	Other Related Party Transactions ​ The Company leases a commercial office building which houses the Company’s Richmond, Virginia operations under the terms of a non-cancelable lease from an entity with which it is affiliated. As a result of being deemed the owner for accounting purposes, the building is recorded as an asset and the related financing obligation is recorded as a liability on the accompanying consolidated balance sheets. Since the arrangement did not qualify for sale-lease back treatment upon completion of the asset’s construction, the Company continues to reduce the obligation over the lease term as payments are made and depreciates the asset over its useful life. Both the financing obligation and the lease had initial 10 -year terms which started in 2007 . In 2015 , the term of the lease and financing obligation were extended until 2026 . The arrangements provide for 2% fixed annual rent increases. The Chairman of the Board and Chief Executive Officer of the Company owns a plane that the Company periodically leases. Total fees paid by the Company for the use of this plane were $314,000 , $246,000 , and $211,000 for 2017 , 2016 , and 2015 , respectively. Upon the Company’s initial public offering in 2014, the Company entered into a consulting agreement (the “Consulting Agreement”) with one of the Company's Independent Directors. Under the terms of the Consulting Agreement, the director received $150,000 in each of 2015 and 2016 and $37,500 in 2017, prior to the termination of the Consulting Agreement effective March 31, 2017, for investment and other business consulting services.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 22, 2018 , the Board of Directors declared a cash dividend of $0.30 per share. The dividend is payable on March 30, 2018 to shareholders of record on March 12, 2018 . On February 21, 2018 , the Board of Directors granted awards under the 2014 LTIP and the 2014 Director Plan to the Company’s employees and directors. RSUs for 145,141 shares were awarded with a fair value on the date of grant of $39.71 per share. The RSUs vest over one to three year periods, depending on the award. In response to the Tax Act, we plan to make changes to the Company’s structure in 2018 to minimize the impact of BEAT that include the formation of Carolina Re, Ltd. (“Carolina Re”), a Bermuda-domiciled, wholly owned subsidiary of James River Group, Inc. We expect Carolina Re to be a Class 3A reinsurer and make an irrevocable election to be taxed as a U.S. domestic corporation under Section 953(d) of the Code. Effective January 1, 2018, we will discontinue ceding 70% of our U.S.-written premiums to JRG Re and will instead cede 70% of our U.S.-written premiums to Carolina Re. We also expect Carolina Re will enter into a stop loss reinsurance agreement with JRG Re. While Carolina Re will be subject to U.S. corporate income tax, we do not expect that it will be subject to BEAT in fiscal year 2018 as the Company is expected to have sufficient regular U.S. income tax liability compared to the BEAT liability. The applicability of BEAT depends on a number of factors, and it is unclear whether we will be subject to BEAT in future periods.

Unaudited Selected Quarterly Fi

Unaudited Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Unaudited Selected Quarterly Financial Data	Unaudited Selected Quarterly Financial Data The following is a summary of the unaudited quarterly results of operations: ​ 2017 Quarter 2017 ​ First Second Third Fourth Year ​ (in thousands, except per share data) Gross written premiums $ 224,179 $ 281,475 $ 338,351 $ 237,900 ​ $ 1,081,905 Total revenues 176,402 202,394 220,866 217,963 ​ 817,625 Net income 18,450 14,541 10,351 224 ​ 43,566 Comprehensive income (loss) 22,484 21,249 11,067 (2,015 ) ​ 52,785 Earnings per share: ​ ​ ​ ​ ​ ​ Basic $ 0.63 $ 0.49 $ 0.35 $ 0.01 ​ $ 1.48 Diluted $ 0.61 $ 0.48 $ 0.34 $ 0.01 ​ $ 1.44 For the second quarter of 2017, the Casualty Reinsurance segment was impacted by a correction of the accrual for a contingent commission liability and related commission expense. As a result of this correction, net income for the second quarter of 2017 was reduced by $2.0 million . 23. Unaudited Selected Quarterly Financial Data (continued) 2016 Quarter 2016 First Second Third Fourth Year (in thousands, except per share data) Gross written premiums $ 133,071 $ 170,671 $ 260,166 $ 173,490 $ 737,398 Total revenues 131,329 134,511 151,365 169,022 586,227 Net income 12,837 14,596 21,366 25,672 74,471 Comprehensive income (loss) 28,457 27,388 19,694 (3,127 ) 72,412 Earnings per share: ​ ​ ​ ​ ​ Basic $ 0.44 $ 0.50 $ 0.73 $ 0.88 $ 2.56 Diluted $ 0.43 $ 0.49 $ 0.71 $ 0.85 $ 2.49

SCHEDULE I - Summary of Investm

SCHEDULE I - Summary of 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 Investments - Other than Investments in Related Parties	Summary of Investments—Other than Investments in Related Parties Type of Investment Cost or Amortized Cost Fair Value Amount at which shown on Balance Sheet (1) ​ (in thousands) Fixed maturity securities, available-for-sale: ​ ​ ​ State and municipal $ 139,382 $ 144,366 $ 144,366 Residential mortgage-backed 160,379 158,661 158,661 Corporate 408,857 413,721 413,721 Commercial mortgage and asset-backed 182,595 182,611 182,611 Obligations of U.S. government corporations and agencies 35,948 35,847 35,847 U.S. Treasury securities and obligations guaranteed by the U.S. government 79,476 78,874 78,874 Redeemable preferred stock 2,025 2,018 2,018 Total fixed maturity securities, available-for-sale 1,008,662 1,016,098 1,016,098 Fixed maturity securities, trading 3,801 3,808 3,808 Equity securities, available-for-sale ​ ​ ​ Preferred Stock 59,102 66,281 66,281 Common Stock 16,216 16,241 16,241 Total equity securities, available-for-sale 75,318 82,522 82,522 Bank loan participations, held-for-investment, net of allowance 238,214 236,532 238,214 Short-term investments 35,804 35,804 35,804 Other invested assets ​ ​ 26,367 Total invested assets ​ ​ $ 1,402,813 (1) Differences between the amounts in this column and the amounts in the consolidated balance sheet are due to this schedule excluding investments in related parties. ​

SCHEDULE II - Condensed Financi

SCHEDULE II - Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Statements of financial information of parent company only (Parent Company)	Condensed Financial Information of Registrant Balance Sheets (Parent Company) ​ December 31, ​ 2017 2016 ​ (in thousands) Assets ​ ​ Cash and cash equivalents $ 2,849 $ 2,969 Investment in subsidiaries 910,610 897,729 Other assets 2,351 2,415 Total assets $ 915,810 $ 903,113 Liabilities and shareholders’ equity Liabilities: Accrued expenses $ 1,406 $ 1,428 Senior debt 83,300 73,300 Junior subordinated debt 15,928 15,928 Notes payable to subsidiary 100,000 100,000 Due to subsidiaries 20,043 18,549 Other liabilities 434 687 Total liabilities 221,111 209,892 Commitments and contingent liabilities — — Shareholders’ equity: Class A common shares 6 6 Additional paid-in capital 636,149 636,856 Retained earnings 48,198 55,232 Accumulated other comprehensive income 10,346 1,127 Total shareholders’ equity 694,699 693,221 Total liabilities and shareholders’ equity $ 915,810 $ 903,113 See accompanying notes. SCHEDULE II​ JAMES RIVER GROUP HOLDINGS, LTD. AND SUBSIDIARIES Condensed Financial Information of Registrant Statements of Income and Comprehensive Income (Parent Company) ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Revenues: Other income $ 49 $ 44 $ 40 Total revenues 49 44 40 Expenses: Other operating expenses 10,375 9,467 8,254 Other expenses 640 293 284 Interest expense 4,130 3,974 3,687 Total expenses 15,145 13,734 12,225 Loss before equity in net income of subsidiaries (15,096 ) (13,690 ) (12,185 ) Equity in net income of subsidiaries 58,662 88,161 65,682 Net income $ 43,566 $ 74,471 $ 53,497 Other comprehensive income (loss): Equity in other comprehensive earnings (losses) of subsidiaries 9,219 (2,059 ) (15,170 ) Total comprehensive income $ 52,785 $ 72,412 $ 38,327 See accompanying notes. SCHEDULE II​ JAMES RIVER GROUP HOLDINGS, LTD. AND SUBSIDIARIES Condensed Financial Information of Registrant Statements of Cash Flows (Parent Company) ​ Year Ended December 31, ​ 2017 2016 2015 Operating activities Net income $ 43,566 $ 74,471 $ 53,497 Adjustments to reconcile net income to net cash (used in) provided by operating activities: Provision for depreciation and amortization 176 157 157 Share based compensation expense 7,688 5,492 3,735 Equity in undistributed earnings of subsidiaries (23,662 ) (88,161 ) (65,682 ) Changes in operating assets and liabilities: 684 (1,889 ) 9,532 Net cash (used in) provided by operating activities 28,452 (9,930 ) 1,239 Investing activities Dividends from subsidiaries 20,000 80,000 52,250 Net cash provided by investing activities 20,000 80,000 52,250 Financing activities Dividends paid (50,832 ) (65,988 ) (47,405 ) Senior debt issuance 10,000 — 10,000 Senior debt repayments — — (10,000 ) Debt issue costs paid — (442 ) — Issuances of common shares under equity incentive plans 1,708 2,260 1,730 Common share repurchases (9,448 ) (4,907 ) (6,461 ) Net cash used in financing activities (48,572 ) (69,077 ) (52,136 ) Change in cash and cash equivalents (120 ) 993 1,353 Cash and cash equivalents at beginning of period 2,969 1,976 623 Cash and cash equivalents at end of period $ 2,849 $ 2,969 $ 1,976 Supplemental information Interest paid $ 4,612 $ 4,676 $ 4,337 ​ See accompanying notes. SCHEDULE II​ JAMES RIVER GROUP HOLDINGS, LTD. AND SUBSIDIARIES Condensed Financial Information of Registrant Notes to Condensed Financial Statements 1. Accounting Policies Organization James River Group Holdings, Ltd. (the “Company”) is an exempted holding company registered in Bermuda, organized for the purpose of acquiring and managing insurance and reinsurance entities. Basis of Presentation The accompanying condensed financial statements have been prepared using the equity method. Under the equity method, the investment in consolidated subsidiaries is stated at cost plus equity in undistributed earnings of consolidated subsidiaries since the date of acquisition. These condensed financial statements should be read in conjunction with the Company’s consolidated financial statements. Estimates and Assumptions Preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying disclosures. Those estimates are inherently subject to change, and actual results may ultimately differ from those estimates. Adopted Accounting Standards Effective January 1, 2017, the Company adopted Accounting Standards Update (“ASU”) 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting using the prospective method. The guidance requires that all tax effects related to share-based payments be made through the income statement at the time of settlement as opposed to excess tax benefits being recognized in additional paid-in capital under the previous guidance . The ASU also removes the requirement to delay recognition of a tax benefit until it reduces current taxes payable. Additionally, all tax related cash flows resulting from share-based payments are now reported as operating activities on the statement of cash flows, a change from the previous requirement to present tax benefits as an inflow from financing activities and an outflow from operating activities. For the years ended December 31, 2016 and 2015 , net cash provided by operating activities increased by $3.2 million and $3.6 million , respectively, and net cash used in financing activities increased by $3.2 million and $3.6 million , respectively, for the adoption of ASU 2016-09. The Company has elected to recognize forfeitures as they occur in accordance with ASU 2016-09. The adoption of ASU 2016-09 did not materially impact the Company’s income statements.

SCHEDULE III - Supplementary In

SCHEDULE III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
Supplementary Insurance Information	Supplementary Insurance Information (in thousands) ​ Deferred Reserve Unearned Net Net Losses Amortization Other Net December 31, 2017 ​ ​ ​ ​ ​ ​ ​ ​ ​ Excess and Surplus Lines $ 17,050 $ 759,043 $ 149,047 $ 463,521 $ 15,014 $ 371,717 $ 45,158 $ 79,136 $ 469,891 Specialty Admitted (725 ) 271,446 73,649 68,110 2,532 44,863 5,605 20,081 60,957 Casualty Reinsurance 56,040 261,860 195,418 209,478 31,507 138,797 65,238 72,446 235,778 Corporate and Other — — — — 12,066 — — 25,330 — Total $ 72,365 $ 1,292,349 $ 418,114 $ 741,109 $ 61,119 $ 555,377 $ 116,001 $ 196,993 $ 766,626 December 31, 2016 Excess and Surplus Lines $ 14,808 $ 575,280 $ 137,290 $ 301,404 $ 18,051 $ 188,768 $ 46,669 $ 75,467 $ 316,922 Specialty Admitted 1,767 128,795 84,156 52,281 2,542 30,897 5,968 18,512 55,803 Casualty Reinsurance 48,214 239,790 169,117 161,978 27,257 105,756 48,987 56,416 184,983 Corporate and Other — — — — 4,788 — — 20,433 — Total $ 64,789 $ 943,865 $ 390,563 $ 515,663 $ 52,638 $ 325,421 $ 101,624 $ 170,828 $ 557,708 December 31, 2015 Excess and Surplus Lines $ 15,266 $ 468,438 $ 114,369 $ 240,878 $ 13,427 $ 131,221 $ 42,069 $ 65,233 $ 253,285 Specialty Admitted 2,184 76,179 40,622 42,206 2,316 25,623 4,455 15,509 44,917 Casualty Reinsurance 43,304 240,705 146,113 178,121 22,706 122,172 51,226 58,507 172,830 Corporate and Other — — — — 6,386 — — 18,554 — Total $ 60,754 $ 785,322 $ 301,104 $ 461,205 $ 44,835 $ 279,016 $ 97,750 $ 157,803 $ 471,032

SCHEDULE IV - Reinsurance

SCHEDULE IV - Reinsurance	12 Months Ended
	Dec. 31, 2017
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Reinsurance	Reinsurance ​ Direct Amount Ceded to Other Companies Assumed from Other Companies Net Amount Percentage of Amount Assumed to Net ​ (in thousands) Year Ended December 31, 2017 ​ ​ ​ ​ ​ Excess and Surplus Lines Written Premiums $ 530,077 $ 60,229 $ 43 $ 469,891 — Specialty Admitted Written Premiums 313,642 255,473 2,788 60,957 4.6 % Casualty Reinsurance Written Premiums — (423 ) 235,355 235,778 99.8 % Total Written Premiums $ 843,719 $ 315,279 $ 238,186 $ 766,626 31.1 % Year Ended December 31, 2016 ​ ​ ​ ​ Excess and Surplus Lines Written Premiums $ 370,802 $ 53,922 $ 42 $ 316,922 — Specialty Admitted Written Premiums 178,898 126,418 3,323 55,803 6.0 % Casualty Reinsurance Written Premiums — (650 ) 184,333 184,983 99.6 % Total Written Premiums $ 549,700 $ 179,690 $ 187,698 $ 557,708 33.7 % Year Ended December 31, 2015 ​ ​ ​ ​ Excess and Surplus Lines Written Premiums $ 308,713 $ 55,432 $ 4 $ 253,285 — Specialty Admitted Written Premiums 87,103 46,061 3,875 44,917 8.6 % Casualty Reinsurance Written Premiums — (331 ) 172,499 172,830 99.8 % Total Written Premiums $ 395,816 $ 101,162 $ 176,378 $ 471,032 37.4 %

SCHEDULE V - Valuation and Qual

SCHEDULE V - Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Valuation and Qualifying Accounts	Valuation and Qualifying Accounts ​ Balance at Beginning of Period Additions Amounts Charged to Expense Deductions Amounts Written Off or Disposals Balance at End of Period ​ (in thousands) Year Ended December 31, 2017 ​ ​ ​ ​ Allowance for Doubtful Accounts $ 2,136 $ 1,029 $ (408 ) $ 2,757 Allowance for Credit Losses on Bank Loans 943 2,424 (148 ) 3,219 Total $ 3,079 $ 3,453 $ (556 ) $ 5,976 Year Ended December 31, 2016 ​ ​ ​ ​ Allowance for Doubtful Accounts $ 2,778 $ 814 $ (1,456 ) $ 2,136 Allowance for Credit Losses on Bank Loans 4,310 (791 ) (2,576 ) 943 Total $ 7,088 $ 23 $ (4,032 ) $ 3,079 Year Ended December 31, 2015 ​ ​ ​ ​ Allowance for Doubtful Accounts $ 1,985 $ 1,061 $ (268 ) $ 2,778 Allowance for Credit Losses on Bank Loans 752 3,896 (338 ) 4,310 Total $ 2,737 $ 4,957 $ (606 ) $ 7,088

SCHEDULE VI - Supplementary Inf

SCHEDULE VI - 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	Supplementary Information Concerning Property Casualty Insurance Operations ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Deferred policy acquisition costs $ 72,365 $ 64,789 $ 60,754 Reserve for losses and loss adjustment expenses 1,292,349 943,865 785,322 Unearned premiums 418,114 390,563 301,104 Net earned premiums 741,109 515,663 461,205 Net investment income 61,119 52,638 44,835 Losses and loss adjustment expenses incurred: Current year 533,905 349,137 295,334 Prior year 21,472 (23,716 ) (16,318 ) Total losses and loss adjustment expenses incurred 555,377 325,421 279,016 Amortization of policy acquisition costs 116,001 101,624 97,750 Paid losses and loss adjustment expenses, net of reinsurance 326,680 217,827 214,524 Net written premiums 766,626 557,708 471,032

Accounting Policies (Policies)

Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The consolidated financial statements are prepared in accordance with U.S. generally accepted accounting principles (“GAAP”), which vary in some respects from statutory accounting practices (“SAP”) which are prescribed or permitted by the various state insurance departments in the U.S. or by insurance regulators in Bermuda. The accompanying consolidated financial statements include the accounts and operations of the Company and its subsidiaries. Intercompany transactions and balances have been eliminated.
Estimates and Assumptions	Estimates and Assumptions Preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying disclosures. Those estimates are inherently subject to change, and actual results may ultimately differ from those estimates.
Fixed Maturity and Equity Securities	Fixed Maturity and Equity Securities Fixed maturity and equity securities classified as “available-for-sale” are carried at fair value, and unrealized gains and losses on such securities, net of any deferred taxes, are reported as a separate component of accumulated other comprehensive income. Fixed maturity securities purchased for short-term resale are classified as “trading” and are carried at fair value with unrealized gains and losses included in earnings as a component of net investment income. The Company does not have any securities classified as “held-to-maturity”. Fair value generally represents quoted market value prices for securities traded in the public marketplace or prices analytically determined using bid or closing prices for securities not traded in the public marketplace. 1. Accounting Policies (continued) Premiums and discounts on mortgage-backed securities and asset-backed securities are amortized or accrued using the constant yield method which considers anticipated prepayments at the date of purchase. To the extent that the estimated lives of such securities change as a result of changes in estimated prepayment rates, the adjustments are included in net investment income using the retrospective method. Realized investment gains or losses are determined on a specific identification basis. Interest income is recognized as earned, and dividend income is recognized on the ex-dividend date. The Company evaluates its available-for-sale investments regularly to determine whether there are declines in value that are other-than-temporary. The Company’s outside investment managers assist the Company in this evaluation. When the Company determines that a security has experienced an other-than-temporary impairment, the impairment loss is recognized as a realized investment loss. The factors that the Company considers in evaluating whether such an other-than-temporary impairment has occurred include the amount and percentage that fair value is below amortized cost or cost and the length of time that fair value has been below amortized cost or cost. For fixed maturity securities, the Company considers the credit quality rating of the security, with a special emphasis on securities downgraded below investment grade. Management does not intend to sell available-for-sale fixed maturity securities in an unrealized loss position, and it is not “more likely than not” that the Company will be required to sell these securities before a recovery in fair value to their amortized cost basis occurs. For equity securities, management evaluates the near-term prospects of these investments in relation to the severity and duration of the impairment, and the Company’s ability and intent to hold these investments until a recovery of fair value occurs.
Bank Loan Participations Held-for-Investment and Allowance for Credit Losses	Bank Loan Participations Held-for-Investment and Allowance for Credit Losses Bank loan participations held-for-investment are managed by a specialized outside investment manager and are generally stated at their outstanding unpaid principal balances net of unamortized premiums or discounts and net of any allowance for credit losses. Interest income is accrued on the unpaid principal balance. Discounts and premiums are amortized to income using the interest method. Generally, the accrual of interest on a bank loan participation is discontinued when the contractual payment of principal or interest has become 90 days past due or management has serious doubts about further collectability of principal or interest. A bank loan participation may remain on accrual status if it is in the process of collection and is either guaranteed or well secured. Interest received on nonaccrual loans generally is reported as investment income. There were no bank loans on nonaccrual status at December 31, 2017 or 2016 . Generally, bank loan participations are restored to accrual status when the obligation is brought current, has performed in accordance with the contractual terms for a reasonable period of time, and the ultimate collectability of the total contractual principal and interest is no longer in doubt. ​ The allowance for credit losses is maintained at a level believed adequate by management to absorb estimated probable credit losses. Management’s periodic evaluation of the adequacy of the allowance is based on consultations and advice of the Company’s specialized investment manager, known and inherent risks in the portfolio, adverse situations that may affect the borrower’s ability to repay, the estimated value of any underlying collateral, current economic conditions, and other relevant factors. When an observable market price for a loan is available, the Company has recorded an allowance equal to the difference between the fair value and the amortized cost of bank loans that it has determined to be impaired as a practical expedient for an estimate of probable future cash flows to be collected on those bank loans. If an observable market price for a loan is not available, the Company records an allowance equal to the difference between the present value of expected future cash flows discounted at the loan’s effective interest rate and the amortized cost of the loan. Bank loans are charged off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote.
Other Invested Assets	Other Invested Assets Other invested assets at December 31, 2017 and 2016 include the Company’s interests in private debt and equity investments. The investments are primarily focused in renewable energy, limited partnerships, and bank holding companies. Equity interests in various limited liability companies (“LLCs”) and limited partnerships are accounted for under the equity method, as the Company has determined that the equity method best reflects its economic interest in the underlying equity investment. For certain note agreements, original discounts and commitment fees received are recognized over the terms of the notes under the effective interest method.
Short-Term Investments	Short-Term Investments Short-term investments are carried at cost, which approximates fair value. Short-term investments have maturities greater than three months but less than one year at the date of purchase.
Cash Equivalents	Cash Equivalents The Company considers highly liquid investments with maturities of three months or less at the date of purchase to be cash equivalents.
Direct Written Premiums	Direct Written Premiums Direct written premiums are earned on a pro rata basis over the terms of the policies, generally 12 months. The portion of premiums written applicable to the unexpired terms of the policies in force is recorded as unearned premiums. Policies are accounted for on an individual basis, with no aggregation by counterparty.
Assumed Reinsurance Premiums	Assumed Reinsurance Premiums Assumed reinsurance written premiums include amounts reported by brokers and ceding companies, supplemented by the Company’s own estimates of premiums when reports have not been received. Premiums on the Company’s excess of loss and pro rata reinsurance contracts are estimated when the business is underwritten. For excess of loss contracts, the deposit premium, as defined in the contract, is generally recorded as an estimate of premiums written at the inception date of the treaty. Estimates of premiums written under pro rata contracts are recorded in the period in which the underlying risks are expected to begin and are based on information provided by the brokers and the ceding companies. Reinsurance premium estimates are reviewed by management periodically. Any adjustment to these estimates is recorded in the period in which it becomes known. ​ Reinsurance premiums assumed are earned over the terms of the underlying policies or reinsurance contracts. Contracts and policies written on a “losses occurring” basis cover claims that may occur during the term of the contract or policy, which is typically 12 months. Accordingly, the premium is earned evenly over the term. Contracts which are written on a “risks attaching” basis cover claims which attach to the underlying insurance policies written during the terms of such contracts. Premiums earned on such contracts usually extend beyond the original term of the reinsurance contract, typically resulting in recognition of premiums earned over a 24-month period in proportion to the level of underlying exposure. Contracts are accounted for on an individual basis, with no aggregation by counterparty.
Premiums Receivable and Agents' Balances, Net	Premiums Receivable and Agents’ Balances, Net Premiums receivable and agents’ balances are carried at face value net of any allowance for doubtful accounts. The allowance for doubtful accounts represents an estimate of amounts considered uncollectible based on the Company’s assessment of the collectability of receivables that are past due. Receivables greater than 90 days past due were $2.6 million and $2.0 million at December 31, 2017 and 2016 , respectively. The allowance for doubtful accounts was $2.8 million and $2.1 million at December 31, 2017 and 2016 , respectively. Bad debt expense was $1.0 million for the year ended December 31, 2017 , $813,000 for the year ended December 31, 2016 , and $1.1 million for the year ended December 31, 2015 . Receivables written off against the allowance for doubtful accounts totaled $408,000 for the year ended December 31, 2017 , $1.5 million for the year ended December 31, 2016 , and $268,000 for the year ended December 31, 2015 . Account balances are charged off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote.
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs Costs which are incrementally or directly related to the successful acquisition of new or renewal insurance business are deferred. These deferred costs are primarily commissions to agents, ceding commissions paid on reinsurance assumed, premium taxes, and the portion of underwriting fixed compensation and payroll related fringe benefits directly related to an insurance contract that has been acquired, net of ceding commissions related to reinsurance ceded. Amortization of such policy acquisition costs is charged to expense in proportion to premium earned over the estimated policy life. To the extent that unearned premiums on existing policies are not adequate to cover projected related costs and expenses, deferred policy acquisition costs are charged to earnings. The Company considers anticipated investment income in determining whether a premium deficiency exists.
Reinsurance and Adjustable Features of Insurance and Reinsurance Contracts	Reinsurance and Adjustable Features of Insurance and Reinsurance Contracts Certain premiums and losses are ceded to other insurance companies or assumed from other insurance companies under various excess of loss and quota-share reinsurance contracts. The Company enters into ceded reinsurance contracts to limit its exposure to large losses, to limit exposure on new lines of insurance written by the Company, and to provide additional capacity for growth. Premiums, commissions, and losses and loss adjustment expenses on reinsured business are accounted for on a basis consistent with that used in accounting for the original policies issued and the terms of the reinsurance contracts. Reinsurance recoverables and prepaid reinsurance premiums are reported as assets. Other amounts payable to insurance companies and reinsurers or receivable from insurance companies and reinsurers are netted where the right of offset exists. The Company receives ceding commissions in connection with certain ceded reinsurance. The ceding commissions are recorded as a reduction of other operating expenses. Certain reinsurance contracts of the Casualty Reinsurance segment include provisions that adjust premiums or acquisition expenses based upon the experience under the contracts. Premiums written and earned, as well as related acquisition expenses are recorded based upon the projected experience under the contracts. The Company’s Specialty Admitted Insurance segment writes insurance under specialty admitted fronting and program arrangements. The fronting and program arrangements may contain contractual provisions that adjust acquisition expenses based upon loss experience under the contracts. The specialty admitted fronting and program arrangements are significantly reinsured. These reinsurance contracts may also contain provisions that adjust premiums or acquisition expenses based upon the loss experience under the contracts.
Other Income	Other Income Other income is principally comprised of fee income earned on policies for which the Company has no exposure to underwriting risk. Fee income of $17.0 million , $10.1 million , and $3.2 million is included in other income for the years ended December 31, 2017 , 2016 , and 2015 , respectively. Fees are earned on a pro rata basis over the service period of the underlying business. Policies are accounted for on an individual basis, with no aggregation by counterparty.
Income Taxes	Income Taxes Deferred tax assets and deferred tax liabilities are provided for the future tax consequences attributable to temporary differences between the financial statement carrying amounts of assets and liabilities and their respective U.S. tax bases. Deferred tax assets and liabilities are measured using enacted U.S. corporate tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. Deferred tax assets are reduced by a valuation allowance only when management believes it is more likely than not that some, or all, of the deferred tax assets will not be realized.
Goodwill	Goodwill Goodwill is tested annually for impairment in the fourth quarter of each calendar year, or more frequently if events or changes in circumstances indicate that the carrying amount of the Company’s reporting units, including goodwill, may exceed their fair values. The Company first assesses qualitative factors in determining whether it is necessary to perform the quantitative goodwill impairment test. If management determines that it is more likely than not that the fair value of a reporting unit is less than the carrying value based on qualitative factors then they will perform the quantitative goodwill impairment test. For the quantitative goodwill impairment testing, the fair value of the reporting units is determined using a combination of a market approach and an income approach which projects the future cash flows produced by the reporting units and discounts those cash flows to their present value. The projection of future cash flows is necessarily dependent upon assumptions on the future levels of income as well as business trends, prospects, market, and economic conditions. The results of the two approaches are weighted to determine the fair value of each reporting unit. When the fair value is less than the carrying value of the net assets of the reporting unit, including goodwill, an impairment loss is charged to operations. To determine the amount of any goodwill impairment, the implied fair value of reporting unit goodwill is compared to the carrying amount of that goodwill. The implied fair value of goodwill is determined in the same manner as the amount of goodwill recognized in a business combination is determined. That is, the fair value of a reporting unit is assigned to all of the assets and liabilities of that unit (including any unrecognized intangible assets) as if the reporting unit had been acquired in a business combination. The excess of the fair value of a reporting unit over the amounts assigned to its assets and liabilities is the implied fair value of goodwill.
Intangible Assets, Net	Intangible Assets, Net Intangible assets are initially recognized and measured at fair value. Specifically identified intangible assets with indefinite lives include trademarks and state insurance licenses and authorities. Other specifically identified intangible assets with lives ranging from 7.0 to 27.5 years represent relationships with brokers. These intangible assets are amortized on a straight-line basis over their estimated useful lives. Intangible assets with indefinite useful lives are reviewed for impairment at least annually. In evaluating whether there has been impairment to the intangible asset, management determines the fair value of the intangible asset and compares the resulting fair value to the carrying value of the intangible asset. If the carrying value exceeds the fair value, the intangible asset is written down to fair value, and the impairment is reported through earnings. The Company evaluates intangible assets with definite lives for impairment when impairment indicators are noted.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets with finite lives are tested for impairment whenever recognized events or changes in circumstances indicate the carrying value of these assets may not be recoverable. If indicators of impairment are present, fair value is calculated using estimated future cash flows expected to be generated from the use of those assets. An impairment loss is recognized only if the carrying amount of a long-lived asset or asset group is not recoverable and exceeds its fair value. The carrying amount of a long-lived asset or asset group is not recoverable if it exceeds the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the asset or asset group. That assessment is based on the carrying amount of the asset or asset group at the date it is tested for recoverability. An impairment loss is measured as the amount by which the carrying amount of a long-lived asset or asset group exceeds its fair value.
Property and Equipment, Net	Property and Equipment, Net Property and equipment, which is included in “other assets” in the accompanying consolidated balance sheets, is reported at cost less accumulated depreciation and is depreciated principally on a straight-line basis over the estimated useful lives of the depreciable assets, generally three to ten years . In the event the Company has been deemed the owner for accounting purposes of construction projects in lease arrangements, the estimated construction costs incurred to date are recorded as assets in property and equipment, net and included in “other assets” in the accompanying consolidated balance sheets. Upon occupancy of facilities under lease, the Company assesses whether arrangements qualify for sales recognition under the sale-leaseback accounting guidance. If the Company continues to be the deemed owner for accounting purposes, the cost of the building is depreciated over its estimated useful life.
Reserve for Losses and Loss Adjustment Expenses	Reserve for Losses and Loss Adjustment Expenses The reserve for losses and loss adjustment expenses represents the estimated ultimate cost of all reported and unreported losses and loss adjustment expenses incurred and unpaid at the balance sheet date. The Company does not discount this reserve. The process of estimating the reserve for losses and loss adjustment expenses requires a high degree of judgment and is subject to a number of variables. The reserve for losses and loss adjustment expenses is estimated using individual case-basis valuations and statistical analyses. Those estimates are subject to the effects of trends in loss severity and frequency. The Company utilizes various actuarially-accepted reserving methodologies in determining the continuum of expected outcomes for its reserves. These methodologies utilize various inputs, including management’s initial expected loss ratio (the ratio of losses and loss adjustment expenses incurred to net earned premiums), expected reporting patterns and payment patterns for losses and loss adjustment expenses (based on insurance industry data and the Company’s own experience), and the Company’s actual paid and reported losses and loss adjustment expenses. An internal actuary reviews these results and (after applying appropriate professional judgment and other actuarial techniques that are considered necessary) presents recommendations to the Company’s management. Management uses this information and its judgment to make decisions on the final recorded reserve for losses and loss adjustment expenses. Management believes that the use of judgment is necessary to arrive at a best estimate for the reserve for losses and loss adjustment expenses given the long-tailed nature of the business generally written by the Company and the limited operating experience of the Casualty Reinsurance segment, the fronting and program business in the Specialty Admitted Insurance segment, and the commercial auto business in the Excess and Surplus Lines segment. 1. Accounting Policies (continued) Catastrophes of significant magnitude, including hurricanes and earthquakes, involve complex coverage issues. In estimating the reserve for losses and loss adjustment expenses for these catastrophes, management uses case reserve estimates based on information obtained from site inspections by the Company’s adjustors and the terms of coverage provided in the policies. Management estimates reserves for incurred but not reported claims for these catastrophes using judgment based on an assessment of the Company’s property insurance exposures where the catastrophes occur and the Company’s progress in settling claims. Although management believes that the reserve for losses and loss adjustment expenses is reasonable, it is possible that the Company’s actual incurred losses and loss adjustment expenses will not develop in a manner consistent with the assumptions inherent in the determination of these reserves. Specifically, the Company’s actual ultimate loss ratio could differ from management’s initial expected loss ratio and/or the Company’s actual reporting patterns for losses could differ from the expected reporting patterns. Accordingly, the ultimate settlement of losses and the related loss adjustment expenses may vary significantly from the estimates included in the Company’s consolidated financial statements. These estimates are reviewed continually by management and are adjusted as necessary as experience develops or new information becomes known; such adjustments are included in current operations.
Share Based Compensation	Share Based Compensation The Company expenses the fair value of share equity awards over the vesting period of the award on a straight-line basis. The Black-Scholes-Merton option pricing model is used to value the options granted (see Note 11). As the share based compensation expense is incurred, a corresponding increase to additional paid-in capital in shareholders’ equity is recognized. Share based compensation expense is reflected in “other operating expenses” in the accompanying consolidated statements of income and comprehensive income.
Financing Obligations	Financing Obligations In a lease arrangement where the Company made a minority investment in a partnership that was involved in the construction of a building, the Company was deemed the owner for accounting purposes during the construction period. The Company recorded an asset for the amount of the total project costs and the related financing obligation is included in “other liabilities” in the accompanying consolidated balance sheets. Once construction was completed, the Company determined the arrangement did not qualify for sale-lease back treatment. Accordingly, the Company continues to reduce the obligation over the lease term as payments are made and depreciates the asset over its useful life. The Company does not report rent expense for the portion of the rent payment determined to be related to the assets which are owned for accounting purposes. Rather, this portion of the rent payment under the lease is recognized as a reduction of the financing obligation and as interest expense.
Variable Interest Entities	Variable Interest Entities Entities that do not have sufficient equity at risk to allow the entity to finance its activities without additional financial support or in which the equity investors, as a group, do not have the characteristic of a controlling financial interest are referred to as variable interest entities (“VIE”). A VIE is consolidated by the variable interest holder that is determined to have the controlling financial interest (primary beneficiary) as a result of having both the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance and the obligation to absorb losses or right to receive benefits from the VIE that could potentially be significant to the VIE. The Company determines whether it is the primary beneficiary of an entity subject to consolidation based on a qualitative assessment of the VIE’s capital structure, contractual terms, nature of the VIE’s operations and purpose, and the Company’s relative exposure to the related risks of the VIE on the date it becomes initially involved in the VIE. The Company reassesses its VIE determination with respect to an entity on an ongoing basis. The Company holds interests in VIEs through certain equity method investments included in “other invested assets” in the accompanying consolidated balance sheets. The Company has determined that it should not consolidate any of the VIEs as it is not the primary beneficiary in any of the relationships. Although the investments resulted in the Company holding variable interests in the entities, they did not empower the Company to direct the activities that most significantly impact the economic performance of the entities.
Earnings Per Share	Earnings Per Share Basic earnings per share excludes dilution and is computed by dividing net income by the weighted-average number of common shares outstanding for the period. Diluted earnings per share reflects the dilution that could occur if securities or other contracts to issue common shares or common share equivalents were exercised or converted into common shares as calculated 1. Accounting Policies (continued) using the treasury stock method. When inclusion of common share equivalents increases the earnings per share or reduces the loss per share, the effect on earnings is anti-dilutive, and the diluted net earnings or net loss per share is computed excluding these common share equivalents.
Adopted Accounting Standards	Adopted Accounting Standards Effective January 1, 2017, the Company adopted Accounting Standards Update (“ASU”) 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting using the prospective method. The guidance requires that all tax effects related to share-based payments be made through the income statement at the time of settlement as opposed to excess tax benefits being recognized in additional paid-in capital under the previous guidance . The ASU also removes the requirement to delay recognition of a tax benefit until it reduces current taxes payable. Additionally, all tax related cash flows resulting from share-based payments are now reported as operating activities on the statement of cash flows, a change from the previous requirement to present tax benefits as an inflow from financing activities and an outflow from operating activities. For the years ended December 31, 2016 and 2015 , net cash provided by operating activities increased by $3.2 million and $3.6 million , respectively, and net cash used in financing activities increased by $3.2 million and $3.6 million , respectively, for the adoption of ASU 2016-09. The Company has elected to recognize forfeitures as they occur in accordance with ASU 2016-09. The adoption of ASU 2016-09 did not materially impact the Company’s consolidated income statements. Prospective Accounting Standards In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) . The guidance applies to all companies that either enter into contracts with customers to transfer goods or services or enter into contracts for the transfer of nonfinancial assets, unless those contracts are within the scope of other standards, such as insurance contracts. Under this guidance, a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. Judgments required in adopting this update may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate 1. Accounting Policies (continued) performance obligation. ASU 2014-09 becomes effective for the Company during the first quarter of 2018 and must be applied retrospectively. The Company has completed its evaluation of ASU 2014-09 and determined that adopting this standard will have no impact on reported fee income. As part of this evaluation, the Company completed a review of its contracts and assessed the impact the new standard will have on its disclosures and internal controls. The Company will adopt this ASU in the first quarter of 2018, with no cumulative effect of initially applying the update. In January 2016, the FASB issued ASU 2016-01, Financial Instruments – Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities . Among other things, this ASU will require equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income. This ASU is effective for interim and annual reporting periods beginning after December 15, 2017. Upon adoption on January 1, 2018, the Company expects to make a $4.7 million cumulative-effect adjustment to increase retained earnings and reduce accumulated other comprehensive income. Adoption of this ASU is not expected to have a material impact on the Company's financial position, cash flows, or total comprehensive income, but will have an impact on the Company's results of operations, as changes in fair value of equity instruments will be presented in net income rather than other comprehensive income. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) . Under current guidance for lessees, leases are only included on the balance sheet if certain criteria, classifying the agreement as a capital lease, are met. This update will require the recognition of a right-of-use asset and a corresponding lease liability, discounted to the present value, for all leases that extend beyond 12 months. This ASU is effective for annual and interim reporting periods beginning after December 15, 2018. Upon adoption, leases will be recognized and measured at the beginning of the earliest period presented using a modified retrospective approach. The Company is currently evaluating ASU 2016-02 to determine the potential impact that adopting this standard will have on its consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. Current GAAP delays the recognition of credit losses until it is probable a loss has been incurred. The update will require financial assets measured at amortized cost, such as bank loan participations held for investment, to be presented at the net amount expected to be collected by means of an allowance for credit losses that runs through net income. Credit losses relating to available-for-sale debt securities will also be recorded through an allowance for credit losses, with the amount of the allowance limited to the amount by which fair value is below amortized cost. This ASU is effective for annual and interim reporting periods beginning after December 15, 2019. Upon adoption, this ASU will be applied using the modified-retrospective approach, by which a cumulative-effect adjustment will be made to retained earnings as of the beginning of the first reporting period presented. The Company has not yet completed the analysis of how adopting this ASU will affect the Company’s financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments to address the diversity in practice of how certain cash receipts and payments are classified in the statement of cash flows. The update addresses specific issues, including distributions received from equity method investees and the classification of cash receipts and payments that have aspects of more than one class of cash flows. This ASU is effective for annual and interim reporting periods beginning after December 15, 2017. Upon adoption, the update will be applied using the retrospective transition method. The Company does not expect that the adoption of this ASU will have a material impact on its statement of cash flows. In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income as a result of the enactment of the Tax Cuts and Jobs Act of 2017 (TCJA) on December 22, 2017. Current guidance requires the effect of a change in tax laws or rates on deferred tax balances to be reported in income from continuing operations in the accounting period that includes the period of enactment, even if the related income tax effects were originally charged or credited to accumulated other comprehensive income. This ASU allows for the option to reclassify the stranded tax effects resulting from the implementation of the TCJA out of accumulated other comprehensive income and into retained earnings. This ASU is effective for annual and interim reporting periods beginning after December 15, 2018. Early adoption is permitted. As the adoption of ASU 2016-01 in 2018 will result in the reclassification of the entire unrealized balance on equity securities from accumulated other comprehensive income into retained earnings, only the stranded tax effects on the unrealized balances of our fixed income securities will be impacted by the adoption of ASU 2018-02.

Accounting Policies (Tables)

Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of reconciliation of numerator and denominator of basic and diluted earnings per share	The following represents a reconciliation of the numerator and denominator of the basic and diluted earnings per share computations contained in the consolidated financial statements. ​ Net Income Weighted-Average Earnings ​ (in thousands, except per share data) Year ended December 31, 2017 ​ ​ ​ Basic $ 43,566 29,461,717 $ 1.48 Common share equivalents — 811,432 (0.04 ) Diluted $ 43,566 30,273,149 $ 1.44 Year ended December 31, 2016 ​ ​ ​ Basic $ 74,471 29,063,075 $ 2.56 Common share equivalents — 831,303 (0.07 ) Diluted $ 74,471 29,894,378 $ 2.49 Year ended December 31, 2015 ​ ​ ​ Basic $ 53,497 28,662,051 $ 1.87 Common share equivalents — 672,867 (0.05 ) Diluted $ 53,497 29,334,918 $ 1.82

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Schedule of summary of available-for-sale investments	The Company's available-for-sale investments are summarized as follows: ​ Cost or Gross Gross Fair ​ (in thousands) December 31, 2017 Fixed maturity securities: State and municipal $ 139,382 $ 5,587 $ (603 ) $ 144,366 Residential mortgage-backed 160,379 723 (2,441 ) 158,661 Corporate 408,857 7,503 (2,639 ) 413,721 Commercial mortgage and asset-backed 182,595 714 (698 ) 182,611 Obligations of U.S. government corporations and agencies 35,948 — (101 ) 35,847 U.S. Treasury securities and obligations guaranteed by the U.S. government 79,476 37 (639 ) 78,874 Redeemable preferred stock 2,025 — (7 ) 2,018 Total fixed maturity securities 1,008,662 14,564 (7,128 ) 1,016,098 Equity securities 75,318 7,830 (626 ) 82,522 Total investments available-for-sale $ 1,083,980 $ 22,394 $ (7,754 ) $ 1,098,620 December 31, 2016 Fixed maturity securities: State and municipal $ 101,793 $ 5,032 $ (984 ) $ 105,841 Residential mortgage-backed 152,703 1,070 (2,975 ) 150,798 Corporate 379,727 4,382 (5,661 ) 378,448 Commercial mortgage and asset-backed 167,967 906 (826 ) 168,047 Obligations of U.S. government corporations and agencies 64,823 276 (85 ) 65,014 U.S. Treasury securities and obligations guaranteed by the U.S. government 71,174 131 (185 ) 71,120 Redeemable preferred stock 2,025 — (216 ) 1,809 Total fixed maturity securities 940,212 11,797 (10,932 ) 941,077 Equity securities 74,553 4,503 (2,655 ) 76,401 Total investments available-for-sale $ 1,014,765 $ 16,300 $ (13,587 ) $ 1,017,478
Schedule of summary of available-for-sale investments by contractual maturity	The amortized cost and fair value of available-for-sale investments in fixed maturity securities at December 31, 2017 are summarized, by contractual maturity, as follows: ​ Amortized Fair ​ (in thousands) One year or less $ 66,852 $ 66,770 After one year through five years 274,095 274,421 After five years through ten years 195,333 195,995 After ten years 127,383 135,622 Residential mortgage-backed 160,379 158,661 Commercial mortgage and asset-backed 182,595 182,611 Redeemable preferred stock 2,025 2,018 Total $ 1,008,662 $ 1,016,098
Schedule of gross unrealized losses and fair value for available-for-sale securities	The following table shows the Company’s gross unrealized losses and fair value for available-for-sale securities aggregated by investment category and the length of time that individual securities have been in a continuous unrealized loss position: ​ Less Than 12 Months 12 Months or More Total ​ Fair Gross Fair Gross Fair Gross ​ (in thousands) December 31, 2017 Fixed maturity securities: State and municipal $ 40,117 $ (318 ) $ 10,662 $ (285 ) $ 50,779 $ (603 ) Residential mortgage-backed 50,447 (261 ) 84,193 (2,180 ) 134,640 (2,441 ) Corporate 113,464 (846 ) 66,954 (1,793 ) 180,418 (2,639 ) Commercial mortgage and asset-backed 53,965 (244 ) 25,299 (454 ) 79,264 (698 ) Obligations of U.S. government corporations and agencies 3,024 (1 ) 32,154 (100 ) 35,178 (101 ) U.S. Treasury securities and obligations guaranteed by the U.S. government 50,760 (430 ) 26,707 (209 ) 77,467 (639 ) Redeemable preferred stock 2,018 (7 ) — — 2,018 (7 ) Total fixed maturity securities 313,795 (2,107 ) 245,969 (5,021 ) 559,764 (7,128 ) Equity securities 5,859 (65 ) 5,665 (561 ) 11,524 (626 ) Total investments available-for-sale $ 319,654 $ (2,172 ) $ 251,634 $ (5,582 ) $ 571,288 $ (7,754 ) December 31, 2016 Fixed maturity securities: State and municipal $ 28,398 $ (984 ) $ — $ — $ 28,398 $ (984 ) Residential mortgage-backed 93,242 (1,548 ) 32,330 (1,427 ) 125,572 (2,975 ) Corporate 199,841 (4,212 ) 8,477 (1,449 ) 208,318 (5,661 ) Commercial mortgage and asset-backed 47,990 (799 ) 7,195 (27 ) 55,185 (826 ) Obligations of U.S. government corporations and agencies 50,573 (85 ) — — 50,573 (85 ) U.S. Treasury securities and obligations guaranteed by the U.S. government 48,989 (185 ) — — 48,989 (185 ) Redeemable preferred stock 1,809 (216 ) — — 1,809 (216 ) Total fixed maturity securities 470,842 (8,029 ) 48,002 (2,903 ) 518,844 (10,932 ) Equity securities 21,345 (1,071 ) 6,558 (1,584 ) 27,903 (2,655 ) Total investments available-for-sale $ 492,187 $ (9,100 ) $ 54,560 $ (4,487 ) $ 546,747 $ (13,587 )
Schedule of major categories of the company' s net investment income	Major categories of the Company’s net investment income are summarized as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Fixed maturity securities $ 26,833 $ 25,917 $ 24,178 Bank loan participations 17,388 14,486 13,432 Equity securities 5,045 5,617 4,444 Other invested assets 14,079 9,536 5,947 Cash, cash equivalents, short-term investments, and other 1,708 824 672 Trading gains (losses) (4 ) 18 (9 ) Gross investment income 65,049 56,398 48,664 Investment expense (3,930 ) (3,760 ) (3,829 ) Net investment income $ 61,119 $ 52,638 $ 44,835
Schedule of summary of realized gains and losses	The Company’s realized gains and losses on investments are summarized as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Fixed maturity securities: Gross realized gains $ 840 $ 1,916 $ 2,197 Gross realized losses (512 ) (106 ) (826 ) ​ 328 1,810 1,371 Equity securities: Gross realized gains 429 4,761 1,041 Gross realized losses (1,591 ) — (10 ) ​ (1,162 ) 4,761 1,031 Bank loan participations: Gross realized gains 2,407 2,827 1,269 Gross realized losses (3,557 ) (1,832 ) (8,258 ) ​ (1,150 ) 995 (6,989 ) Short-term investments and other: Gross realized gains 1 3 54 Gross realized losses (6 ) (4 ) (14 ) ​ (5 ) (1 ) 40 Total $ (1,989 ) $ 7,565 $ (4,547 )
Schedule of available-for-sale gross unrealized gains or losses	The following table summarizes the change in the Company’s available-for-sale gross unrealized gains or losses by investment type: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Change in gross unrealized gains (losses): Fixed maturity securities $ 6,571 $ (1,350 ) $ (16,832 ) Equity securities 5,356 (2,433 ) 724 Total $ 11,927 $ (3,783 ) $ (16,108 )
Schedule of other invested assets	The Company invests selectively in private debt and equity opportunities. These investments, which together comprise the Company’s other invested assets, are primarily focused in renewable energy, limited partnerships, and bank holding companies. ​ Carrying Value Investment Income ​ Year Ended ​ December 31, ​ 2017 2016 2017 2016 2015 ​ (in thousands) (in thousands) Category: Renewable energy LLCs (a) $ 32,063 $ 27,067 $ 10,578 $ 3,480 $ 3,936 Renewable energy notes receivable (b) 7,278 — 526 450 3,136 Limited partnerships (c) 26,367 23,852 2,632 5,263 (1,468 ) Bank holding companies (d) 4,500 4,500 343 343 343 Total other invested assets $ 70,208 $ 55,419 $ 14,079 $ 9,536 $ 5,947 (a) The Company’s Corporate and Other segment owns equity interests ranging from 2.6% to 32.8% in various LLCs whose principal objective is capital appreciation and income generation from owning and operating renewable energy production facilities (wind and solar). The LLCs are managed by an affiliate of the Company’s largest shareholder and the Company’s Chairman and Chief Executive Officer has invested in certain of these LLCs. The equity method is used to account for the Company’s LLC investments. Income for the LLCs primarily reflects adjustments to the carrying values of investments in renewable energy projects to their determined fair values. The fair value adjustments are included in revenues for the LLCs. Expenses for the LLCs are not significant and are comprised of administrative and interest expenses. The Company received cash distributions from these investments totaling $5.6 million and $2.4 million for the years ended December 31, 2017 and 2016 , respectively. ​ (b) The Company has invested in notes receivable for renewable energy projects. In 2017 , the Company invested $7.3 million in a note with an affiliate of the Company's largest shareholder. Interest on the note, which matures in 2021, is fixed at 15% . The Company is committed to investing another $1.5 million in the note and commitment fees of 2.5% are earned on the unfunded commitment. Interest income on the note was $526,000 for the year ended December 31, 2017 . In 2016, the outstanding balance of a $6.5 million note was fully repaid. Income prior to repayment of the note was $450,000 for the year ended December 31, 2016 . During the year ended December 31, 2015, the Company invested a total of $36.3 million in these notes and received maturities and repayments totaling $30.8 million . ​ (c) The Company owns investments in limited partnerships that invest in concentrated portfolios including publicly-traded small cap equities, loans of middle market private equity sponsored companies, equity tranches of collateralized loan obligations (CLOs), and tranches of distressed home loans. Income from the partnerships is recognized under the equity method of accounting. The Company’s Corporate and Other segment held investments in limited partnerships of $3.0 million and $2.6 million at December 31, 2017 and 2016 , and recognized investment income of $394,000 and $455,000 and investment losses of $510,000 for the years ended December 31, 2017 , 2016 and 2015 , respectively. The Chairman and Chief Executive Officer of the Company is an investor in one limited partnership held by the Corporate and Other segment. The Company’s Excess and Surplus Lines segment holds investments in limited partnerships of $23.4 million and $21.2 million at December 31, 2017 and 2016 , respectively. Investment income of $2.2 million and $4.8 million and investment losses of $958,000 were recognized on the investments for the years ended December 31, 2017 , 2016 , and 2015 , respectively. At December 31, 2017 , the Company’s Excess and Surplus Lines segment has outstanding commitments to invest another $625,000 in a limited partnership that invests in loans of middle market private equity sponsored companies and another $5.0 million in a limited partnership that invests in tranches of distressed home loans. The Chairman and former Chief Executive Officer of the Company and one of the Company's directors are investors in one limited partnership held by the Excess and Surplus Lines segment. 2. Investments (continued) ​ (d) The Company holds $4.5 million of subordinated notes issued by a bank holding company. Interest on the notes, which mature in 2023, is fixed at 7.6% per annum. Interest income on the notes was $343,000 in each of the years ended December 31, 2017 , 2016 and 2015 . The Company's Chairman and former Chief Executive Officer was previously the Lead Independent Director of the bank holding company and an investor in the bank holding company. Additionally, one of the Company’s directors is a former investor in the bank holding company and is currently a lender to the bank holding company.

Deferred Policy Acquisition C40

Deferred Policy Acquisition Costs (Tables)	12 Months Ended
	Dec. 31, 2017
Deferred Policy Acquisition Costs Disclosures [Abstract]
Schedule of deferred policy acquisition costs	​ An analysis of deferred policy acquisition costs is as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Balance at beginning of period $ 64,789 $ 60,754 $ 60,202 Policy acquisition costs deferred: ​ ​ ​ Commissions 107,387 92,736 90,342 Underwriting and other issue expenses 16,190 12,923 7,960 ​ 123,577 105,659 98,302 Amortization of policy acquisition costs (116,001 ) (101,624 ) (97,750 ) Net change 7,576 4,035 552 Balance at end of period $ 72,365 $ 64,789 $ 60,754

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of gross carrying amounts and accumulated amortization	The gross carrying amounts and accumulated amortization for each major specifically identifiable intangible asset class were as follows: ​ December 31, ​ 2017 2016 Weighted- Average Life (Years) Gross Accumulated Gross Accumulated ​ ​ (in thousands) Trademarks Indefinite $ 22,200 $ — $ 22,200 $ — Insurance licenses and authorities Indefinite 9,164 — 9,164 — Identifiable intangibles not subject to amortization ​ 31,364 — 31,364 — Broker relationships 24.6 11,611 4,641 11,611 4,044 Identifiable intangible assets subject to amortization ​ 11,611 4,641 11,611 4,044 ​ ​ $ 42,975 $ 4,641 $ 42,975 $ 4,044
Schedule of future estimated amortization	Future estimated amortization of specifically identifiable intangible assets as of December 31, 2017 is as follows ( in thousands) : 2018 $ 597 2019 597 2020 538 2021 363 2022 363 Thereafter 4,512 Total $ 6,970
Schedule of acquired indefinite-lived intangible assets	The table below summarizes the changes in the net carrying values of intangible assets by segment for 2017: December 31, 2016 December 31, 2017 ​ Net Carrying Amortization Impairment Net Carrying ​ (in thousands) Excess and Surplus Lines Trademarks $ 19,700 $ — $ — $ 19,700 Insurance licenses and authorities 4,900 — — 4,900 Broker relationships 6,689 (362 ) — 6,327 ​ 31,289 (362 ) — 30,927 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — — 4,265 Broker relationships 877 (235 ) — 642 ​ 7,642 (235 ) — 7,407 Total identifiable intangible assets $ 38,931 $ (597 ) $ — $ 38,334   ​ The table below summarizes the changes in the net carrying values of intangible assets by segment for 2016: December 31, 2015 December 31, 2016 ​ Net Carrying Amortization Impairment Net Carrying ​ (in thousands) Excess and Surplus Lines Trademarks $ 19,700 $ — $ — $ 19,700 Insurance licenses and authorities 4,900 — — 4,900 Broker relationships 7,051 (362 ) — 6,689 ​ 31,651 (362 ) — 31,289 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — — 4,265 Broker relationships 1,112 (235 ) — 877 ​ 7,877 (235 ) — 7,642 Total identifiable intangible assets $ 39,528 $ (597 ) $ — $ 38,931
Schedule of acquired finite-lived intangible assets	The table below summarizes the changes in the net carrying values of intangible assets by segment for 2017: December 31, 2016 December 31, 2017 ​ Net Carrying Amortization Impairment Net Carrying ​ (in thousands) Excess and Surplus Lines Trademarks $ 19,700 $ — $ — $ 19,700 Insurance licenses and authorities 4,900 — — 4,900 Broker relationships 6,689 (362 ) — 6,327 ​ 31,289 (362 ) — 30,927 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — — 4,265 Broker relationships 877 (235 ) — 642 ​ 7,642 (235 ) — 7,407 Total identifiable intangible assets $ 38,931 $ (597 ) $ — $ 38,334   ​ The table below summarizes the changes in the net carrying values of intangible assets by segment for 2016: December 31, 2015 December 31, 2016 ​ Net Carrying Amortization Impairment Net Carrying ​ (in thousands) Excess and Surplus Lines Trademarks $ 19,700 $ — $ — $ 19,700 Insurance licenses and authorities 4,900 — — 4,900 Broker relationships 7,051 (362 ) — 6,689 ​ 31,651 (362 ) — 31,289 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — — 4,265 Broker relationships 1,112 (235 ) — 877 ​ 7,877 (235 ) — 7,642 Total identifiable intangible assets $ 39,528 $ (597 ) $ — $ 38,931

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	Property and equipment, net of accumulated depreciation, consists of the following: ​ December 31, ​ 2017 2016 ​ (in thousands) Building, leased for which the Company has been deemed the owner for accounting purposes (Note 21) $ 30,902 $ 30,902 Electronic data processing hardware and software 6,156 4,460 Furniture and equipment 2,659 2,706 Property and equipment, cost basis 39,717 38,068 Accumulated depreciation (14,716 ) (13,388 ) Property and equipment, net $ 25,001 $ 24,680

Reserve for Losses and Loss A43

Reserve for Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Abstract]
Schedule of reconciliation of beginning and ending reserve balances for losses and loss adjustment expenses	The following table provides a reconciliation of the beginning and ending reserve balances for losses and loss adjustment expenses, net of reinsurance, to the gross amounts reported in the consolidated balance sheets: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period $ 761,128 $ 653,534 $ 589,042 Add: Incurred losses and loss adjustment expenses net of reinsurance: ​ ​ ​ Current year 533,905 349,137 295,334 Prior years 21,472 (23,716 ) (16,318 ) Total incurred losses and loss and adjustment expenses 555,377 325,421 279,016 Deduct: Loss and loss adjustment expense payments net of reinsurance: ​ ​ ​ Current year 103,205 39,473 31,957 Prior years 223,475 178,354 182,567 Total loss and loss adjustment expense payments 326,680 217,827 214,524 Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period 989,825 761,128 653,534 Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period 302,524 182,737 131,788 Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period $ 1,292,349 $ 943,865 $ 785,322
Schedule of Incurred losses and loss adjustment expenses, net of reinsurance	Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 120,760 $ 119,736 $ 116,817 $ 112,227 $ 107,927 $ 102,122 $ 99,169 $ 98,765 $ 96,687 $ 95,930 2009 114,834 110,783 106,480 98,502 86,691 81,764 83,431 83,846 85,470 2010 78,424 80,569 78,117 73,035 69,080 69,964 70,294 70,913 2011 111,190 119,927 114,473 106,564 106,381 106,130 106,643 2012 97,908 98,672 97,829 96,497 97,306 99,619 2013 96,729 96,064 85,433 81,009 82,830 2014 114,942 104,092 90,267 82,232 2015 126,443 113,417 104,847 2016 138,507 125,093 2017 144,349 Total $ 997,926   6. Reserve for Losses and Loss Adjustment Expenses (continued) Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 16,922 $ 35,228 $ 53,566 $ 69,585 $ 80,385 $ 83,937 $ 87,630 $ 91,750 $ 91,768 $ 92,332 2009 29,860 41,687 51,731 61,548 67,293 71,245 76,091 79,014 81,496 2010 13,673 26,418 35,812 45,641 52,071 57,371 61,307 64,214 2011 27,684 53,109 72,732 81,696 90,884 94,998 98,684 2012 6,944 33,757 49,604 63,216 74,869 82,545 2013 3,867 14,509 30,382 44,421 59,641 2014 3,412 16,969 28,212 43,891 2015 4,048 17,164 34,801 2016 5,180 22,852 2017 5,290 Total $ 585,746 All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance $ 9,033 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 421,213 Excess and Surplus Lines — Commercial Auto Incurred losses and adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 1,255 $ 1,300 $ 1,451 $ 1,351 $ 1,301 2014 20,487 14,071 17,233 18,953 2015 30,109 33,113 35,149 2016 74,340 109,286 2017 207,355 Total $ 372,044 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 60 $ 1,182 $ 1,285 $ 1,291 $ 1,275 2014 6,166 8,645 12,679 16,359 2015 8,356 15,234 24,282 2016 18,295 54,054 2017 41,467 Total $ 137,437 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 234,607 6. Reserve for Losses and Loss Adjustment Expenses (continued) Specialty Admitted — Individual Risk Workers’ Compensation Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 37,602 $ 36,641 $ 38,654 $ 38,807 $ 40,330 $ 40,552 $ 39,321 $ 39,320 $ 39,131 $ 39,001 2009 28,691 28,526 27,535 28,116 27,795 26,171 26,169 26,232 26,194 2010 27,209 28,736 30,464 30,373 28,963 28,938 27,590 27,098 2011 37,834 41,421 40,154 38,999 38,311 37,455 36,594 2012 32,116 32,420 31,490 29,689 28,255 28,174 2013 12,525 13,668 12,786 11,578 10,907 2014 16,638 16,652 14,620 13,890 2015 20,938 21,274 19,741 2016 21,678 20,299 2017 24,869 Total $ 246,767 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 10,479 $ 22,664 $ 28,436 $ 31,432 $ 33,885 $ 36,778 $ 37,905 $ 37,933 $ 37,965 $ 37,976 2009 7,277 16,945 21,095 22,646 24,231 24,192 24,350 24,418 25,660 2010 7,157 16,245 21,805 23,898 25,210 25,477 26,345 26,352 2011 10,123 23,127 29,021 33,204 34,240 34,287 34,334 2012 9,222 20,308 24,755 26,435 26,897 26,932 2013 4,487 8,723 9,846 10,246 10,263 2014 4,633 10,648 12,041 12,236 2015 6,604 13,285 15,118 2016 4,664 10,227 2017 6,546 Total $ 205,644 All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance $ 357 Outstanding losses and loss adjustment expenses assumed from involuntary workers’ compensation pools $ 4,830 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 46,310 Specialty Admitted — Fronting and Programs Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 104 $ 80 $ 52 $ 52 $ 52 2014 3,460 3,468 3,818 3,425 2015 7,136 9,632 9,358 2016 11,542 15,670 2017 21,229 Total $ 49,734 6. Reserve for Losses and Loss Adjustment Expenses (continued) Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2013 $ 28 $ 52 $ 52 $ 52 $ 52 2014 883 1,687 2,369 2,728 2015 2,058 4,666 6,165 2016 1,894 5,123 2017 1,223 Total $ 15,291 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 34,443 Casualty Reinsurance Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 1,418 $ 3,215 $ 2,897 $ 2,859 $ 3,199 $ 3,473 $ 3,462 $ 3,467 $ 3,470 $ 3,488 2009 34,587 28,244 24,125 26,458 27,078 27,116 26,989 26,931 26,980 2010 64,413 60,476 61,068 62,714 61,344 60,949 60,978 61,619 2011 114,908 103,123 97,366 97,812 98,993 99,282 101,276 2012 148,251 132,388 131,281 135,594 136,813 139,978 2013 133,230 130,361 131,352 134,446 137,801 2014 118,881 115,927 114,636 116,981 2015 119,157 108,870 108,699 2016 112,759 105,533 2017 134,628 Total $ 936,983 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2008 $ 320 $ 616 $ 999 $ 1,595 $ 2,087 $ 2,478 $ 2,706 $ 2,929 $ 3,029 $ 3,074 2009 6,487 9,926 12,956 16,466 19,672 21,646 23,024 23,796 24,649 2010 21,918 31,500 38,430 44,921 49,263 52,761 54,659 57,013 2011 48,688 61,922 68,616 78,164 87,267 90,287 94,627 2012 73,124 81,859 97,215 113,943 121,026 128,567 2013 59,756 75,094 93,902 108,396 119,256 2014 41,421 58,601 76,302 89,899 2015 40,021 53,986 68,002 2016 36,268 50,905 2017 47,739 Total $ 683,731 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 253,252
Schedule of reserve for losses and loss adjustment expenses	The reconciliation of the net incurred and paid claims development tables to the reserve for losses and loss adjustment expenses in the consolidated balance sheet at December 31, 2017 is as follows (in thousands): E&S – excluding commercial auto $ 421,213 E&S – commercial auto 234,607 Specialty Admitted – individual risk workers’ compensation 46,310 Specialty Admitted – fronting and programs 34,443 Casualty Reinsurance 253,252 Net reserve for losses and loss adjustment expenses 989,825 Reinsurance recoverables on unpaid losses 302,524 Gross reserve for losses and loss adjustment expenses $ 1,292,349
Schedule of average annual percentage payouts of incurred claims by age, net of reinsurance	The following is unaudited supplementary information about average annual percentage payouts of incurred claims by age, net of reinsurance, for the Excess and Surplus Lines segment and the Specialty Admitted Insurance segments as of December 31, 2017 . The Specialty Admitted Insurance segment’s first full year of writing fronting and programs business was 2014, so the average annual percentage payouts for fronting and programs only shows four years of payout information. ​ Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 E&S – excluding commercial auto 10.3 % 16.6 % 17.1 % 17.5 % 14.4 % 7.7 % 5.5 % 3.3 % 2.5 % 1.1 % E&S – commercial auto 21.6 % 40.4 % 15.1 % 13.9 % 7.1 % Specialty Admitted – individual risk workers’ compensation 28.0 % 32.7 % 13.6 % 6.1 % 3.4 % 1.4 % 1.0 % 0.1 % 0.5 % 0.0 % Specialty Admitted – fronting and programs 12.2 % 13.4 % 4.4 % 0.7 % Casualty Reinsurance 28.8 % 21.1 % 14.8 % 10.3 % 7.3 % 5.0 % 3.5 % 2.4 % 1.7 % 1.3 %
Schedule of incurred but not reported liabilities and claims frequency	The table below provides information on IBNR liabilities and claims frequency for: (1) the Excess and Surplus Lines segment split between commercial auto and all non commercial auto, and (2) the Specialty Admitted Insurance segment split between individual risk workers’ compensation and fronting and programs: 6. Reserve for Losses and Loss Adjustment Expenses (continued) Excess and Surplus Lines — Excluding Commercial Auto Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2008 $ 87,112 $ 3,169 2,283 2009 58,371 2,831 1,677 2010 60,883 3,563 1,367 2011 55,514 5,834 1,443 2012 82,142 8,692 1,725 2013 82,830 14,382 2,223 2014 82,232 23,241 2,024 2015 104,847 46,275 2,292 2016 125,093 77,940 2,468 2017 144,349 123,432 2,052 Excess and Surplus Lines — Commercial Auto Accident Year Incurred Losses and Loss Adj Expenses IBNR Cumulative # of Reported Claims ​ ($ in thousands) 2013 $ 1,301 $ 26 54 2014 18,953 791 7,748 2015 35,149 5,098 41,640 2016 109,286 20,321 87,920 2017 207,355 105,097 125,863 Specialty Admitted - Individual Risk Workers’ Compensation Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2008 $ 39,001 $ 663 2,012 2009 26,194 534 1,490 2010 27,098 700 1,604 2011 36,594 1,632 1,814 2012 28,174 809 1,323 2013 10,907 502 540 2014 13,890 1,506 850 2015 19,741 3,431 975 2016 20,299 6,409 831 2017 24,803 4,617 1,036   6. Reserve for Losses and Loss Adjustment Expenses (continued) Specialty Admitted — Fronting and Programs Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2013 $ 52 $ — 22 2014 3,041 384 857 2015 8,081 1,277 1,347 2016 12,050 3,620 2,730 2017 8,523 12,706 5,421

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Schedule of premiums written, premiums earned, and losses and loss adjustment expenses	Premiums written, premiums earned, and losses and loss adjustment expenses incurred are summarized as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Written premiums: ​ ​ ​ Direct $ 843,719 $ 549,700 $ 395,816 Assumed 238,186 187,698 176,378 Ceded (315,279 ) (179,690 ) (101,162 ) Net $ 766,626 $ 557,708 $ 471,032 Earned premiums: ​ ​ ​ Direct $ 842,182 $ 483,166 $ 367,340 Assumed 173,472 164,771 181,326 Ceded (274,545 ) (132,274 ) (87,461 ) Net $ 741,109 $ 515,663 $ 461,205 Losses and loss adjustment expenses: ​ ​ ​ Direct $ 626,318 $ 314,920 $ 201,755 Assumed 142,818 108,029 127,276 Ceded (213,759 ) (97,528 ) (50,015 ) Net $ 555,377 $ 325,421 $ 279,016

Junior Subordinated Debt (Table

Junior Subordinated Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Subordinated Borrowings [Abstract]
Schedule of Junior Subordinated Debt	The following table summarizes the nature and terms of the Junior Subordinated Debt and Trust Preferred Securities: ​ James River Capital Trust I James River James River Capital Trust III James River Franklin ​ ($ in thousands) Issue date May 26, December 15, June 15, December 11, January 10, Principal amount of Trust Preferred Securities $7,000 $15,000 $20,000 $54,000 $30,000 Principal amount of Junior Subordinated Debt $7,217 $15,464 $20,619 $55,670 $30,928 Carrying amount of Junior Subordinated Debt net of repurchases $7,217 $15,464 $20,619 $44,827 $15,928 Maturity date of Junior Subordinated Debt, unless accelerated earlier May 24, December 15, June 15, December 15, March 15, Trust common stock $217 $464 $619 $1,670 $928 Interest rate, per annum Three-Month LIBOR plus 4.0% Three-Month LIBOR plus 3.4% Three-Month LIBOR plus 3.0% Three-Month LIBOR plus 3.1% Three-Month LIBOR plus 4.0%

Capital Stock (Tables)

Capital Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Schedule of cash dividends declared	The Board of Directors declared the following cash dividends in 2017 , 2016 , and 2015 : Date of Declaration Dividend per Common Share Payable to Shareholders of Record on Payment Date Total Amount 2017 February 14, 2017 $0.30 March 13, 2017 March 31, 2017 $8.9 May 2, 2017 $0.30 June 12, 2017 June 30, 2017 $8.9 August 1, 2017 $0.30 September 11, 2017 September 29, 2017 $8.9 November 1, 2017 $0.30 December 15, 2017 December 28, 2017 $9.0 November 1, 2017 $0.50 December 15, 2017 December 28, 2017 $15.0 Total $1.70 $50.7 2016 ​ ​ ​ ​ February 16, 2016 $0.20 March 14, 2016 March 28, 2016 $5.8 May 3, 2016 $0.20 June 13, 2016 June 30, 2016 $5.9 August 3, 2016 $0.20 September 12, 2016 September 30, 2016 $5.9 November 1, 2016 $0.30 December 16, 2016 December 29, 2016 $8.9 November 1, 2016 $1.35 December 16, 2016 December 29, 2016 $39.8 Total $2.25 ​ ​ $66.3 2015 February 17, 2015 $0.16 March 16, 2015 March 31, 2015 $4.6 May 5, 2015 $0.16 June 15, 2015 June 30, 2015 $4.6 August 5, 2015 $0.16 September 14, 2015 September 30, 2015 $4.7 November 3, 2015 $0.16 December 14, 2015 December 28, 2015 $4.7 November 3, 2015 $1.00 December 14, 2015 December 28, 2015 $29.2 Total $1.64 $47.8

Equity Awards (Tables)

Equity Awards (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of summary of option activity	The following table summarizes the option activity for the periods ended December 31, 2017 , 2016 , and 2015 : ​ Year Ended December 31, ​ 2017 2016 2015 ​ Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Outstanding: ​ ​ ​ ​ ​ ​ Beginning of year 2,234,699 $ 22.84 2,058,085 $ 18.11 3,104,768 $ 17.27 Granted 205,244 $ 42.24 706,203 $ 32.07 10,627 $ 24.32 Exercised (898,218 ) $ 18.53 (496,550 ) $ 16.02 (1,047,500 ) $ 15.66 Forfeited (62,489 ) $ 30.80 (33,039 ) $ 27.68 (9,810 ) $ 21.00 End of year 1,479,236 $ 27.81 2,234,699 $ 22.84 2,058,085 $ 18.11 Exercisable, end of year 846,371 $ 22.35 1,207,479 $ 18.14 1,217,903 $ 16.85
Schedule of value of the options granted was estimated at the date of grant using the Black-Scholes-Merton	The value of the options granted was estimated at the date of grant using the Black-Scholes-Merton option pricing model using the following assumptions: ​ Year Ended December 31, ​ 2017 2016 2015 Range of risk-free interest rates 1.97 % 1.23 % 1.54 % Dividend yield 2.85 % 2.50 % 4.00 % Expected share price volatility 27.39 % 25.00 % 25.00 % Expected life 5.0 years 5.0 years 5.0 years
Schedule of summary of RSU activity	The following table summarizes the RSU activity for the years ended December 31, 2017 , 2016 , and 2015 : ​ Year Ended December 31, ​ 2017 2016 2015 ​ Shares Weighted- Average Grant Date Fair Value Shares Weighted- Average Grant Date Fair Value Shares Weighted- Average Grant Date Fair Value Unvested, beginning of year 196,800 $ 24.38 234,922 $ 21.00 340,474 $ 21.00 Granted 137,034 $ 42.20 60,291 $ 32.03 — $ — Vested (132,764 ) $ 24.24 (98,413 ) $ 21.00 (105,552 ) $ 21.00 Forfeited (22,188 ) $ 26.06 — $ — — $ — Unvested, end of year 178,882 $ 37.93 196,800 $ 24.38 234,922 $ 21.00
Schedule of summary of amount of expense and related tax benefit	The amount of expense and related tax benefit is summarized below: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Share based compensation expense $ 7,688 $ 5,492 $ 3,735 U.S. tax benefit on share based compensation expense $ 2,093 $ 1,532 $ 1,025

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of weighted-average tax rate as well as a reconciliation to total tax expense	A reconciliation of the difference between the Company’s Federal income tax provision on U.S. income and the expected Federal tax provision on U.S. income using the weighted-average tax rate as well as a reconciliation to total tax expense is as follows: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Federal income tax expense at applicable statutory rates $ 17,541 $ 6,401 $ 5,317 Tax-exempt investment income (586 ) (643 ) (858 ) Dividends received deduction (792 ) (880 ) (867 ) Excess tax benefits on share based compensation (2,114 ) — — Effect of tax rate reduction on deferred tax liability (3,498 ) — — Other (90 ) (6 ) 379 Federal income tax expense $ 10,461 $ 4,872 $ 3,971 U.S. state income tax expense (benefit) 65 — (192 ) U.S. dividend withholding tax 1,053 — 2,500 Total income tax expense $ 11,579 $ 4,872 $ 6,279
Schedule of deferred tax asset at the current prevailing corporate income tax rate	The significant components of the net deferred tax liability at the corporate income tax rate of 21% for the year ended December 31, 2017 and 35% for December 31, 2016 are summarized as follows: ​ December 31, ​ 2017 2016 ​ (in thousands) Deferred tax assets: ​ ​ Accrued compensation expenses $ 1,712 $ 1,604 Reserve for losses and loss adjustment expenses 3,036 4,810 Unearned premiums 1,709 2,859 Share based compensation 1,143 2,281 Allowance for doubtful accounts 579 748 Deferred policy acquisition costs 1,509 1,983 Property and equipment 1,601 2,649 AMT credit — 464 Invested asset impairments 791 291 Other 1,564 2,946 Total deferred tax assets 13,644 20,635 Deferred tax liabilities: Intangible assets 7,521 12,554 Net unrealized gains 2,578 1,586 Deferred gain on extinguishment of debt 63 212 Equity method investments 8,348 8,668 Other 381 517 Total deferred tax liabilities 18,891 23,537 Net deferred tax liabilities $ (5,247 ) $ (2,902 )

Other Operating Expenses and 49

Other Operating Expenses and Other Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Other Operating Expenses And Other Expenses [Abstract]
Schedule of other operating expenses	Other operating expenses consist of the following: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Amortization of policy acquisition costs $ 116,001 $ 101,624 $ 97,750 Other underwriting expenses of the insurance segments 55,662 48,771 41,499 Other operating expenses of the Corporate and Other segment 25,330 20,433 18,554 Total $ 196,993 $ 170,828 $ 157,803

Commitments and Contingent Li50

Commitments and Contingent Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum payments under non-cancelable operating leases	As of December 31, 2017 , future minimum payments under non-cancelable operating leases for office space are as follows (in thousands): 2018 $ 4,025 2019 3,644 2020 3,342 2021 3,154 2022 2,768 Thereafter 7,658 ​ $ 24,591

Other Comprehensive Income (L51

Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of components of comprehensive income	The following table summarizes the components of other comprehensive (loss) income: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Unrealized gains (losses) arising during the period, before U.S. income taxes $ 11,091 $ 2,790 $ (13,707 ) U.S. income taxes (2,374 ) (414 ) 517 Unrealized gains (losses) arising during the period, net of U.S. income taxes 8,717 2,376 (13,190 ) Less reclassification adjustment: Net realized investment (losses) gains (835 ) 6,572 2,402 U.S. income taxes 333 (2,137 ) (422 ) Reclassification adjustment for investment (losses) gains realized in net income (502 ) 4,435 1,980 Other comprehensive income (loss) $ 9,219 $ (2,059 ) $ (15,170 )

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Schedule of summary of company's segment results	Segment results are reported prior to the effects of the intercompany reinsurance agreements between the Company’s insurance subsidiaries. All gross written premiums and net earned premiums for all periods presented were generated from policies issued to U.S. based insureds. ​ Excess and Surplus Lines Specialty Admitted Insurance Casualty Reinsurance Corporate and Other Total ​ (in thousands) As of and for the Year Ended December 31, 2017 ​ ​ ​ ​ ​ ​ Gross written premiums $ 530,120 $ 316,430 $ 235,355 $ — ​ $ 1,081,905 Net earned premiums 463,521 68,110 209,478 — ​ 741,109 Segment revenues 493,853 70,366 240,751 12,655 ​ 817,625 Net investment income 15,014 2,532 31,507 12,066 ​ 61,119 Interest expense — — — 8,974 ​ 8,974 Underwriting profit (loss) of operating segments 29,693 3,166 (1,765 ) — ​ 31,094 Segment goodwill 181,831 — — — ​ 181,831 Segment assets 843,486 439,416 1,379,866 93,927 ​ 2,756,695 17. Segment Information (continued) ​ ​ Excess and Surplus Lines Specialty Admitted Insurance Casualty Reinsurance Corporate and Other Total ​ As of and for the Year Ended December 31, 2016 ​ ​ ​ ​ ​ Gross written premiums $ 370,844 $ 182,221 $ 184,333 $ — $ 737,398 Net earned premiums 301,404 52,281 161,978 — 515,663 Segment revenues 331,090 55,412 190,064 9,661 586,227 Net investment income 18,051 2,542 27,257 4,788 52,638 Interest expense — — — 8,448 8,448 Underwriting profit (loss) of operating segments 47,235 2,872 (194 ) — 49,913 Segment goodwill 181,831 — — — 181,831 Segment assets 740,144 300,519 1,195,230 110,640 2,346,533 As of and for the Year Ended December 31, 2015 ​ ​ ​ ​ ​ Gross written premiums $ 308,717 $ 90,978 $ 172,499 $ — $ 572,194 Net earned premiums 240,878 42,206 178,121 — 461,205 Segment revenues 255,529 44,791 197,586 7,015 504,921 Net investment income 13,427 2,316 22,706 6,386 44,835 Interest expense — — — 6,999 6,999 Underwriting profit (loss) of operating segments 47,607 1,074 (2,558 ) — 46,123 Segment goodwill 181,831 — — — 181,831 Segment assets 671,143 173,094 1,108,278 102,982 2,055,497
Schedule of underwriting profit (loss) of operating segments by individual segment and reconciliation to consolidated income before taxes	The following table reconciles the underwriting profit (loss) of insurance segments by individual segment to income before taxes: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Underwriting profit (loss) of the operating segments: ​ ​ ​ ​ ​ Excess and Surplus Lines $ 29,693 $ 47,235 $ 47,607 Specialty Admitted Insurance 3,166 2,872 1,074 Casualty Reinsurance (1,765 ) (194 ) (2,558 ) Total underwriting profit of operating segments 31,094 49,913 46,123 Other operating expenses of the Corporate and Other segment (25,330 ) (20,433 ) (18,554 ) Underwriting profit 5,764 29,480 27,569 Net investment income 61,119 52,638 44,835 Net realized investment (losses) gains (1,989 ) 7,565 (4,547 ) Other income 361 295 245 Other expenses (539 ) (1,590 ) (730 ) Interest expense (8,974 ) (8,448 ) (6,999 ) Amortization of intangible assets (597 ) (597 ) (597 ) Income before income taxes $ 55,145 $ 79,343 $ 59,776
Schedule of gross written premiums by segment and underwriting division	Gross written premiums by segment and underwriting division are presented below: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Commercial Auto $ 247,960 ​ $ 110,050 ​ $ 73,770 Manufacturers and Contractors 85,719 ​ 83,279 ​ 78,315 Excess Casualty 51,160 ​ 43,574 ​ 32,458 General Casualty 38,097 ​ 36,858 ​ 30,972 Energy 29,704 ​ 29,709 ​ 30,623 Allied Health 19,181 ​ 14,413 ​ 13,513 Excess Property 14,447 ​ 14,083 ​ 12,498 Life Sciences 12,981 ​ 11,132 ​ 8,917 Small Business 11,307 ​ 9,104 ​ 6,916 Professional Liability 6,326 ​ 8,361 ​ 10,046 Environmental 7,920 ​ 5,321 ​ 4,437 Medical Professionals 2,297 ​ 2,739 ​ 3,585 Sports and Entertainment 3,021 ​ 2,221 ​ 2,667 Total Excess and Surplus Lines segment 530,120 ​ 370,844 ​ 308,717 Specialty Admitted Insurance segment 316,430 ​ 182,221 ​ 90,978 Casualty Reinsurance segment 235,355 ​ 184,333 ​ 172,499 Total $ 1,081,905 ​ $ 737,398 ​ $ 572,194

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of assets measured at fair value on a recurring basis	Assets measured at fair value on a recurring basis as of December 31, 2017 are summarized below: Fair Value Measurements Using ​ Quoted Prices in Active Markets for Identical Assets Level 1 Significant Other Observable Inputs Level 2 ​ Significant Unobservable Inputs Level 3 Total ​ (in thousands) Available-for-sale securities ​ ​ ​ ​ Fixed maturity securities: ​ ​ ​ ​ State and municipal $ — $ 144,366 $ — $ 144,366 Residential mortgage-backed — 158,661 — 158,661 Corporate — 413,721 — 413,721 Commercial mortgage and asset-backed — 177,931 4,680 182,611 Obligations of U.S. government corporations and agencies — 35,847 — 35,847 U.S. Treasury securities and obligations guaranteed by the U.S. government 78,265 609 — 78,874 Redeemable preferred stock — 2,018 — 2,018 Total fixed maturity securities 78,265 933,153 4,680 1,016,098 Equity securities: Preferred stock — 66,281 — 66,281 Common stock 15,507 734 — 16,241 Total equity securities 15,507 67,015 — 82,522 Total available-for-sale securities $ 93,772 $ 1,000,168 $ 4,680 $ 1,098,620 Trading securities: Fixed maturity securities $ — $ 3,808 $ — $ 3,808 Short-term investments $ 1,000 $ 35,804 $ — $ 36,804 18. Fair Value Measurements (continued) Assets measured at fair value on a recurring basis as of December 31, 2016 are summarized below: ​ Fair Value Measurements Using ​ Quoted Prices in Active Markets for Identical Assets Level 1 Significant Other Observable Inputs Level 2 ​ Significant Unobservable Inputs Level 3 Total ​ ​ (in thousands) Available-for-sale securities ​ ​ ​ ​ ​ ​ Fixed maturity securities: ​ ​ ​ ​ ​ ​ State and municipal $ — ​ $ 105,841 ​ $ — $ 105,841 Residential mortgage-backed — ​ 150,798 ​ — 150,798 Corporate — ​ 378,448 ​ — 378,448 Commercial mortgage and asset-backed — ​ 163,047 ​ 5,000 168,047 Obligations of U.S. government corporations and agencies — ​ 65,014 ​ — 65,014 U.S. Treasury securities and obligations guaranteed by the U.S. government 70,465 ​ 655 ​ — 71,120 Redeemable preferred stock — ​ 1,809 ​ — 1,809 Total fixed maturity securities 70,465 ​ 865,612 ​ 5,000 941,077 Equity securities: Preferred stock — ​ 64,827 ​ — 64,827 Common stock 10,840 ​ 734 ​ — 11,574 Total equity securities 10,840 ​ 65,561 ​ — 76,401 Total available-for-sale securities $ 81,305 ​ $ 931,173 ​ $ 5,000 $ 1,017,478 Trading securities: Fixed maturity securities $ 1,250 ​ $ 3,813 ​ $ — $ 5,063 Short-term investments $ 1,100 ​ $ 49,744 ​ $ — $ 50,844
Schedule of fair value on a recurring basis using significant unobservable inputs (Level 3)	A reconciliation of the beginning and ending balances of available-for-sale fixed maturity securities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) is shown below: ​ Year Ended December 31, ​ 2017 2016 2015 ​ (in thousands) Beginning balance $ 5,000 $ 5,000 ​ $ — Transfers out of Level 3 — — ​ — Transfers in to Level 3 — — ​ — Purchases — — ​ 5,000 Sales — — ​ — Maturities and calls (320 ) — — Amortization of discount — — ​ — Total gains or losses (realized/unrealized): — — ​ — Included in earnings — — ​ — Included in other comprehensive income — — ​ — Ending balance $ 4,680 $ 5,000 ​ $ 5,000
Schedule of assets measured at fair value on a nonrecurring basis	Assets measured at fair value on a nonrecurring basis are summarized below: ​ Fair Value Measurements Using ​ Quoted Prices in Active Markets for Identical Assets Level 1 Significant Other Observable Inputs Level 2 Significant Unobservable Inputs Level 3 ​ Total ​ (in thousands) December 31, 2017 ​ ​ ​ ​ ​ ​ ​ Bank loan participations held-for-investment $ — $ — ​ $ 5,111 ​ $ 5,111 December 31, 2016 Bank loan participations held-for-investment $ — $ — ​ $ 4,849 ​ $ 4,849
Schedule of carrying value and fair value	The carrying values and fair values of financial instruments are summarized below: ​ December 31, ​ 2017 2016 ​ Carrying Value Fair Value Carrying Value Fair Value ​ (in thousands) Assets ​ ​ ​ ​ ​ Available-for-sale: ​ ​ ​ ​ Fixed maturity securities $ 1,016,098 ​ $ 1,016,098 $ 941,077 $ 941,077 Equity securities 82,522 ​ 82,522 76,401 76,401 Trading: ​ ​ ​ ​ ​ Fixed maturity securities 3,808 ​ 3,808 5,063 5,063 Bank loan participations held-for-investment 238,214 ​ 236,532 203,526 203,123 Cash and cash equivalents 163,495 ​ 163,495 109,784 109,784 Short-term investments 36,804 ​ 36,804 50,844 50,844 Other invested assets – notes receivable 11,778 ​ 17,104 4,500 6,008 Liabilities ​ ​ ​ ​ ​ Senior debt 98,300 ​ 97,884 88,300 85,404 Junior subordinated debt 104,055 ​ 116,569 104,055 99,397

Statutory Matters (Tables)

Statutory Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Statutory Matters [Abstract]
Schedule of statutory accounting	Combined net income, statutory capital and surplus and minimum required statutory capital and surplus, as determined in accordance with statutory accounting practices, for the U.S. insurance subsidiaries as of December 31, 2017 , 2016 , and 2015 and for the years then ended are summarized as follows: 19. Statutory Matters (continued) ​ ​ 2017 2016 2015 ​ (in thousands) Statutory net income $ 31,881 $ 13,468 $ 14,747 Statutory capital and surplus 219,132 184,859 176,884 Minimum required statutory capital and surplus 25,000 25,000 25,000

Unaudited Selected Quarterly 55

Unaudited Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Schedule of unaudited quarterly results of operations	The following is a summary of the unaudited quarterly results of operations: ​ 2017 Quarter 2017 ​ First Second Third Fourth Year ​ (in thousands, except per share data) Gross written premiums $ 224,179 $ 281,475 $ 338,351 $ 237,900 ​ $ 1,081,905 Total revenues 176,402 202,394 220,866 217,963 ​ 817,625 Net income 18,450 14,541 10,351 224 ​ 43,566 Comprehensive income (loss) 22,484 21,249 11,067 (2,015 ) ​ 52,785 Earnings per share: ​ ​ ​ ​ ​ ​ Basic $ 0.63 $ 0.49 $ 0.35 $ 0.01 ​ $ 1.48 Diluted $ 0.61 $ 0.48 $ 0.34 $ 0.01 ​ $ 1.44 For the second quarter of 2017, the Casualty Reinsurance segment was impacted by a correction of the accrual for a contingent commission liability and related commission expense. As a result of this correction, net income for the second quarter of 2017 was reduced by $2.0 million . 23. Unaudited Selected Quarterly Financial Data (continued) 2016 Quarter 2016 First Second Third Fourth Year (in thousands, except per share data) Gross written premiums $ 133,071 $ 170,671 $ 260,166 $ 173,490 $ 737,398 Total revenues 131,329 134,511 151,365 169,022 586,227 Net income 12,837 14,596 21,366 25,672 74,471 Comprehensive income (loss) 28,457 27,388 19,694 (3,127 ) 72,412 Earnings per share: ​ ​ ​ ​ ​ Basic $ 0.44 $ 0.50 $ 0.73 $ 0.88 $ 2.56 Diluted $ 0.43 $ 0.49 $ 0.71 $ 0.85 $ 2.49

Accounting Policies - Narrative

Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017USD ($)Companyloanshares	Dec. 31, 2016USD ($)loan	Dec. 31, 2015USD ($)	Mar. 31, 2018USD ($)
Accounting Policies [Line Items]
Number of bank loans on nonaccrual status | loan	0	0
Allowance for doubtful accounts	$ 2,800,000	$ 2,100,000
Bad debt expense	1,000,000	813,000	$ 1,100,000
Allowance for doubtful accounts receivables written off	408,000	1,500,000	268,000
Fee income	17,000,000	10,100,000	3,200,000
Investment in variable interest entities	$ 32,100,000	27,100,000
Common share equivalents excluded from calculations of diluted earnings per share | shares	174,380
Increase in net cash provided by operating activities		3,200,000	3,600,000
Increase in net cash provided by financing activities		3,200,000	$ 3,600,000
Minimum
Accounting Policies [Line Items]
Intangible assets, useful life (in years)	7 years
Property and equipment, estimated useful lives (in years)	3 years
Maximum
Accounting Policies [Line Items]
Intangible assets, useful life (in years)	27 years 6 months
Property and equipment, estimated useful lives (in years)	10 years
Receivables greater than 90 days past due
Accounting Policies [Line Items]
Premiums receivable, past due	$ 2,600,000	$ 2,000,000
United States
Accounting Policies [Line Items]
Number of insurance companies | Company	6
Subsequent event | Retained Earnings | ASU 2016-01
Accounting Policies [Line Items]
Increase (decrease) from reclassification				$ 4,700,000
Subsequent event | Retained Earnings | ASU 2018-02
Accounting Policies [Line Items]
Increase (decrease) from reclassification				(710,000)
Subsequent event | AOCI | ASU 2016-01
Accounting Policies [Line Items]
Increase (decrease) from reclassification				4,700,000
Subsequent event | AOCI | ASU 2018-02
Accounting Policies [Line Items]
Increase (decrease) from reclassification				$ (710,000,000)

Accounting Policies - Schedule

Accounting Policies - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Income	$ 224	$ 10,351	$ 14,541	$ 18,450	$ 25,672	$ 21,366	$ 14,596	$ 12,837	$ 43,566	$ 74,471	$ 53,497
Weighted average common shares
Basic (in shares)									29,461,717	29,063,075	28,662,051
Common share equivalents (in shares)									811,432	831,303	672,867
Diluted (in shares)									30,273,149	29,894,378	29,334,918
Earnings per share
Basic earnings per share (in dollars per share)	$ 0.01	$ 0.35	$ 0.49	$ 0.63	$ 0.88	$ 0.73	$ 0.50	$ 0.44	$ 1.48	$ 2.56	$ 1.87
Common share equivalents (in dollars per share)									(0.04)	(0.07)	(0.05)
Diluted earnings per share (in dollars per share)	$ 0.01	$ 0.34	$ 0.48	$ 0.61	$ 0.85	$ 0.71	$ 0.49	$ 0.43	$ 1.44	$ 2.49	$ 1.82

Investments - Schedule of Avail

Investments - Schedule of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 1,083,980	$ 1,014,765
Gross Unrealized Gains	22,394	16,300
Gross Unrealized Losses	(7,754)	(13,587)
Fair Value	1,098,620	1,017,478
Fixed maturity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	1,008,662	940,212
Gross Unrealized Gains	14,564	11,797
Gross Unrealized Losses	(7,128)	(10,932)
Fair Value	1,016,098	941,077
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	139,382	101,793
Gross Unrealized Gains	5,587	5,032
Gross Unrealized Losses	(603)	(984)
Fair Value	144,366	105,841
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	160,379	152,703
Gross Unrealized Gains	723	1,070
Gross Unrealized Losses	(2,441)	(2,975)
Fair Value	158,661	150,798
Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	408,857	379,727
Gross Unrealized Gains	7,503	4,382
Gross Unrealized Losses	(2,639)	(5,661)
Fair Value	413,721	378,448
Commercial mortgage and asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	182,595	167,967
Gross Unrealized Gains	714	906
Gross Unrealized Losses	(698)	(826)
Fair Value	182,611	168,047
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	35,948	64,823
Gross Unrealized Gains	0	276
Gross Unrealized Losses	(101)	(85)
Fair Value	35,847	65,014
U.S. Treasury securities and obligations guaranteed by the U.S. government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	79,476	71,174
Gross Unrealized Gains	37	131
Gross Unrealized Losses	(639)	(185)
Fair Value	78,874	71,120
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,025	2,025
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(216)
Fair Value	2,018	1,809
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	75,318	74,553
Gross Unrealized Gains	7,830	4,503
Gross Unrealized Losses	(626)	(2,655)
Fair Value	$ 82,522	$ 76,401

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost and Fair Value of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
One year or less	$ 66,852
After one year through five years	274,095
After five years through ten years	195,333
After ten years	127,383
Residential mortgage-backed	160,379
Commercial mortgage and asset-backed	182,595
Redeemable preferred stock	2,025
Total, Amortized Cost	1,008,662	$ 940,212
Fair Value
One year or less	66,770
After one year through five years	274,421
After five years through ten years	195,995
After ten years	135,622
Residential mortgage-backed	158,661
Commercial mortgage and asset-backed	182,611
Redeemable preferred stock	2,018
Total, Fair Value	$ 1,016,098	$ 941,077

Investments - Schedule of Gross

Investments - Schedule of Gross Unrealized Losses and Fair Values of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	$ 319,654	$ 492,187
Less Than 12 Months, Gross Unrealized Losses	(2,172)	(9,100)
12 Months or More, Fair Value	251,634	54,560
12 Months or More, Gross Unrealized Losses	(5,582)	(4,487)
Total, Fair Value	571,288	546,747
Total, Gross Unrealized Losses	(7,754)	(13,587)
Fixed maturity securities:
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	313,795	470,842
Less Than 12 Months, Gross Unrealized Losses	(2,107)	(8,029)
12 Months or More, Fair Value	245,969	48,002
12 Months or More, Gross Unrealized Losses	(5,021)	(2,903)
Total, Fair Value	559,764	518,844
Total, Gross Unrealized Losses	(7,128)	(10,932)
State and municipal
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	40,117	28,398
Less Than 12 Months, Gross Unrealized Losses	(318)	(984)
12 Months or More, Fair Value	10,662	0
12 Months or More, Gross Unrealized Losses	(285)	0
Total, Fair Value	50,779	28,398
Total, Gross Unrealized Losses	(603)	(984)
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	50,447	93,242
Less Than 12 Months, Gross Unrealized Losses	(261)	(1,548)
12 Months or More, Fair Value	84,193	32,330
12 Months or More, Gross Unrealized Losses	(2,180)	(1,427)
Total, Fair Value	134,640	125,572
Total, Gross Unrealized Losses	(2,441)	(2,975)
Corporate
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	113,464	199,841
Less Than 12 Months, Gross Unrealized Losses	(846)	(4,212)
12 Months or More, Fair Value	66,954	8,477
12 Months or More, Gross Unrealized Losses	(1,793)	(1,449)
Total, Fair Value	180,418	208,318
Total, Gross Unrealized Losses	(2,639)	(5,661)
Commercial mortgage and asset-backed
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	53,965	47,990
Less Than 12 Months, Gross Unrealized Losses	(244)	(799)
12 Months or More, Fair Value	25,299	7,195
12 Months or More, Gross Unrealized Losses	(454)	(27)
Total, Fair Value	79,264	55,185
Total, Gross Unrealized Losses	(698)	(826)
Obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	3,024	50,573
Less Than 12 Months, Gross Unrealized Losses	(1)	(85)
12 Months or More, Fair Value	32,154	0
12 Months or More, Gross Unrealized Losses	(100)	0
Total, Fair Value	35,178	50,573
Total, Gross Unrealized Losses	(101)	(85)
U.S. Treasury securities and obligations guaranteed by the U.S. government
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	50,760	48,989
Less Than 12 Months, Gross Unrealized Losses	(430)	(185)
12 Months or More, Fair Value	26,707	0
12 Months or More, Gross Unrealized Losses	(209)	0
Total, Fair Value	77,467	48,989
Total, Gross Unrealized Losses	(639)	(185)
Redeemable preferred stock
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	2,018	1,809
Less Than 12 Months, Gross Unrealized Losses	(7)	(216)
12 Months or More, Fair Value	0	0
12 Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	2,018	1,809
Total, Gross Unrealized Losses	(7)	(216)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Less Than 12 Months, Fair Value	5,859	21,345
Less Than 12 Months, Gross Unrealized Losses	(65)	(1,071)
12 Months or More, Fair Value	5,665	6,558
12 Months or More, Gross Unrealized Losses	(561)	(1,584)
Total, Fair Value	11,524	27,903
Total, Gross Unrealized Losses	$ (626)	$ (2,655)

Investments - Narrative (Detail

Investments - Narrative (Details)	1 Months Ended	12 Months Ended
	Mar. 31, 2015USD ($)	Dec. 31, 2017USD ($)issuerDirectorloan	Dec. 31, 2016USD ($)loan	Dec. 31, 2015USD ($)Bondloan	Dec. 31, 2014USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale securities held in an unrealized loss position | issuer		166
Total fair value of unrealized loss position securities		$ 571,288,000	$ 546,747,000
Total gross unrealized losses of unrealized loss position securities		7,754,000	13,587,000
Total available-for-sale securities		1,098,620,000	1,017,478,000
Gross unrealized gains		22,394,000	16,300,000
Impairment loss		1,500,000
Allowance for credit losses		3,200,000	943,000	$ 4,300,000
Carrying value of loan		238,214,000	203,526,000
Unpaid principal on impaired loans		8,400,000	7,600,000	11,100,000
Carrying value of impaired loans		$ 5,100,000	$ 6,500,000	$ 6,300,000
Number of impaired loans | loan		5	5	5
Net Investment Income		$ 61,119,000	$ 52,638,000	$ 44,835,000
Available-for-sale, at fair value (amortized cost: 2017 – $1,008,662; 2016 – $940,212)		1,016,098,000	941,077,000
Investment as collateral for outstanding letters of credit		472,000,000
Investments on deposit with state insurance departments		16,500,000	16,400,000
Investment Income		14,079,000	9,536,000	5,947,000
Bank Holding Company
Schedule of Available-for-sale Securities [Line Items]
Interest income		343,000	343,000	343,000
Investment in certificate of deposit		1,000,000
Available-for-sale fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Total fair value of unrealized loss position securities		559,764,000	518,844,000
Total gross unrealized losses of unrealized loss position securities		7,128,000	10,932,000
Total available-for-sale securities		1,016,098,000	941,077,000
Gross unrealized gains		14,564,000	11,797,000
Gross realized gains		840,000	1,916,000	2,197,000
Puerto Rico loan
Schedule of Available-for-sale Securities [Line Items]
Allowance for credit losses		759,000	177,000	414,000
Carrying value of loan		0	1,700,000	3,900,000
Unpaid principal on impaired loans		807,000	2,000,000	4,600,000
Oil and gas companies loan
Schedule of Available-for-sale Securities [Line Items]
Allowance for credit losses		2,300,000	545,000	3,900,000
Carrying value of loan		7,900,000
Unpaid principal on impaired loans		6,900,000	2,200,000	5,800,000
Carrying value of impaired loans		4,600,000	$ 1,600,000	$ 1,700,000
Unrealzed loss on investments		$ 180,000
Number of loans held | loan		4
Number of impaired loans | loan		3	1	2
Non-energy sector loan
Schedule of Available-for-sale Securities [Line Items]
Allowance for credit losses		$ 145,000	$ 221,000	$ 34,000
Unpaid principal on impaired loans		706,000	3,500,000	722,000
Carrying value of impaired loans		$ 561,000	$ 3,200,000	$ 689,000
Number of impaired loans | loan		1	3	1
Impaired bank loans
Schedule of Available-for-sale Securities [Line Items]
Average recorded investment in impaired bank loans		$ 5,800,000	$ 6,400,000	$ 6,700,000
Net Investment Income		300,000	297,000	229,000
Net realized gain (loss) on changes in fair value of impaired bank loans		(2,400,000)	415,000	$ (3,400,000)
Bank loan participations
Schedule of Available-for-sale Securities [Line Items]
Unamortized discount		1,100,000	2,900,000
Unamortized premium		$ 3,000	14,000
Standard & Poor's, BBB- Rating | Available-for-sale fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Percentage of available for sale securities		99.40%
Standard & Poor's, Below BBB- Rating | Available-for-sale fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale securities		$ 6,500,000
Gross unrealized gains		305,000
Standard & Poor's, Below BBB- Rating | Impaired Puerto Rico municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Number of impaired Puerto Rico municipal bonds held | Bond				2
Impairment loss				$ 660,000
Net realized gain on sale				22,000
Renewable Energy Bridge Financing Notes
Schedule of Available-for-sale Securities [Line Items]
Interest income		526,000
Proceeds from other investments maturities and repayments			6,500,000	30,800,000
Investment Income		526,000	450,000	3,136,000
Trading gains (losses)
Schedule of Available-for-sale Securities [Line Items]
Trading securities, realized gain (loss)		(4,000)	18,000	(9,000)
Trading securities, net trading losses related to securities				(7,000)
Surviving Bank Holding Company
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, at fair value (amortized cost: 2017 – $1,008,662; 2016 – $940,212)		4,700,000
Surviving Bank Holding Company | Bank Holding Company
Schedule of Available-for-sale Securities [Line Items]
Dividend income			299,000	66,000
Gross realized gains		409,000	3,600,000
Interest income		$ 3,000	$ 4,000	$ 5,000
First Wind | Available-for-sale fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Carrying value of loan					$ 4,600,000
Gross realized gains	$ 845,000
Available-for-sale, at fair value (amortized cost: 2017 – $1,008,662; 2016 – $940,212)					$ 12,600,000
Number of directors | Director		2

Investments - Summary of Net In

Investments - Summary of Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Gross investment income	$ 65,049	$ 56,398	$ 48,664
Investment expense	(3,930)	(3,760)	(3,829)
Net investment income	61,119	52,638	44,835
Fixed maturity securities:
Net Investment Income [Line Items]
Gross investment income	26,833	25,917	24,178
Bank loan participations
Net Investment Income [Line Items]
Gross investment income	17,388	14,486	13,432
Equity securities
Net Investment Income [Line Items]
Gross investment income	5,045	5,617	4,444
Other invested assets
Net Investment Income [Line Items]
Gross investment income	14,079	9,536	5,947
Cash, cash equivalents, short-term investments, and other
Net Investment Income [Line Items]
Gross investment income	1,708	824	672
Trading gains (losses)
Net Investment Income [Line Items]
Gross investment income	$ (4)	$ 18	$ (9)

Investments - Summary of Realiz

Investments - Summary of Realized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Realized gains and losses on investments	$ (1,989)	$ 7,565	$ (4,547)
Fixed maturity securities:
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	840	1,916	2,197
Gross realized losses	(512)	(106)	(826)
Realized gains and losses on investments	328	1,810	1,371
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	429	4,761	1,041
Gross realized losses	(1,591)	0	(10)
Realized gains and losses on investments	(1,162)	4,761	1,031
Bank loan participations
Schedule of Available-for-sale Securities [Line Items]
Realized gains and losses on investments	(1,150)	995	(6,989)
Gross realized gains	2,407	2,827	1,269
Gross realized losses	(3,557)	(1,832)	(8,258)
Short-term investments and other:
Schedule of Available-for-sale Securities [Line Items]
Realized gains and losses on investments	(5)	(1)	40
Gross realized gains	1	3	54
Gross realized losses	$ (6)	$ (4)	$ (14)

Investments - Summary of Availa

Investments - Summary of Available-for-Sale Gross Unrealized Gains or Losses by Investment Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, change in gross unrealized gains (losses)	$ 11,927	$ (3,783)	$ (16,108)
Fixed maturity securities:
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, change in gross unrealized gains (losses)	6,571	(1,350)	(16,832)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, change in gross unrealized gains (losses)	$ 5,356	$ (2,433)	$ 724

Investments - Summary of Privat

Investments - Summary of Private Debt and Equity Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Total other invested assets	$ 70,208	$ 55,419
Investment Income	14,079	9,536	$ 5,947
Distributions from equity method investments	7,333	3,467	2,885
Equity in net income of subsidiaries	13,209	8,743	2,468
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets	32,063	27,067
Investment Income	10,578	3,480	3,936
Renewable Energy Bridge Financing Notes
Net Investment Income [Line Items]
Total other invested assets	7,278	0	36,300
Investment Income	$ 526	450	3,136
Investment interest rate	15.00%
Outstanding commitment to invest	$ 1,500
Commitment fee percentage	2.50%
Interest income	$ 526
Proceeds from other investments maturities and repayments		6,500	30,800
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets	26,367	23,852
Investment Income	2,632	5,263	(1,468)
Bank holding companies
Net Investment Income [Line Items]
Total other invested assets	4,500	4,500
Investment Income	343	343	343
Corporate and other segment | Limited partnerships
Net Investment Income [Line Items]
Carrying value of investments	3,000	2,600
Equity in net income of subsidiaries	394	455	(510)
Excess and Surplus Lines | Limited partnerships
Net Investment Income [Line Items]
Carrying value of investments	23,400	21,200
Equity in net income of subsidiaries	2,200	4,800	(958)
Investment in LLC | Corporate and other segment
Net Investment Income [Line Items]
Distributions from equity method investments	$ 5,600	2,400
Bank Holding Company
Net Investment Income [Line Items]
Investment interest rate	7.60%
Interest income	$ 343	343	343
Investment in subordinated notes	4,500
Equity securities | Excess and Surplus Lines | Limited partnerships
Net Investment Income [Line Items]
Commitment to investment in limited partnership	625
Residential mortgage-backed | Excess and Surplus Lines | Limited partnerships
Net Investment Income [Line Items]
Commitment to investment in limited partnership	$ 5,000
Minimum | Investment in LLC | Corporate and other segment
Net Investment Income [Line Items]
Ownership percentage	2.60%
Maximum | Investment in LLC | Corporate and other segment
Net Investment Income [Line Items]
Ownership percentage	32.80%
Surviving Bank Holding Company | Bank Holding Company
Net Investment Income [Line Items]
Interest income	$ 3	$ 4	$ 5

Deferred Policy Acquisition C66

Deferred Policy Acquisition Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at beginning of period	$ 64,789	$ 60,754	$ 60,202
Commissions	107,387	92,736	90,342
Underwriting and other issue expenses	16,190	12,923	7,960
Policy acquisition costs deferred	123,577	105,659	98,302
Amortization of policy acquisition costs	(116,001)	(101,624)	(97,750)
Net change	7,576	4,035	552
Balance at end of period	$ 72,365	$ 64,789	$ 60,754

Goodwill and Intangible Asset67

Goodwill and Intangible Assets - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 11, 2007
Business Acquisition [Line Items]
Goodwill impairment	$ 0	$ 0	$ 0
Goodwill	181,831,000	181,831,000	181,831,000
Accumulated goodwill impairment losses	99,600,000	99,600,000
Impairment of intangible assets	0	0	0
Amortization of intangible assets	597,000	597,000	597,000
Excess and Surplus Lines
Business Acquisition [Line Items]
Goodwill	181,831,000	181,831,000	181,831,000
Amortization of intangible assets	362,000	362,000	362,000
Specialty Admitted Insurance
Business Acquisition [Line Items]
Goodwill	0	0	0
Amortization of intangible assets	$ 235,000	$ 235,000	$ 235,000
James River Group, Inc.
Business Acquisition [Line Items]
Percentage of outstanding shares acquired				100.00%

Goodwill and Intangible Asset68

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Acquired Intangible Assets [Line Items]
Identifiable intangibles not subject to amortization	$ 31,364	$ 31,364
Identifiable intangible assets subject to amortization
Gross Carrying Amount	11,611	11,611
Accumulated Amortization	4,641	4,044
Total intangible assets, gross	42,975	42,975
Broker relationships
Identifiable intangible assets subject to amortization
Gross Carrying Amount	11,611	11,611
Accumulated Amortization	$ 4,641	4,044
Weighted-Average Life (in years)	24 years 7 months 6 days
Trademarks
Acquired Intangible Assets [Line Items]
Identifiable intangibles not subject to amortization	$ 22,200	22,200
Insurance licenses and authorities
Acquired Intangible Assets [Line Items]
Identifiable intangibles not subject to amortization	$ 9,164	$ 9,164

Goodwill and Intangible Asset69

Goodwill and Intangible Assets - Schedule of Future Estimated Amortization (Details) $ in Thousands	Dec. 31, 2017USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,018	$ 597
2,019	597
2,020	538
2,021	363
2,022	363
Thereafter	4,512
Total	$ 6,970

Goodwill and Intangible Asset70

Goodwill and Intangible Assets - Summary of Changes in Net Carrying Value of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible Assets [Roll Forward]
Net Carrying Value	$ 38,931	$ 39,528
Amortization	(597)	(597)	$ (597)
Impairment Losses	0	0
Net Carrying Value	38,334	38,931	39,528
Excess and Surplus Lines
Intangible Assets [Roll Forward]
Net Carrying Value	31,289	31,651
Amortization	(362)	(362)	(362)
Impairment Losses	0	0
Net Carrying Value	30,927	31,289	31,651
Excess and Surplus Lines | Trademarks
Intangible Assets [Roll Forward]
Net Carrying Value	19,700	19,700
Amortization	0	0
Impairment Losses	0	0
Net Carrying Value	19,700	19,700	19,700
Excess and Surplus Lines | Insurance licenses and authorities
Intangible Assets [Roll Forward]
Net Carrying Value	4,900	4,900
Amortization	0	0
Impairment Losses	0	0
Net Carrying Value	4,900	4,900	4,900
Excess and Surplus Lines | Broker relationships
Intangible Assets [Roll Forward]
Net Carrying Value	6,689	7,051
Amortization	(362)	(362)
Impairment Losses	0	0
Net Carrying Value	6,327	6,689	7,051
Specialty Admitted Insurance
Intangible Assets [Roll Forward]
Net Carrying Value	7,642	7,877
Amortization	(235)	(235)	(235)
Impairment Losses	0	0
Net Carrying Value	7,407	7,642	7,877
Specialty Admitted Insurance | Trademarks
Intangible Assets [Roll Forward]
Net Carrying Value	2,500	2,500
Amortization	0	0
Impairment Losses	0	0
Net Carrying Value	2,500	2,500	2,500
Specialty Admitted Insurance | Insurance licenses and authorities
Intangible Assets [Roll Forward]
Net Carrying Value	4,265	4,265
Amortization	0	0
Impairment Losses	0	0
Net Carrying Value	4,265	4,265	4,265
Specialty Admitted Insurance | Broker relationships
Intangible Assets [Roll Forward]
Net Carrying Value	877	1,112
Amortization	(235)	(235)
Impairment Losses	0	0
Net Carrying Value	$ 642	$ 877	$ 1,112

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	$ 39,717	$ 38,068
Accumulated depreciation	(14,716)	(13,388)
Property and equipment, net	25,001	24,680
Building, leased for which the Company has been deemed the owner for accounting purposes (Note 21)
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	30,902	30,902
Electronic data processing hardware and software
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	6,156	4,460
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	$ 2,659	$ 2,706

Reserve for Losses and Loss A72

Reserve for Losses and Loss Adjustment Expenses - Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) [Abstract]
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period	$ 761,128	$ 653,534	$ 589,042
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	533,905	349,137	295,334
Prior years	21,472	(23,716)	(16,318)
Total incurred losses and loss and adjustment expenses	555,377	325,421	279,016
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	103,205	39,473	31,957
Prior years	223,475	178,354	182,567
Total loss and loss adjustment expense payments	326,680	217,827	214,524
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	989,825	761,128	653,534
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	302,524	182,737	131,788
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	$ 1,292,349	$ 943,865	$ 785,322

Reserve for Losses and Loss A73

Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred	$ (21,472)	$ 23,716	$ 16,318
Excess and Surplus Lines
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred	(20,000)	24,100	25,400
Excess and Surplus Lines | Accident years 2014 through 2016
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred	18,600
Excess and Surplus Lines | Accident year 2016
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred	(38,700)
Excess and Surplus Lines | Accident year 2015
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred		10,000
Excess and Surplus Lines | Accident year 2014
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred		10,700	17,300
Excess and Surplus Lines | Accident year 2013
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred		4,500	10,500
Specialty Admitted Insurance | Accident years 2010 through 2014
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred		3,800
Specialty Admitted Insurance | Accident years 2011 through 2013
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred			3,500
Specialty Admitted Insurance | Accident years 2010 through 2015
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred	2,700
Casualty Reinsurance
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred		$ (4,200)	$ (12,600)
Casualty Reinsurance | Accident years 2010 through 2013
Favorable development on prior year loss reserves
Prior year - Losses and loss adjustment expenses incurred	$ (4,200)

Reserve for Losses and Loss A74

Reserve for Losses and Loss Adjustment Expenses - Incurred Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Casualty Reinsurance
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 936,983
Cumulative paid losses and loss adjustment expenses, net of reinsurance	683,731
Total outstanding losses and loss adjustment expenses, net of reinsurance	253,252
Casualty Reinsurance | Accident year 2008
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	3,488	$ 3,470	$ 3,467	$ 3,462	$ 3,473	$ 3,199	$ 2,859	$ 2,897	$ 3,215	$ 1,418
Cumulative paid losses and loss adjustment expenses, net of reinsurance	3,074	3,029	2,929	2,706	2,478	2,087	1,595	999	616	320
Casualty Reinsurance | Accident year 2009
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	26,980	26,931	26,989	27,116	27,078	26,458	24,125	28,244	34,587
Cumulative paid losses and loss adjustment expenses, net of reinsurance	24,649	23,796	23,024	21,646	19,672	16,466	12,956	9,926	6,487
Casualty Reinsurance | Accident year 2010
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	61,619	60,978	60,949	61,344	62,714	61,068	60,476	64,413
Cumulative paid losses and loss adjustment expenses, net of reinsurance	57,013	54,659	52,761	49,263	44,921	38,430	31,500	21,918
Casualty Reinsurance | Accident year 2011
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	101,276	99,282	98,993	97,812	97,366	103,123	114,908
Cumulative paid losses and loss adjustment expenses, net of reinsurance	94,627	90,287	87,267	78,164	68,616	61,922	48,688
Casualty Reinsurance | Accident year 2012
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	139,978	136,813	135,594	131,281	132,388	148,251
Cumulative paid losses and loss adjustment expenses, net of reinsurance	128,567	121,026	113,943	97,215	81,859	73,124
Casualty Reinsurance | Accident year 2013
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	137,801	134,446	131,352	130,361	133,230
Cumulative paid losses and loss adjustment expenses, net of reinsurance	119,256	108,396	93,902	75,094	59,756
Casualty Reinsurance | Accident year 2014
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	116,981	114,636	115,927	118,881
Cumulative paid losses and loss adjustment expenses, net of reinsurance	89,899	76,302	58,601	41,421
Casualty Reinsurance | Accident year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	108,699	108,870	119,157
Cumulative paid losses and loss adjustment expenses, net of reinsurance	68,002	53,986	40,021
Casualty Reinsurance | Accident year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	105,533	112,759
Cumulative paid losses and loss adjustment expenses, net of reinsurance	50,905	36,268
Casualty Reinsurance | Accident year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	134,628
Cumulative paid losses and loss adjustment expenses, net of reinsurance	47,739
Fronting and programs | Specialty Admitted Insurance
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	49,734
Cumulative paid losses and loss adjustment expenses, net of reinsurance	15,291
Total outstanding losses and loss adjustment expenses, net of reinsurance	34,443
Fronting and programs | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	52	52	52	80	104
Cumulative paid losses and loss adjustment expenses, net of reinsurance	52	52	52	52	28
Fronting and programs | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	3,425	3,818	3,468	3,460
Cumulative paid losses and loss adjustment expenses, net of reinsurance	2,728	2,369	1,687	883
Fronting and programs | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	9,358	9,632	7,136
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,165	4,666	2,058
Fronting and programs | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	15,670	11,542
Cumulative paid losses and loss adjustment expenses, net of reinsurance	5,123	1,894
Fronting and programs | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	21,229
Cumulative paid losses and loss adjustment expenses, net of reinsurance	1,223
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	246,767
Cumulative paid losses and loss adjustment expenses, net of reinsurance	205,644
All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance	357
Total outstanding losses and loss adjustment expenses, net of reinsurance	46,310
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2008
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	39,001	39,131	39,320	39,321	40,552	40,330	38,807	38,654	36,641	37,602
Cumulative paid losses and loss adjustment expenses, net of reinsurance	37,976	37,965	37,933	37,905	36,778	33,885	31,432	28,436	22,664	10,479
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2009
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	26,194	26,232	26,169	26,171	27,795	28,116	27,535	28,526	28,691
Cumulative paid losses and loss adjustment expenses, net of reinsurance	25,660	24,418	24,350	24,192	24,231	22,646	21,095	16,945	7,277
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2010
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	27,098	27,590	28,938	28,963	30,373	30,464	28,736	27,209
Cumulative paid losses and loss adjustment expenses, net of reinsurance	26,352	26,345	25,477	25,210	23,898	21,805	16,245	7,157
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2011
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	36,594	37,455	38,311	38,999	40,154	41,421	37,834
Cumulative paid losses and loss adjustment expenses, net of reinsurance	34,334	34,287	34,240	33,204	29,021	23,127	10,123
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2012
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	28,174	28,255	29,689	31,490	32,420	32,116
Cumulative paid losses and loss adjustment expenses, net of reinsurance	26,932	26,897	26,435	24,755	20,308	9,222
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	10,907	11,578	12,786	13,668	12,525
Cumulative paid losses and loss adjustment expenses, net of reinsurance	10,263	10,246	9,846	8,723	4,487
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	13,890	14,620	16,652	16,638
Cumulative paid losses and loss adjustment expenses, net of reinsurance	12,236	12,041	10,648	4,633
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	19,741	21,274	20,938
Cumulative paid losses and loss adjustment expenses, net of reinsurance	15,118	13,285	6,604
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	20,299	21,678
Cumulative paid losses and loss adjustment expenses, net of reinsurance	10,227	4,664
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	24,869
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,546
Commercial auto | Excess and Surplus Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	372,044
Cumulative paid losses and loss adjustment expenses, net of reinsurance	137,437
Total outstanding losses and loss adjustment expenses, net of reinsurance	234,607
Commercial auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	1,301	1,351	1,451	1,300	1,255
Cumulative paid losses and loss adjustment expenses, net of reinsurance	1,275	1,291	1,285	1,182	60
Commercial auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	18,953	17,233	14,071	20,487
Cumulative paid losses and loss adjustment expenses, net of reinsurance	16,359	12,679	8,645	6,166
Commercial auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	35,149	33,113	30,109
Cumulative paid losses and loss adjustment expenses, net of reinsurance	24,282	15,234	8,356
Commercial auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	109,286	74,340
Cumulative paid losses and loss adjustment expenses, net of reinsurance	54,054	18,295
Commercial auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	207,355
Cumulative paid losses and loss adjustment expenses, net of reinsurance	41,467
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	997,926
Cumulative paid losses and loss adjustment expenses, net of reinsurance	585,746
All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance	9,033
Total outstanding losses and loss adjustment expenses, net of reinsurance	421,213
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2008
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	95,930	96,687	98,765	99,169	102,122	107,927	112,227	116,817	119,736	120,760
Cumulative paid losses and loss adjustment expenses, net of reinsurance	92,332	91,768	91,750	87,630	83,937	80,385	69,585	53,566	35,228	$ 16,922
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2009
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	85,470	83,846	83,431	81,764	86,691	98,502	106,480	110,783	114,834
Cumulative paid losses and loss adjustment expenses, net of reinsurance	81,496	79,014	76,091	71,245	67,293	61,548	51,731	41,687	$ 29,860
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2010
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	70,913	70,294	69,964	69,080	73,035	78,117	80,569	78,424
Cumulative paid losses and loss adjustment expenses, net of reinsurance	64,214	61,307	57,371	52,071	45,641	35,812	26,418	$ 13,673
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2011
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	106,643	106,130	106,381	106,564	114,473	119,927	111,190
Cumulative paid losses and loss adjustment expenses, net of reinsurance	98,684	94,998	90,884	81,696	72,732	53,109	$ 27,684
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2012
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	99,619	97,306	96,497	97,829	98,672	97,908
Cumulative paid losses and loss adjustment expenses, net of reinsurance	82,545	74,869	63,216	49,604	33,757	$ 6,944
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	82,830	81,009	85,433	96,064	96,729
Cumulative paid losses and loss adjustment expenses, net of reinsurance	59,641	44,421	30,382	14,509	$ 3,867
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	82,232	90,267	104,092	114,942
Cumulative paid losses and loss adjustment expenses, net of reinsurance	43,891	28,212	16,969	$ 3,412
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	104,847	113,417	126,443
Cumulative paid losses and loss adjustment expenses, net of reinsurance	34,801	17,164	$ 4,048
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	125,093	138,507
Cumulative paid losses and loss adjustment expenses, net of reinsurance	22,852	$ 5,180
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	144,349
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 5,290

Reserve for Losses and Loss A75

Reserve for Losses and Loss Adjustment Expenses - Cumulative Paid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Casualty Reinsurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 683,731
Total outstanding losses and loss adjustment expenses, net of reinsurance	253,252
Casualty Reinsurance | Accident year 2008
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	3,074	$ 3,029	$ 2,929	$ 2,706	$ 2,478	$ 2,087	$ 1,595	$ 999	$ 616	$ 320
Casualty Reinsurance | Accident year 2009
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	24,649	23,796	23,024	21,646	19,672	16,466	12,956	9,926	6,487
Casualty Reinsurance | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	57,013	54,659	52,761	49,263	44,921	38,430	31,500	21,918
Casualty Reinsurance | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	94,627	90,287	87,267	78,164	68,616	61,922	48,688
Casualty Reinsurance | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	128,567	121,026	113,943	97,215	81,859	73,124
Casualty Reinsurance | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	119,256	108,396	93,902	75,094	59,756
Casualty Reinsurance | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	89,899	76,302	58,601	41,421
Casualty Reinsurance | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	68,002	53,986	40,021
Casualty Reinsurance | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	50,905	36,268
Casualty Reinsurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	47,739
Specialty Admitted Insurance | Fronting and programs
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	15,291
Total outstanding losses and loss adjustment expenses, net of reinsurance	34,443
Specialty Admitted Insurance | Fronting and programs | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	52	52	52	52	28
Specialty Admitted Insurance | Fronting and programs | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	2,728	2,369	1,687	883
Specialty Admitted Insurance | Fronting and programs | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,165	4,666	2,058
Specialty Admitted Insurance | Fronting and programs | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	5,123	1,894
Specialty Admitted Insurance | Fronting and programs | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	1,223
Specialty Admitted Insurance | Individual Risk Workers' Compensation
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	205,644
All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance	357
Outstanding losses and loss adjustment expenses assumed from involuntary workers compensation pools	4,830
Total outstanding losses and loss adjustment expenses, net of reinsurance	46,310
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2008
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	37,976	37,965	37,933	37,905	36,778	33,885	31,432	28,436	22,664	10,479
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2009
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	25,660	24,418	24,350	24,192	24,231	22,646	21,095	16,945	7,277
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	26,352	26,345	25,477	25,210	23,898	21,805	16,245	7,157
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	34,334	34,287	34,240	33,204	29,021	23,127	10,123
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	26,932	26,897	26,435	24,755	20,308	9,222
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	10,263	10,246	9,846	8,723	4,487
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	12,236	12,041	10,648	4,633
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	15,118	13,285	6,604
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	10,227	4,664
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,546
Excess and Surplus Lines | Excluding Commercial Auto
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	585,746
All outstanding losses and loss adjustment expenses prior to 2008, net of reinsurance	9,033
Total outstanding losses and loss adjustment expenses, net of reinsurance	421,213
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2008
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	92,332	91,768	91,750	87,630	83,937	80,385	69,585	53,566	35,228	$ 16,922
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2009
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	81,496	79,014	76,091	71,245	67,293	61,548	51,731	41,687	$ 29,860
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	64,214	61,307	57,371	52,071	45,641	35,812	26,418	$ 13,673
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	98,684	94,998	90,884	81,696	72,732	53,109	$ 27,684
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	82,545	74,869	63,216	49,604	33,757	$ 6,944
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	59,641	44,421	30,382	14,509	3,867
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	43,891	28,212	16,969	3,412
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	34,801	17,164	4,048
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	22,852	5,180
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	5,290
Excess and Surplus Lines | Commercial Auto
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	137,437
Total outstanding losses and loss adjustment expenses, net of reinsurance	234,607
Excess and Surplus Lines | Commercial Auto | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	1,275	1,291	1,285	1,182	$ 60
Excess and Surplus Lines | Commercial Auto | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	16,359	12,679	8,645	$ 6,166
Excess and Surplus Lines | Commercial Auto | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	24,282	15,234	$ 8,356
Excess and Surplus Lines | Commercial Auto | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	54,054	$ 18,295
Excess and Surplus Lines | Commercial Auto | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 41,467

Reserve for Losses and Loss A76

Reserve for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	$ 989,825	$ 761,128	$ 653,534	$ 589,042
Reinsurance recoverable on unpaid losses	302,524	182,737	131,788
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	1,292,349	$ 943,865	$ 785,322
Excess and Surplus Lines | Excluding commercial auto
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	421,213
Excess and Surplus Lines | Commercial auto
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	234,607
Specialty Admitted Insurance | Individual risk workers' compensation
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	46,310
Specialty Admitted Insurance | Fronting and programs
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	34,443
Casualty Reinsurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	$ 253,252

Reserve for Losses and Loss A77

Reserve for Losses and Loss Adjustment Expenses - Average Annual Percentage Payouts of Incurred Claims (Details)	Dec. 31, 2017
E&S | Excluding commercial auto
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	10.30%
Year 2	16.60%
Year 3	17.10%
Year 4	17.50%
Year 5	14.40%
Year 6	7.70%
Year 7	5.50%
Year 8	3.30%
Year 9	2.50%
Year 10	1.10%
E&S | Commercial auto
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	21.60%
Year 2	40.40%
Year 3	15.10%
Year 4	13.90%
Year 5	7.10%
Specialty Admitted Insurance | Individual risk workers' compensation
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	28.00%
Year 2	32.70%
Year 3	13.60%
Year 4	6.10%
Year 5	3.40%
Year 6	1.40%
Year 7	1.00%
Year 8	0.10%
Year 9	0.50%
Year 10	0.00%
Specialty Admitted Insurance | Fronting and programs
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	12.20%
Year 2	13.40%
Year 3	4.40%
Year 4	0.70%
Casualty Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	28.80%
Year 2	21.10%
Year 3	14.80%
Year 4	10.30%
Year 5	7.30%
Year 6	5.00%
Year 7	3.50%
Year 8	2.40%
Year 9	1.70%
Year 10	1.30%

Reserve for Losses and Loss A78

Reserve for Losses and Loss Adjustment Expenses - IBNR Liabilities and Claims Frequency (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 555,377	$ 325,421	$ 279,016
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2008
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	87,112
IBNR	$ 3,169
Cumulative Number of Reported Claims | Claim	2,283
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2009
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 58,371
IBNR	$ 2,831
Cumulative Number of Reported Claims | Claim	1,677
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 60,883
IBNR	$ 3,563
Cumulative Number of Reported Claims | Claim	1,367
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 55,514
IBNR	$ 5,834
Cumulative Number of Reported Claims | Claim	1,443
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 82,142
IBNR	$ 8,692
Cumulative Number of Reported Claims | Claim	1,725
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 82,830
IBNR	$ 14,382
Cumulative Number of Reported Claims | Claim	2,223
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 82,232
IBNR	$ 23,241
Cumulative Number of Reported Claims | Claim	2,024
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 104,847
IBNR	$ 46,275
Cumulative Number of Reported Claims | Claim	2,292
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 125,093
IBNR	$ 77,940
Cumulative Number of Reported Claims | Claim	2,468
Excess and Surplus Lines | Excluding Commercial Auto | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 144,349
IBNR	$ 123,432
Cumulative Number of Reported Claims | Claim	2,052
Excess and Surplus Lines | Commercial Auto | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 1,301
IBNR	$ 26
Cumulative Number of Reported Claims | Claim	54
Excess and Surplus Lines | Commercial Auto | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 18,953
IBNR	$ 791
Cumulative Number of Reported Claims | Claim	7,748
Excess and Surplus Lines | Commercial Auto | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 35,149
IBNR	$ 5,098
Cumulative Number of Reported Claims | Claim	41,640
Excess and Surplus Lines | Commercial Auto | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 109,286
IBNR	$ 20,321
Cumulative Number of Reported Claims | Claim	87,920
Excess and Surplus Lines | Commercial Auto | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 207,355
IBNR	$ 105,097
Cumulative Number of Reported Claims | Claim	125,863
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2008
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 39,001
IBNR	$ 663
Cumulative Number of Reported Claims | Claim	2,012
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2009
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 26,194
IBNR	$ 534
Cumulative Number of Reported Claims | Claim	1,490
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 27,098
IBNR	$ 700
Cumulative Number of Reported Claims | Claim	1,604
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 36,594
IBNR	$ 1,632
Cumulative Number of Reported Claims | Claim	1,814
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 28,174
IBNR	$ 809
Cumulative Number of Reported Claims | Claim	1,323
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 10,907
IBNR	$ 502
Cumulative Number of Reported Claims | Claim	540
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 13,890
IBNR	$ 1,506
Cumulative Number of Reported Claims | Claim	850
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 19,741
IBNR	$ 3,431
Cumulative Number of Reported Claims | Claim	975
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 20,299
IBNR	$ 6,409
Cumulative Number of Reported Claims | Claim	831
Specialty Admitted Insurance | Individual Risk Workers' Compensation | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 24,803
IBNR	$ 4,617
Cumulative Number of Reported Claims | Claim	1,036
Specialty Admitted Insurance | Fronting and programs | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 52
IBNR	$ 0
Cumulative Number of Reported Claims | Claim	22
Specialty Admitted Insurance | Fronting and programs | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 3,041
IBNR	$ 384
Cumulative Number of Reported Claims | Claim	857
Specialty Admitted Insurance | Fronting and programs | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 8,081
IBNR	$ 1,277
Cumulative Number of Reported Claims | Claim	1,347
Specialty Admitted Insurance | Fronting and programs | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 12,050
IBNR	$ 3,620
Cumulative Number of Reported Claims | Claim	2,730
Specialty Admitted Insurance | Fronting and programs | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 8,523
IBNR	$ 12,706
Cumulative Number of Reported Claims | Claim	5,421

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	$ 302,524	$ 182,737	$ 131,788
Reinsurance recoverable on paid losses	$ 11,292	2,877
Percentage of reinsurance recoverables on unpaid losses for 3 largest reinsurers	49.70%
Prepaid reinsurance premiums ceded	$ 91,979	$ 90,147
Percentage of prepaid reinsurance premiums ceded for 3 largest reinsurers	54.30%
Largest reinsurer one
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	$ 88,800
Prepaid reinsurance premiums ceded	27,900
Largest reinsurer two
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	37,800
Prepaid reinsurance premiums ceded	11,200
Largest reinsurer three
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	23,800
Prepaid reinsurance premiums ceded	$ 10,900

Reinsurance - Summary of Premiu

Reinsurance - Summary of Premiums Written, Premiums Earned, and Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Written premiums:
Direct	$ 843,719	$ 549,700	$ 395,816
Assumed	238,186	187,698	176,378
Ceded	(315,279)	(179,690)	(101,162)
Net written premiums	766,626	557,708	471,032
Earned premiums:
Direct	842,182	483,166	367,340
Assumed	173,472	164,771	181,326
Ceded	(274,545)	(132,274)	(87,461)
Net earned premiums	741,109	515,663	461,205
Losses and loss adjustment expenses:
Direct	626,318	314,920	201,755
Assumed	142,818	108,029	127,276
Ceded	(213,759)	(97,528)	(50,015)
Net	$ 555,377	$ 325,421	$ 279,016

Senior Debt (Details)

Senior Debt (Details) - USD ($)	Dec. 31, 2017	Aug. 02, 2017	May 26, 2004
Senior revolving credit facility
Line of Credit Facility [Line Items]
Current total capacity	$ 215,000,000
Senior revolving credit facility | JRG Re
Line of Credit Facility [Line Items]
Maximum borrowing capacity	102,500,000	$ 100,000,000
Senior revolving credit facility | Surviving Bank Holding Company
Line of Credit Facility [Line Items]
Commitment to lend	25,000,000
Senior revolving credit facility | Secured | JRG Re
Line of Credit Facility [Line Items]
Letters of credit outstanding	98,500,000
Senior revolving credit facility | Unsecured
Line of Credit Facility [Line Items]
Maximum borrowing capacity	112,500,000
Amount outstanding on line of credit	73,300,000
Senior revolving credit facility | Unsecured | JRG Re
Line of Credit Facility [Line Items]
Amount outstanding on line of credit	10,000,000
Secured letters of credit | Secured | JRG Re
Line of Credit Facility [Line Items]
Amount outstanding on line of credit	$ 6,900,000
Senior Debt
Line of Credit Facility [Line Items]
Principal amount of Senior Debt			$ 15,000,000
LIBOR | Senior revolving credit facility | Unsecured | JRG Re
Line of Credit Facility [Line Items]
Margin above LIBOR, percentage rate	1.50%
LIBOR | Senior Debt
Line of Credit Facility [Line Items]
Margin above LIBOR, percentage rate			3.85%

Junior Subordinated Debt - Summ

Junior Subordinated Debt - Summary of Junior Subordinated Debt and Trust Preferred Securities (Details) - Junior Subordinated Debt $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
James River Capital Trust I
Debt Instrument [Line Items]
Issue date	May 26, 2004
Principal amount of Trust Preferred Securities	$ 7,000
Principal amount of Junior Subordinated Debt	7,217
Carrying amount of Junior Subordinated Debt net of repurchases	$ 7,217
Maturity date of Junior Subordinated Debt, unless accelerated earlier	May 24, 2034
Trust common stock	$ 217
Interest rate, per annum	Three-Month LIBOR plus 4.0%
James River Capital Trust II
Debt Instrument [Line Items]
Issue date	Dec. 15, 2004
Principal amount of Trust Preferred Securities	$ 15,000
Principal amount of Junior Subordinated Debt	15,464
Carrying amount of Junior Subordinated Debt net of repurchases	$ 15,464
Maturity date of Junior Subordinated Debt, unless accelerated earlier	Dec. 15, 2034
Trust common stock	$ 464
Interest rate, per annum	Three-Month LIBOR plus 3.4%
James River Capital Trust III
Debt Instrument [Line Items]
Issue date	Jun. 15, 2006
Principal amount of Trust Preferred Securities	$ 20,000
Principal amount of Junior Subordinated Debt	20,619
Carrying amount of Junior Subordinated Debt net of repurchases	$ 20,619
Maturity date of Junior Subordinated Debt, unless accelerated earlier	Jun. 15, 2036
Trust common stock	$ 619
Interest rate, per annum	Three-Month LIBOR plus 3.0%
James River Capital Trust IV
Debt Instrument [Line Items]
Issue date	Dec. 11, 2007
Principal amount of Trust Preferred Securities	$ 54,000
Principal amount of Junior Subordinated Debt	55,670
Carrying amount of Junior Subordinated Debt net of repurchases	$ 44,827
Maturity date of Junior Subordinated Debt, unless accelerated earlier	Dec. 15, 2037
Trust common stock	$ 1,670
Interest rate, per annum	Three-Month LIBOR plus 3.1%
Franklin Holdings II (Bermuda) Capital Trust I
Debt Instrument [Line Items]
Issue date	Jan. 10, 2008
Principal amount of Trust Preferred Securities	$ 30,000
Principal amount of Junior Subordinated Debt	30,928
Carrying amount of Junior Subordinated Debt net of repurchases	$ 15,928
Maturity date of Junior Subordinated Debt, unless accelerated earlier	Mar. 15, 2038
Trust common stock	$ 928
Interest rate, per annum	Three-Month LIBOR plus 4.0%

Junior Subordinated Debt - Narr

Junior Subordinated Debt - Narrative (Details)	12 Months Ended
	Dec. 31, 2017
Subordinated Borrowings [Abstract]
Debt redemption percentage	100.00%

Capital Stock - Narrative (Deta

Capital Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 09, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized	200,000,000	200,000,000		200,000,000
Common stock, par value (in dollars per share)	$ 0.0002	$ 0.0002		$ 0.0002
Common stock, shares issued	29,696,682	29,257,566
Undesignated preferred stock, shares authorized	20,000,000	20,000,000
Undesignated stock par value (in dollars per share)	$ 0.00125	$ 0.00125		$ 0.00125
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares outstanding	29,696,682	29,257,566	28,941,547
Common shares reserved for future issuance	4,385,213
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock unit dividend equivalents	$ 450	$ 664	$ 558
Dividends payable	$ 434	$ 666	$ 385

Capital Stock - Summary of Cash

Capital Stock - Summary of Cash Dividends Declared (Details) - Dividends Declared - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share	$ 1.70	$ 2.25	$ 1.64
Total Amount	$ 50.7	$ 66.3	$ 47.8
February 14, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share	$ 0.30
Total Amount	$ 8.9
May 2, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share	$ 0.30
Total Amount	$ 8.9
August 1, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share	$ 0.30
Total Amount	$ 8.9
November 1, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share	$ 0.30
Total Amount	$ 9
November 1, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share	$ 0.50
Total Amount	$ 15
February 16, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share		$ 0.20
Total Amount		$ 5.8
May 3, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share		$ 0.20
Total Amount		$ 5.9
August 3, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share		$ 0.20
Total Amount		$ 5.9
November 1, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share		$ 0.30
Total Amount		$ 8.9
November 1, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share		$ 1.35
Total Amount		$ 39.8
February 17, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share			$ 0.16
Total Amount			$ 4.6
May 5, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share			$ 0.16
Total Amount			$ 4.6
August 5, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share			$ 0.16
Total Amount			$ 4.7
November 3, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share			$ 0.16
Total Amount			$ 4.7
November 3, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend per Common Share			$ 1
Total Amount			$ 29.2

Equity Awards - Narrative (Deta

Equity Awards - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Feb. 14, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual life of option		7 years
Total intrinsic value of options exercised		$ 20.4	$ 10.2	$ 12.8
Aggregate intrinsic value of options outstanding		18.5	41.8	31.8
Aggregate intrinsic value of options exercisable		$ 14.9	$ 28.3	$ 20.3
Fair value of shares used in calculation of intrinsic values		$ 40.01	$ 41.55	$ 33.54
Weighted-average remaining contractual life of options outstanding (in years)		4 years 6 months
Weighted-average remaining contractual life of options exercisable (in years)		3 years 9 months 10 days
Weighted-average fair value of options granted		$ 8.21	$ 5.55	$ 4.31
Options | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period (in years)		3 years
Options | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period (in years)		4 years
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of shares vested		$ 5.5	$ 3.8	$ 3.5
Unrecognized share based compensation expense		$ 6.9
Weighted-average period of unrecognized share based compensation expense (in years)		1 year 10 months 24 days
RSUs | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period (in years)		1 year
RSUs | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period (in years)		5 years
2014 Long Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares available for issuance		4,171,150
Number of shares available for grant		1,862,291
2014 Long Term Incentive Plan | RSUs | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period (in years)	1 year
2014 Long Term Incentive Plan | RSUs | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period (in years)	3 years
2014 Non-Employee Director Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares available for issuance		50,000
Number of shares available for grant		32,994

Equity Awards - Summary of Opti

Equity Awards - Summary of Option Activity (Details) - Options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Beginning of year (in shares)	2,234,699	2,058,085	3,104,768
Granted (in shares)	205,244	706,203	10,627
Exercised (in shares)	(898,218)	(496,550)	(1,047,500)
Forfeited (in shares)	(62,489)	(33,039)	(9,810)
End of year (in shares)	1,479,236	2,234,699	2,058,085
Exercisable, end of year (in shares)	846,371	1,207,479	1,217,903
Weighted- Average Exercise Price
Beginning of year (in dollars per share)	$ 22.84	$ 18.11	$ 17.27
Granted (in dollars per share)	42.24	32.07	24.32
Exercised (in dollars per share)	18.53	16.02	15.66
Forfeited (in dollars per share)	30.80	27.68	21
End of year (in dollars per share)	27.81	22.84	18.11
Exercisable, end of year	$ 22.35	$ 18.14	$ 16.85

Equity Awards - Summary of Assu

Equity Awards - Summary of Assumptions for Options Granted (Details) - Options	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of risk-free interest rates	1.97%	1.23%	1.54%
Dividend yield	2.85%	2.50%	4.00%
Expected share price volatility	27.39%	25.00%	25.00%
Expected life (in years)	5 years	5 years	5 years

Equity Awards - Summary of RSU

Equity Awards - Summary of RSU Activity (Details) - RSUs - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Unvested, beginning of year (in shares)	196,800	234,922	340,474
Granted (in shares)	137,034	60,291	0
Vested (in shares)	(132,764)	(98,413)	(105,552)
Forfeited (in shares)	(22,188)	0	0
Unvested, end of year (in shares)	178,882	196,800	234,922
Weighted- Average Grant Date Fair Value
Unvested, beginning of year (in dollars per share)	$ 24.38	$ 21	$ 21
Granted (in dollars per share)	42.20	32.03	0
Vested (in dollars per share)	24.24	21	21
Forfeited (in dollars per share)	26.06	0	0
Unvested, end of year (in dollars per share)	$ 37.93	$ 24.38	$ 21

Equity Awards - Summary of Comp

Equity Awards - Summary of Compensation Expense and Related Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share based compensation expense	$ 7,688	$ 5,492	$ 3,735
U.S. tax benefit on share based compensation expense	$ 2,093	$ 1,532	$ 1,025

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Examination [Line Items]
Percentage of dividend withholding tax	5.00%
Dividend distributions to intermediate holding company	$ 21,100,000	$ 0	$ 50,000,000
Remittance of dividend withholding tax	$ 1,100,000		2,500,000
Federal statutory tax rates	35.00%
U.S. income before Federal income taxes	$ 50,100,000	18,300,000	15,200,000
Percentage of excise tax on reinsurance premiums paid	1.00%
Payment of federal excise taxes	$ 3,400,000	2,600,000	1,900,000
Excise taxes paid	$ 2,200,000	$ 1,600,000	$ 1,700,000
Bermuda
Income Tax Examination [Line Items]
Federal statutory tax rates	0.00%

Income Taxes - Summary of Incom

Income Taxes - Summary of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal income tax expense at applicable statutory rates	$ 17,541	$ 6,401	$ 5,317
Tax-exempt investment income	(586)	(643)	(858)
Dividends received deduction	(792)	(880)	(867)
Excess tax benefits on share based compensation	(2,114)	0	0
Effect of tax rate reduction on deferred tax liability	(3,498)	0	0
Other	(90)	(6)	379
Federal income tax expense	10,461	4,872	3,971
U.S. state income tax expense (benefit)	65	0	(192)
U.S. dividend withholding tax	1,053	0	2,500
Total income tax expense	$ 11,579	$ 4,872	$ 6,279

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrued compensation expenses	$ 1,712	$ 1,604
Reserve for losses and loss adjustment expenses	3,036	4,810
Unearned premiums	1,709	2,859
Share based compensation	1,143	2,281
Allowance for doubtful accounts	579	748
Deferred policy acquisition costs	1,509	1,983
Property and equipment	1,601	2,649
AMT credit	0	464
Invested asset impairments	791	291
Other	1,564	2,946
Total deferred tax assets	13,644	20,635
Deferred tax liabilities:
Intangible assets	7,521	12,554
Net unrealized gains	2,578	1,586
Deferred gain on extinguishment of debt	63	212
Equity method investments	8,348	8,668
Other	381	517
Total deferred tax liabilities	18,891	23,537
Net deferred tax liabilities	$ (5,247)	$ (2,902)

Other Operating Expenses and 94

Other Operating Expenses and Other Expenses - Summary of Other Operating Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Operating Expenses And Other Expenses [Abstract]
Amortization of policy acquisition costs	$ 116,001	$ 101,624	$ 97,750
Other underwriting expenses of the insurance segments	55,662	48,771	41,499
Other operating expenses of the Corporate and Other segment	25,330	20,433	18,554
Total	$ 196,993	$ 170,828	$ 157,803

Other Operating Expenses and 95

Other Operating Expenses and Other Expenses - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Operating Expenses And Other Expenses [Abstract]
Total other expenses	$ 539,000	$ 1,590,000	$ 730,000
Legal Fees	$ 535,000
Severance expenses		$ 1,500,000	170,000
Related party leasing agreement expense			276,000
Legal, tax, and other professional services expense			$ 284,000

Employee Benefits (Details)

Employee Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Benefits and Share-based Compensation [Abstract]
Maximum matching employer contribution	6.00%
Expense associated with Savings Plans	$ 2.5	$ 1.9	$ 1.6

Commitments and Contingent Li97

Commitments and Contingent Liabilities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contingent Liabilities [Line Items]
Rental expense	$ 4,200,000	$ 3,700,000	$ 2,600,000
Future minimum lease payments for building constructed	$ 19,300,000
Lease term (in years)	10 years
Total payments under lease	$ 24,591,000
Policy management software
Contingent Liabilities [Line Items]
Lease term (in years)		5 years
Total payments under lease		$ 880,000
Lease payment per year		$ 440,000
JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Total amount deposited in trust accounts	265,000,000
JRG Reinsurance Company, Ltd. | Letter of credit 100 million, 50.2 million issued
Contingent Liabilities [Line Items]
Letters of credit facility, amount	100,000,000
Amount of letters of credit issued	47,400,000
Assets deposited for securing letters of credit	93,900,000
JRG Reinsurance Company, Ltd. | Letter of credit 102.5 million
Contingent Liabilities [Line Items]
Letters of credit facility, amount	102,500,000
Amount of letters of credit issued	98,500,000
Assets deposited for securing letters of credit	102,900,000
JRG Reinsurance Company, Ltd. | Letter of credit 100 million, 10.2 million issued
Contingent Liabilities [Line Items]
Letters of credit facility, amount	100,000,000
Amount of letters of credit issued	6,900,000
Assets deposited for securing letters of credit	10,200,000
Captive insurance company affiliate of the insured group
Contingent Liabilities [Line Items]
Cash equivalent collateral held in the collateral trust arrangement	$ 780,900,000

Commitments and Contingent Li98

Commitments and Contingent Liabilities - Schedule of Future Minimum Payments for Non-cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,018	$ 4,025
2,019	3,644
2,020	3,342
2,021	3,154
2,022	2,768
Thereafter	7,658
Total	$ 24,591

Other Comprehensive Income (L99

Other Comprehensive Income (Loss) - Summary of Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Unrealized gains (losses) arising during the period, before U.S. income taxes	$ 11,091	$ 2,790	$ (13,707)
U.S. income taxes	(2,374)	(414)	517
Unrealized gains (losses) arising during the period, net of U.S. income taxes	8,717	2,376	(13,190)
Less reclassification adjustment:
Net realized investment (losses) gains	(835)	6,572	2,402
U.S. income taxes	333	(2,137)	(422)
Reclassification adjustment for investment (losses) gains realized in net income	(502)	4,435	1,980
Other comprehensive income (loss)	$ 9,219	$ (2,059)	$ (15,170)

Other Comprehensive Income (100

Other Comprehensive Income (Loss) - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Realized Investment Gain Losses [Line Items]
Net realized investment (losses) gains	$ (835,000)	$ 6,572,000	$ 2,402,000
Bank loan participations
Realized Investment Gain Losses [Line Items]
Net realized investment (losses) gains	$ (1,150,000)	$ 995,000	$ (7,000,000)

Segment Information - Narrative

Segment Information - Narrative (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)Underwriting_divisionSegment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									4
Fee income									$ 17,000	$ 10,100	$ 3,200
Number of underwriting divisions | Underwriting_division									15
Gross written premiums	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 173,490	$ 260,166	$ 170,671	$ 133,071	$ 1,081,905	737,398	572,194
Broker one
Segment Reporting Information [Line Items]
Gross written premiums percentage									47.90%
Broker two
Segment Reporting Information [Line Items]
Gross written premiums percentage									13.30%
Agency one
Segment Reporting Information [Line Items]
Consolidated gross written premiums percentage									16.90%
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Number of underwriting divisions | Underwriting_division									13
Gross written premiums									$ 530,120	370,844	308,717
Excess and Surplus Lines | Broker one
Segment Reporting Information [Line Items]
Gross written premiums									$ 253,700
Consolidated gross written premiums percentage									23.50%
Excess and Surplus Lines | Broker two
Segment Reporting Information [Line Items]
Gross written premiums									$ 70,300
Consolidated gross written premiums percentage									6.50%
Excess and Surplus Lines | Insured one
Segment Reporting Information [Line Items]
Gross written premiums									$ 216,100
Gross written premiums percentage									40.80%
Excess and Surplus Lines | Rasier LLC
Segment Reporting Information [Line Items]
Fee income									$ 16,900
Consolidated gross written premiums percentage									20.00%
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Number of underwriting divisions | Underwriting_division									1
Gross written premiums									$ 316,430	182,221	90,978
Specialty Admitted Insurance | Agency one
Segment Reporting Information [Line Items]
Gross written premiums									$ 183,000
Gross written premiums percentage									57.80%
Casualty Reinsurance
Segment Reporting Information [Line Items]
Number of underwriting divisions | Underwriting_division									1
Gross written premiums									$ 235,355	$ 184,333	$ 172,499
Casualty Reinsurance | Broker one
Segment Reporting Information [Line Items]
Gross written premiums									$ 56,100
Consolidated gross written premiums percentage									5.20%
Gross written premiums percentage									23.80%
Casualty Reinsurance | Broker two
Segment Reporting Information [Line Items]
Gross written premiums									$ 55,000
Consolidated gross written premiums percentage									5.10%
Gross written premiums percentage									23.40%
Casualty Reinsurance | Broker three
Segment Reporting Information [Line Items]
Gross written premiums									$ 38,100
Consolidated gross written premiums percentage									3.50%
Gross written premiums percentage									16.20%
Casualty Reinsurance | Broker four
Segment Reporting Information [Line Items]
Gross written premiums									$ 33,400
Consolidated gross written premiums percentage									3.10%
Gross written premiums percentage									14.20%
Casualty Reinsurance | Unaffiliated ceding company one
Segment Reporting Information [Line Items]
Gross written premiums									$ 120,300
Consolidated gross written premiums percentage									11.10%
Gross written premiums percentage									51.10%
Casualty Reinsurance | Unaffiliated ceding company two
Segment Reporting Information [Line Items]
Gross written premiums									$ 48,400
Consolidated gross written premiums percentage									4.50%
Gross written premiums percentage									20.60%

Segment Information - Schedule

Segment Information - Schedule of Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 173,490	$ 260,166	$ 170,671	$ 133,071	$ 1,081,905	$ 737,398	$ 572,194
Net earned premiums									741,109	515,663	461,205
Segment revenues	217,963	$ 220,866	$ 202,394	$ 176,402	169,022	$ 151,365	$ 134,511	$ 131,329	817,625	586,227	504,921
Net investment income									61,119	52,638	44,835
Interest expense									8,974	8,448	6,999
Underwriting profit (loss) of operating segments									31,094	49,913	46,123
Segment goodwill	181,831				181,831				181,831	181,831	181,831
Segment assets	2,756,695				2,346,533				2,756,695	2,346,533	2,055,497
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums									530,120	370,844	308,717
Net earned premiums									463,521	301,404	240,878
Segment revenues									493,853	331,090	255,529
Net investment income									15,014	18,051	13,427
Interest expense									0	0	0
Underwriting profit (loss) of operating segments									29,693	47,235	47,607
Segment goodwill	181,831				181,831				181,831	181,831	181,831
Segment assets	843,486				740,144				843,486	740,144	671,143
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums									316,430	182,221	90,978
Net earned premiums									68,110	52,281	42,206
Segment revenues									70,366	55,412	44,791
Net investment income									2,532	2,542	2,316
Interest expense									0	0	0
Underwriting profit (loss) of operating segments									3,166	2,872	1,074
Segment goodwill	0				0				0	0	0
Segment assets	439,416				300,519				439,416	300,519	173,094
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									235,355	184,333	172,499
Net earned premiums									209,478	161,978	178,121
Segment revenues									240,751	190,064	197,586
Net investment income									31,507	27,257	22,706
Interest expense									0	0	0
Underwriting profit (loss) of operating segments									(1,765)	(194)	(2,558)
Segment goodwill	0				0				0	0	0
Segment assets	1,379,866				1,195,230				1,379,866	1,195,230	1,108,278
Corporate and other segment
Segment Reporting Information [Line Items]
Gross written premiums									0	0	0
Net earned premiums									0	0	0
Segment revenues									12,655	9,661	7,015
Net investment income									12,066	4,788	6,386
Interest expense									8,974	8,448	6,999
Underwriting profit (loss) of operating segments									0	0	0
Segment goodwill	0				0				0	0	0
Segment assets	$ 93,927				$ 110,640				$ 93,927	$ 110,640	$ 102,982

Segment Information - Reconcili

Segment Information - Reconciliation of Underwriting Profit (Loss) of Insurance Segments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	$ 31,094,000	$ 49,913,000	$ 46,123,000
Other operating expenses	(25,330,000)	(20,433,000)	(18,554,000)
Underwriting profit	5,764,000	29,480,000	27,569,000
Net investment income	61,119,000	52,638,000	44,835,000
Net realized investment (losses) gains	(1,989,000)	7,565,000	(4,547,000)
Other income	361,000	295,000	245,000
Other expenses	(539,000)	(1,590,000)	(730,000)
Interest expense	(8,974,000)	(8,448,000)	(6,999,000)
Amortization of intangible assets	(597,000)	(597,000)	(597,000)
Income before income taxes	55,145,000	79,343,000	59,776,000
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	29,693,000	47,235,000	47,607,000
Net investment income	15,014,000	18,051,000	13,427,000
Interest expense	0	0	0
Amortization of intangible assets	(362,000)	(362,000)	(362,000)
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	3,166,000	2,872,000	1,074,000
Net investment income	2,532,000	2,542,000	2,316,000
Interest expense	0	0	0
Amortization of intangible assets	(235,000)	(235,000)	(235,000)
Casualty Reinsurance
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	(1,765,000)	(194,000)	(2,558,000)
Net investment income	31,507,000	27,257,000	22,706,000
Interest expense	0	0	0
Corporate and other segment
Segment Reporting Information [Line Items]
Underwriting profit (loss) of operating segments	0	0	0
Net investment income	12,066,000	4,788,000	6,386,000
Interest expense	$ (8,974,000)	$ (8,448,000)	$ (6,999,000)

Segment Information - Gross Wri

Segment Information - Gross Written Premiums (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total gross written premiums	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 173,490	$ 260,166	$ 170,671	$ 133,071	$ 1,081,905	$ 737,398	$ 572,194
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Total gross written premiums									530,120	370,844	308,717
Excess and Surplus Lines | Commercial Auto
Segment Reporting Information [Line Items]
Total gross written premiums									247,960	110,050	73,770
Excess and Surplus Lines | Manufacturers and Contractors
Segment Reporting Information [Line Items]
Total gross written premiums									85,719	83,279	78,315
Excess and Surplus Lines | Excess Casualty
Segment Reporting Information [Line Items]
Total gross written premiums									51,160	43,574	32,458
Excess and Surplus Lines | General Casualty
Segment Reporting Information [Line Items]
Total gross written premiums									38,097	36,858	30,972
Excess and Surplus Lines | Energy
Segment Reporting Information [Line Items]
Total gross written premiums									29,704	29,709	30,623
Excess and Surplus Lines | Allied Health
Segment Reporting Information [Line Items]
Total gross written premiums									19,181	14,413	13,513
Excess and Surplus Lines | Excess Property
Segment Reporting Information [Line Items]
Total gross written premiums									14,447	14,083	12,498
Excess and Surplus Lines | Life Sciences
Segment Reporting Information [Line Items]
Total gross written premiums									12,981	11,132	8,917
Excess and Surplus Lines | Small Business
Segment Reporting Information [Line Items]
Total gross written premiums									11,307	9,104	6,916
Excess and Surplus Lines | Professional Liability
Segment Reporting Information [Line Items]
Total gross written premiums									6,326	8,361	10,046
Excess and Surplus Lines | Environmental
Segment Reporting Information [Line Items]
Total gross written premiums									7,920	5,321	4,437
Excess and Surplus Lines | Medical Professionals
Segment Reporting Information [Line Items]
Total gross written premiums									2,297	2,739	3,585
Excess and Surplus Lines | Sports and Entertainment
Segment Reporting Information [Line Items]
Total gross written premiums									3,021	2,221	2,667
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Total gross written premiums									316,430	182,221	90,978
Casualty Reinsurance
Segment Reporting Information [Line Items]
Total gross written premiums									$ 235,355	$ 184,333	$ 172,499

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	$ 1,098,620	$ 1,017,478
Trading securities:
Fixed maturity securities	3,808	5,063
Short-term investments	36,804	50,844
Fixed maturity securities:
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	1,016,098	941,077
State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	144,366	105,841
Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	158,661	150,798
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	413,721	378,448
Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	182,611	168,047
Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	35,847	65,014
U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	78,874	71,120
Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	2,018	1,809
Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	82,522	76,401
Recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	1,098,620	1,017,478
Trading securities:
Fixed maturity securities	3,808	5,063
Short-term investments	36,804	50,844
Recurring basis | Fixed maturity securities:
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	1,016,098	941,077
Recurring basis | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	144,366	105,841
Recurring basis | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	158,661	150,798
Recurring basis | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	413,721	378,448
Recurring basis | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	182,611	168,047
Recurring basis | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	35,847	65,014
Recurring basis | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	78,874	71,120
Recurring basis | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	2,018	1,809
Recurring basis | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	82,522	76,401
Recurring basis | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	66,281	64,827
Recurring basis | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	16,241	11,574
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	93,772	81,305
Trading securities:
Fixed maturity securities	0	1,250
Short-term investments	1,000	1,100
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Fixed maturity securities:
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	78,265	70,465
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	78,265	70,465
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	15,507	10,840
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	15,507	10,840
Recurring basis | Significant Other Observable Inputs Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	1,000,168	931,173
Trading securities:
Fixed maturity securities	3,808	3,813
Short-term investments	35,804	49,744
Recurring basis | Significant Other Observable Inputs Level 2 | Fixed maturity securities:
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	933,153	865,612
Recurring basis | Significant Other Observable Inputs Level 2 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	144,366	105,841
Recurring basis | Significant Other Observable Inputs Level 2 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	158,661	150,798
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	413,721	378,448
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	177,931	163,047
Recurring basis | Significant Other Observable Inputs Level 2 | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	35,847	65,014
Recurring basis | Significant Other Observable Inputs Level 2 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	609	655
Recurring basis | Significant Other Observable Inputs Level 2 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	2,018	1,809
Recurring basis | Significant Other Observable Inputs Level 2 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	67,015	65,561
Recurring basis | Significant Other Observable Inputs Level 2 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	66,281	64,827
Recurring basis | Significant Other Observable Inputs Level 2 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	734	734
Recurring basis | Significant Unobservable Inputs Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	4,680	5,000
Trading securities:
Fixed maturity securities	0	0
Short-term investments	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Fixed maturity securities:
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	4,680	5,000
Recurring basis | Significant Unobservable Inputs Level 3 | State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Residential mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Commercial mortgage and asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	4,680	5,000
Recurring basis | Significant Unobservable Inputs Level 3 | Obligations of U.S. government corporations and agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Redeemable preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Preferred stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Common stock
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total available-for-sale securities	$ 0	$ 0

Fair Value Measurements - Su106

Fair Value Measurements - Summary of Available-for-Sale Fixed Maturity Securities Measured at Fair Value on a Recurring Basis (Details) - Available-for-sale fixed maturity securities - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5,000	$ 5,000	$ 0
Transfers out of Level 3	0	0	0
Transfers in to Level 3	0	0	0
Purchases	0	0	5,000
Sales	0	0	0
Maturities and calls	(320)	0	0
Amortization of discount	0	0	0
Total gains or losses (realized/unrealized) - Included in earnings	0	0	0
Total gains or losses (realized/unrealized) - Included in other comprehensive income	0	0	0
Ending balance	$ 4,680	$ 5,000	$ 5,000

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unpaid principal on bank loan participations for which external sources were unavailable to determine fair value	$ 807,000	$ 2,300,000
Carrying value of bank loan participations for which external sources were unavailable to determine fair value	0	2,000,000
Available-for-sale fixed maturity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of available-for-sale fixed maturity security	$ 4,680,000	$ 5,000,000	$ 5,000,000	$ 0

Fair Value Measurements - Su108

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Nonecurring Basis (Details) - Nonrecurring basis - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank loan participations held-for-investment	$ 5,111	$ 4,849
Quoted Prices in Active Markets for Identical Assets Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank loan participations held-for-investment	0	0
Significant Other Observable Inputs Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank loan participations held-for-investment	0	0
Significant Unobservable Inputs Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank loan participations held-for-investment	$ 5,111	$ 4,849

Fair Value Measurements - Su109

Fair Value Measurements - Summary of the Carrying Value and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale:
Fixed maturity securities	$ 1,016,098	$ 941,077
Equity securities	82,522	76,401
Trading securities:
Bank loan participations held-for-investment, at amortized cost, net of allowance	238,214	203,526
Cash and cash equivalents	163,495	109,784	$ 106,406	$ 73,383
Short-term investments	36,804	50,844
Liabilities
Senior debt	98,300	88,300
Carrying Value
Available-for-sale:
Fixed maturity securities	1,016,098	941,077
Equity securities	82,522	76,401
Trading securities:
Fixed maturity securities	3,808	5,063
Bank loan participations held-for-investment, at amortized cost, net of allowance	238,214	203,526
Cash and cash equivalents	163,495	109,784
Short-term investments	36,804	50,844
Other invested assets – notes receivable	11,778	4,500
Liabilities
Senior debt	98,300	88,300
Junior subordinated debt	104,055	104,055
Fair Value
Available-for-sale:
Fixed maturity securities	1,016,098	941,077
Equity securities	82,522	76,401
Trading securities:
Fixed maturity securities	3,808	5,063
Bank loan participations held-for-investment, at amortized cost, net of allowance	236,532	203,123
Cash and cash equivalents	163,495	109,784
Short-term investments	36,804	50,844
Other invested assets – notes receivable	17,104	6,008
Liabilities
Senior debt	97,884	85,404
Junior subordinated debt	$ 116,569	$ 99,397

Statutory Matters - Schedule of

Statutory Matters - Schedule of Statutory Accounting (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Matters [Abstract]
Statutory net income	$ 31,881	$ 13,468	$ 14,747
Statutory capital and surplus	219,132	184,859	176,884
Minimum required statutory capital and surplus	$ 25,000	$ 25,000	$ 25,000

Statutory Matters - Narrative (

Statutory Matters - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statutory Accounting Practices [Line Items]
Minimum statutory solvency margin	$ 25,000	$ 25,000	$ 25,000
Actual statutory capital and surplus	219,132	184,859	176,884
Statutory net income	31,881	13,468	14,747
Shareholders' equity	694,699	693,221	681,038	$ 687,921
JRG Re
Statutory Accounting Practices [Line Items]
Shareholders' equity	409,700
JRG Re | BERMUDA
Statutory Accounting Practices [Line Items]
Minimum statutory solvency margin	109,900
Actual statutory capital and surplus	409,200
Statutory net income	$ 20,100	$ 74,700	$ 54,300
JRG Re | Minimum | BERMUDA
Statutory Accounting Practices [Line Items]
Minimum liquidity ratio	75.00%

Dividend Restrictions (Details)

Dividend Restrictions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Line Items]
Retained earnings	$ 48,198	$ 55,232
JRG Re
Statutory Accounting Practices [Line Items]
Dividend limitations percentage of statutory surplus	10.00%
Maximum amount of dividends available without regulatory approval	$ 37,000
Percentage of dividend withholding tax for distributions	5.00%
Percentage of dividend distribution tax	30.00%
Maximum amount of dividends and return of capital available	$ 102,300

Other Related Party Transact113

Other Related Party Transactions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Lease term (in years)	10 years
Increase in fixed annual rent	2.00%
Chairman of the Board and Chief Executive Officer
Related Party Transaction [Line Items]
Total fees paid for the use of plane	$ 314,000	$ 246,000	$ 211,000
Independent Directors | Consulting Agreement
Related Party Transaction [Line Items]
Investment and other business consulting services fees	$ 37,500	$ 150,000	$ 150,000

Subsequent Events (Details)

Subsequent Events (Details) - $ / shares	1 Months Ended	12 Months Ended
	Feb. 14, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 22, 2018
RSUs
Subsequent Event [Line Items]
Number of awards granted		137,034	60,291	0
Fair value on grant date		$ 42.20	$ 32.03	$ 0
RSUs | Minimum
Subsequent Event [Line Items]
Vesting period (in years)		1 year
RSUs | Maximum
Subsequent Event [Line Items]
Vesting period (in years)		5 years
2014 Long Term Incentive Plan | RSUs
Subsequent Event [Line Items]
Number of awards granted	145,141
Fair value on grant date	$ 39.71
2014 Long Term Incentive Plan | RSUs | Minimum
Subsequent Event [Line Items]
Vesting period (in years)	1 year
2014 Long Term Incentive Plan | RSUs | Maximum
Subsequent Event [Line Items]
Vesting period (in years)	3 years
Subsequent event
Subsequent Event [Line Items]
Dividends payable, per common share					$ 0.30

Unaudited Selected Quarterly115

Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 173,490	$ 260,166	$ 170,671	$ 133,071	$ 1,081,905	$ 737,398	$ 572,194
Total revenues	217,963	220,866	202,394	176,402	169,022	151,365	134,511	131,329	817,625	586,227	504,921
Net income	224	10,351	14,541	18,450	25,672	21,366	14,596	12,837	43,566	74,471	53,497
Comprehensive income (loss)	$ (2,015)	$ 11,067	$ 21,249	$ 22,484	$ (3,127)	$ 19,694	$ 27,388	$ 28,457	$ 52,785	$ 72,412	$ 38,327
Earnings per share:
Basic earnings per share (in dollars per share)	$ 0.01	$ 0.35	$ 0.49	$ 0.63	$ 0.88	$ 0.73	$ 0.50	$ 0.44	$ 1.48	$ 2.56	$ 1.87
Diluted earnings per share (in dollars per share)	$ 0.01	$ 0.34	$ 0.48	$ 0.61	$ 0.85	$ 0.71	$ 0.49	$ 0.43	$ 1.44	$ 2.49	$ 1.82
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									$ 235,355	$ 184,333	$ 172,499
Total revenues									$ 240,751	$ 190,064	$ 197,586
Commission expense | Casualty Reinsurance
Segment Reporting Information [Line Items]
Net income			$ (2,000)

SCHEDULE I - Summary of Inve116

SCHEDULE I - Summary of Investments - Other than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2017USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amount at which shown on Balance Sheet	$ 1,402,813
Bank loan participations, held-for-investment, net of allowance
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	238,214
Fair Value	236,532
Amount at which shown on Balance Sheet	238,214
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	35,804
Fair Value	35,804
Amount at which shown on Balance Sheet	35,804
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amount at which shown on Balance Sheet	26,367
Available-for-sale Securities | Fixed maturity securities:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	1,008,662
Fair Value	1,016,098
Amount at which shown on Balance Sheet	1,016,098
Available-for-sale Securities | State and municipal
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	139,382
Fair Value	144,366
Amount at which shown on Balance Sheet	144,366
Available-for-sale Securities | Residential mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	160,379
Fair Value	158,661
Amount at which shown on Balance Sheet	158,661
Available-for-sale Securities | Corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	408,857
Fair Value	413,721
Amount at which shown on Balance Sheet	413,721
Available-for-sale Securities | Commercial mortgage and asset-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	182,595
Fair Value	182,611
Amount at which shown on Balance Sheet	182,611
Available-for-sale Securities | Obligations of U.S. government corporations and agencies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	35,948
Fair Value	35,847
Amount at which shown on Balance Sheet	35,847
Available-for-sale Securities | U.S. Treasury securities and obligations guaranteed by the U.S. government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	79,476
Fair Value	78,874
Amount at which shown on Balance Sheet	78,874
Available-for-sale Securities | Redeemable preferred stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	2,025
Fair Value	2,018
Amount at which shown on Balance Sheet	2,018
Available-for-sale Securities | Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	75,318
Fair Value	82,522
Amount at which shown on Balance Sheet	82,522
Available-for-sale Securities | Preferred stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	59,102
Fair Value	66,281
Amount at which shown on Balance Sheet	66,281
Available-for-sale Securities | Common stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	16,216
Fair Value	16,241
Amount at which shown on Balance Sheet	16,241
Trading Securities | Fixed maturity securities:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or Amortized Cost	3,801
Fair Value	3,808
Amount at which shown on Balance Sheet	$ 3,808

SCHEDULE II - Condensed Fina117

SCHEDULE II - Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets [Abstract]
Cash and cash equivalents	$ 163,495	$ 109,784	$ 106,406	$ 73,383
Other assets	83,598	64,980
Total assets	2,756,695	2,346,533	2,055,497
Liabilities:
Accrued expenses	39,295	36,884
Senior debt	98,300	88,300
Junior subordinated debt	104,055	104,055
Other liabilities	48,368	46,844
Total liabilities	2,061,996	1,653,312
Commitments and contingent liabilities	0	0
Shareholders’ equity:
Class A common shares	6	6
Additional paid-in capital	636,149	636,856
Retained earnings	48,198	55,232
Accumulated other comprehensive income	10,346	1,127
Total shareholders’ equity	694,699	693,221	681,038	687,921
Total liabilities and shareholders’ equity	2,756,695	2,346,533
Parent Company
Assets [Abstract]
Cash and cash equivalents	2,849	2,969	$ 1,976	$ 623
Investment in subsidiaries	910,610	897,729
Other assets	2,351	2,415
Total assets	915,810	903,113
Liabilities:
Accrued expenses	1,406	1,428
Senior debt	83,300	73,300
Junior subordinated debt	15,928	15,928
Notes payable to subsidiary	100,000	100,000
Due to subsidiaries	20,043	18,549
Other liabilities	434	687
Total liabilities	221,111	209,892
Commitments and contingent liabilities	0	0
Shareholders’ equity:
Class A common shares	6	6
Additional paid-in capital	636,149	636,856
Retained earnings	48,198	55,232
Accumulated other comprehensive income	10,346	1,127
Total shareholders’ equity	694,699	693,221
Total liabilities and shareholders’ equity	$ 915,810	$ 903,113

SCHEDULE II - Condensed Fina118

SCHEDULE II - Condensed Financial Information of Registrant (Details 1) - USD ($)	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:
Other income									$ 17,386,000	$ 10,361,000	$ 3,428,000
Total revenues	$ 217,963,000	$ 220,866,000	$ 202,394,000	$ 176,402,000	$ 169,022,000	$ 151,365,000	$ 134,511,000	$ 131,329,000	817,625,000	586,227,000	504,921,000
Expenses:
Other operating expenses									196,993,000	170,828,000	157,803,000
Other expenses									539,000	1,590,000	730,000
Interest expense									8,974,000	8,448,000	6,999,000
Total expenses									762,480,000	506,884,000	445,145,000
Loss before equity in net income of subsidiaries									55,145,000	79,343,000	59,776,000
Equity in net income of subsidiaries									13,209,000	8,743,000	2,468,000
Net income	224,000	10,351,000	14,541,000	18,450,000	25,672,000	21,366,000	14,596,000	12,837,000	43,566,000	74,471,000	53,497,000
Other comprehensive (loss) income:
Other comprehensive loss									9,219,000	(2,059,000)	(15,170,000)
Total comprehensive income	$ (2,015,000)	$ 11,067,000	$ 21,249,000	$ 22,484,000	$ (3,127,000)	$ 19,694,000	$ 27,388,000	$ 28,457,000	52,785,000	72,412,000	38,327,000
Parent Company
Revenues:
Other income									49,000	44,000	40,000
Total revenues									49,000	44,000	40,000
Expenses:
Other operating expenses									10,375,000	9,467,000	8,254,000
Other expenses									640,000	293,000	284,000
Interest expense									4,130,000	3,974,000	3,687,000
Total expenses									15,145,000	13,734,000	12,225,000
Loss before equity in net income of subsidiaries									(15,096,000)	(13,690,000)	(12,185,000)
Equity in net income of subsidiaries									58,662,000	88,161,000	65,682,000
Net income									43,566,000	74,471,000	53,497,000
Other comprehensive (loss) income:
Other comprehensive loss									9,219,000	(2,059,000)	(15,170,000)
Total comprehensive income									$ 52,785,000	$ 72,412,000	$ 38,327,000

SCHEDULE II - Condensed Fina119

SCHEDULE II - Condensed Financial Information of Registrant (Details 2) - 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	$ 224	$ 10,351	$ 14,541	$ 18,450	$ 25,672	$ 21,366	$ 14,596	$ 12,837	$ 43,566	$ 74,471	$ 53,497
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Provision for depreciation and amortization									2,123	2,414	2,004
Share based compensation expense									7,688	5,492	3,735
Net cash provided by operating activities									207,816	154,349	119,971
Investing activities
Net cash used in investing activities									(104,741)	(80,764)	(34,163)
Financing activities
Dividends paid									(50,832)	(65,988)	(47,405)
Senior debt issuance									10,000	0	10,000
Senior debt repayments									0	0	(10,000)
Issuances of common shares under equity incentive plans									1,708	2,260	1,730
Common share repurchases									(9,448)	(4,907)	(6,461)
Net cash used in financing activities									(49,364)	(70,207)	(52,785)
Increase in cash and cash equivalents									53,711	3,378	33,023
Cash and cash equivalents at beginning of year				109,784				106,406	109,784	106,406	73,383
Cash and cash equivalents at end of year	163,495				109,784				163,495	109,784	106,406
Supplemental information
Interest paid									8,909	8,121	7,342
Parent Company
Operating activities
Net income									43,566	74,471	53,497
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Provision for depreciation and amortization									176	157	157
Share based compensation expense									7,688	5,492	3,735
Equity in undistributed earnings of subsidiaries									(23,662)	(88,161)	(65,682)
Changes in operating assets and liabilities:									684	(1,889)	9,532
Net cash provided by operating activities									28,452	(9,930)	1,239
Investing activities
Dividends from subsidiaries									20,000	80,000	52,250
Net cash used in investing activities									20,000	80,000	52,250
Financing activities
Dividends paid									(50,832)	(65,988)	(47,405)
Senior debt issuance									10,000	0	10,000
Senior debt repayments									0	0	(10,000)
Debt issue costs paid									0	(442)	0
Issuances of common shares under equity incentive plans									1,708	2,260	1,730
Common share repurchases									(9,448)	(4,907)	(6,461)
Net cash used in financing activities									(48,572)	(69,077)	(52,136)
Increase in cash and cash equivalents									(120)	993	1,353
Cash and cash equivalents at beginning of year				$ 2,969				$ 1,976	2,969	1,976	623
Cash and cash equivalents at end of year	$ 2,849				$ 2,969				2,849	2,969	1,976
Supplemental information
Interest paid									$ 4,612	$ 4,676	$ 4,337

SCHEDUEL II - Condensed Financi

SCHEDUEL II - Condensed Financial Information of Registrant - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Increase in net cash provided by operating activities	$ 3.2	$ 3.6
Increase in net cash provided by financing activities	$ 3.2	$ 3.6

SCHEDULE III - Supplementary121

SCHEDULE III - Supplementary Insurance Information (Details) - 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	$ 72,365	$ 64,789	$ 60,754
Reserve for Losses and Loss Adjustment Expenses	1,292,349	943,865	785,322
Unearned Premiums	418,114	390,563	301,104
Net Earned Premiums	741,109	515,663	461,205
Net Investment Income	61,119	52,638	44,835
Losses and Loss Adjustment Expenses	555,377	325,421	279,016
Amortization of Policy Acquisition Costs	116,001	101,624	97,750
Other Operating Expenses	196,993	170,828	157,803
Net Written Premiums	766,626	557,708	471,032
Excess and Surplus Lines
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	17,050	14,808	15,266
Reserve for Losses and Loss Adjustment Expenses	759,043	575,280	468,438
Unearned Premiums	149,047	137,290	114,369
Net Earned Premiums	463,521	301,404	240,878
Net Investment Income	15,014	18,051	13,427
Losses and Loss Adjustment Expenses	371,717	188,768	131,221
Amortization of Policy Acquisition Costs	45,158	46,669	42,069
Other Operating Expenses	79,136	75,467	65,233
Net Written Premiums	469,891	316,922	253,285
Specialty Admitted Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	(725)	1,767	2,184
Reserve for Losses and Loss Adjustment Expenses	271,446	128,795	76,179
Unearned Premiums	73,649	84,156	40,622
Net Earned Premiums	68,110	52,281	42,206
Net Investment Income	2,532	2,542	2,316
Losses and Loss Adjustment Expenses	44,863	30,897	25,623
Amortization of Policy Acquisition Costs	5,605	5,968	4,455
Other Operating Expenses	20,081	18,512	15,509
Net Written Premiums	60,957	55,803	44,917
Casualty Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	56,040	48,214	43,304
Reserve for Losses and Loss Adjustment Expenses	261,860	239,790	240,705
Unearned Premiums	195,418	169,117	146,113
Net Earned Premiums	209,478	161,978	178,121
Net Investment Income	31,507	27,257	22,706
Losses and Loss Adjustment Expenses	138,797	105,756	122,172
Amortization of Policy Acquisition Costs	65,238	48,987	51,226
Other Operating Expenses	72,446	56,416	58,507
Net Written Premiums	235,778	184,983	172,830
Corporate and other segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	0	0	0
Reserve for Losses and Loss Adjustment Expenses	0	0	0
Unearned Premiums	0	0	0
Net Earned Premiums	0	0	0
Net Investment Income	12,066	4,788	6,386
Losses and Loss Adjustment Expenses	0	0	0
Amortization of Policy Acquisition Costs	0	0	0
Other Operating Expenses	25,330	20,433	18,554
Net Written Premiums	$ 0	$ 0	$ 0

SCHEDULE IV - Reinsurance (Deta

SCHEDULE IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Written Premiums	$ 843,719	$ 549,700	$ 395,816
Ceded to Other Companies Written Premiums	315,279	179,690	101,162
Assumed from Other Companies Written Premiums	238,186	187,698	176,378
Net written premiums	$ 766,626	$ 557,708	$ 471,032
Percentage of Amount Assumed to Net	31.10%	33.70%	37.40%
Excess and Surplus Lines
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Written Premiums	$ 530,077	$ 370,802	$ 308,713
Ceded to Other Companies Written Premiums	60,229	53,922	55,432
Assumed from Other Companies Written Premiums	43	42	4
Net written premiums	$ 469,891	$ 316,922	$ 253,285
Percentage of Amount Assumed to Net	0.00%	0.00%	0.00%
Specialty Admitted Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Written Premiums	$ 313,642	$ 178,898	$ 87,103
Ceded to Other Companies Written Premiums	255,473	126,418	46,061
Assumed from Other Companies Written Premiums	2,788	3,323	3,875
Net written premiums	$ 60,957	$ 55,803	$ 44,917
Percentage of Amount Assumed to Net	4.60%	6.00%	8.60%
Casualty Reinsurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Written Premiums	$ 0	$ 0	$ 0
Ceded to Other Companies Written Premiums	(423)	(650)	(331)
Assumed from Other Companies Written Premiums	235,355	184,333	172,499
Net written premiums	$ 235,778	$ 184,983	$ 172,830
Percentage of Amount Assumed to Net	99.80%	99.60%	99.80%

SCHEDULE V - Valuation and Q123

SCHEDULE V - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 3,079	$ 7,088	$ 2,737
Additions Amounts Charged to Expense	3,453	23	4,957
Deductions Amounts Written Off or Disposals	556	(4,032)	(606)
Balance at End of Period	5,976	3,079	7,088
Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	2,136	2,778	1,985
Additions Amounts Charged to Expense	1,029	814	1,061
Deductions Amounts Written Off or Disposals	408	(1,456)	(268)
Balance at End of Period	2,757	2,136	2,778
Allowance for Credit Losses on Bank Loans
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	943	4,310	752
Additions Amounts Charged to Expense	2,424	(791)	3,896
Deductions Amounts Written Off or Disposals	148	(2,576)	(338)
Balance at End of Period	$ 3,219	$ 943	$ 4,310

SCHEDULE VI - Supplementary 124

SCHEDULE VI - Supplementary Information Concerning Property Casualty Insurance Operations (Details) - 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	$ 72,365	$ 64,789	$ 60,754
Reserve for losses and loss adjustment expenses	1,292,349	943,865	785,322
Unearned premiums	418,114	390,563	301,104
Net earned premiums	741,109	515,663	461,205
Net investment income	61,119	52,638	44,835
Current year	533,905	349,137	295,334
Prior year - Losses and loss adjustment expenses incurred	21,472	(23,716)	(16,318)
Total incurred losses and loss and adjustment expenses	555,377	325,421	279,016
Amortization of policy acquisition costs	116,001	101,624	97,750
Paid losses and loss adjustment expenses, net of reinsurance	326,680	217,827	214,524
Net written premiums	$ 766,626	$ 557,708	$ 471,032