James River (JRVR) 10-K 27 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 25, 2019	Jun. 30, 2018
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 Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Well-known Seasoned Issuer	Yes
Entity Common Stock, Shares Outstanding		30,067,545
Entity Public Float			$ 1,143,957,071
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2,018

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fixed maturity securities:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	$ 1,184,202	$ 1,016,098
Trading, at fair value (amortized cost: 2018 – $0; 2017 – $3,801)	0	3,808
Equity securities, at fair value (cost: 2018 – $77,152; 2017 – $75,318)	78,385	82,522
Bank loan participations held-for-investment, at amortized cost, net of allowance	260,972	238,214
Short-term investments	81,966	36,804
Other invested assets	72,321	70,208
Total invested assets	1,677,846	1,447,654
Cash and cash equivalents	172,457	163,495
Accrued investment income	11,110	8,381
Premiums receivable and agents’ balances, net	307,899	352,436
Reinsurance recoverable on unpaid losses	467,371	302,524
Reinsurance recoverable on paid losses	18,344	11,292
Prepaid reinsurance premiums	112,498	91,979
Deferred policy acquisition costs	54,450	72,365
Intangible assets, net	37,537	38,334
Goodwill	181,831	181,831
Deferred tax assets, net	1,054	0
Income taxes receivable	0	2,806
Other assets	94,379	83,598
Total assets	3,136,776	2,756,695
Liabilities:
Reserve for losses and loss adjustment expenses	1,661,459	1,292,349
Unearned premiums	386,473	418,114
Payables to reinsurers	61,662	56,268
Senior debt	118,300	98,300
Junior subordinated debt	104,055	104,055
Accrued expenses	51,792	39,295
Deferred tax liabilities, net	0	5,247
Income taxes payable	1,915	0
Other liabilities	41,879	48,368
Total liabilities	2,427,535	2,061,996
Commitments and contingent liabilities	0	0
Shareholders’ equity:
Common Shares – $0.0002 par value; 200,000,000 shares authorized. 2018 and 2017: 29,988,460 and 29,696,682 shares issued and outstanding, respectively	6	6
Preferred Shares – 2018 and 2017: $0.00125 par value; 20,000,000 shares authorized; no shares issued and outstanding	0	0
Additional paid-in capital	645,310	636,149
Retained earnings	79,753	48,198
Accumulated other comprehensive (loss) income	(15,828)	10,346
Total shareholders’ equity	709,241	694,699
Total liabilities and shareholders’ equity	$ 3,136,776	$ 2,756,695

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Available-for-sale debt securities, amortized cost (in dollars)	$ 1,199,409	$ 1,008,662
Trading securities, debt, amortized cost (in dollars)	0	3,801
Equity Securities, FV-NI, Cost	$ 77,512
Available-for-sale equity securities, amortized cost (in dollars)		$ 75,318
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	29,988,460	29,696,682
Common stock, shares outstanding	29,988,460	29,696,682
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 ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Gross written premiums	$ 1,166,773	$ 1,081,905	$ 737,398
Ceded written premiums	(404,101)	(315,279)	(179,690)
Net written premiums	762,672	766,626	557,708
Change in net unearned premiums	52,726	(25,517)	(42,045)
Net earned premiums	815,398	741,109	515,663
Net investment income	61,256	61,119	52,638
Net realized and unrealized (losses) gains on investments	(5,479)	(1,989)	7,565
Other income	14,424	17,386	10,361
Total revenues	885,599	817,625	586,227
Expenses:
Losses and loss adjustment expenses	600,276	555,377	325,421
Other operating expenses	201,035	196,993	170,828
Other expenses	1,300	539	1,590
Interest expense	11,553	8,974	8,448
Amortization of intangible assets	597	597	597
Total expenses	814,761	762,480	506,884
Income before income taxes	70,838	55,145	79,343
Income tax expense (benefit):
Current	12,867	11,943	(221)
Deferred	(5,859)	(364)	5,093
Total income tax expense	7,008	11,579	4,872
Net income	63,830	43,566	74,471
Other comprehensive (losses) income:
Net unrealized (losses) gains, net of taxes of $(444) in 2018, $2,707 in 2017, and $(1,723) in 2016	(22,203)	9,219	(2,059)
Total comprehensive income	$ 41,627	$ 52,785	$ 72,412
Per share data:
Basic earnings per share (in dollars per share)	$ 2.14	$ 1.48	$ 2.56
Diluted earnings per share (in dollars per share)	2.11	1.44	2.49
Dividend declared per share (in dollars per share)	$ 1.2	$ 1.70	$ 2.25
Weighted-average common shares outstanding:
Basic (in shares)	29,887,990	29,461,717	29,063,075
Diluted (in shares)	30,307,101	30,273,149	29,894,378

Consolidated Statements of In_2

Consolidated Statements of Income and Comprehensive Income (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Tax on net unrealized gains (losses)	$ (444)	$ 2,707	$ (1,723)

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 (Loss) Income
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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 15,633,000
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 63,830,000				63,830,000
Net income | ASU 2016-01	(4,700,000)
Other comprehensive loss	(22,203,000)					(22,203,000)
Dividends	(36,246,000)				(36,246,000)
Exercise of stock options and related excess tax benefits	4,045,000			4,045,000
Exercise of stock options and related excess tax benefits (in shares)		237,319
Vesting of RSUs and related excess tax benefits	(1,112,000)			(1,112,000)
Vesting of RSUs and related excess tax benefits (in shares)		54,459
Compensation expense under share incentive plans	6,228,000			6,228,000
Balances at Dec. 31, 2018	$ 709,241,000	$ 6,000	$ 0	$ 645,310,000	$ 79,753,000	$ (15,828,000)
Balances (in shares) at Dec. 31, 2018	29,988,460	29,988,460

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net income	$ 63,830,000	$ 43,566,000	$ 74,471,000
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred policy acquisition costs	(93,188,000)	(123,577,000)	(105,659,000)
Amortization of policy acquisition costs	111,103,000	116,001,000	101,624,000
Net realized investment losses (gains)	5,479,000	1,989,000	(7,565,000)
Impairment of intangible assets	200,000	0	0
Distributions from equity method investments	7,499,000	7,333,000	3,467,000
Income from equity method investments	(3,540,000)	(13,209,000)	(8,743,000)
Trading securities purchases, sales, and maturities, net	3,804,000	1,250,000	0
Deferred U.S. federal income tax (benefit) expense	(5,859,000)	(364,000)	5,093,000
Provision for depreciation and amortization	4,215,000	2,123,000	2,414,000
Share based compensation expense	6,228,000	7,688,000	5,492,000
Excess tax benefits from equity incentive plan transactions	(2,699,000)	(2,115,000)	3,191,000
Change in operating assets and liabilities:
Reserve for losses and loss adjustment expenses	369,110,000	348,484,000	158,543,000
Unearned premiums	(31,641,000)	27,551,000	89,459,000
Premiums receivable and agents’ balances	44,537,000	(87,121,000)	(88,630,000)
Reinsurance balances	(187,024,000)	(113,665,000)	(68,497,000)
Payable to insurance companies	(2,029,000)	445,000	(950,000)
Other	3,000	(8,563,000)	(9,361,000)
Net cash provided by operating activities	290,028,000	207,816,000	154,349,000
Securities available-for-sale:
Purchases – fixed maturity securities	(418,011,000)	(270,014,000)	(300,135,000)
Sales – fixed maturity securities	95,793,000	77,781,000	110,124,000
Maturities and calls – fixed maturity securities	129,139,000	121,890,000	135,472,000
Purchases – equity securities	(7,507,000)	(5,540,000)	(3,680,000)
Sales – equity securities	5,578,000	3,522,000	14,850,000
Bank loan participations:
Purchases	(203,328,000)	(240,799,000)	(156,638,000)
Sales	123,114,000	138,214,000	51,077,000
Maturities	58,742,000	69,740,000	97,097,000
Other invested assets:
Purchases	(6,993,000)	(8,913,000)	(2,365,000)
Return of capital	921,000	0	226,000
Maturities and repayments	0	0	6,500,000
Securities receivable or payable, net	2,268,000	(2,525,000)	1,018,000
Short-term investments, net	(45,162,000)	14,040,000	(31,574,000)
Other	(1,326,000)	(2,137,000)	(2,736,000)
Net cash used in investing activities	(266,772,000)	(104,741,000)	(80,764,000)
Financing activities
Senior debt issuances	20,000,000	10,000,000	0
Dividends paid	(36,123,000)	(50,832,000)	(65,988,000)
Issuances of common shares under equity incentive plans	5,172,000	1,708,000	2,260,000
Common share repurchases	(2,239,000)	(9,448,000)	(4,907,000)
Other financing activities	(1,104,000)	(792,000)	(1,572,000)
Net cash used in financing activities	(14,294,000)	(49,364,000)	(70,207,000)
Change in cash and cash equivalents	8,962,000	53,711,000	3,378,000
Cash and cash equivalents at beginning of year	163,495,000	109,784,000	106,406,000
Cash and cash equivalents at end of year	172,457,000	163,495,000	109,784,000
Supplemental information
Income taxes paid (refunded), net	8,174,000	9,848,000	(59,000)
Interest paid	$ 11,269,000	$ 8,909,000	$ 8,121,000

Accounting Policies

Accounting Policies	12 Months Ended
	Dec. 31, 2018
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 five insurance companies based in the United States (“U.S.”) focused on specialty insurance niches and two Bermuda-based reinsurance companies 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, 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, primarily provides non-catastrophe casualty reinsurance to U.S. third parties and to the Company’s U.S.-based insurance subsidiaries. • Carolina Re Ltd ("Carolina Re") was formed in 2018 and provides reinsurance to the Company's U.S.-based insurance subsidiaries. Carolina Re is also the cedent on a stop loss reinsurance treaty with JRG Re. ​ 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 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. 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 fixed maturity securities 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 and the length of time that fair value has been below amortized cost. In addition, 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. Effective January 1, 2018, with the adoption of ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities , equity securities (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) are measured at fair value with changes in fair value recognized in net investment income. Prior to the adoption of ASU 2016-01, changes in the fair value of equity securities were recognized net of taxes as a component of accumulated other comprehensive income. 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, 2018 or 2017 . 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, 2018 and 2017 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 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 $4.5 million and $2.6 million at December 31, 2018 and 2017 , respectively. The allowance for doubtful accounts was $3.9 million and $2.8 million at December 31, 2018 and 2017 , respectively. Bad debt expense was $1.5 million for the year ended December 31, 2018 , $1.0 million for the year ended December 31, 2017 , and $813,000 for the year ended December 31, 2016 . Receivables written off against the allowance for doubtful accounts totaled $313,000 for the year ended December 31, 2018 , $408,000 for the year ended December 31, 2017 , and $1.5 million for the year ended December 31, 2016 . 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. 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 loss experience under the contracts. Premiums written and earned, as well as related acquisition expenses are recorded based upon the projected loss 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 $13.9 million , $17.0 million , and $10.1 million is included in other income for the years ended December 31, 2018 , 2017 , and 2016 , 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 basis. 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. 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. 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). Forfeitures of share-based awards are recognized as they occur. 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. Upon adoption of ASU 2016-02, Leases (Topic 842) on January 1, 2019 , the Company will derecognize assets of $22.6 million and liabilities of $30.9 million associated with the above lease that was designated as build-to-suit under the previous guidance, and record a cumulative-effect adjustment to retained earnings of $8.3 million . The lease will be classified as an operating lease under the new standard. The Company will record a right-of-use asset and lease liability for the lease under the new standard. 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 $29.8 million and $32.1 million as of December 31, 2018 and 2017 , 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 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, 2018 ​ ​ ​ Basic $ 63,830 29,887,990 $ 2.14 Common share equivalents — 419,111 (0.03 ) Diluted $ 63,830 30,307,101 $ 2.11 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 Common share equivalents relate to our outstanding equity awards (stock options and RSUs). For the years ended December 31, 2018 and 2017 , common share equivalents of 180,329 shares and 174,380 shares, respectively, are excluded from the calculations of diluted earnings per share as their effects are anti-dilutive. For the year ended December 31, 2016, all common share equivalents are dilutive. Adopted Accounting Standards Effective January 1, 2018, the Company adopted Accounting Standards Update (“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 recognizes 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 included 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 performance obligation. The adoption of ASU 2014-09 had no impact on reported fee income and there was no cumulative effect of initially applying the update. Effective January 1, 2018, the Company adopted ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The Company's available-for-sale fixed maturity securities are summarized as follows: ​ Cost or Gross Gross Fair ​ (in thousands) December 31, 2018 Fixed maturity securities: State and municipal $ 147,160 $ 3,422 $ (1,287 ) $ 149,295 Residential mortgage-backed 208,869 577 (5,337 ) 204,109 Corporate 534,024 1,516 (10,772 ) 524,768 Commercial mortgage and asset-backed 199,528 310 (2,813 ) 197,025 U.S. Treasury securities and obligations guaranteed by the U.S. government 107,803 235 (845 ) 107,193 Redeemable preferred stock 2,025 — (213 ) 1,812 Total fixed maturity securities, available-for-sale $ 1,199,409 $ 6,060 $ (21,267 ) $ 1,184,202 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, available-for-sale $ 1,008,662 $ 14,564 $ (7,128 ) $ 1,016,098 The amortized cost and fair value of available-for-sale investments in fixed maturity securities at December 31, 2018 are summarized, by contractual maturity, as follows: ​ Amortized Fair ​ (in thousands) One year or less $ 52,312 $ 52,100 After one year through five years 406,647 403,126 After five years through ten years 213,673 209,155 After ten years 116,355 116,875 Residential mortgage-backed 208,869 204,109 Commercial mortgage and asset-backed 199,528 197,025 Redeemable preferred stock 2,025 1,812 Total $ 1,199,409 $ 1,184,202 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, 2018 Fixed maturity securities: State and municipal $ 19,733 $ (284 ) $ 47,018 $ (1,003 ) $ 66,751 $ (1,287 ) Residential mortgage-backed 49,180 (743 ) 105,778 (4,594 ) 154,958 (5,337 ) Corporate 243,384 (5,089 ) 155,902 (5,683 ) 399,286 (10,772 ) Commercial mortgage and asset-backed 106,423 (1,229 ) 51,805 (1,584 ) 158,228 (2,813 ) U.S. Treasury securities and obligations guaranteed by the U.S. government 17,618 (51 ) 54,201 (794 ) 71,819 (845 ) Redeemable preferred stock 1,812 (213 ) — — 1,812 (213 ) Total fixed maturity securities, available-for-sale $ 438,150 $ (7,609 ) $ 414,704 $ (13,658 ) $ 852,854 $ (21,267 ) 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, available-for-sale $ 313,795 $ (2,107 ) $ 245,969 $ (5,021 ) $ 559,764 $ (7,128 ) The Company held securities of 222 issuers that were in an unrealized loss position at December 31, 2018 with a total fair value of $852.9 million and gross unrealized losses of $21.3 million . None of the fixed maturity securities with unrealized losses has ever missed, or been delinquent on, a scheduled principal or interest payment. At December 31, 2018 , 100.0% 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. Management concluded that none of the fixed maturity securities with an unrealized loss at December 31, 2018 , 2017 , and 2016 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. At December 31, 2017 , 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. 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 and 2016 experienced an other-than-temporary impairment. 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, 2017 and 2016 , the Company held a participation in a loan issued by a company that produces and supplies power to Puerto Rico through a power purchase agreement with Puerto Rico Electric Power Authority (“PREPA”), a public corporation and governmental agency of the Commonwealth of Puerto Rico. Management concluded that the loan was impaired at December 31, 2016 and established an allowance for credit losses on the loan of $177,000 . After recording this impairment, the loan had a carrying value of $1.7 million at December 31, 2016 and unpaid principal of $2.0 million . At December 31, 2017 , management concluded that the loan was still impaired and established an allowance for credit losses of $759,000 to reduce the loan's carrying value to zero at December 31, 2017 . The unpaid principal on the loan was $807,000 at December 31, 2017 . In the first quarter of 2018, the full outstanding principal on the loan was repaid and the Company recognized a realized gain of $807,000 on the repayment. Management concluded that none of the loans in the Company's bank loan portfolio were impaired as of December 31, 2018 . At December 31, 2017 , the aggregate allowance for credit losses was $3.2 million 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, 2017 and 2016 , impairments in the bank loan portfolio largely reflect the impact of declining energy prices on the market values of loans to oil and gas companies in the energy sector. The average recorded investment in impaired bank loans was $2.6 million , $5.8 million , and $6.4 million during the years ended December 31, 2018 , 2017 , and 2016 , respectively, and investment income of $125,000 , $300,000 , and $297,000 was recognized during the time that the loans were impaired. The Company recorded realized losses of $858,000 and $2.4 million , and realized gains of $415,000 , during the years ended December 31, 2018 , 2017 , and 2016 , respectively, for changes in the fair value of impaired bank loans. At December 31, 2018 , unamortized discounts on bank loan participations were $1.8 million , and unamortized premiums were $17,000 . At December 31, 2017 , unamortized discounts on bank loan participations were $1.1 million , and unamortized premiums were $3,000 . Major categories of the Company’s net investment income are summarized as follows: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Fixed maturity securities $ 34,129 $ 26,833 $ 25,917 Bank loan participations 18,279 17,388 14,486 Equity securities 5,240 5,045 5,617 Other invested assets 5,165 14,079 9,536 Cash, cash equivalents, short-term investments, and other 2,681 1,708 824 Trading (losses) gains (4 ) (4 ) 18 Gross investment income 65,490 65,049 56,398 Investment expense (4,234 ) (3,930 ) (3,760 ) Net investment income $ 61,256 $ 61,119 $ 52,638   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, 2018 included $4,000 of net trading losses, all relating to securities sold during 2018 . 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 . The Company’s net realized and unrealized losses and gains on investments are summarized as follows: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Fixed maturity securities: Gross realized gains $ 422 $ 840 $ 1,916 Gross realized losses (976 ) (512 ) (106 ) ​ (554 ) 328 1,810 Equity securities: Gross realized gains — 429 4,761 Gross realized losses (62 ) (1,591 ) — Changes in fair values of equity securities (5,970 ) — — ​ (6,032 ) (1,162 ) 4,761 Bank loan participations: Gross realized gains 2,279 2,407 2,827 Gross realized losses (1,166 ) (3,557 ) (1,832 ) ​ 1,113 (1,150 ) 995 Short-term investments and other: Gross realized gains — 1 3 Gross realized losses (6 ) (6 ) (4 ) ​ (6 ) (5 ) (1 ) Total $ (5,479 ) $ (1,989 ) $ 7,565 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, ​ 2018 2017 2016 ​ (in thousands) Change in gross unrealized gains (losses): Fixed maturity securities $ (22,643 ) $ 6,571 $ (1,350 ) Equity securities — 5,356 (2,433 ) Total $ (22,643 ) $ 11,927 $ (3,783 )   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 ​ December 31, Year Ended December 31, ​ 2018 2017 2018 2017 2016 ​ (in thousands) (in thousands) Renewable energy LLCs (a) $ 29,795 $ 32,063 $ 2,974 $ 10,578 $ 3,480 Renewable energy notes receivable (b) 8,750 7,278 1,282 526 450 Limited partnerships (c) 29,276 26,367 566 2,632 5,263 Bank holding companies (d) 4,500 4,500 343 343 343 Total other invested assets $ 72,321 $ 70,208 $ 5,165 $ 14,079 $ 9,536 (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 entity for which two of our directors serve as officers, and the Company’s Non-Executive Chairman 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 $5.8 million and $5.6 million for the years ended December 31, 2018 and 2017 , respectively. ​ (b) The Company's Corporate and Other segment has invested in notes receivable for renewable energy projects. At December 31, 2018 , the Company holds one $8.8 million note issued by an entity for which two of our directors serve as officers . The amount invested in the note was $7.3 million at December 31, 2017 . Interest on the note, which matures in 2021, is fixed at 15% . Interest income on the note was $1.3 million and $526,000 for the years ended December 31, 2018 and 2017 , respectively. 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 . ​ (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 an investment in a limited partnership with a carrying value of $3.1 million and $3.0 million at December 31, 2018 and 2017 , and recognized investment income of $70,000 , $394,000 , and $455,000 for the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company’s Excess and Surplus Lines segment holds investments in limited partnerships of $26.2 million and $23.4 million at December 31, 2018 and 2017 , respectively. Investment income of $496,000 , $2.2 million , and $4.8 million were recognized on these investments for the years ended December 31, 2018 , 2017 , and 2016 , respectively. At December 31, 2018 , the Company’s Excess and Surplus Lines segment has outstanding commitments to invest another $625,000 in these limited partnerships. (d) The Company's Corporate and Other segment holds $4.5 million of subordinated notes issued by a bank holding company for which the Company’s Non-Executive Chairman was previously the Lead Independent Director and an investor and for which one of the Company’s directors was an investor and is currently a lender (the "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, 2018 , 2017 and 2016 . The Company previously held common shares issued by the Bank Holding Company. Dividend income of $299,000 was recorded on the shares for the year ended December 31, 2016 . 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, 2018 and 2017 , 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 $4.2 million at December 31, 2018 , is classified as an available-for-sale fixed maturity. The Company maintains fixed maturity securities, short-term investments, accrued investment income, and cash and cash equivalents amounting to $471.4 million at December 31, 2018 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, 2018 and 2017 , cash and investments with a fair value of $25.5 million and $16.5 million , respectively, were on deposit with state insurance departments to satisfy regulatory requirements. At December 31, 2018 , 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, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Balance at beginning of period $ 72,365 $ 64,789 $ 60,754 Policy acquisition costs deferred: Commissions 72,473 107,387 92,736 Underwriting and other issue expenses 20,715 16,190 12,923 ​ 93,188 123,577 105,659 Amortization of policy acquisition costs (111,103 ) (116,001 ) (101,624 ) Net change (17,915 ) 7,576 4,035 Balance at end of period $ 54,450 $ 72,365 $ 64,789

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2018
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 2018 , 2017 and 2016 indicated that no impairment of goodwill had occurred. The carrying amount of goodwill at December 31, 2018 and 2017 was $181.8 million . Accumulated goodwill impairment losses were $99.6 million at December 31, 2018 and 2017 . The most recent goodwill impairment losses occurred in 2010. Specifically identifiable intangible assets were acquired in the Merger. During the fourth quarters of 2018 , 2017 and 2016 , the indefinite-lived intangible assets for trademarks and insurance licenses and authorities were tested for impairment. Intangible assets for broker relationships that have specific lives and are subject to amortization were also reviewed for impairment. In the Specialty Admitted Insurance segment, Falls Lake General Insurance Company was merged into Falls Lake National in the fourth quarter of 2018. In connection with this merger, Falls Lake General Insurance Company surrendered its licenses to the various state insurance departments and reduced the carrying value of its intangible asset for "State Licenses" to $0 . This caused a $200,000 impairment in 2018 in the Specialty Admitted Insurance segment. There were no impairments recognized in 2017 or 2016 . The gross carrying amounts and accumulated amortization for each major specifically identifiable intangible asset class were as follows: ​ December 31, ​ 2018 2017 Weighted- Average Life (Years) Gross Accumulated Gross Accumulated ​ ​ (in thousands) Trademarks Indefinite $ 22,200 $ — $ 22,200 $ — Insurance licenses and authorities Indefinite 8,964 — 9,164 — Identifiable intangibles not subject to amortization ​ 31,164 — 31,364 — Broker relationships 24.6 11,611 5,238 11,611 4,641 Identifiable intangible assets subject to amortization ​ 11,611 5,238 11,611 4,641 ​ ​ $ 42,775 $ 5,238 $ 42,975 $ 4,641 Future estimated amortization of specifically identifiable intangible assets as of December 31, 2018 is as follows ( in thousands) : 2019 $ 597 2020 538 2021 363 2022 363 2023 363 Thereafter 4,149 Total $ 6,373 The table below summarizes the changes in the net carrying values of intangible assets by segment for 2018 : December 31, 2017 December 31, 2018 ​ 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 6,327 (362 ) — 5,965 ​ 30,927 (362 ) — 30,565 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — (200 ) 4,065 Broker relationships 642 (235 ) — 407 ​ 7,407 (235 ) (200 ) 6,972 Total identifiable intangible assets $ 38,334 $ (597 ) $ (200 ) $ 37,537   ​ 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 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, 2016 .

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net ​ Property and equipment, net of accumulated depreciation, is included in "other assets" on the consolidated balance sheets and consists of the following: ​ December 31, ​ 2018 2017 ​ (in thousands) Building, leased for which the Company has been deemed the owner for accounting purposes (Note 21) $ 30,971 $ 30,902 Electronic data processing hardware and software 7,423 6,156 Furniture and equipment 2,662 2,659 Property and equipment, cost basis 41,056 39,717 Accumulated depreciation (17,745 ) (14,716 ) Property and equipment, net $ 23,311 $ 25,001

Reserve for Losses and Loss Adj

Reserve for Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2018
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 actuaries perform a study on each of these parameters annually and make 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 development data 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. 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, ​ 2018 2017 2016 ​ (in thousands) Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period $ 989,825 $ 761,128 $ 653,534 Add: Incurred losses and loss adjustment expenses net of reinsurance: ​ ​ ​ Current year 582,604 533,905 349,137 Prior years 17,672 21,472 (23,716 ) Total incurred losses and loss and adjustment expenses 600,276 555,377 325,421 Deduct: Loss and loss adjustment expense payments net of reinsurance: ​ ​ ​ Current year 86,355 103,205 39,473 Prior years 309,658 223,475 178,354 Total loss and loss adjustment expense payments 396,013 326,680 217,827 Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period 1,194,088 989,825 761,128 Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period 467,371 302,524 182,737 Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period $ 1,661,459 $ 1,292,349 $ 943,865 The foregoing reconciliation shows that $17.7 million of adverse development was experienced in 2018 on the reserve for losses and loss adjustment expenses held at December 31, 2017. This adverse reserve development included $15.0 million of adverse development in the Excess and Surplus Lines segment, including $20.7 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 $5.7 million of favorable development in other Excess and Surplus Lines underwriting divisions primarily from favorable development in the Excess Property underwriting division related to the 2017 hurricanes. Favorable reserve development in the Specialty Admitted Insurance segment was $5.6 million and primarily came from accident years 2014 through 2016, as loss emergence on our workers’ compensation business written prior to 2016 continued to develop more favorably than we had anticipated. In addition, $8.2 million of adverse development occurred in the Casualty Reinsurance segment, with a majority of this adverse development coming from accident years at least four years old and treaties the Company has since non-renewed. 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 continued 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 casualty business continued 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 following tables present incurred and paid losses and loss adjustment expenses, net of reinsurance as of December 31, 2018 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, 2017 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 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 114,834 $ 110,783 $ 106,480 $ 98,502 $ 86,691 $ 81,764 $ 83,431 $ 83,846 $ 85,470 $ 85,807 2010 78,424 80,569 78,117 73,035 69,080 69,964 70,294 70,913 71,312 2011 111,190 119,927 114,473 106,564 106,381 106,130 106,643 106,536 2012 97,908 98,672 97,829 96,497 97,306 99,619 101,271 2013 96,729 96,064 85,433 81,009 82,830 83,855 2014 114,942 104,092 90,267 82,232 84,074 2015 126,443 113,417 104,847 102,434 2016 138,507 125,093 126,050 2017 144,349 131,897 2018 167,004 Total $ 1,060,240   Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 29,860 $ 41,687 $ 51,731 $ 61,548 $ 67,293 $ 71,245 $ 76,091 $ 79,014 $ 81,496 $ 82,560 2010 13,673 26,418 35,812 45,641 52,071 57,371 61,307 64,214 65,246 2011 27,684 53,109 72,732 81,696 90,884 94,998 98,684 99,798 2012 6,944 33,757 49,604 63,216 74,869 82,545 88,812 2013 3,867 14,509 30,382 44,421 59,641 66,553 2014 3,412 16,969 28,212 43,891 58,774 2015 4,048 17,164 34,801 55,911 2016 5,180 22,852 46,045 2017 5,290 22,956 2018 6,000 Total $ 592,655 All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance (33 claims outstanding) $ 12,187 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 479,772 Excess and Surplus Lines — Commercial Auto Incurred losses and adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 1,255 $ 1,300 $ 1,451 $ 1,351 $ 1,301 $ 1,277 2014 20,487 14,071 17,233 18,953 19,779 2015 30,109 33,113 35,149 36,139 2016 74,340 109,286 126,791 2017 207,355 208,743 2018 255,881 Total $ 648,610 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 60 $ 1,182 $ 1,285 $ 1,291 $ 1,275 $ 1,275 2014 6,166 8,645 12,679 16,359 18,678 2015 8,356 15,234 24,282 31,592 2016 18,295 54,054 89,381 2017 41,467 107,377 2018 45,136 Total $ 293,439 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 355,171 Specialty Admitted — Individual Risk Workers’ Compensation Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 28,691 $ 28,526 $ 27,535 $ 28,116 $ 27,795 $ 26,171 $ 26,169 $ 26,232 $ 26,194 $ 25,660 2010 27,209 28,736 30,464 30,373 28,963 28,938 27,590 27,098 27,099 2011 37,834 41,421 40,154 38,999 38,311 37,455 36,594 36,593 2012 32,116 32,420 31,490 29,689 28,255 28,174 28,186 2013 12,525 13,668 12,786 11,578 10,907 10,909 2014 16,638 16,652 14,620 13,890 12,704 2015 20,938 21,274 19,741 18,376 2016 21,678 20,299 18,050 2017 24,869 22,071 2018 16,432 Total $ 216,080 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 7,277 $ 16,945 $ 21,095 $ 22,646 $ 24,231 $ 24,192 $ 24,350 $ 24,418 $ 25,660 $ 25,660 2010 7,157 16,245 21,805 23,898 25,210 25,477 26,345 26,352 26,358 2011 10,123 23,127 29,021 33,204 34,240 34,287 34,334 34,614 2012 9,222 20,308 24,755 26,435 26,897 26,932 26,963 2013 4,487 8,723 9,846 10,246 10,263 10,309 2014 4,633 10,648 12,041 12,236 12,282 2015 6,604 13,285 15,118 15,889 2016 4,664 10,227 12,135 2017 6,546 12,782 2018 4,497 Total $ 181,489 All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance (4 claims outstanding) $ 1,056 Outstanding losses and loss adjustment expenses assumed from involuntary workers’ compensation pools $ 4,966 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 40,613 Specialty Admitted — Fronting and Programs Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 104 $ 80 $ 52 $ 52 $ 52 $ 52 2014 3,460 3,468 3,818 3,425 3,228 2015 7,136 9,632 9,358 8,974 2016 11,542 15,670 14,682 2017 21,229 24,271 2018 21,758 Total $ 72,965 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 28 $ 52 $ 52 $ 52 $ 52 $ 52 2014 883 1,687 2,369 2,728 2,854 2015 2,058 4,666 6,165 6,919 2016 1,894 5,123 6,888 2017 1,223 6,682 2018 $ 885 Total $ 24,280 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 48,685 Casualty Reinsurance Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 34,587 $ 28,244 $ 24,125 $ 26,458 $ 27,078 $ 27,116 $ 26,989 $ 26,931 $ 26,980 $ 27,168 2010 64,413 60,476 61,068 62,714 61,344 60,949 60,978 61,619 62,605 2011 114,908 103,123 97,366 97,812 98,993 99,282 101,276 103,196 2012 148,251 132,388 131,281 135,594 136,813 139,978 143,305 2013 133,230 130,361 131,352 134,446 137,801 143,124 2014 118,881 115,927 114,636 116,981 121,200 2015 119,157 108,870 108,699 109,117 2016 112,759 105,533 103,544 2017 134,628 128,472 2018 121,529 Total $ 1,063,260 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 6,487 $ 9,926 $ 12,956 $ 16,466 $ 19,672 $ 21,646 $ 23,024 $ 23,796 $ 24,649 $ 25,209 2010 21,918 31,500 38,430 44,921 49,263 52,761 54,659 57,013 58,707 2011 48,688 61,922 68,616 78,164 87,267 90,287 94,627 97,715 2012 73,124 81,859 97,215 113,943 121,026 128,567 133,606 2013 59,756 75,094 93,902 108,396 119,256 127,732 2014 41,421 58,601 76,302 89,899 101,366 2015 40,021 53,986 68,002 80,208 2016 36,268 50,905 65,409 2017 47,739 72,891 2018 30,903 Total $ 793,746 All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance $ 333 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 269,847 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, 2018 is as follows (in thousands): E&S – excluding commercial auto $ 479,772 E&S – commercial auto 355,171 Specialty Admitted – individual risk workers’ compensation 40,613 Specialty Admitted – fronting and programs 48,685 Casualty Reinsurance 269,847 Net reserve for losses and loss adjustment expenses 1,194,088 Reinsurance recoverables on unpaid losses 467,371 Gross reserve for losses and loss adjustment expenses $ 1,661,459 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, 2018 . 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 five 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 10.0 % 16.0 % 17.4 % 18.0 % 14.5 % 7.9 % 5.6 % 3.4 % 1.1 % 2.3 % E&S – commercial auto 21.0 % 38.9 % 15.6 % 14.3 % 6.5 % 3.7 % Specialty Admitted – individual risk workers’ compensation 28.7 % 33.8 % 13.8 % 6.2 % 3.3 % 1.3 % 1.0 % 0.2 % 0.5 % 0.0 % Specialty Admitted – fronting and programs 7.7 % 16.7 % 7.2 % 1.5 % 0.2 % Casualty Reinsurance 27.5 % 17.7 % 12.5 % 8.7 % 6.1 % 4.5 % 3.4 % 2.7 % 2.2 % 1.9 % 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: Excess and Surplus Lines — Excluding Commercial Auto Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2009 $ 85,807 $ 2,188 1,698 2010 71,312 2,890 1,380 2011 106,536 3,919 1,465 2012 101,271 4,339 1,765 2013 83,855 10,248 2,298 2014 84,074 12,440 2,106 2015 102,434 21,215 2,434 2016 126,050 44,649 2,754 2017 131,897 79,836 2,630 2018 167,004 135,018 3,242 Excess and Surplus Lines — Commercial Auto Accident Year Incurred Losses and Loss Adj Expenses IBNR Cumulative # of Reported Claims ​ ($ in thousands) 2013 $ 1,277 $ 1 54 2014 19,779 653 7,761 2015 36,139 1,761 41,723 2016 126,791 9,206 88,746 2017 208,743 23,777 132,191 2018 255,881 155,760 91,798 Specialty Admitted - Individual Risk Workers’ Compensation Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2009 $ 25,660 $ — $ 1,490 2010 27,099 699 1,604 2011 36,593 1,786 1,814 2012 28,186 1,020 1,323 2013 10,909 510 540 2014 12,704 422 850 2015 18,376 2,177 975 2016 18,050 5,010 833 2017 22,071 6,434 1,087 2018 16,432 5,422 1,170   Specialty Admitted — Fronting and Programs Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2013 $ 52 $ — 22 2014 3,228 238 856 2015 8,974 1,076 1,355 2016 14,682 3,203 2,792 2017 24,271 10,083 6,572 2018 21,758 16,058 5,874 Casualty Reinsurance Accident Year Incurred Losses IBNR ​ ($ in thousands) 2009 $ 27,168 $ 431 2010 62,605 937 2011 103,196 1,427 2012 143,305 2,397 2013 143,124 3,330 2014 121,200 5,450 2015 109,117 14,595 2016 103,544 26,675 2017 128,472 52,785 2018 121,529 55,052 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, 2018
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 it is fronting. At December 31, 2018 , the Company had reinsurance recoverables on unpaid losses of $467.4 million and reinsurance recoverables on paid losses of $18.3 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, 2018 , reinsurance recoverables on unpaid losses from the Company’s three largest reinsurers were $151.6 million , $47.9 million , and $44.1 million , representing 52.1% of the total balance. At December 31, 2018 , prepaid reinsurance premiums ceded to three reinsurers totaled $30.8 million , $13.4 million , and $9.6 million , representing 47.8% of the total balance. Premiums written, premiums earned, and losses and loss adjustment expenses incurred are summarized as follows: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Written premiums: ​ ​ ​ Direct $ 1,027,222 $ 843,719 $ 549,700 Assumed 139,551 238,186 187,698 Ceded (404,101 ) (315,279 ) (179,690 ) Net $ 762,672 $ 766,626 $ 557,708 Earned premiums: ​ ​ ​ Direct $ 990,221 $ 842,182 $ 483,166 Assumed 208,192 173,472 164,771 Ceded (383,015 ) (274,545 ) (132,274 ) Net $ 815,398 $ 741,109 $ 515,663 Losses and loss adjustment expenses: ​ ​ ​ Direct $ 769,490 $ 626,318 $ 314,920 Assumed 131,346 142,818 108,029 Ceded (300,560 ) (213,759 ) (97,528 ) Net $ 600,276 $ 555,377 $ 325,421

Senior Debt

Senior Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Senior Debt	Senior Debt ​ The Company has a $215.0 million senior revolving credit facility (as amended or amended and restated, the "2013 Facility”). The 2013 Facility is comprised of the following at December 31, 2018: • A $102.5 million secured revolving facility utilized by JRG Re to issue letters of credit for the benefit of third-party reinsureds. This portion of our credit facility is secured by our investment securities. At December 31, 2018 , the Company had $75.5 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 which is currently 1.5% , which is subject to change according to terms in the credit agreement. At December 31, 2018 and 2017 , 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 2013 Facility. The 2013 Facility has been amended from time to time since its inception in 2013. 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 entered into a First Amendment to the 2013 Facility, which among other things, modified the financial covenants and increased the amount of additional debt which the Company may incur under new financings, subject to compliance with certain conditions. A subsidiary of the Bank Holding Company is one of the lenders for the Facility, with a $25.0 million commitment allocation on the total $215.0 million 2013 Facility. The 2013 Facility contains certain financial and other covenants (including risk-based capital, minimum shareholders’ equity levels, maximum ratios of total debt outstanding to total capitalization and minimum fixed charge coverage ratios) with which the Company was in compliance, at December 31, 2018 . On August 2, 2017, the Company and its wholly-owned subsidiary, JRG Re, together as borrowers, entered into a credit agreement (the "2017 Facility") that provides the Company 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 the borrowers' option, on a secured or ​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. Obligations under the 2017 Facility will carry a variable rate of interest subject to terms in the credit agreement and will mature 30 days after notice of termination from the lender. 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, 2018 , unsecured loans of $30.0 million and secured letters of credit totaling $7.6 million were outstanding under the facility. 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. The 2017 Facility contains certain financial and other covenants which we are in compliance with at December 31, 2018 . 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.​ The terms of the Senior Debt contain certain covenants, with which we are in compliance at December 31, 2018 , and which, among other things, restrict our ability to assume senior indebtedness secured by our U.S. holding company's common stock or its subsidiaries' capital stock or to issue shares of its subsidiaries' capital stock. Interest payable is included in “accrued expenses” in the accompanying consolidated balance sheets.

Junior Subordinated Debt

Junior Subordinated Debt	12 Months Ended
	Dec. 31, 2018
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.   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, 2018 . Interest payable is included in “accrued expenses” on the accompanying consolidated balance sheets.

Capital Stock

Capital Stock	12 Months Ended
	Dec. 31, 2018
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 ( 29,988,460 shares issued and outstanding at December 31, 2018 ) and 20,000,000 undesignated preferred shares, par value $0.00125 per share ( no shares issued or outstanding). Share activity in 2017 and 2018 includes issuances from stock option exercises and RSU vesting, increasing the number of common shares outstanding from 29,257,566 at December 31, 2016 to 29,696,682 at December 31, 2017 and 29,988,460 at December 31, 2018 . The Company has 4,321,463 common shares reserved for future issuance upon exercise or vesting of equity awards, as applicable. The Board of Directors declared the following cash dividends in 2018 , 2017 , and 2016 : Date of Declaration Dividend per Common Share Payable to Shareholders of Record on Payment Date Total Amount 2018 February 22, 2018 $0.30 March 12, 2018 March 30, 2018 $ 9,049,476 May 1, 2018 $0.30 June 11, 2018 June 29, 2018 $ 9,066,023 August 1, 2018 $0.30 September 10, 2018 September 28, 2018 $ 9,080,519 November 7, 2018 $0.30 December 14, 2018 December 28, 2018 $ 9,088,542 Total $1.20 $ 36,284,560 2017 ​ ​ ​ February 14, 2017 $0.30 March 13, 2017 March 31, 2017 $ 8,878,147 May 2, 2017 $0.30 June 12, 2017 June 30, 2017 $ 8,917,471 August 1, 2017 $0.30 September 11, 2017 September 29, 2017 $ 8,943,279 November 1, 2017 $0.30 December 15, 2017 December 28, 2017 $ 8,974,196 November 1, 2017 $0.50 December 15, 2017 December 28, 2017 $ 14,956,994 Total $1.70 $ 50,670,087 2016 February 16, 2016 $0.20 March 14, 2016 March 28, 2016 $ 5,848,561 May 3, 2016 $0.20 June 13, 2016 June 30, 2016 $ 5,873,582 August 3, 2016 $0.20 September 12, 2016 September 30, 2016 $ 5,882,342 November 1, 2016 $0.30 December 16, 2016 December 29, 2016 $ 8,847,431 November 1, 2016 $1.35 December 16, 2016 December 29, 2016 $ 39,813,440 Total $2.25 $ 66,265,356 Included in the dividends are $391,000 , $450,000 and $664,000 of dividend equivalents on RSUs, of which $557,000 , $434,000 and $666,000 were payable as of December 31, 2018 , 2017 , and 2016 , respectively.

Equity Awards

Equity Awards	12 Months Ended
	Dec. 31, 2018
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, RSUs, 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, 2018 , 1,724,865 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, RSUs, 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, 2018 , 21,663 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). Options The following table summarizes the option activity: ​ Year Ended December 31, ​ 2018 2017 2016 ​ Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Outstanding: ​ ​ ​ ​ ​ ​ Beginning of year 1,479,236 $ 27.81 2,234,699 $ 22.84 2,058,085 $ 18.11 Granted — $ — 205,244 $ 42.24 706,203 $ 32.07 Exercised (308,025 ) $ 22.01 (898,218 ) $ 18.53 (496,550 ) $ 16.02 Forfeited (55,887 ) $ 35.69 (62,489 ) $ 30.80 (33,039 ) $ 27.68 End of year 1,115,324 $ 29.02 1,479,236 $ 27.81 2,234,699 $ 22.84 Exercisable, end of year 814,421 $ 26.46 846,371 $ 22.35 1,207,479 $ 18.14 All of the outstanding options vest over two to 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 equal to the fair value of the underlying shares at the date of grant. 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 2018 , 2017 and 2016 was $4.6 million , $20.4 million and $10.2 million , respectively. The aggregate intrinsic value of options outstanding at December 31, 2018 , 2017 and 2016 was $9.4 million , $18.5 million and $41.8 million , respectively. The aggregate intrinsic value of options exercisable at December 31, 2018 , 2017 and 2016 was $8.5 million , $14.9 million and $28.3 million , respectively. The fair value used for calculating intrinsic values was $36.54 , $40.01 and $41.55 at December 31, 2018 , 2017 and 2016 , respectively. The weighted-average remaining contractual life of the options outstanding and options exercisable at December 31, 2018 is 3.6 years and 3.3 years , respectively. There were no options granted in 2018 . The weighted-average fair value of options granted during 2017 and 2016 was $8.21 and $5.55 , 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 Risk-free interest rate 1.97 % 1.23 % Dividend yield 2.85 % 2.50 % Expected share price volatility 27.39 % 25.00 % Expected life 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 was based upon dividends expected to be declared over the life of the options at the date of grant. In 2017, the share price volatility assumption was based upon the Company's 100-day realized volatility. In years prior to 2017, 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 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 RSU activity: ​ Year Ended December 31, ​ 2018 2017 2016 ​ 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 178,882 $ 37.93 196,800 $ 24.38 234,922 $ 21.00 Granted 227,481 $ 39.74 137,034 $ 42.20 60,291 $ 32.03 Vested (83,384 ) $ 37.61 (132,764 ) $ 24.24 (98,413 ) $ 21.00 Forfeited (22,837 ) $ 40.21 (22,188 ) $ 26.06 — $ — Unvested, end of year 300,142 $ 39.22 178,882 $ 37.93 196,800 $ 24.38 The vesting period of RSUs granted to employees range from one to five years and vest ratably over the respective vesting period, with the majority vesting 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 2018 , 2017 and 2016 was $3.2 million , $5.5 million and $3.8 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, ​ 2018 2017 2016 ​ (in thousands) Share based compensation expense $ 6,228 $ 7,688 $ 5,492 U.S. tax benefit on share based compensation expense $ 716 $ 2,093 $ 1,532 As of December 31, 2018 , the Company had $8.4 million of unrecognized share based compensation expense expected to be charged to earnings over a weighted-average period of 1.8 years .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes ​ Under current Bermuda law, James River Group Holdings, Ltd. and its Bermuda based subsidiaries, JRG Re and Carolina Re, are not required to pay any Bermuda taxes on their income or capital gains. Those companies have 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 2018 or 2016. The Company’s U.S. subsidiaries are subject to federal, state and local corporate income taxes, and other taxes applicable to U.S. corporations. In addition, Carolina Re is subject to Federal income taxes as a result of its irrevocable election to be taxed as a U.S. domestic corporation under Section 953(d) of the Code. The Company’s U.S.-domiciled subsidiaries and Carolina Re file a consolidated U.S. federal income tax return. The 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, 2015. Financial results for 2017 reflect provisional amounts related to the December 2017 enactment of the Tax Act. The Company did not record any amounts related to the changes in loss reserve discounting required by the Tax Act during 2017. These changes required the IRS to publish new discount factors based on loss payment patterns and interest rates determined under the Tax Act. During 2018, factors were published that allowed us to adjust our current and deferred tax liabilities based on the provisions of the Tax Act. The Tax Act has specific transition provisions associated with reserve discounting. The initial impact of the adjustments in 2018 was an increase to our deferred tax asset for the additional discount as of December 31, 2017 of $8.8 million offset by an increase to our deferred tax liability of $8.8 million representing the 8 year transition provision required by the Tax Act. During 2018, $1.1 million of this transition provision was recognized in our current provision and adjusted out of our deferred tax liability. There were other provisions of the Tax Act for which the Company finalized its estimates during 2018, and the impact of these was immaterial. 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 21% have been used in 2018 for Bermuda and the U.S., respectively. A Federal statutory rate of 35% has been used for the U.S. in 2017 and 2016. U.S. deferred taxes were remeasured at 21% in 2017, down from 35% in 2016. U.S. income before Federal income taxes was $37.5 million , $50.1 million , and $18.3 million for the years ending December 31, 2018 , 2017 , and 2016 , 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, ​ 2018 2017 2016 ​ (in thousands) Federal income tax expense at applicable statutory rates $ 7,875 $ 17,541 $ 6,401 Tax-exempt investment income (272 ) (586 ) (643 ) Dividends received deduction (307 ) (792 ) (880 ) Excess tax benefits on share based compensation (567 ) (2,114 ) — Effect of tax rate reduction on deferred tax liability 220 (3,498 ) — Other 145 (90 ) (6 ) Federal income tax expense $ 7,094 $ 10,461 $ 4,872 U.S. state income tax (benefit) expense (86 ) 65 — U.S. dividend withholding tax — 1,053 — Total income tax expense $ 7,008 $ 11,579 $ 4,872 The significant components of net deferred tax assets (liabilities) at the corporate income tax rate of 21% for the years ended December 31, 2018 and 2017 are summarized as follows: ​ December 31, ​ 2018 2017 ​ (in thousands) Deferred tax assets: ​ ​ Accrued compensation expenses $ 2,456 $ 1,712 Reserve for losses and loss adjustment expenses 7,482 3,036 Unearned premiums 5,711 1,709 Share based compensation 1,352 1,143 Allowance for doubtful accounts 829 579 Deferred policy acquisition costs — 1,509 Property and equipment 1,909 1,601 Invested asset impairments — 791 Other 1,903 1,564 Total deferred tax assets 21,642 13,644 Deferred tax liabilities: Intangible assets 7,461 7,521 Net unrealized gains 887 2,578 Deferred policy acquisition costs 3,491 — Equity method investments 8,582 8,348 Other 167 444 Total deferred tax liabilities 20,588 18,891 Net deferred tax assets (liabilities) $ 1,054 $ (5,247 ) 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, 2018 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, 2018 or 2017 . 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, 2018 , 2017 , and 2016 , the Company paid $823,000 , $3.4 million , and $2.6 million , respectively, of federal excise taxes on its intercompany reinsurance transactions. The Company also paid excise taxes of $2.1 million , $2.2 million , and $1.6 million for the years ended December 31, 2018 , 2017 , and 2016 , 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, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Amortization of policy acquisition costs $ 111,103 $ 116,001 $ 101,624 Other underwriting expenses of the insurance segments 63,029 55,662 48,771 Other operating expenses of the Corporate and Other segment 26,903 25,330 20,433 Total $ 201,035 $ 196,993 $ 170,828 Other expenses consist of the following: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Employee severance $ 1,386 $ 147 $ 1,492 Income on leased building the Company is deemed to own for accounting purposes (see Note 21) (623 ) (248 ) (196 ) Legal and professional services related to secondary share offerings 337 535 281 Other — 105 13 Impairment of intangible asset 200 — — Total $ 1,300 $ 539 $ 1,590

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2018
Employee Benefits and Share-based Compensation, Noncash [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, 2018 , 2017 , and 2016 , the expense associated with the Savings Plans totaled $2.9 million , $2.5 million , and $1.9 million , respectively.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	12 Months Ended
	Dec. 31, 2018
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 $4.6 million , $4.2 million , and $3.7 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. As of December 31, 2018 , future minimum payments under non-cancelable operating leases for office space are as follows (in thousands): 2019 $ 3,946 2020 3,790 2021 3,507 2022 2,639 2023 2,376 Thereafter 4,705 ​ $ 20,963 Included in the future minimum lease payments is $15.2 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, 2018 was $440,000 and total remaining payments of $468,000 will be paid in 2019. 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, $52.6 million of letters of credit were issued through December 31, 2018 which were secured by deposits of $65.7 million . Under the 2013 Facility, $75.5 million of letters of credit were issued through December 31, 2018 which were secured by deposits of $94.0 million . Under the 2017 Facility, $7.6 million of letters of credit were issued through December 31, 2018 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 $301.5 million at December 31, 2018 (see Note 2). The Company is a party 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, 2018 , the cash equivalent collateral held in the collateral trust arrangement was approximately $1,099.2 million , which exceeds the amount of claims receivable and unpaid reported losses and loss adjustment expenses outstanding. The Company has 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 (Loss) Inco

Other Comprehensive (Loss) Income	12 Months Ended
	Dec. 31, 2018
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive (Loss) Income	Other Comprehensive (Loss) Income The following table summarizes the components of other comprehensive (loss) income: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Unrealized (losses) gains arising during the period, before U.S. income taxes $ (23,201 ) $ 11,091 $ 2,790 U.S. income taxes 483 (2,374 ) (414 ) Unrealized (losses) gains arising during the period, net of U.S. income taxes (22,718 ) 8,717 2,376 Less reclassification adjustment: Net realized investment (losses) gains (554 ) (835 ) 6,572 U.S. income taxes 39 333 (2,137 ) Reclassification adjustment for investment (losses) gains realized in net income (515 ) (502 ) 4,435 Other comprehensive (loss) income $ (22,203 ) $ 9,219 $ (2,059 ) In addition to the $554,000 and $835,000 of realized investment losses and $6.6 million of realized investment gains on available-for-sale fixed maturity securities for the years ended December 31, 2018 , 2017 , and 2016 , the Company recognized $1.1 million of realized investment gains, $1.2 million of realized investment losses, and $995,000 of realized investment gains in the respective years on its investments in bank loan participations. 2018 also includes net realized losses of $6.0 million for the change in net unrealized gains on equity securities in accordance with the Company's adoption of ASU 2016-01 effective January 1, 2018.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
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 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 and unrealized (losses) gains on investments. 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. Gross fee income of the Excess and Surplus Lines segment are included in that segment’s underwriting profit (loss). Gross fee income of $13.9 million , $17.0 million and $10.1 million was included in underwriting profit for the years ended December 31, 2018 , 2017 and 2016 , 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, 2018 ​ ​ ​ ​ ​ ​ Gross written premiums $ 656,538 $ 374,346 $ 135,889 $ — ​ $ 1,166,773 Net earned premiums 555,684 55,146 204,568 — ​ 815,398 Segment revenues 580,785 56,717 243,178 4,919 ​ 885,599 Net investment income 14,456 3,262 38,838 4,700 ​ 61,256 Interest expense — — — 11,553 ​ 11,553 Underwriting profit of operating segments 42,834 6,972 5,103 — ​ 54,909 Segment goodwill 181,831 — — — ​ 181,831 Segment assets 972,111 633,689 1,453,754 77,222 ​ 3,136,776 ​ 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 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 The following table reconciles the underwriting profit (loss) of insurance segments by individual segment to income before taxes: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Underwriting profit (loss) of the operating segments: ​ ​ ​ ​ ​ Excess and Surplus Lines $ 42,834 $ 29,693 $ 47,235 Specialty Admitted Insurance 6,972 3,166 2,872 Casualty Reinsurance 5,103 (1,765 ) (194 ) Total underwriting profit of operating segments 54,909 31,094 49,913 Other operating expenses of the Corporate and Other segment (26,903 ) (25,330 ) (20,433 ) Underwriting profit 28,006 5,764 29,480 Net investment income 61,256 61,119 52,638 Net realized and unrealized (losses) gains on investments (5,479 ) (1,989 ) 7,565 Other income 505 361 295 Other expenses (1,300 ) (539 ) (1,590 ) Interest expense (11,553 ) (8,974 ) (8,448 ) Amortization of intangible assets (597 ) (597 ) (597 ) Income before income taxes $ 70,838 $ 55,145 $ 79,343 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, ​ 2018 2017 2016 ​ (in thousands) Commercial Auto $ 322,126 ​ $ 247,960 ​ $ 110,050 Manufacturers and Contractors 79,160 ​ 85,719 ​ 83,279 Excess Casualty 66,452 ​ 51,160 ​ 43,574 General Casualty 54,127 ​ 38,097 ​ 36,858 Energy 33,942 ​ 29,704 ​ 29,709 Allied Health 30,450 ​ 19,181 ​ 14,413 Excess Property 16,963 ​ 14,447 ​ 14,083 Life Sciences 16,636 ​ 12,981 ​ 11,132 Small Business 14,808 ​ 11,307 ​ 9,104 Professional Liability 5,916 ​ 6,326 ​ 8,361 Environmental 10,499 ​ 7,920 ​ 5,321 Medical Professionals 1,774 ​ 2,297 ​ 2,739 Sports and Entertainment 3,685 ​ 3,021 ​ 2,221 Total Excess and Surplus Lines segment 656,538 ​ 530,120 ​ 370,844 Specialty Admitted Insurance segment 374,346 ​ 316,430 ​ 182,221 Casualty Reinsurance segment 135,889 ​ 235,355 ​ 184,333 Total $ 1,166,773 ​ $ 1,081,905 ​ $ 737,398 The Company does business with two brokers that generated $358.3 million and $81.4 million of gross written premiums for the Excess and Surplus Lines segment for the year ended December 31, 2018 , representing 30.7% (BB&T Insurance Services) and 7.0% of consolidated gross written premiums and 54.6% and 12.4% 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, 2018 . The Company does business with one insured (Rasier LLC) that generated $294.3 million of gross written premiums and $13.9 million of gross fee income for the Excess and Surplus Lines segment for the year ended December 31, 2018 , representing 25.2% of consolidated gross written premiums and 44.8% 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 (Atlas General Insurance Services) that generated $201.7 million of gross written premiums for the Specialty Admitted Insurance segment for the year ended December 31, 2018 , representing 17.3% of the consolidated gross written premiums and 53.9% 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, 2018 . The Company does business with two brokers that generated $54.0 million and $42.3 million of gross written premiums for the Casualty Reinsurance segment for the year ended December 31, 2018 , representing 4.6% and 3.6% of consolidated gross written premiums and 39.8% and 31.1% 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, 2018 . The Casualty Reinsurance segment assumed business from three unaffiliated ceding companies that generated $83.4 million , $22.1 million , and $20.8 million of gross written premiums for the year ended December 31, 2018 , representing 7.1% , 1.9% , and 1.8% of consolidated gross written premiums and 61.4% , 16.2% , and 15.3% of the Casualty Reinsurance segment’s gross written premiums, respectively.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
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, 2016. 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 its 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. Assets measured at fair value on a recurring basis as of December 31, 2018 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) Fixed maturity securities, available-for-sale: ​ ​ ​ ​ State and municipal $ — $ 149,295 $ — $ 149,295 Residential mortgage-backed — 204,109 — 204,109 Corporate — 524,768 — 524,768 Commercial mortgage and asset-backed — 192,797 4,228 197,025 U.S. Treasury securities and obligations guaranteed by the U.S. government 106,651 542 — 107,193 Redeemable preferred stock — 1,812 — 1,812 Total fixed maturity securities, available-for-sale $ 106,651 $ 1,073,323 $ 4,228 $ 1,184,202 Equity securities: Preferred stock $ — $ 60,740 $ — $ 60,740 Common stock 16,674 757 214 17,645 Total equity securities $ 16,674 $ 61,497 $ 214 $ 78,385 Short-term investments $ 1,250 $ 80,716 $ — $ 81,966 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) Fixed maturity securities, available-for-sale: ​ ​ ​ ​ ​ ​ 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, available-for-sale $ 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 Trading securities: Fixed maturity securities $ — $ 3,808 $ — $ 3,808 Short-term investments $ 1,000 $ 35,804 $ — $ 36,804 A reconciliation of the beginning and ending balances of available-for-sale fixed maturity securities and equity securities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) is shown below: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Beginning balance $ 4,680 $ 5,000 ​ $ 5,000 Transfers out of Level 3 — — ​ — Transfers in to Level 3 — — ​ — Purchases 214 — ​ — Sales — — ​ — Maturities and calls (452 ) (320 ) — Amortization of discount — — ​ — Total gains or losses (realized/unrealized): ​ Included in earnings — — ​ — Included in other comprehensive income — — ​ — Ending balance $ 4,442 $ 4,680 ​ $ 5,000 The Company held one available-for-sale fixed maturity security at December 31, 2018 and 2017 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 $4.2 million for the security at December 31, 2018 and $4.7 million at December 31, 2017 . At December 31, 2018 , the Company held one equity security 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 $214,000 for the security at December 31, 2018 . 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 2018 , 2017 or 2016 . 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, 2018 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, 2018 . 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, 2018 ​ ​ ​ ​ ​ ​ ​ Bank loan participations held-for-investment $ — $ — ​ $ — ​ $ — December 31, 2017 Bank loan participations held-for-investment $ — $ — ​ $ 5,111 ​ $ 5,111 Management concluded that none of the bank loan participations held-for-investment were impaired as of December 31, 2018 . At December 31, 2017 , bank loan participations held for investment that were determined to be impaired were written down to their fair value of $5.1 million . 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, 2018 , there were no investments for which external sources were unavailable to determine fair value. 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. The carrying values and fair values of financial instruments are summarized below: ​ December 31, ​ 2018 2017 ​ Carrying Value Fair Value Carrying Value Fair Value ​ (in thousands) Assets ​ ​ ​ ​ ​ Fixed maturity securities, available-for-sale $ 1,184,202 ​ $ 1,184,202 $ 1,016,098 $ 1,016,098 Equity securities 78,385 ​ 78,385 82,522 82,522 Fixed maturity securities, trading — ​ — 3,808 3,808 Bank loan participations held-for-investment 260,972 ​ 250,697 238,214 236,532 Cash and cash equivalents 172,457 ​ 172,457 163,495 163,495 Short-term investments 81,966 ​ 81,966 36,804 36,804 Other invested assets – notes receivable 13,250 ​ 18,687 11,778 17,104 Liabilities ​ ​ ​ Senior debt 118,300 ​ 118,317 98,300 97,884 Junior subordinated debt 104,055 ​ 117,057 104,055 116,569 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, 2018 and 2017 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, 2018 and 2017 , respectively. The fair values of bank loan participations held-for-investment, senior debt, and Junior Subordinated Debt at December 31, 2018 and 2017 were determined using inputs to the valuation methodology that are unobservable (Level 3).

Statutory Matters

Statutory Matters	12 Months Ended
	Dec. 31, 2018
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, 2018 , 2017 , and 2016 and for the years then ended are summarized as follows: ​ 2018 2017 2016 ​ (in thousands) Statutory net income $ 6,770 $ 31,881 $ 13,468 Statutory capital and surplus 241,668 219,132 184,859 Minimum required statutory capital and surplus 24,850 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, 2018 , the insurance subsidiaries’ adjusted capital and surplus exceeds their authorized control level RBC. Bermuda The Company has two Bermuda-based insurance subsidiaries: JRG Re, a Class 3B insurer and Carolina Re, a Class 3A insurer. Under the Bermuda Insurance Act 1978 and related regulations, an insurer must maintain minimum statutory capital and surplus at the greater of a minimum solvency margin (“MSM”) and the Enhanced Capital Requirement (“ECR”), which is the higher of the MSM and capital calculated by the Bermuda Solvency Capital Requirement (“BSCR”) model or an approved internal model. The combined Bermuda insurers minimum statutory solvency margin required at December 31, 2018 was approximately $161.3 million ( 2017 : $109.9 million ). Actual combined statutory capital and surplus at December 31, 2018 was $439.4 million ( 2017 : $409.2 million ). The insurers had combined statutory net income of $86.5 million for 2018 , $20.1 million for 2017 , and $74.7 million for 2016 . The ECR for the year ended December 31, 2017 was $275.4 million . The BSCR models for the year ended December 31, 2018 will not be filed with the Bermuda Monetary Authority until April 30, 2019. The Company believes that the minimum statutory capital and surplus requirements will be met. The insurers must also 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, quoted alternative investments, accrued interest income, premiums receivable, losses recoverable from reinsurers, and funds withheld. The relevant liabilities include total insurance provisions and other liabilities less deferred income taxes and letters of credit, guarantees and other instruments. As of December 31, 2018 the minimum liquidity ratio requirements were met.

Dividend Restrictions

Dividend Restrictions	12 Months Ended
	Dec. 31, 2018
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 2019 without regulatory approval is $24.2 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 The Bermuda Insurance Act of 1978 prohibits an insurer 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. An insurer can declare or pay dividends without prior regulatory approval up to 25% of the total statutory capital and surplus. At December 31, 2018 , the maximum combined amount of dividends that can be paid without prior regulatory approval was approximately $109.8 million . However, this dividend amount is subject to annual enhanced solvency requirement calculations. As of December 31, 2018 , JRG Holdings had consolidated retained earnings of $79.8 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, 2018
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. Upon adoption of ASU 2016-02, Leases (Topic 842) on January 1, 2019 , the Company will derecognize assets of $22.6 million and liabilities of $30.9 million associated with the above lease that was designated as build-to-suit under the previous guidance, and record a cumulative-effect adjustment to retained earnings of $8.3 million . The lease will be classified as an operating lease under the new standard. The Company will record a right-of-use asset and lease liability for the lease under the new standard. The Non-Executive Chairman of the Board and former 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 $123,000 , $314,000 , and $246,000 for 2018 , 2017 , and 2016 , 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 2016 and $37,500 in 2017 for investment and other business consulting services. The Consulting Agreement was terminated on March 31, 2017.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 20, 2019 , the Board of Directors declared a cash dividend of $0.30 per share. The dividend is payable on March 29, 2019 to shareholders of record on March 11, 2019 . On February 20, 2019 , the Board of Directors granted awards under the 2014 LTIP and the 2014 Director Plan to the Company’s employees and directors. RSUs for 167,295 shares were awarded with a fair value on the date of grant of $42.07 per share. The RSUs vest over one to three year periods, depending on the award. On January 31, 2019, the Company repaid $20.0 million of unsecured borrowings on its 2017 Facility, reducing the balance of unsecured loans outstanding under the facility to $10.0 million .

Unaudited Selected Quarterly Fi

Unaudited Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2018
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: ​ 2018 Quarter 2018 ​ First Second Third Fourth Year ​ (in thousands, except per share data) Gross written premiums $ 298,116 $ 293,378 $ 279,969 $ 295,310 ​ $ 1,166,773 Total revenues 218,344 228,041 224,692 214,522 ​ 885,599 Net income 15,633 16,984 19,581 11,632 ​ 63,830 Comprehensive (loss) income (2,914 ) 10,426 15,192 18,923 ​ 41,627 Earnings per share: ​ ​ ​ ​ ​ ​ Basic (1) $ 0.53 $ 0.57 $ 0.65 $ 0.39 ​ $ 2.14 Diluted (1) $ 0.52 $ 0.56 $ 0.64 $ 0.38 ​ $ 2.11 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 (1) $ 0.63 $ 0.49 $ 0.35 $ 0.01 $ 1.48 Diluted (1) $ 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 . (1) Since the weighted-average shares for the quarter are calculated independently of the weighted-average shares for the year, quarterly earnings per share may not total to annual earnings per share.

SCHEDULE I - Summary of Investm

SCHEDULE I - Summary of Investments - Other than Investments in Related Parties	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-15, Insurance Companies, 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 $ 147,160 $ 149,295 $ 149,295 Residential mortgage-backed 208,869 204,109 204,109 Corporate 534,024 524,768 524,768 Commercial mortgage and asset-backed 199,528 197,025 197,025 U.S. Treasury securities and obligations guaranteed by the U.S. government 107,803 107,193 107,193 Redeemable preferred stock 2,025 1,812 1,812 Total fixed maturity securities, available-for-sale 1,199,409 1,184,202 1,184,202 Equity securities, available-for-sale Preferred Stock 57,689 60,740 60,740 Common Stock 19,463 17,645 17,645 Total equity securities 77,152 78,385 78,385 Bank loan participations, held-for-investment, net of allowance 260,972 250,697 260,972 Short-term investments 81,966 81,966 81,966 Other invested assets 29,276 Total invested assets $ 1,634,801 (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, 2018
Condensed Financial Information Disclosure [Abstract]
Statements of financial information of parent company only (Parent Company)	Condensed Financial Information of Registrant Balance Sheets (Parent Company) ​ December 31, ​ 2018 2017 ​ (in thousands) Assets ​ ​ Cash and cash equivalents $ 240 $ 2,849 Investment in subsidiaries 882,407 910,610 Due from subsidiaries 5,775 — Other assets 1,579 2,351 Total assets $ 890,001 $ 915,810 Liabilities and shareholders’ equity Liabilities: Accrued expenses $ 2,322 $ 1,406 Senior debt 103,300 83,300 Junior subordinated debt 15,928 15,928 Notes payable to subsidiary 40,000 100,000 Due to subsidiaries 18,605 20,043 Other liabilities 605 434 Total liabilities 180,760 221,111 Commitments and contingent liabilities — — Shareholders’ equity: Class A common shares 6 6 Additional paid-in capital 645,310 636,149 Retained earnings 79,753 48,198 Accumulated other comprehensive (loss) income (15,828 ) 10,346 Total shareholders’ equity 709,241 694,699 Total liabilities and shareholders’ equity $ 890,001 $ 915,810 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, ​ 2018 2017 2016 ​ (in thousands) Revenues: Other income $ 58 $ 49 $ 44 Total revenues 58 49 44 Expenses: Other operating expenses 13,768 10,375 9,467 Other expenses 337 640 293 Interest expense 5,122 4,130 3,974 Total expenses 19,227 15,145 13,734 Loss before equity in net income of subsidiaries (19,169 ) (15,096 ) (13,690 ) Equity in net income of subsidiaries 82,999 58,662 88,161 Net income $ 63,830 $ 43,566 $ 74,471 Other comprehensive (loss) income: Equity in other comprehensive (losses) earnings of subsidiaries (22,203 ) 9,219 (2,059 ) Total comprehensive income $ 41,627 $ 52,785 $ 72,412 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, ​ 2018 2017 2016 Operating activities Net income $ 63,830 $ 43,566 $ 74,471 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Provision for depreciation and amortization 176 176 157 Share based compensation expense 6,228 7,688 5,492 Equity in undistributed earnings of subsidiaries 6,000 (23,662 ) (88,161 ) Changes in operating assets and liabilities: (5,653 ) 684 (1,889 ) Net cash provided by (used in) operating activities 70,581 28,452 (9,930 ) Investing activities Dividends from subsidiaries — 20,000 80,000 Net cash provided by investing activities — 20,000 80,000 Financing activities Dividends paid (36,123 ) (50,832 ) (65,988 ) Senior debt issuance 20,000 10,000 — Senior debt repayments — — — Subsidiary note repayments (60,000 ) — — Debt issue costs paid — — (442 ) Issuances of common shares under equity incentive plans 5,172 1,708 2,260 Common share repurchases (2,239 ) (9,448 ) (4,907 ) Net cash used in financing activities (73,190 ) (48,572 ) (69,077 ) Change in cash and cash equivalents (2,609 ) (120 ) 993 Cash and cash equivalents at beginning of period 2,849 2,969 1,976 Cash and cash equivalents at end of period $ 240 $ 2,849 $ 2,969 Supplemental information Interest paid $ 5,052 $ 4,612 $ 4,676 ​ 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, 2018, the Company adopted ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities . Among other things, this ASU requires 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. Upon adoption on January 1, 2018, the Company made a $4.7 million cumulative-effect adjustment to increase retained earnings and reduce accumulated other comprehensive income. The adoption of ASU 2016-01 did not materially impact the Company's financial position, cash flows, or total comprehensive income. The Company's results of operations were impacted as changes in fair value of equity instruments are now presented in net income rather than other comprehensive (loss) income. For the year ended December 31, 2018 , the respective impact on net income was a reduction of $4.7 million . Effective January 1, 2018, the Company adopted ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. This update was issued as a result of the enactment of Public Law No. 115-97, informally titled the Tax Cuts and Jobs Act (the "Tax Act"), on December 22, 2017. The ASU allows for the option to reclassify the stranded tax effects resulting from the implementation of the Tax Act out of accumulated other comprehensive income and into retained earnings. As the adoption of ASU 2016-01 in 2018 resulted 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 fixed income securities were impacted by the adoption of ASU 2018-02. The reclassification resulted in a $711,000 decrease to the Company's retained earnings with a corresponding increase to accumulated other comprehensive income in the first quarter of 2018.

SCHEDULE III - Supplementary In

SCHEDULE III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Abstract]
Supplementary Insurance Information	Supplementary Insurance Information (in thousands) ​ Deferred Reserve Unearned Net Net Losses Amortization Other Net December 31, 2018 ​ ​ ​ ​ ​ ​ ​ ​ ​ Excess and Surplus Lines $ 16,736 $ 960,562 $ 175,231 $ 555,684 $ 14,456 $ 437,904 $ 48,299 $ 88,865 $ 571,098 Specialty Admitted (1,829 ) 426,315 84,659 55,146 3,262 32,623 (1,792 ) 15,551 55,840 Casualty Reinsurance 39,543 274,582 126,583 204,568 38,838 129,749 64,596 69,716 135,734 Corporate and Other — — — — 4,700 — — 26,903 — Total $ 54,450 $ 1,661,459 $ 386,473 $ 815,398 $ 61,256 $ 600,276 $ 111,103 $ 201,035 $ 762,672 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

SCHEDULE IV - Reinsurance

SCHEDULE IV - Reinsurance	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [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, 2018 ​ ​ ​ ​ ​ Excess and Surplus Lines Written Premiums $ 656,538 $ 85,440 $ — $ 571,098 — Specialty Admitted Written Premiums 370,684 318,506 3,662 55,840 6.6 % Casualty Reinsurance Written Premiums — 155 135,889 135,734 100.1 % Total Written Premiums $ 1,027,222 $ 404,101 $ 139,551 $ 762,672 18.3 % 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 %

SCHEDULE V - Valuation and Qual

SCHEDULE V - Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-09, 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, 2018 ​ ​ ​ ​ Allowance for Doubtful Accounts $ 2,757 $ 1,504 $ (313 ) $ 3,948 Allowance for Credit Losses on Bank Loans 3,219 950 (4,169 ) — Total $ 5,976 $ 2,454 $ (4,482 ) $ 3,948 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

SCHEDULE VI - Supplementary Inf

SCHEDULE VI - Supplementary Information Concerning Property Casualty Insurance Operations	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Supplementary Information Concerning Property Casualty Insurance Operations	Supplementary Information Concerning Property Casualty Insurance Operations ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Deferred policy acquisition costs $ 54,450 $ 72,365 $ 64,789 Reserve for losses and loss adjustment expenses 1,661,459 1,292,349 943,865 Unearned premiums 386,473 418,114 390,563 Net earned premiums 815,398 741,109 515,663 Net investment income 61,256 61,119 52,638 Losses and loss adjustment expenses incurred: Current year 582,604 533,905 349,137 Prior year 17,672 21,472 (23,716 ) Total losses and loss adjustment expenses incurred 600,276 555,377 325,421 Amortization of policy acquisition costs 111,103 116,001 101,624 Paid losses and loss adjustment expenses, net of reinsurance 396,013 326,680 217,827 Net written premiums 762,672 766,626 557,708

Accounting Policies (Policies)

Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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 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. 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 fixed maturity securities 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 and the length of time that fair value has been below amortized cost. In addition, 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. Effective January 1, 2018, with the adoption of ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities , equity securities (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) are measured at fair value with changes in fair value recognized in net investment income. Prior to the adoption of ASU 2016-01, changes in the fair value of equity securities were recognized net of taxes as a component of accumulated other comprehensive income.
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, 2018 or 2017 . 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, 2018 and 2017 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 $4.5 million and $2.6 million at December 31, 2018 and 2017 , respectively. The allowance for doubtful accounts was $3.9 million and $2.8 million at December 31, 2018 and 2017 , respectively. Bad debt expense was $1.5 million for the year ended December 31, 2018 , $1.0 million for the year ended December 31, 2017 , and $813,000 for the year ended December 31, 2016 . Receivables written off against the allowance for doubtful accounts totaled $313,000 for the year ended December 31, 2018 , $408,000 for the year ended December 31, 2017 , and $1.5 million for the year ended December 31, 2016 . 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 loss experience under the contracts. Premiums written and earned, as well as related acquisition expenses are recorded based upon the projected loss 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 Other income is principally comprised of fee income earned on policies for which the Company has no exposure to underwriting risk. Fee income of $13.9 million , $17.0 million , and $10.1 million is included in other income for the years ended December 31, 2018 , 2017 , and 2016 , 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 basis. 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. 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). Forfeitures of share-based awards are recognized as they occur. 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. Upon adoption of ASU 2016-02, Leases (Topic 842) on January 1, 2019 , the Company will derecognize assets of $22.6 million and liabilities of $30.9 million associated with the above lease that was designated as build-to-suit under the previous guidance, and record a cumulative-effect adjustment to retained earnings of $8.3 million . The lease will be classified as an operating lease under the new standard. The Company will record a right-of-use asset and lease liability for the lease under the new standard.
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 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 and Prospective Accounting Standards	Adopted Accounting Standards Effective January 1, 2018, the Company adopted Accounting Standards Update (“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 recognizes 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 included 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 performance obligation. The adoption of ASU 2014-09 had no impact on reported fee income and there was no cumulative effect of initially applying the update. Effective January 1, 2018, the Company adopted ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities . Among other things, this ASU requires 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. Upon adoption on January 1, 2018, the Company made a $4.7 million cumulative-effect adjustment to increase retained earnings and reduce accumulated other comprehensive income. The adoption of ASU 2016-01 did not materially impact the Company's financial position, cash flows, or total comprehensive income. The Company's results of operations were impacted as changes in fair value of equity instruments are now presented in net income rather than other comprehensive (loss) income. For the year ended December 31, 2018 , the respective impact on net income was a reduction of $4.7 million ( $0.16 reduction in basic and diluted earnings per share). Effective January 1, 2018, the Company adopted ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. This update was issued as a result of the enactment of Public Law No. 115-97, informally titled the Tax Cuts and Jobs Act (the "Tax Act"), on December 22, 2017. The ASU allows for the option to reclassify the stranded tax effects resulting from the implementation of the Tax Act out of accumulated other comprehensive income and into retained earnings. As the adoption of ASU 2016-01 in 2018 resulted 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 fixed income securities were impacted by the adoption of ASU 2018-02. The reclassification resulted in a $711,000 decrease to the Company's retained earnings with a corresponding increase to accumulated other comprehensive income in the first quarter of 2018. Prospective Accounting Standards 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 they are designated as capital leases. 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. In the third quarter of 2018, the FASB issued ASU 2018-10 to clarify certain aspects of the guidance and ASU 2018-11, which provides an optional alternative transition method to initially apply the new leases standard at the adoption date (collectively, with ASU 2016-02, Topic 842). Topic 842 now allows for the use of either the modified retrospective adoption method or the alternative transition method. The Company has completed its evaluation and will adopt the new standard on January 1, 2019 using a modified retrospective transition method, applying the transition provisions at the beginning of the period of adoption. The Company will elect the package of practical expedients permitted under the transition guidance within the new standard and will not elect to use hindsight in determining the lease term. The new standard will not be applied to leases with an initial term of 12 months or less. Upon adoption of the new standard, the Company will derecognize assets of $22.6 million and liabilities of $30.9 million associated with a lease that was designated as build-to-suit under the previous guidance, and record a cumulative-effect adjustment to retained earnings of $8.3 million . The lease will be classified as an operating lease under the new standard. The Company will record right-of-use assets of $17.2 million and lease liabilities of $17.8 million at adoption of the new standard. The new standard will not materially impact the Company's results of operations or cash flows, and will not impact compliance under the covenants of our current credit agreements. 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.

Accounting Policies (Tables)

Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 ​ ​ ​ Basic $ 63,830 29,887,990 $ 2.14 Common share equivalents — 419,111 (0.03 ) Diluted $ 63,830 30,307,101 $ 2.11 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

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Schedule of summary of available-for-sale investments	The Company's available-for-sale fixed maturity securities are summarized as follows: ​ Cost or Gross Gross Fair ​ (in thousands) December 31, 2018 Fixed maturity securities: State and municipal $ 147,160 $ 3,422 $ (1,287 ) $ 149,295 Residential mortgage-backed 208,869 577 (5,337 ) 204,109 Corporate 534,024 1,516 (10,772 ) 524,768 Commercial mortgage and asset-backed 199,528 310 (2,813 ) 197,025 U.S. Treasury securities and obligations guaranteed by the U.S. government 107,803 235 (845 ) 107,193 Redeemable preferred stock 2,025 — (213 ) 1,812 Total fixed maturity securities, available-for-sale $ 1,199,409 $ 6,060 $ (21,267 ) $ 1,184,202 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, available-for-sale $ 1,008,662 $ 14,564 $ (7,128 ) $ 1,016,098
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, 2018 are summarized, by contractual maturity, as follows: ​ Amortized Fair ​ (in thousands) One year or less $ 52,312 $ 52,100 After one year through five years 406,647 403,126 After five years through ten years 213,673 209,155 After ten years 116,355 116,875 Residential mortgage-backed 208,869 204,109 Commercial mortgage and asset-backed 199,528 197,025 Redeemable preferred stock 2,025 1,812 Total $ 1,199,409 $ 1,184,202
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, 2018 Fixed maturity securities: State and municipal $ 19,733 $ (284 ) $ 47,018 $ (1,003 ) $ 66,751 $ (1,287 ) Residential mortgage-backed 49,180 (743 ) 105,778 (4,594 ) 154,958 (5,337 ) Corporate 243,384 (5,089 ) 155,902 (5,683 ) 399,286 (10,772 ) Commercial mortgage and asset-backed 106,423 (1,229 ) 51,805 (1,584 ) 158,228 (2,813 ) U.S. Treasury securities and obligations guaranteed by the U.S. government 17,618 (51 ) 54,201 (794 ) 71,819 (845 ) Redeemable preferred stock 1,812 (213 ) — — 1,812 (213 ) Total fixed maturity securities, available-for-sale $ 438,150 $ (7,609 ) $ 414,704 $ (13,658 ) $ 852,854 $ (21,267 ) 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, available-for-sale $ 313,795 $ (2,107 ) $ 245,969 $ (5,021 ) $ 559,764 $ (7,128 )
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, ​ 2018 2017 2016 ​ (in thousands) Fixed maturity securities $ 34,129 $ 26,833 $ 25,917 Bank loan participations 18,279 17,388 14,486 Equity securities 5,240 5,045 5,617 Other invested assets 5,165 14,079 9,536 Cash, cash equivalents, short-term investments, and other 2,681 1,708 824 Trading (losses) gains (4 ) (4 ) 18 Gross investment income 65,490 65,049 56,398 Investment expense (4,234 ) (3,930 ) (3,760 ) Net investment income $ 61,256 $ 61,119 $ 52,638
Schedule of summary of realized gains and losses	The Company’s net realized and unrealized losses and gains on investments are summarized as follows: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Fixed maturity securities: Gross realized gains $ 422 $ 840 $ 1,916 Gross realized losses (976 ) (512 ) (106 ) ​ (554 ) 328 1,810 Equity securities: Gross realized gains — 429 4,761 Gross realized losses (62 ) (1,591 ) — Changes in fair values of equity securities (5,970 ) — — ​ (6,032 ) (1,162 ) 4,761 Bank loan participations: Gross realized gains 2,279 2,407 2,827 Gross realized losses (1,166 ) (3,557 ) (1,832 ) ​ 1,113 (1,150 ) 995 Short-term investments and other: Gross realized gains — 1 3 Gross realized losses (6 ) (6 ) (4 ) ​ (6 ) (5 ) (1 ) Total $ (5,479 ) $ (1,989 ) $ 7,565
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, ​ 2018 2017 2016 ​ (in thousands) Change in gross unrealized gains (losses): Fixed maturity securities $ (22,643 ) $ 6,571 $ (1,350 ) Equity securities — 5,356 (2,433 ) Total $ (22,643 ) $ 11,927 $ (3,783 )
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 ​ December 31, Year Ended December 31, ​ 2018 2017 2018 2017 2016 ​ (in thousands) (in thousands) Renewable energy LLCs (a) $ 29,795 $ 32,063 $ 2,974 $ 10,578 $ 3,480 Renewable energy notes receivable (b) 8,750 7,278 1,282 526 450 Limited partnerships (c) 29,276 26,367 566 2,632 5,263 Bank holding companies (d) 4,500 4,500 343 343 343 Total other invested assets $ 72,321 $ 70,208 $ 5,165 $ 14,079 $ 9,536 (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 entity for which two of our directors serve as officers, and the Company’s Non-Executive Chairman 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 $5.8 million and $5.6 million for the years ended December 31, 2018 and 2017 , respectively. ​ (b) The Company's Corporate and Other segment has invested in notes receivable for renewable energy projects. At December 31, 2018 , the Company holds one $8.8 million note issued by an entity for which two of our directors serve as officers . The amount invested in the note was $7.3 million at December 31, 2017 . Interest on the note, which matures in 2021, is fixed at 15% . Interest income on the note was $1.3 million and $526,000 for the years ended December 31, 2018 and 2017 , respectively. 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 . ​ (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 an investment in a limited partnership with a carrying value of $3.1 million and $3.0 million at December 31, 2018 and 2017 , and recognized investment income of $70,000 , $394,000 , and $455,000 for the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company’s Excess and Surplus Lines segment holds investments in limited partnerships of $26.2 million and $23.4 million at December 31, 2018 and 2017 , respectively. Investment income of $496,000 , $2.2 million , and $4.8 million were recognized on these investments for the years ended December 31, 2018 , 2017 , and 2016 , respectively. At December 31, 2018 , the Company’s Excess and Surplus Lines segment has outstanding commitments to invest another $625,000 in these limited partnerships. (d) The Company's Corporate and Other segment holds $4.5 million of subordinated notes issued by a bank holding company for which the Company’s Non-Executive Chairman was previously the Lead Independent Director and an investor and for which one of the Company’s directors was an investor and is currently a lender (the "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, 2018 , 2017 and 2016 .

Deferred Policy Acquisition C_2

Deferred Policy Acquisition Costs (Tables)	12 Months Ended
	Dec. 31, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Balance at beginning of period $ 72,365 $ 64,789 $ 60,754 Policy acquisition costs deferred: Commissions 72,473 107,387 92,736 Underwriting and other issue expenses 20,715 16,190 12,923 ​ 93,188 123,577 105,659 Amortization of policy acquisition costs (111,103 ) (116,001 ) (101,624 ) Net change (17,915 ) 7,576 4,035 Balance at end of period $ 54,450 $ 72,365 $ 64,789

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2018
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, ​ 2018 2017 Weighted- Average Life (Years) Gross Accumulated Gross Accumulated ​ ​ (in thousands) Trademarks Indefinite $ 22,200 $ — $ 22,200 $ — Insurance licenses and authorities Indefinite 8,964 — 9,164 — Identifiable intangibles not subject to amortization ​ 31,164 — 31,364 — Broker relationships 24.6 11,611 5,238 11,611 4,641 Identifiable intangible assets subject to amortization ​ 11,611 5,238 11,611 4,641 ​ ​ $ 42,775 $ 5,238 $ 42,975 $ 4,641
Schedule of future estimated amortization	Future estimated amortization of specifically identifiable intangible assets as of December 31, 2018 is as follows ( in thousands) : 2019 $ 597 2020 538 2021 363 2022 363 2023 363 Thereafter 4,149 Total $ 6,373
Schedule of acquired indefinite-lived intangible assets	The table below summarizes the changes in the net carrying values of intangible assets by segment for 2018 : December 31, 2017 December 31, 2018 ​ 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 6,327 (362 ) — 5,965 ​ 30,927 (362 ) — 30,565 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — (200 ) 4,065 Broker relationships 642 (235 ) — 407 ​ 7,407 (235 ) (200 ) 6,972 Total identifiable intangible assets $ 38,334 $ (597 ) $ (200 ) $ 37,537   ​ 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
Schedule of acquired finite-lived intangible assets	The table below summarizes the changes in the net carrying values of intangible assets by segment for 2018 : December 31, 2017 December 31, 2018 ​ 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 6,327 (362 ) — 5,965 ​ 30,927 (362 ) — 30,565 Specialty Admitted Insurance Trademarks 2,500 — — 2,500 Insurance licenses and authorities 4,265 — (200 ) 4,065 Broker relationships 642 (235 ) — 407 ​ 7,407 (235 ) (200 ) 6,972 Total identifiable intangible assets $ 38,334 $ (597 ) $ (200 ) $ 37,537   ​ 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

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	Property and equipment, net of accumulated depreciation, is included in "other assets" on the consolidated balance sheets and consists of the following: ​ December 31, ​ 2018 2017 ​ (in thousands) Building, leased for which the Company has been deemed the owner for accounting purposes (Note 21) $ 30,971 $ 30,902 Electronic data processing hardware and software 7,423 6,156 Furniture and equipment 2,662 2,659 Property and equipment, cost basis 41,056 39,717 Accumulated depreciation (17,745 ) (14,716 ) Property and equipment, net $ 23,311 $ 25,001

Reserve for Losses and Loss A_2

Reserve for Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period $ 989,825 $ 761,128 $ 653,534 Add: Incurred losses and loss adjustment expenses net of reinsurance: ​ ​ ​ Current year 582,604 533,905 349,137 Prior years 17,672 21,472 (23,716 ) Total incurred losses and loss and adjustment expenses 600,276 555,377 325,421 Deduct: Loss and loss adjustment expense payments net of reinsurance: ​ ​ ​ Current year 86,355 103,205 39,473 Prior years 309,658 223,475 178,354 Total loss and loss adjustment expense payments 396,013 326,680 217,827 Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period 1,194,088 989,825 761,128 Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period 467,371 302,524 182,737 Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period $ 1,661,459 $ 1,292,349 $ 943,865
Schedule of incurred losses and loss adjustment expenses, net of reinsurance	Excess and Surplus Lines — Excluding Commercial Auto Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 114,834 $ 110,783 $ 106,480 $ 98,502 $ 86,691 $ 81,764 $ 83,431 $ 83,846 $ 85,470 $ 85,807 2010 78,424 80,569 78,117 73,035 69,080 69,964 70,294 70,913 71,312 2011 111,190 119,927 114,473 106,564 106,381 106,130 106,643 106,536 2012 97,908 98,672 97,829 96,497 97,306 99,619 101,271 2013 96,729 96,064 85,433 81,009 82,830 83,855 2014 114,942 104,092 90,267 82,232 84,074 2015 126,443 113,417 104,847 102,434 2016 138,507 125,093 126,050 2017 144,349 131,897 2018 167,004 Total $ 1,060,240   Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 29,860 $ 41,687 $ 51,731 $ 61,548 $ 67,293 $ 71,245 $ 76,091 $ 79,014 $ 81,496 $ 82,560 2010 13,673 26,418 35,812 45,641 52,071 57,371 61,307 64,214 65,246 2011 27,684 53,109 72,732 81,696 90,884 94,998 98,684 99,798 2012 6,944 33,757 49,604 63,216 74,869 82,545 88,812 2013 3,867 14,509 30,382 44,421 59,641 66,553 2014 3,412 16,969 28,212 43,891 58,774 2015 4,048 17,164 34,801 55,911 2016 5,180 22,852 46,045 2017 5,290 22,956 2018 6,000 Total $ 592,655 All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance (33 claims outstanding) $ 12,187 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 479,772 Excess and Surplus Lines — Commercial Auto Incurred losses and adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 1,255 $ 1,300 $ 1,451 $ 1,351 $ 1,301 $ 1,277 2014 20,487 14,071 17,233 18,953 19,779 2015 30,109 33,113 35,149 36,139 2016 74,340 109,286 126,791 2017 207,355 208,743 2018 255,881 Total $ 648,610 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 60 $ 1,182 $ 1,285 $ 1,291 $ 1,275 $ 1,275 2014 6,166 8,645 12,679 16,359 18,678 2015 8,356 15,234 24,282 31,592 2016 18,295 54,054 89,381 2017 41,467 107,377 2018 45,136 Total $ 293,439 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 355,171 Specialty Admitted — Individual Risk Workers’ Compensation Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 28,691 $ 28,526 $ 27,535 $ 28,116 $ 27,795 $ 26,171 $ 26,169 $ 26,232 $ 26,194 $ 25,660 2010 27,209 28,736 30,464 30,373 28,963 28,938 27,590 27,098 27,099 2011 37,834 41,421 40,154 38,999 38,311 37,455 36,594 36,593 2012 32,116 32,420 31,490 29,689 28,255 28,174 28,186 2013 12,525 13,668 12,786 11,578 10,907 10,909 2014 16,638 16,652 14,620 13,890 12,704 2015 20,938 21,274 19,741 18,376 2016 21,678 20,299 18,050 2017 24,869 22,071 2018 16,432 Total $ 216,080 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 7,277 $ 16,945 $ 21,095 $ 22,646 $ 24,231 $ 24,192 $ 24,350 $ 24,418 $ 25,660 $ 25,660 2010 7,157 16,245 21,805 23,898 25,210 25,477 26,345 26,352 26,358 2011 10,123 23,127 29,021 33,204 34,240 34,287 34,334 34,614 2012 9,222 20,308 24,755 26,435 26,897 26,932 26,963 2013 4,487 8,723 9,846 10,246 10,263 10,309 2014 4,633 10,648 12,041 12,236 12,282 2015 6,604 13,285 15,118 15,889 2016 4,664 10,227 12,135 2017 6,546 12,782 2018 4,497 Total $ 181,489 All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance (4 claims outstanding) $ 1,056 Outstanding losses and loss adjustment expenses assumed from involuntary workers’ compensation pools $ 4,966 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 40,613 Specialty Admitted — Fronting and Programs Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 104 $ 80 $ 52 $ 52 $ 52 $ 52 2014 3,460 3,468 3,818 3,425 3,228 2015 7,136 9,632 9,358 8,974 2016 11,542 15,670 14,682 2017 21,229 24,271 2018 21,758 Total $ 72,965 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2013 2014 2015 2016 2017 2018 2013 $ 28 $ 52 $ 52 $ 52 $ 52 $ 52 2014 883 1,687 2,369 2,728 2,854 2015 2,058 4,666 6,165 6,919 2016 1,894 5,123 6,888 2017 1,223 6,682 2018 $ 885 Total $ 24,280 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 48,685 Casualty Reinsurance Incurred losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 34,587 $ 28,244 $ 24,125 $ 26,458 $ 27,078 $ 27,116 $ 26,989 $ 26,931 $ 26,980 $ 27,168 2010 64,413 60,476 61,068 62,714 61,344 60,949 60,978 61,619 62,605 2011 114,908 103,123 97,366 97,812 98,993 99,282 101,276 103,196 2012 148,251 132,388 131,281 135,594 136,813 139,978 143,305 2013 133,230 130,361 131,352 134,446 137,801 143,124 2014 118,881 115,927 114,636 116,981 121,200 2015 119,157 108,870 108,699 109,117 2016 112,759 105,533 103,544 2017 134,628 128,472 2018 121,529 Total $ 1,063,260 Cumulative paid losses and loss adjustment expenses, net of reinsurance (in thousands) Accident Year 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2009 $ 6,487 $ 9,926 $ 12,956 $ 16,466 $ 19,672 $ 21,646 $ 23,024 $ 23,796 $ 24,649 $ 25,209 2010 21,918 31,500 38,430 44,921 49,263 52,761 54,659 57,013 58,707 2011 48,688 61,922 68,616 78,164 87,267 90,287 94,627 97,715 2012 73,124 81,859 97,215 113,943 121,026 128,567 133,606 2013 59,756 75,094 93,902 108,396 119,256 127,732 2014 41,421 58,601 76,302 89,899 101,366 2015 40,021 53,986 68,002 80,208 2016 36,268 50,905 65,409 2017 47,739 72,891 2018 30,903 Total $ 793,746 All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance $ 333 Total outstanding losses and loss adjustment expenses, net of reinsurance $ 269,847
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, 2018 is as follows (in thousands): E&S – excluding commercial auto $ 479,772 E&S – commercial auto 355,171 Specialty Admitted – individual risk workers’ compensation 40,613 Specialty Admitted – fronting and programs 48,685 Casualty Reinsurance 269,847 Net reserve for losses and loss adjustment expenses 1,194,088 Reinsurance recoverables on unpaid losses 467,371 Gross reserve for losses and loss adjustment expenses $ 1,661,459
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, 2018 . 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 five 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 10.0 % 16.0 % 17.4 % 18.0 % 14.5 % 7.9 % 5.6 % 3.4 % 1.1 % 2.3 % E&S – commercial auto 21.0 % 38.9 % 15.6 % 14.3 % 6.5 % 3.7 % Specialty Admitted – individual risk workers’ compensation 28.7 % 33.8 % 13.8 % 6.2 % 3.3 % 1.3 % 1.0 % 0.2 % 0.5 % 0.0 % Specialty Admitted – fronting and programs 7.7 % 16.7 % 7.2 % 1.5 % 0.2 % Casualty Reinsurance 27.5 % 17.7 % 12.5 % 8.7 % 6.1 % 4.5 % 3.4 % 2.7 % 2.2 % 1.9 %
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: Excess and Surplus Lines — Excluding Commercial Auto Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2009 $ 85,807 $ 2,188 1,698 2010 71,312 2,890 1,380 2011 106,536 3,919 1,465 2012 101,271 4,339 1,765 2013 83,855 10,248 2,298 2014 84,074 12,440 2,106 2015 102,434 21,215 2,434 2016 126,050 44,649 2,754 2017 131,897 79,836 2,630 2018 167,004 135,018 3,242 Excess and Surplus Lines — Commercial Auto Accident Year Incurred Losses and Loss Adj Expenses IBNR Cumulative # of Reported Claims ​ ($ in thousands) 2013 $ 1,277 $ 1 54 2014 19,779 653 7,761 2015 36,139 1,761 41,723 2016 126,791 9,206 88,746 2017 208,743 23,777 132,191 2018 255,881 155,760 91,798 Specialty Admitted - Individual Risk Workers’ Compensation Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2009 $ 25,660 $ — $ 1,490 2010 27,099 699 1,604 2011 36,593 1,786 1,814 2012 28,186 1,020 1,323 2013 10,909 510 540 2014 12,704 422 850 2015 18,376 2,177 975 2016 18,050 5,010 833 2017 22,071 6,434 1,087 2018 16,432 5,422 1,170   Specialty Admitted — Fronting and Programs Accident Year Incurred Losses IBNR Cumulative # of ​ ($ in thousands) 2013 $ 52 $ — 22 2014 3,228 238 856 2015 8,974 1,076 1,355 2016 14,682 3,203 2,792 2017 24,271 10,083 6,572 2018 21,758 16,058 5,874 Casualty Reinsurance Accident Year Incurred Losses IBNR ​ ($ in thousands) 2009 $ 27,168 $ 431 2010 62,605 937 2011 103,196 1,427 2012 143,305 2,397 2013 143,124 3,330 2014 121,200 5,450 2015 109,117 14,595 2016 103,544 26,675 2017 128,472 52,785 2018 121,529 55,052

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Written premiums: ​ ​ ​ Direct $ 1,027,222 $ 843,719 $ 549,700 Assumed 139,551 238,186 187,698 Ceded (404,101 ) (315,279 ) (179,690 ) Net $ 762,672 $ 766,626 $ 557,708 Earned premiums: ​ ​ ​ Direct $ 990,221 $ 842,182 $ 483,166 Assumed 208,192 173,472 164,771 Ceded (383,015 ) (274,545 ) (132,274 ) Net $ 815,398 $ 741,109 $ 515,663 Losses and loss adjustment expenses: ​ ​ ​ Direct $ 769,490 $ 626,318 $ 314,920 Assumed 131,346 142,818 108,029 Ceded (300,560 ) (213,759 ) (97,528 ) Net $ 600,276 $ 555,377 $ 325,421

Junior Subordinated Debt (Table

Junior Subordinated Debt (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018
Stockholders' Equity Note [Abstract]
Schedule of cash dividends declared	The Board of Directors declared the following cash dividends in 2018 , 2017 , and 2016 : Date of Declaration Dividend per Common Share Payable to Shareholders of Record on Payment Date Total Amount 2018 February 22, 2018 $0.30 March 12, 2018 March 30, 2018 $ 9,049,476 May 1, 2018 $0.30 June 11, 2018 June 29, 2018 $ 9,066,023 August 1, 2018 $0.30 September 10, 2018 September 28, 2018 $ 9,080,519 November 7, 2018 $0.30 December 14, 2018 December 28, 2018 $ 9,088,542 Total $1.20 $ 36,284,560 2017 ​ ​ ​ February 14, 2017 $0.30 March 13, 2017 March 31, 2017 $ 8,878,147 May 2, 2017 $0.30 June 12, 2017 June 30, 2017 $ 8,917,471 August 1, 2017 $0.30 September 11, 2017 September 29, 2017 $ 8,943,279 November 1, 2017 $0.30 December 15, 2017 December 28, 2017 $ 8,974,196 November 1, 2017 $0.50 December 15, 2017 December 28, 2017 $ 14,956,994 Total $1.70 $ 50,670,087 2016 February 16, 2016 $0.20 March 14, 2016 March 28, 2016 $ 5,848,561 May 3, 2016 $0.20 June 13, 2016 June 30, 2016 $ 5,873,582 August 3, 2016 $0.20 September 12, 2016 September 30, 2016 $ 5,882,342 November 1, 2016 $0.30 December 16, 2016 December 29, 2016 $ 8,847,431 November 1, 2016 $1.35 December 16, 2016 December 29, 2016 $ 39,813,440 Total $2.25 $ 66,265,356

Equity Awards (Tables)

Equity Awards (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of summary of option activity	The following table summarizes the option activity: ​ Year Ended December 31, ​ 2018 2017 2016 ​ Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Shares Weighted- Average Exercise Price Outstanding: ​ ​ ​ ​ ​ ​ Beginning of year 1,479,236 $ 27.81 2,234,699 $ 22.84 2,058,085 $ 18.11 Granted — $ — 205,244 $ 42.24 706,203 $ 32.07 Exercised (308,025 ) $ 22.01 (898,218 ) $ 18.53 (496,550 ) $ 16.02 Forfeited (55,887 ) $ 35.69 (62,489 ) $ 30.80 (33,039 ) $ 27.68 End of year 1,115,324 $ 29.02 1,479,236 $ 27.81 2,234,699 $ 22.84 Exercisable, end of year 814,421 $ 26.46 846,371 $ 22.35 1,207,479 $ 18.14
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 Risk-free interest rate 1.97 % 1.23 % Dividend yield 2.85 % 2.50 % Expected share price volatility 27.39 % 25.00 % Expected life 5.0 years 5.0 years
Schedule of summary of RSU activity	The following table summarizes RSU activity: ​ Year Ended December 31, ​ 2018 2017 2016 ​ 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 178,882 $ 37.93 196,800 $ 24.38 234,922 $ 21.00 Granted 227,481 $ 39.74 137,034 $ 42.20 60,291 $ 32.03 Vested (83,384 ) $ 37.61 (132,764 ) $ 24.24 (98,413 ) $ 21.00 Forfeited (22,837 ) $ 40.21 (22,188 ) $ 26.06 — $ — Unvested, end of year 300,142 $ 39.22 178,882 $ 37.93 196,800 $ 24.38
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, ​ 2018 2017 2016 ​ (in thousands) Share based compensation expense $ 6,228 $ 7,688 $ 5,492 U.S. tax benefit on share based compensation expense $ 716 $ 2,093 $ 1,532

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Federal income tax expense at applicable statutory rates $ 7,875 $ 17,541 $ 6,401 Tax-exempt investment income (272 ) (586 ) (643 ) Dividends received deduction (307 ) (792 ) (880 ) Excess tax benefits on share based compensation (567 ) (2,114 ) — Effect of tax rate reduction on deferred tax liability 220 (3,498 ) — Other 145 (90 ) (6 ) Federal income tax expense $ 7,094 $ 10,461 $ 4,872 U.S. state income tax (benefit) expense (86 ) 65 — U.S. dividend withholding tax — 1,053 — Total income tax expense $ 7,008 $ 11,579 $ 4,872
Schedule of deferred tax asset at the current prevailing corporate income tax rate	The significant components of net deferred tax assets (liabilities) at the corporate income tax rate of 21% for the years ended December 31, 2018 and 2017 are summarized as follows: ​ December 31, ​ 2018 2017 ​ (in thousands) Deferred tax assets: ​ ​ Accrued compensation expenses $ 2,456 $ 1,712 Reserve for losses and loss adjustment expenses 7,482 3,036 Unearned premiums 5,711 1,709 Share based compensation 1,352 1,143 Allowance for doubtful accounts 829 579 Deferred policy acquisition costs — 1,509 Property and equipment 1,909 1,601 Invested asset impairments — 791 Other 1,903 1,564 Total deferred tax assets 21,642 13,644 Deferred tax liabilities: Intangible assets 7,461 7,521 Net unrealized gains 887 2,578 Deferred policy acquisition costs 3,491 — Equity method investments 8,582 8,348 Other 167 444 Total deferred tax liabilities 20,588 18,891 Net deferred tax assets (liabilities) $ 1,054 $ (5,247 )

Other Operating Expenses and _2

Other Operating Expenses and Other Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Other Operating Expenses And Other Expenses [Abstract]
Schedule of other operating expenses	Other operating expenses consist of the following: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Amortization of policy acquisition costs $ 111,103 $ 116,001 $ 101,624 Other underwriting expenses of the insurance segments 63,029 55,662 48,771 Other operating expenses of the Corporate and Other segment 26,903 25,330 20,433 Total $ 201,035 $ 196,993 $ 170,828
Schedule of other expenses	Other expenses consist of the following: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Employee severance $ 1,386 $ 147 $ 1,492 Income on leased building the Company is deemed to own for accounting purposes (see Note 21) (623 ) (248 ) (196 ) Legal and professional services related to secondary share offerings 337 535 281 Other — 105 13 Impairment of intangible asset 200 — — Total $ 1,300 $ 539 $ 1,590

Commitments and Contingent Li_2

Commitments and Contingent Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum payments under non-cancelable operating leases	As of December 31, 2018 , future minimum payments under non-cancelable operating leases for office space are as follows (in thousands): 2019 $ 3,946 2020 3,790 2021 3,507 2022 2,639 2023 2,376 Thereafter 4,705 ​ $ 20,963

Other Comprehensive (Loss) In_2

Other Comprehensive (Loss) Income (Tables)	12 Months Ended
	Dec. 31, 2018
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, ​ 2018 2017 2016 ​ (in thousands) Unrealized (losses) gains arising during the period, before U.S. income taxes $ (23,201 ) $ 11,091 $ 2,790 U.S. income taxes 483 (2,374 ) (414 ) Unrealized (losses) gains arising during the period, net of U.S. income taxes (22,718 ) 8,717 2,376 Less reclassification adjustment: Net realized investment (losses) gains (554 ) (835 ) 6,572 U.S. income taxes 39 333 (2,137 ) Reclassification adjustment for investment (losses) gains realized in net income (515 ) (502 ) 4,435 Other comprehensive (loss) income $ (22,203 ) $ 9,219 $ (2,059 )

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 ​ ​ ​ ​ ​ ​ Gross written premiums $ 656,538 $ 374,346 $ 135,889 $ — ​ $ 1,166,773 Net earned premiums 555,684 55,146 204,568 — ​ 815,398 Segment revenues 580,785 56,717 243,178 4,919 ​ 885,599 Net investment income 14,456 3,262 38,838 4,700 ​ 61,256 Interest expense — — — 11,553 ​ 11,553 Underwriting profit of operating segments 42,834 6,972 5,103 — ​ 54,909 Segment goodwill 181,831 — — — ​ 181,831 Segment assets 972,111 633,689 1,453,754 77,222 ​ 3,136,776 ​ 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 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
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, ​ 2018 2017 2016 ​ (in thousands) Underwriting profit (loss) of the operating segments: ​ ​ ​ ​ ​ Excess and Surplus Lines $ 42,834 $ 29,693 $ 47,235 Specialty Admitted Insurance 6,972 3,166 2,872 Casualty Reinsurance 5,103 (1,765 ) (194 ) Total underwriting profit of operating segments 54,909 31,094 49,913 Other operating expenses of the Corporate and Other segment (26,903 ) (25,330 ) (20,433 ) Underwriting profit 28,006 5,764 29,480 Net investment income 61,256 61,119 52,638 Net realized and unrealized (losses) gains on investments (5,479 ) (1,989 ) 7,565 Other income 505 361 295 Other expenses (1,300 ) (539 ) (1,590 ) Interest expense (11,553 ) (8,974 ) (8,448 ) Amortization of intangible assets (597 ) (597 ) (597 ) Income before income taxes $ 70,838 $ 55,145 $ 79,343
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, ​ 2018 2017 2016 ​ (in thousands) Commercial Auto $ 322,126 ​ $ 247,960 ​ $ 110,050 Manufacturers and Contractors 79,160 ​ 85,719 ​ 83,279 Excess Casualty 66,452 ​ 51,160 ​ 43,574 General Casualty 54,127 ​ 38,097 ​ 36,858 Energy 33,942 ​ 29,704 ​ 29,709 Allied Health 30,450 ​ 19,181 ​ 14,413 Excess Property 16,963 ​ 14,447 ​ 14,083 Life Sciences 16,636 ​ 12,981 ​ 11,132 Small Business 14,808 ​ 11,307 ​ 9,104 Professional Liability 5,916 ​ 6,326 ​ 8,361 Environmental 10,499 ​ 7,920 ​ 5,321 Medical Professionals 1,774 ​ 2,297 ​ 2,739 Sports and Entertainment 3,685 ​ 3,021 ​ 2,221 Total Excess and Surplus Lines segment 656,538 ​ 530,120 ​ 370,844 Specialty Admitted Insurance segment 374,346 ​ 316,430 ​ 182,221 Casualty Reinsurance segment 135,889 ​ 235,355 ​ 184,333 Total $ 1,166,773 ​ $ 1,081,905 ​ $ 737,398

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 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) Fixed maturity securities, available-for-sale: ​ ​ ​ ​ State and municipal $ — $ 149,295 $ — $ 149,295 Residential mortgage-backed — 204,109 — 204,109 Corporate — 524,768 — 524,768 Commercial mortgage and asset-backed — 192,797 4,228 197,025 U.S. Treasury securities and obligations guaranteed by the U.S. government 106,651 542 — 107,193 Redeemable preferred stock — 1,812 — 1,812 Total fixed maturity securities, available-for-sale $ 106,651 $ 1,073,323 $ 4,228 $ 1,184,202 Equity securities: Preferred stock $ — $ 60,740 $ — $ 60,740 Common stock 16,674 757 214 17,645 Total equity securities $ 16,674 $ 61,497 $ 214 $ 78,385 Short-term investments $ 1,250 $ 80,716 $ — $ 81,966 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) Fixed maturity securities, available-for-sale: ​ ​ ​ ​ ​ ​ 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, available-for-sale $ 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 Trading securities: Fixed maturity securities $ — $ 3,808 $ — $ 3,808 Short-term investments $ 1,000 $ 35,804 $ — $ 36,804
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 and equity securities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) is shown below: ​ Year Ended December 31, ​ 2018 2017 2016 ​ (in thousands) Beginning balance $ 4,680 $ 5,000 ​ $ 5,000 Transfers out of Level 3 — — ​ — Transfers in to Level 3 — — ​ — Purchases 214 — ​ — Sales — — ​ — Maturities and calls (452 ) (320 ) — Amortization of discount — — ​ — Total gains or losses (realized/unrealized): ​ Included in earnings — — ​ — Included in other comprehensive income — — ​ — Ending balance $ 4,442 $ 4,680 ​ $ 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, 2018 ​ ​ ​ ​ ​ ​ ​ Bank loan participations held-for-investment $ — $ — ​ $ — ​ $ — December 31, 2017 Bank loan participations held-for-investment $ — $ — ​ $ 5,111 ​ $ 5,111
Schedule of carrying value and fair value	The carrying values and fair values of financial instruments are summarized below: ​ December 31, ​ 2018 2017 ​ Carrying Value Fair Value Carrying Value Fair Value ​ (in thousands) Assets ​ ​ ​ ​ ​ Fixed maturity securities, available-for-sale $ 1,184,202 ​ $ 1,184,202 $ 1,016,098 $ 1,016,098 Equity securities 78,385 ​ 78,385 82,522 82,522 Fixed maturity securities, trading — ​ — 3,808 3,808 Bank loan participations held-for-investment 260,972 ​ 250,697 238,214 236,532 Cash and cash equivalents 172,457 ​ 172,457 163,495 163,495 Short-term investments 81,966 ​ 81,966 36,804 36,804 Other invested assets – notes receivable 13,250 ​ 18,687 11,778 17,104 Liabilities ​ ​ ​ Senior debt 118,300 ​ 118,317 98,300 97,884 Junior subordinated debt 104,055 ​ 117,057 104,055 116,569

Statutory Matters (Tables)

Statutory Matters (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 , 2017 , and 2016 and for the years then ended are summarized as follows: ​ 2018 2017 2016 ​ (in thousands) Statutory net income $ 6,770 $ 31,881 $ 13,468 Statutory capital and surplus 241,668 219,132 184,859 Minimum required statutory capital and surplus 24,850 25,000 25,000

Unaudited Selected Quarterly _2

Unaudited Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Schedule of unaudited quarterly results of operations	The following is a summary of the unaudited quarterly results of operations: ​ 2018 Quarter 2018 ​ First Second Third Fourth Year ​ (in thousands, except per share data) Gross written premiums $ 298,116 $ 293,378 $ 279,969 $ 295,310 ​ $ 1,166,773 Total revenues 218,344 228,041 224,692 214,522 ​ 885,599 Net income 15,633 16,984 19,581 11,632 ​ 63,830 Comprehensive (loss) income (2,914 ) 10,426 15,192 18,923 ​ 41,627 Earnings per share: ​ ​ ​ ​ ​ ​ Basic (1) $ 0.53 $ 0.57 $ 0.65 $ 0.39 ​ $ 2.14 Diluted (1) $ 0.52 $ 0.56 $ 0.64 $ 0.38 ​ $ 2.11 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 (1) $ 0.63 $ 0.49 $ 0.35 $ 0.01 $ 1.48 Diluted (1) $ 0.61 $ 0.48 $ 0.34 $ 0.01 $ 1.44

Accounting Policies - Narrative

Accounting Policies - Narrative (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018USD ($)companyloan	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)loan	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)companyloan$ / sharesshares	Dec. 31, 2017USD ($)loanshares	Dec. 31, 2016USD ($)	Jan. 01, 2019USD ($)	Jan. 01, 2018USD ($)
Accounting Policies [Line Items]
Number of bank loans on nonaccrual status | loan	0				0				0	0
Allowance for doubtful accounts	$ 3,900				$ 2,800				$ 3,900	$ 2,800
Bad debt expense									1,500	1,000	$ 813
Allowance for doubtful accounts receivables written off									313	408	1,500
Fee income									13,900	17,000	10,100
Assets	3,136,776				2,756,695				3,136,776	2,756,695	2,346,533
Liabilities	2,427,535				2,061,996				2,427,535	2,061,996
Investment in variable interest entities	29,800				32,100				$ 29,800	$ 32,100
Common share equivalents excluded from calculations of diluted earnings per share | shares									180,329	174,380
Net income	11,632	$ 19,581	$ 16,984	$ 15,633	224	$ 10,351	$ 14,541	$ 18,450	$ 63,830	$ 43,566	74,471
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	$ 4,500				$ 2,600				$ 4,500	2,600
United States
Accounting Policies [Line Items]
Number of insurance companies | company	5								5
ASU 2016-01
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU													$ 0
Net income									$ (4,700)
Reduction in basic and diluted earnings per share (in dollars per share) | $ / shares									$ 0.16
ASU 2018-02
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU													0
Retained Earnings
Accounting Policies [Line Items]
Net income									$ 63,830	$ 43,566	$ 74,471
Retained Earnings | ASU 2016-01
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU													4,682
Retained Earnings | ASU 2018-02
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU													$ (711)
AOCI | ASU 2016-01
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU				4,700
AOCI | ASU 2018-02
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU				$ 711
Subsequent event | ASU 2016-02
Accounting Policies [Line Items]
Cumulative effect of adoption of ASU												$ 8,300
Assets												(22,600)
Liabilities												(30,900)
Right-of-use assets												17,200
Lease liabilities												$ 17,800

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Income	$ 11,632	$ 19,581	$ 16,984	$ 15,633	$ 224	$ 10,351	$ 14,541	$ 18,450	$ 63,830	$ 43,566	$ 74,471
Weighted average common shares
Basic (in shares)									29,887,990	29,461,717	29,063,075
Common share equivalents (in shares)									419,111	811,432	831,303
Diluted (in shares)									30,307,101	30,273,149	29,894,378
Earnings per share
Basic earnings per share (in dollars per share)	$ 0.39	$ 0.65	$ 0.57	$ 0.53	$ 0.01	$ 0.35	$ 0.49	$ 0.63	$ 2.14	$ 1.48	$ 2.56
Common share equivalents (in dollars per share)									(0.03)	(0.04)	(0.07)
Diluted earnings per share (in dollars per share)	$ 0.38	$ 0.64	$ 0.56	$ 0.52	$ 0.01	$ 0.34	$ 0.48	$ 0.61	$ 2.11	$ 1.44	$ 2.49

Investments - Schedule of Avail

Investments - Schedule of Available-for-Sale Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 1,199,409	$ 1,008,662
Fair Value	1,184,202	1,016,098
Fixed maturity securities:
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,199,409	1,008,662
Gross Unrealized Gains	6,060	14,564
Gross Unrealized Losses	(21,267)	(7,128)
Fair Value	1,184,202	1,016,098
State and municipal
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	147,160	139,382
Gross Unrealized Gains	3,422	5,587
Gross Unrealized Losses	(1,287)	(603)
Fair Value	149,295	144,366
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	208,869	160,379
Gross Unrealized Gains	577	723
Gross Unrealized Losses	(5,337)	(2,441)
Fair Value	204,109	158,661
Corporate
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	534,024	408,857
Gross Unrealized Gains	1,516	7,503
Gross Unrealized Losses	(10,772)	(2,639)
Fair Value	524,768	413,721
Commercial mortgage and asset-backed
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	199,528	182,595
Gross Unrealized Gains	310	714
Gross Unrealized Losses	(2,813)	(698)
Fair Value	197,025	182,611
Obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost		35,948
Gross Unrealized Gains		0
Gross Unrealized Losses		(101)
Fair Value		35,847
U.S. Treasury securities and obligations guaranteed by the U.S. government
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	107,803	79,476
Gross Unrealized Gains	235	37
Gross Unrealized Losses	(845)	(639)
Fair Value	107,193	78,874
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	2,025	2,025
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(213)	(7)
Fair Value	$ 1,812	$ 2,018

Investments - Schedule of Amort

Investments - Schedule of Amortized Cost and Fair Value of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
One year or less	$ 52,312
After one year through five years	406,647
After five years through ten years	213,673
After ten years	116,355
Residential mortgage-backed	208,869
Commercial mortgage and asset-backed	199,528
Redeemable preferred stock	2,025
Cost or Amortized Cost	1,199,409	$ 1,008,662
Fair Value
One year or less	52,100
After one year through five years	403,126
After five years through ten years	209,155
After ten years	116,875
Residential mortgage-backed	204,109
Commercial mortgage and asset-backed	197,025
Redeemable preferred stock	1,812
Total, Fair Value	$ 1,184,202	$ 1,016,098

Investments - Schedule of Gross

Investments - Schedule of Gross Unrealized Losses and Fair Values of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 438,150	$ 313,795
Less Than 12 Months, Unrealized Losses	(7,609)	(2,107)
12 Months or More, Fair Value	414,704	245,969
12 Months or More, Gross Unrealized Losses	(13,658)	(5,021)
Total, Fair Value	852,854	559,764
Total, Gross Unrealized Losses	(21,267)	(7,128)
State and municipal
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	19,733	40,117
Less Than 12 Months, Unrealized Losses	(284)	(318)
12 Months or More, Fair Value	47,018	10,662
12 Months or More, Gross Unrealized Losses	(1,003)	(285)
Total, Fair Value	66,751	50,779
Total, Gross Unrealized Losses	(1,287)	(603)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	49,180	50,447
Less Than 12 Months, Unrealized Losses	(743)	(261)
12 Months or More, Fair Value	105,778	84,193
12 Months or More, Gross Unrealized Losses	(4,594)	(2,180)
Total, Fair Value	154,958	134,640
Total, Gross Unrealized Losses	(5,337)	(2,441)
Corporate
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	243,384	113,464
Less Than 12 Months, Unrealized Losses	(5,089)	(846)
12 Months or More, Fair Value	155,902	66,954
12 Months or More, Gross Unrealized Losses	(5,683)	(1,793)
Total, Fair Value	399,286	180,418
Total, Gross Unrealized Losses	(10,772)	(2,639)
Commercial mortgage and asset-backed
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	106,423	53,965
Less Than 12 Months, Unrealized Losses	(1,229)	(244)
12 Months or More, Fair Value	51,805	25,299
12 Months or More, Gross Unrealized Losses	(1,584)	(454)
Total, Fair Value	158,228	79,264
Total, Gross Unrealized Losses	(2,813)	(698)
Obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value		3,024
Less Than 12 Months, Unrealized Losses		(1)
12 Months or More, Fair Value		32,154
12 Months or More, Gross Unrealized Losses		(100)
Total, Fair Value		35,178
Total, Gross Unrealized Losses		(101)
U.S. Treasury securities and obligations guaranteed by the U.S. government
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	17,618	50,760
Less Than 12 Months, Unrealized Losses	(51)	(430)
12 Months or More, Fair Value	54,201	26,707
12 Months or More, Gross Unrealized Losses	(794)	(209)
Total, Fair Value	71,819	77,467
Total, Gross Unrealized Losses	(845)	(639)
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	1,812	2,018
Less Than 12 Months, Unrealized Losses	(213)	(7)
12 Months or More, Fair Value	0	0
12 Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	1,812	2,018
Total, Gross Unrealized Losses	$ (213)	$ (7)

Investments - Narrative (Detail

Investments - Narrative (Details)	12 Months Ended
	Dec. 31, 2018USD ($)issuer	Dec. 31, 2017USD ($)loan	Dec. 31, 2016USD ($)loan
Debt Securities, Available-for-sale [Line Items]
Number of available-for-sale securities held in an unrealized loss position | issuer	222
Total fair value of unrealized loss position securities	$ 852,854,000	$ 559,764,000
Gross unrealized losses	21,267,000	7,128,000
Impairment loss		1,500,000
Allowance for credit losses		3,200,000	$ 943,000
Carrying value of loan	260,972,000	238,214,000
Unpaid principal on impaired loans		8,400,000	7,600,000
Carrying value of impaired loans		$ 5,100,000	$ 6,500,000
Number of impaired loans | loan		5	5
Net Investment Income	61,256,000	$ 61,119,000	$ 52,638,000
Fixed maturity securities, available-for-sale	1,184,202,000	1,016,098,000
Investment as collateral for outstanding letters of credit	471,400,000
Investments on deposit with state insurance departments	25,500,000	16,500,000
Investment Income	5,165,000	14,079,000	9,536,000
Bank Holding Company
Debt Securities, Available-for-sale [Line Items]
Interest income	343,000	343,000	343,000
Available-for-sale fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale	1,184,202,000	1,016,098,000
Puerto Rico loan
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses		759,000	177,000
Carrying value of loan		0	1,700,000
Unpaid principal on impaired loans		807,000	2,000,000
Impaired bank loans
Debt Securities, Available-for-sale [Line Items]
Average recorded investment in impaired bank loans	2,600,000	5,800,000	6,400,000
Net Investment Income	125,000	300,000	297,000
Net realized gain (loss) on changes in fair value of impaired bank loans	(858,000)	(2,400,000)	415,000
Bank loan participations
Debt Securities, Available-for-sale [Line Items]
Unamortized discount	1,800,000	1,100,000
Unamortized premium	$ 17,000	3,000
Standard & Poor's, BBB- Rating | Available-for-sale fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Percentage of available for sale securities	100.00%
Renewable Energy Bridge Financing Notes
Debt Securities, Available-for-sale [Line Items]
Interest income	$ 1,300,000	526,000
Proceeds from other investments maturities and repayments			6,500,000
Investment Income	1,282,000	526,000	450,000
Trading (losses) gains
Debt Securities, Available-for-sale [Line Items]
Trading securities, realized gain (loss)	(4,000)	(4,000)	18,000
Bank Holding Company
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale	$ 4,200,000
Bank Holding Company | Bank Holding Company
Debt Securities, Available-for-sale [Line Items]
Dividend income			299,000
Gross realized gains		$ 409,000	$ 3,600,000

Investments - Summary of Net In

Investments - Summary of Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Gross investment income	$ 65,490	$ 65,049	$ 56,398
Investment expense	(4,234)	(3,930)	(3,760)
Net investment income	61,256	61,119	52,638
Fixed maturity securities:
Net Investment Income [Line Items]
Gross investment income	34,129	26,833	25,917
Bank loan participations
Net Investment Income [Line Items]
Gross investment income	18,279	17,388	14,486
Equity securities
Net Investment Income [Line Items]
Gross investment income	5,240	5,045	5,617
Other invested assets
Net Investment Income [Line Items]
Gross investment income	5,165	14,079	9,536
Cash, cash equivalents, short-term investments, and other
Net Investment Income [Line Items]
Gross investment income	2,681	1,708	824
Trading (losses) gains
Net Investment Income [Line Items]
Gross investment income	$ (4)	$ (4)	$ 18

Investments - Summary of Realiz

Investments - Summary of Realized Gains and Losses on Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Fixed maturity securities, gross realized gains	$ 422,000	$ 840,000	$ 1,916,000
Fixed maturity securities, gross realized losses	(976,000)	(512,000)	(106,000)
Fixed maturity securities	(554,000)	328,000	1,810,000
Equity securities, gross realized gains	0	429,000	4,761,000
Equity securities, gross realized losses	(62,000)	(1,591,000)	0
Changes in fair values of equity securities	(5,970,000)	0	0
Equity securities	(6,032,000)	(1,162,000)	4,761,000
Bank loan participations, gross realized gain	2,279,000	2,407,000	2,827,000
Bank loan participations, gross realized losses	(1,166,000)	(3,557,000)	(1,832,000)
Bank loan participations	1,113,000	(1,150,000)	995,000
Short-term investments and other, gross realized gains	0	1,000	3,000
Short-term investments and other, gross realized losses	(6,000)	(6,000)	(4,000)
Short-term investments and other	(6,000)	(5,000)	(1,000)
Realized gains and losses on investments	$ (5,479,000)	$ (1,989,000)	$ 7,565,000

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, change in gross unrealized gains (losses)	$ (22,643)	$ 11,927	$ (3,783)
Fixed maturity securities:
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, change in gross unrealized gains (losses)	(22,643)	6,571	(1,350)
Equity securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, change in gross unrealized gains (losses)	$ 0	$ 5,356	$ (2,433)

Investments - Summary of Privat

Investments - Summary of Private Debt and Equity Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Total other invested assets	$ 72,321	$ 70,208
Investment Income	5,165	14,079	$ 9,536
Distributions from equity method investments	7,499	7,333	3,467
Equity in net income of subsidiaries	3,540	13,209	8,743
Renewable energy LLCs
Net Investment Income [Line Items]
Total other invested assets	29,795	32,063
Investment Income	2,974	10,578	3,480
Renewable energy notes receivable
Net Investment Income [Line Items]
Total other invested assets	8,750	7,278
Investment Income	$ 1,282	526	450
Investment interest rate	15.00%
Interest income	$ 1,300	526
Proceeds from other investments maturities and repayments			6,500
Limited partnerships
Net Investment Income [Line Items]
Total other invested assets	29,276	26,367
Investment Income	566	2,632	5,263
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	3,100	3,000
Equity in net income of subsidiaries	70	394	455
Excess and Surplus Lines | Limited partnerships
Net Investment Income [Line Items]
Carrying value of investments	26,200	23,400
Equity in net income of subsidiaries	496	2,200	4,800
Commitment to investment in limited partnership	625
Investment in LLC | Corporate and other segment
Net Investment Income [Line Items]
Distributions from equity method investments	$ 5,800	5,600
Bank Holding Company
Net Investment Income [Line Items]
Investment interest rate	7.60%
Interest income	$ 343	$ 343	$ 343
Investment in subordinated notes	$ 4,500
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%

Deferred Policy Acquisition C_3

Deferred Policy Acquisition Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at beginning of period	$ 72,365	$ 64,789	$ 60,754
Commissions	72,473	107,387	92,736
Underwriting and other issue expenses	20,715	16,190	12,923
Policy acquisition costs deferred	93,188	123,577	105,659
Amortization of policy acquisition costs	(111,103)	(116,001)	(101,624)
Net change	(17,915)	7,576	4,035
Balance at end of period	$ 54,450	$ 72,365	$ 64,789

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	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
Identifiable intangibles not subject to amortization	31,164,000	31,364,000
Impairment of intangible assets	200,000	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
Impairment of intangible assets	200,000
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%
State Licenses
Business Acquisition [Line Items]
Identifiable intangibles not subject to amortization	$ 0

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Acquired Intangible Assets [Line Items]
Identifiable intangibles not subject to amortization	$ 31,164	$ 31,364
Identifiable intangible assets subject to amortization
Gross Carrying Amount	11,611	11,611
Accumulated Amortization	5,238	4,641
Total intangible assets, gross	42,775	42,975
Broker relationships
Identifiable intangible assets subject to amortization
Gross Carrying Amount	11,611	11,611
Accumulated Amortization	$ 5,238	4,641
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	$ 8,964	$ 9,164

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Schedule of Future Estimated Amortization (Details) $ in Thousands	Dec. 31, 2018USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2,019	$ 597
2,020	538
2,021	363
2,022	363
2,023	363
Thereafter	4,149
Total	$ 6,373

Goodwill and Intangible Asset_5

Goodwill and Intangible Assets - Summary of Changes in Net Carrying Value of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Intangible Assets [Roll Forward]
Net Carrying Value	$ 38,334	$ 38,931
Amortization	(597)	(597)	$ (597)
Impairment Losses	(200)	0	0
Net Carrying Value	37,537	38,334	38,931
Excess and Surplus Lines
Intangible Assets [Roll Forward]
Net Carrying Value	30,927	31,289
Amortization	(362)	(362)	(362)
Impairment Losses	0	0
Net Carrying Value	30,565	30,927	31,289
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	6,327	6,689
Amortization	(362)	(362)
Impairment Losses	0	0
Net Carrying Value	5,965	6,327	6,689
Specialty Admitted Insurance
Intangible Assets [Roll Forward]
Net Carrying Value	7,407	7,642
Amortization	(235)	(235)	(235)
Impairment Losses	(200)	0
Net Carrying Value	6,972	7,407	7,642
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	(200)	0
Net Carrying Value	4,065	4,265	4,265
Specialty Admitted Insurance | Broker relationships
Intangible Assets [Roll Forward]
Net Carrying Value	642	877
Amortization	(235)	(235)
Impairment Losses	0	0
Net Carrying Value	$ 407	$ 642	$ 877

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	$ 41,056	$ 39,717
Accumulated depreciation	(17,745)	(14,716)
Property and equipment, net	23,311	25,001
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	30,971	30,902
Electronic data processing hardware and software
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	7,423	6,156
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, cost basis	$ 2,662	$ 2,659

Reserve for Losses and Loss A_3

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, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 989,825	$ 761,128	$ 653,534
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	582,604	533,905	349,137
Prior years	17,672	21,472	(23,716)
Total incurred losses and loss and adjustment expenses	600,276	555,377	325,421
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	86,355	103,205	39,473
Prior years	309,658	223,475	178,354
Total loss and loss adjustment expense payments	396,013	326,680	217,827
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	1,194,088	989,825	761,128
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	467,371	302,524	182,737
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	$ 1,661,459	$ 1,292,349	$ 943,865

Reserve for Losses and Loss A_4

Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	$ (17,672)	$ (21,472)	$ 23,716
Excess and Surplus Lines
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	(15,000)	(20,000)	24,100
Excess and Surplus Lines | Accident year 2017
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	5,700
Excess and Surplus Lines | Accident years 2014 through 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		18,600
Excess and Surplus Lines | Accident year 2015
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			10,000
Excess and Surplus Lines | Accident year 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	(20,700)	(38,700)
Excess and Surplus Lines | Accident year 2013
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			4,500
Excess and Surplus Lines | Accident year 2014
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			10,700
Specialty Admitted Insurance | Accident years 2014 through 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	5,600
Specialty Admitted Insurance | Accident years 2010 through 2014
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			3,800
Specialty Admitted Insurance | Accident years 2010 through 2015
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		2,700
Casualty Reinsurance | Accident years at least four years old
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	$ (8,200)
Casualty Reinsurance | Accident years 2010 through 2013
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		$ (4,200)
Casualty Reinsurance | Accident years 2012 and 2013
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			$ (4,200)

Reserve for Losses and Loss A_5

Reserve for Losses and Loss Adjustment Expenses - Incurred Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	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
Casualty Reinsurance
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	$ 1,063,260
Cumulative paid losses and loss adjustment expenses, net of reinsurance	793,746
All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance	333
Total outstanding losses and loss adjustment expenses, net of reinsurance	269,847
Casualty Reinsurance | Accident year 2009
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	27,168	$ 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	25,209	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	62,605	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	58,707	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	103,196	101,276	99,282	98,993	97,812	97,366	103,123	114,908
Cumulative paid losses and loss adjustment expenses, net of reinsurance	97,715	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	143,305	139,978	136,813	135,594	131,281	132,388	148,251
Cumulative paid losses and loss adjustment expenses, net of reinsurance	133,606	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	143,124	137,801	134,446	131,352	130,361	133,230
Cumulative paid losses and loss adjustment expenses, net of reinsurance	127,732	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	121,200	116,981	114,636	115,927	118,881
Cumulative paid losses and loss adjustment expenses, net of reinsurance	101,366	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	109,117	108,699	108,870	119,157
Cumulative paid losses and loss adjustment expenses, net of reinsurance	80,208	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	103,544	105,533	112,759
Cumulative paid losses and loss adjustment expenses, net of reinsurance	65,409	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	128,472	134,628
Cumulative paid losses and loss adjustment expenses, net of reinsurance	72,891	47,739
Casualty Reinsurance | Accident year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	121,529
Cumulative paid losses and loss adjustment expenses, net of reinsurance	30,903
Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	648,610
Cumulative paid losses and loss adjustment expenses, net of reinsurance	293,439
Total outstanding losses and loss adjustment expenses, net of reinsurance	355,171
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	1,277	1,301	1,351	1,451	1,300	1,255
Cumulative paid losses and loss adjustment expenses, net of reinsurance	1,275	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	19,779	18,953	17,233	14,071	20,487
Cumulative paid losses and loss adjustment expenses, net of reinsurance	18,678	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	36,139	35,149	33,113	30,109
Cumulative paid losses and loss adjustment expenses, net of reinsurance	31,592	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	126,791	109,286	74,340
Cumulative paid losses and loss adjustment expenses, net of reinsurance	89,381	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	208,743	207,355
Cumulative paid losses and loss adjustment expenses, net of reinsurance	107,377	41,467
Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	255,881
Cumulative paid losses and loss adjustment expenses, net of reinsurance	45,136
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	1,060,240
Cumulative paid losses and loss adjustment expenses, net of reinsurance	592,655
All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance	12,187
Total outstanding losses and loss adjustment expenses, net of reinsurance	479,772
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	85,807	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	82,560	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	71,312	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	65,246	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	106,536	106,643	106,130	106,381	106,564	114,473	119,927	111,190
Cumulative paid losses and loss adjustment expenses, net of reinsurance	99,798	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	101,271	99,619	97,306	96,497	97,829	98,672	97,908
Cumulative paid losses and loss adjustment expenses, net of reinsurance	88,812	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	83,855	82,830	81,009	85,433	96,064	96,729
Cumulative paid losses and loss adjustment expenses, net of reinsurance	66,553	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	84,074	82,232	90,267	104,092	114,942
Cumulative paid losses and loss adjustment expenses, net of reinsurance	58,774	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	102,434	104,847	113,417	126,443
Cumulative paid losses and loss adjustment expenses, net of reinsurance	55,911	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	126,050	125,093	138,507
Cumulative paid losses and loss adjustment expenses, net of reinsurance	46,045	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	131,897	144,349
Cumulative paid losses and loss adjustment expenses, net of reinsurance	22,956	5,290
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	167,004
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,000
Fronting and Programs | Specialty Admitted Insurance
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	72,965
Cumulative paid losses and loss adjustment expenses, net of reinsurance	24,280
Total outstanding losses and loss adjustment expenses, net of reinsurance	48,685
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	52	80	104
Cumulative paid losses and loss adjustment expenses, net of reinsurance	52	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	3,228	3,425	3,818	3,468	3,460
Cumulative paid losses and loss adjustment expenses, net of reinsurance	2,854	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	8,974	9,358	9,632	7,136
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,919	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	14,682	15,670	11,542
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,888	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	24,271	21,229
Cumulative paid losses and loss adjustment expenses, net of reinsurance	6,682	1,223
Fronting and Programs | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	21,758
Cumulative paid losses and loss adjustment expenses, net of reinsurance	885
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	216,080
Cumulative paid losses and loss adjustment expenses, net of reinsurance	181,489
All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance	1,056
Total outstanding losses and loss adjustment expenses, net of reinsurance	40,613
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	25,660	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	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	27,099	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	26,358	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	36,593	36,594	37,455	38,311	38,999	40,154	41,421	37,834
Cumulative paid losses and loss adjustment expenses, net of reinsurance	34,614	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	28,186	28,174	28,255	29,689	31,490	32,420	32,116
Cumulative paid losses and loss adjustment expenses, net of reinsurance	26,963	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	10,909	10,907	11,578	12,786	13,668	12,525
Cumulative paid losses and loss adjustment expenses, net of reinsurance	10,309	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	12,704	13,890	14,620	16,652	16,638
Cumulative paid losses and loss adjustment expenses, net of reinsurance	12,282	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	18,376	19,741	21,274	20,938
Cumulative paid losses and loss adjustment expenses, net of reinsurance	15,889	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	18,050	20,299	21,678
Cumulative paid losses and loss adjustment expenses, net of reinsurance	12,135	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	22,071	24,869
Cumulative paid losses and loss adjustment expenses, net of reinsurance	12,782	$ 6,546
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Short-duration insurance contracts, incurred claims and allocated claim adjustment expense, net	16,432
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 4,497

Reserve for Losses and Loss A_6

Reserve for Losses and Loss Adjustment Expenses - Cumulative Paid Losses and Loss Adjustment Expenses (Details) $ in Thousands	Dec. 31, 2018USD ($)Claimclaim	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)
Casualty Reinsurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 793,746
All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance	333
Total outstanding losses and loss adjustment expenses, net of reinsurance	269,847
Casualty Reinsurance | Accident year 2009
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	25,209	$ 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	58,707	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	97,715	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	133,606	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	127,732	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	101,366	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	80,208	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	65,409	50,905	36,268
Casualty Reinsurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	72,891	47,739
Casualty Reinsurance | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	30,903
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	592,655
All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance	12,187
Total outstanding losses and loss adjustment expenses, net of reinsurance	$ 479,772
Claims outstanding | claim	33
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2009
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 82,560	81,496	79,014	76,091	71,245	67,293	61,548	51,731	41,687	29,860
Claims outstanding | Claim	1,698
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 65,246	64,214	61,307	57,371	52,071	45,641	35,812	26,418	13,673
Claims outstanding | Claim	1,380
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 99,798	98,684	94,998	90,884	81,696	72,732	53,109	27,684
Claims outstanding | Claim	1,465
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 88,812	82,545	74,869	63,216	49,604	33,757	6,944
Claims outstanding | Claim	1,765
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 66,553	59,641	44,421	30,382	14,509	3,867
Claims outstanding | Claim	2,298
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 58,774	43,891	28,212	16,969	3,412
Claims outstanding | Claim	2,106
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 55,911	34,801	17,164	4,048
Claims outstanding | Claim	2,434
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 46,045	22,852	5,180
Claims outstanding | Claim	2,754
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 22,956	5,290
Claims outstanding | Claim	2,630
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 6,000
Claims outstanding | Claim	3,242
Fronting and Programs | Specialty Admitted Insurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 24,280
Total outstanding losses and loss adjustment expenses, net of reinsurance	48,685
Fronting and Programs | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 52	52	52	52	52	28
Claims outstanding | Claim	22
Fronting and Programs | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 2,854	2,728	2,369	1,687	883
Claims outstanding | Claim	856
Fronting and Programs | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 6,919	6,165	4,666	2,058
Claims outstanding | Claim	1,355
Fronting and Programs | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 6,888	5,123	1,894
Claims outstanding | Claim	2,792
Fronting and Programs | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 6,682	1,223
Claims outstanding | Claim	6,572
Fronting and Programs | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 885
Claims outstanding | Claim	5,874
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 181,489
All outstanding losses and loss adjustment expenses prior to 2009, net of reinsurance	1,056
Outstanding losses and loss adjustment expenses assumed from involuntary workers compensation pools	4,966
Total outstanding losses and loss adjustment expenses, net of reinsurance	40,613
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2009
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 25,660	25,660	24,418	24,350	24,192	24,231	22,646	21,095	16,945	$ 7,277
Claims outstanding | Claim	1,490
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 26,358	26,352	26,345	25,477	25,210	23,898	21,805	16,245	$ 7,157
Claims outstanding | Claim	1,604
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 34,614	34,334	34,287	34,240	33,204	29,021	23,127	$ 10,123
Claims outstanding | Claim	1,814
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 26,963	26,932	26,897	26,435	24,755	20,308	$ 9,222
Claims outstanding | Claim	1,323
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 10,309	10,263	10,246	9,846	8,723	4,487
Claims outstanding | Claim	540
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 12,282	12,236	12,041	10,648	4,633
Claims outstanding | Claim	850
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 15,889	15,118	13,285	6,604
Claims outstanding | Claim	975
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 12,135	10,227	4,664
Claims outstanding | Claim	833
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 12,782	6,546
Claims outstanding | Claim	1,087
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 4,497
Claims outstanding | Claim	1,170
Commercial Auto | Specialty Admitted Insurance
Claims Development [Line Items]
Claims outstanding | claim	4
Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 293,439
Total outstanding losses and loss adjustment expenses, net of reinsurance	355,171
Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 1,275	1,275	1,291	1,285	1,182	$ 60
Claims outstanding | Claim	54
Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 18,678	16,359	12,679	8,645	$ 6,166
Claims outstanding | Claim	7,761
Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 31,592	24,282	15,234	$ 8,356
Claims outstanding | Claim	41,723
Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 89,381	54,054	$ 18,295
Claims outstanding | Claim	88,746
Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 107,377	$ 41,467
Claims outstanding | Claim	132,191
Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 45,136
Claims outstanding | Claim	91,798

Reserve for Losses and Loss A_7

Reserve for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	$ 1,194,088	$ 989,825	$ 761,128	$ 653,534
Reinsurance recoverable on unpaid losses	467,371	302,524	182,737
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	1,661,459	$ 1,292,349	$ 943,865
Casualty Reinsurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	269,847
Fronting and Programs | Specialty Admitted Insurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	48,685
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	479,772
Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	355,171
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	$ 40,613

Reserve for Losses and Loss A_8

Reserve for Losses and Loss Adjustment Expenses - Average Annual Percentage Payouts of Incurred Claims (Details)	Dec. 31, 2018
Casualty Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	27.50%
Year 2	17.70%
Year 3	12.50%
Year 4	8.70%
Year 5	6.10%
Year 6	4.50%
Year 7	3.40%
Year 8	2.70%
Year 9	2.20%
Year 10	1.90%
Fronting and Programs | Specialty Admitted Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	7.70%
Year 2	16.70%
Year 3	7.20%
Year 4	1.50%
Year 5	0.20%
Excluding Commercial Auto | E&S
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	10.00%
Year 2	16.00%
Year 3	17.40%
Year 4	18.00%
Year 5	14.50%
Year 6	7.90%
Year 7	5.60%
Year 8	3.40%
Year 9	1.10%
Year 10	2.30%
Commercial Auto | E&S
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	21.00%
Year 2	38.90%
Year 3	15.60%
Year 4	14.30%
Year 5	6.50%
Year 6	3.70%
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	28.70%
Year 2	33.80%
Year 3	13.80%
Year 4	6.20%
Year 5	3.30%
Year 6	1.30%
Year 7	1.00%
Year 8	0.20%
Year 9	0.50%
Year 10	0.00%

Reserve for Losses and Loss A_9

Reserve for Losses and Loss Adjustment Expenses - IBNR Liabilities and Claims Frequency (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)Claimclaim	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 600,276	$ 555,377	$ 325,421
Fronting and Programs | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	52
IBNR	$ 0
Cumulative Number of Reported Claims | Claim	22
Fronting and Programs | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 3,228
IBNR	$ 238
Cumulative Number of Reported Claims | Claim	856
Fronting and Programs | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 8,974
IBNR	$ 1,076
Cumulative Number of Reported Claims | Claim	1,355
Fronting and Programs | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 14,682
IBNR	$ 3,203
Cumulative Number of Reported Claims | Claim	2,792
Fronting and Programs | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 24,271
IBNR	$ 10,083
Cumulative Number of Reported Claims | Claim	6,572
Fronting and Programs | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 21,758
IBNR	$ 16,058
Cumulative Number of Reported Claims | Claim	5,874
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2009
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 27,168
IBNR	431
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	62,605
IBNR	937
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	103,196
IBNR	1,427
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	143,305
IBNR	2,397
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	143,124
IBNR	3,330
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	121,200
IBNR	5,450
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	109,117
IBNR	14,595
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	103,544
IBNR	26,675
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	128,472
IBNR	52,785
Individual Risk Workers' Compensation | Casualty Reinsurance | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	121,529
IBNR	55,052
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2009
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	25,660
IBNR	$ 0
Cumulative Number of Reported Claims | Claim	1,490
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 27,099
IBNR	$ 699
Cumulative Number of Reported Claims | Claim	1,604
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 36,593
IBNR	$ 1,786
Cumulative Number of Reported Claims | Claim	1,814
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 28,186
IBNR	$ 1,020
Cumulative Number of Reported Claims | Claim	1,323
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 10,909
IBNR	$ 510
Cumulative Number of Reported Claims | Claim	540
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 12,704
IBNR	$ 422
Cumulative Number of Reported Claims | Claim	850
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 18,376
IBNR	$ 2,177
Cumulative Number of Reported Claims | Claim	975
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 18,050
IBNR	$ 5,010
Cumulative Number of Reported Claims | Claim	833
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 22,071
IBNR	$ 6,434
Cumulative Number of Reported Claims | Claim	1,087
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 16,432
IBNR	$ 5,422
Cumulative Number of Reported Claims | Claim	1,170
Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 1,277
IBNR	$ 1
Cumulative Number of Reported Claims | Claim	54
Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 19,779
IBNR	$ 653
Cumulative Number of Reported Claims | Claim	7,761
Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 36,139
IBNR	$ 1,761
Cumulative Number of Reported Claims | Claim	41,723
Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 126,791
IBNR	$ 9,206
Cumulative Number of Reported Claims | Claim	88,746
Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 208,743
IBNR	$ 23,777
Cumulative Number of Reported Claims | Claim	132,191
Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 255,881
IBNR	$ 155,760
Cumulative Number of Reported Claims | Claim	91,798
Commercial Auto | Specialty Admitted Insurance
Claims Development [Line Items]
Cumulative Number of Reported Claims | claim	4
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Cumulative Number of Reported Claims | claim	33
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2009
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 85,807
IBNR	$ 2,188
Cumulative Number of Reported Claims | Claim	1,698
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2010
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 71,312
IBNR	$ 2,890
Cumulative Number of Reported Claims | Claim	1,380
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2011
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 106,536
IBNR	$ 3,919
Cumulative Number of Reported Claims | Claim	1,465
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2012
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 101,271
IBNR	$ 4,339
Cumulative Number of Reported Claims | Claim	1,765
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 83,855
IBNR	$ 10,248
Cumulative Number of Reported Claims | Claim	2,298
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 84,074
IBNR	$ 12,440
Cumulative Number of Reported Claims | Claim	2,106
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 102,434
IBNR	$ 21,215
Cumulative Number of Reported Claims | Claim	2,434
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 126,050
IBNR	$ 44,649
Cumulative Number of Reported Claims | Claim	2,754
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 131,897
IBNR	$ 79,836
Cumulative Number of Reported Claims | Claim	2,630
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Incurred Losses and Loss Adj Expenses	$ 167,004
IBNR	$ 135,018
Cumulative Number of Reported Claims | Claim	3,242

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	$ 467,371	$ 302,524	$ 182,737
Reinsurance recoverable on paid losses	$ 18,344	11,292
Percentage of reinsurance recoverables on unpaid losses for 3 largest reinsurers	52.10%
Prepaid reinsurance premiums ceded	$ 112,498	$ 91,979
Percentage of prepaid reinsurance premiums ceded for 3 largest reinsurers	47.80%
Largest reinsurer one
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	$ 151,600
Prepaid reinsurance premiums ceded	30,800
Largest reinsurer two
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	47,900
Prepaid reinsurance premiums ceded	13,400
Largest reinsurer three
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid losses	44,100
Prepaid reinsurance premiums ceded	$ 9,600

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Written premiums:
Direct	$ 1,027,222	$ 843,719	$ 549,700
Assumed	139,551	238,186	187,698
Ceded	(404,101)	(315,279)	(179,690)
Net written premiums	762,672	766,626	557,708
Earned premiums:
Direct	990,221	842,182	483,166
Assumed	208,192	173,472	164,771
Ceded	(383,015)	(274,545)	(132,274)
Net earned premiums	815,398	741,109	515,663
Losses and loss adjustment expenses:
Direct	769,490	626,318	314,920
Assumed	131,346	142,818	108,029
Ceded	(300,560)	(213,759)	(97,528)
Net	$ 600,276	$ 555,377	$ 325,421

Senior Debt (Details)

Senior Debt (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Aug. 02, 2017	May 26, 2004
2013 Facility
Line of Credit Facility [Line Items]
Current total capacity	$ 215,000,000
2013 Facility | Bank Holding Company
Line of Credit Facility [Line Items]
Commitment to lend	25,000,000
2013 Facility | Secured | JRG Re
Line of Credit Facility [Line Items]
Maximum borrowing capacity	102,500,000
Letters of credit outstanding	75,500,000
2013 Facility | Unsecured
Line of Credit Facility [Line Items]
Maximum borrowing capacity	112,500,000
Amount outstanding on line of credit	$ 73,300,000	$ 73,300,000
2013 Facility | LIBOR | Unsecured | JRG Re
Line of Credit Facility [Line Items]
Margin above LIBOR, percentage rate	1.50%
2017 Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 100,000,000
2017 Facility | Secured | JRG Re
Line of Credit Facility [Line Items]
Amount outstanding on line of credit	$ 7,600,000
2017 Facility | Unsecured
Line of Credit Facility [Line Items]
Amount outstanding on line of credit	$ 30,000,000
James River Group Senior Debt
Line of Credit Facility [Line Items]
Principal amount of Senior Debt				$ 15,000,000
James River Group Senior Debt | LIBOR
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, 2018USD ($)
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) - Junior Subordinated Debt [Member]	12 Months Ended
	Dec. 31, 2018
Debt Instrument [Line Items]
Debt redemption percentage	100.00%
Deferred interest payment period (in years)	5 years

Capital Stock - Narrative (Deta

Capital Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	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	29,988,460	29,696,682
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	29,988,460	29,696,682	29,257,566
Common shares reserved for future issuance	4,321,463
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock unit dividend equivalents	$ 391	$ 450	$ 664
Dividends payable	$ 557	$ 434	$ 666

Capital Stock - Summary of Cash

Capital Stock - Summary of Cash Dividends Declared (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 28, 2018	Nov. 07, 2018	Sep. 28, 2018	Aug. 01, 2018	Jun. 29, 2018	May 01, 2018	Mar. 30, 2018	Feb. 22, 2018	Dec. 28, 2017	Nov. 01, 2017	Sep. 29, 2017	Aug. 01, 2017	Jun. 30, 2017	May 02, 2017	Mar. 31, 2017	Feb. 14, 2017	Dec. 29, 2016	Nov. 01, 2016	Sep. 30, 2016	Aug. 03, 2016	Jun. 30, 2016	May 03, 2016	Mar. 28, 2016	Feb. 16, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend declared per share (in dollars per share)		$ 0.30		$ 0.30		$ 0.30		$ 0.30				$ 0.30		$ 0.30		$ 0.30				$ 0.20		$ 0.20		$ 0.20	$ 1.2	$ 1.70	$ 2.25
Total Amount	$ 9.1		$ 9.1		$ 9.1		$ 9				$ 8.9		$ 8.9		$ 8.9				$ 5.9		$ 5.9		$ 5.8		$ 36.3	$ 50.7	$ 66.3
November 1, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend declared per share (in dollars per share)										$ 0.30
Total Amount									$ 9
November 1, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend declared per share (in dollars per share)										$ 0.50
Total Amount									$ 15
November 1, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend declared per share (in dollars per share)																		$ 0.30
Total Amount																	$ 8.8
November 1, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend declared per share (in dollars per share)																		$ 1.35
Total Amount																	$ 39.8

Equity Awards - Narrative (Deta

Equity Awards - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual life of option (in years)	7 years
Total intrinsic value of options exercised	$ 4.6	$ 20.4	$ 10.2
Aggregate intrinsic value of options outstanding	9.4	18.5	41.8
Aggregate intrinsic value of options exercisable	$ 8.5	$ 14.9	$ 28.3
Fair value of shares used in calculation of intrinsic values (in dollars per share)	$ 36.54	$ 40.01	$ 41.55
Weighted-average remaining contractual life of options outstanding (in years)	3 years 7 months 24 days
Weighted-average remaining contractual life of options exercisable (in years)	3 years 3 months 29 days
Weighted-average fair value of options granted (in dollars per share)		$ 8.21	$ 5.55
Options | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	2 years
Options | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	4 years
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	3 years
Total fair value of shares vested	$ 3.2	$ 5.5	$ 3.8
Unrecognized share based compensation expense	$ 8.4
Weighted-average period of unrecognized share based compensation expense (in years)	1 year 9 months 18 days
RSUs | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	1 year
RSUs | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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	1,724,865
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	21,663
Non-employee directors | RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)	1 year

Equity Awards - Summary of Opti

Equity Awards - Summary of Option Activity (Details) - Options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Beginning of year (in shares)	1,479,236	2,234,699	2,058,085
Granted (in shares)	0	205,244	706,203
Exercised (in shares)	(308,025)	(898,218)	(496,550)
Forfeited (in shares)	(55,887)	(62,489)	(33,039)
End of year (in shares)	1,115,324	1,479,236	2,234,699
Exercisable, end of year (in shares)	814,421	846,371	1,207,479
Weighted- Average Exercise Price
Beginning of year (in dollars per share)	$ 27.81	$ 22.84	$ 18.11
Granted (in dollars per share)	0	42.24	32.07
Exercised (in dollars per share)	22.01	18.53	16.02
Forfeited (in dollars per share)	35.69	30.80	27.68
End of year (in dollars per share)	29.02	27.81	22.84
Exercisable, end of year	$ 26.46	$ 22.35	$ 18.14

Equity Awards - Summary of Assu

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

Equity Awards - Summary of RSU

Equity Awards - Summary of RSU Activity (Details) - RSUs - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Unvested, beginning of year (in shares)	178,882	196,800	234,922
Granted (in shares)	227,481	137,034	60,291
Vested (in shares)	(83,384)	(132,764)	(98,413)
Forfeited (in shares)	(22,837)	(22,188)	0
Unvested, end of year (in shares)	300,142	178,882	196,800
Weighted- Average Grant Date Fair Value
Unvested, beginning of year (in dollars per share)	$ 37.93	$ 24.38	$ 21
Granted (in dollars per share)	39.74	42.20	32.03
Vested (in dollars per share)	37.61	24.24	21
Forfeited (in dollars per share)	40.21	26.06	0
Unvested, end of year (in dollars per share)	$ 39.22	$ 37.93	$ 24.38

Equity Awards - Summary of Comp

Equity Awards - Summary of Compensation Expense and Related Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share based compensation expense	$ 6,228	$ 7,688	$ 5,492
U.S. tax benefit on share based compensation expense	$ 716	$ 2,093	$ 1,532

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Percentage of dividend withholding tax	5.00%
Dividend distributions to intermediate holding company	$ 0	$ 21,100,000	$ 0
Deferred tax asset for additional discount	8,800,000
Deferred tax liability	8,800,000
Transition provision expense	$ 1,100,000
Remittance of dividend withholding tax		1,100,000
Federal statutory tax rates	21.00%
U.S. income before Federal income taxes	$ 37,500,000	50,100,000	18,300,000
Percentage of excise tax on reinsurance premiums paid	1.00%
Payment of federal excise taxes	$ 823,000	3,400,000	2,600,000
Excise taxes paid	$ 2,100,000	$ 2,200,000	$ 1,600,000
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal income tax expense at applicable statutory rates	$ 7,875	$ 17,541	$ 6,401
Tax-exempt investment income	(272)	(586)	(643)
Dividends received deduction	(307)	(792)	(880)
Excess tax benefits on share based compensation	(567)	(2,114)	0
Effect of tax rate reduction on deferred tax liability	220	(3,498)	0
Other	145	(90)	(6)
Federal income tax expense	7,094	10,461	4,872
U.S. state income tax (benefit) expense	(86)	65	0
U.S. dividend withholding tax	0	1,053	0
Total income tax expense	$ 7,008	$ 11,579	$ 4,872

Income Taxes - Summary of Net D

Income Taxes - Summary of Net Deferred Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Accrued compensation expenses	$ 2,456	$ 1,712
Reserve for losses and loss adjustment expenses	7,482	3,036
Unearned premiums	5,711	1,709
Share based compensation	1,352	1,143
Allowance for doubtful accounts	829	579
Deferred policy acquisition costs	0	1,509
Property and equipment	1,909	1,601
Invested asset impairments	0	791
Other	1,903	1,564
Total deferred tax assets	21,642	13,644
Deferred tax liabilities:
Intangible assets	7,461	7,521
Net unrealized gains	887	2,578
Deferred policy acquisition costs	3,491	0
Equity method investments	8,582	8,348
Other	167	444
Total deferred tax liabilities	20,588	18,891
Net deferred tax assets	$ 1,054
Net deferred tax assets (liabilities)		$ (5,247)

Other Operating Expenses and _3

Other Operating Expenses and Other Expenses - Summary of Other Operating Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Operating Expenses And Other Expenses [Abstract]
Amortization of policy acquisition costs	$ 111,103	$ 116,001	$ 101,624
Other underwriting expenses of the insurance segments	63,029	55,662	48,771
Other operating expenses of the Corporate and Other segment	26,903	25,330	20,433
Total	$ 201,035	$ 196,993	$ 170,828

Other Operating Expenses and _4

Other Operating Expenses and Other Expenses - Summary of Other Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Operating Expenses And Other Expenses [Abstract]
Employee severance	$ 1,386	$ 147	$ 1,492
Income on leased building the Company is deemed to own for accounting purposes (see Note 21)	(623)	(248)	(196)
Legal and professional services related to secondary share offerings	337	535	281
Other	0	105	13
Impairment of intangible asset	200	0	0
Total	$ 1,300	$ 539	$ 1,590

Employee Benefits (Details)

Employee Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Benefits and Share-based Compensation, Noncash [Abstract]
Maximum matching employer contribution	6.00%
Expense associated with Savings Plans	$ 2.9	$ 2.5	$ 1.9

Commitments and Contingent Li_3

Commitments and Contingent Liabilities - Narrative (Details)	12 Months Ended
	Dec. 31, 2018USD ($)letter_of_credit_facility	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Contingent Liabilities [Line Items]
Rental expense	$ 4,600,000	$ 4,200,000	$ 3,700,000
Future minimum lease payments for building constructed	$ 15,200,000
Lease term (in years)	10 years
Total payments under lease	$ 20,963,000
Policy management software
Contingent Liabilities [Line Items]
Lease term (in years)		5 years
Total payments under lease	468,000
Lease payment per year	440,000
Captive insurance company affiliate of the insured group
Contingent Liabilities [Line Items]
Cash equivalent collateral held in the collateral trust arrangement	$ 1,099,200,000
JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Letter of credit facilities | letter_of_credit_facility	3
Total amount deposited in trust accounts	$ 301,500,000
JRG Reinsurance Company, Ltd. | Letter of credit 100 million
Contingent Liabilities [Line Items]
Letters of credit facility, amount	100,000,000
Amount of letters of credit issued	52,600,000
Assets deposited for securing letters of credit	65,700,000
JRG Reinsurance Company, Ltd. | 2013 Facility
Contingent Liabilities [Line Items]
Amount of letters of credit issued	75,500,000
Assets deposited for securing letters of credit	94,000,000
JRG Reinsurance Company, Ltd. | 2017 Facility
Contingent Liabilities [Line Items]
Amount of letters of credit issued	7,600,000
Assets deposited for securing letters of credit	$ 10,200,000

Commitments and Contingent Li_4

Commitments and Contingent Liabilities - Schedule of Future Minimum Payments for Non-cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,018	$ 3,946
2,019	3,790
2,020	3,507
2,021	2,639
2,022	2,376
Thereafter	4,705
Total	$ 20,963

Other Comprehensive (Loss) In_3

Other Comprehensive (Loss) Income - Summary of Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Unrealized (losses) gains arising during the period, before U.S. income taxes	$ (23,201)	$ 11,091	$ 2,790
U.S. income taxes	483	(2,374)	(414)
Unrealized (losses) gains arising during the period, net of U.S. income taxes	(22,718)	8,717	2,376
Less reclassification adjustment:
Net realized investment (losses) gains	(554)	(835)	6,572
U.S. income taxes	39	333	(2,137)
Reclassification adjustment for investment (losses) gains realized in net income	(515)	(502)	4,435
Other comprehensive (loss) income	$ (22,203)	$ 9,219	$ (2,059)

Other Comprehensive (Loss) In_4

Other Comprehensive (Loss) Income - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net realized investment (losses) gains	$ (554,000)	$ (835,000)	$ 6,572,000
Bank loan participations realized investment gains (losses)	1,113,000	(1,150,000)	995,000
Changes in fair values of equity securities	$ (5,970,000)	$ 0	$ 0

Segment Information - Narrative

Segment Information - Narrative (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018USD ($)companybroker	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)companybrokerUnderwriting_divisionSegment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									4
Fee income									$ 13,900	$ 17,000	$ 10,100
Number of underwriting divisions | Underwriting_division									15
Gross written premiums	$ 295,310	$ 279,969	$ 293,378	$ 298,116	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 1,166,773	1,081,905	737,398
Broker one
Segment Reporting Information [Line Items]
Gross written premiums percentage									54.60%
Broker two
Segment Reporting Information [Line Items]
Gross written premiums percentage									12.40%
Agency one
Segment Reporting Information [Line Items]
Consolidated gross written premiums percentage									17.30%
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Number of brokers | broker	2								2
Number of underwriting divisions | Underwriting_division									13
Gross written premiums									$ 656,538	530,120	370,844
Excess and Surplus Lines | Broker one
Segment Reporting Information [Line Items]
Gross written premiums									$ 358,300
Consolidated gross written premiums percentage									30.70%
Excess and Surplus Lines | Broker two
Segment Reporting Information [Line Items]
Gross written premiums									$ 81,400
Consolidated gross written premiums percentage									7.00%
Excess and Surplus Lines | Insured one
Segment Reporting Information [Line Items]
Fee income									$ 13,900
Gross written premiums									$ 294,300
Consolidated gross written premiums percentage									25.20%
Gross written premiums percentage									44.80%
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Number of underwriting divisions | Underwriting_division									1
Gross written premiums									$ 374,346	316,430	182,221
Specialty Admitted Insurance | Agency one
Segment Reporting Information [Line Items]
Gross written premiums									$ 201,700
Gross written premiums percentage									53.90%
Casualty Reinsurance
Segment Reporting Information [Line Items]
Number of brokers | broker	2								2
Number of underwriting divisions | Underwriting_division									1
Gross written premiums									$ 135,889	$ 235,355	$ 184,333
Number of unaffiliated ceding companies | company	3								3
Casualty Reinsurance | Broker one
Segment Reporting Information [Line Items]
Gross written premiums									$ 54,000
Consolidated gross written premiums percentage									4.60%
Gross written premiums percentage									39.80%
Casualty Reinsurance | Broker two
Segment Reporting Information [Line Items]
Gross written premiums									$ 42,300
Consolidated gross written premiums percentage									3.60%
Gross written premiums percentage									31.10%
Casualty Reinsurance | Unaffiliated ceding company one
Segment Reporting Information [Line Items]
Gross written premiums									$ 83,400
Consolidated gross written premiums percentage									7.10%
Gross written premiums percentage									61.40%
Casualty Reinsurance | Unaffiliated ceding company two
Segment Reporting Information [Line Items]
Gross written premiums									$ 22,100
Consolidated gross written premiums percentage									1.90%
Gross written premiums percentage									16.20%
Casualty Reinsurance | Unaffiliated ceding company three
Segment Reporting Information [Line Items]
Gross written premiums									$ 20,800
Consolidated gross written premiums percentage									1.80%
Gross written premiums percentage									15.30%

Segment Information - Schedule

Segment Information - Schedule of Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 295,310	$ 279,969	$ 293,378	$ 298,116	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 1,166,773	$ 1,081,905	$ 737,398
Net earned premiums									815,398	741,109	515,663
Segment revenues	214,522	$ 224,692	$ 228,041	$ 218,344	217,963	$ 220,866	$ 202,394	$ 176,402	885,599	817,625	586,227
Net investment income									61,256	61,119	52,638
Interest expense									11,553	8,974	8,448
Underwriting profit of operating segments									54,909	31,094	49,913
Segment goodwill	181,831				181,831				181,831	181,831	181,831
Segment assets	3,136,776				2,756,695				3,136,776	2,756,695	2,346,533
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums									656,538	530,120	370,844
Net earned premiums									555,684	463,521	301,404
Segment revenues									580,785	493,853	331,090
Net investment income									14,456	15,014	18,051
Interest expense									0	0	0
Underwriting profit of operating segments									42,834	29,693	47,235
Segment goodwill	181,831				181,831				181,831	181,831	181,831
Segment assets	972,111				843,486				972,111	843,486	740,144
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums									374,346	316,430	182,221
Net earned premiums									55,146	68,110	52,281
Segment revenues									56,717	70,366	55,412
Net investment income									3,262	2,532	2,542
Interest expense									0	0	0
Underwriting profit of operating segments									6,972	3,166	2,872
Segment goodwill	0				0				0	0	0
Segment assets	633,689				439,416				633,689	439,416	300,519
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									135,889	235,355	184,333
Net earned premiums									204,568	209,478	161,978
Segment revenues									243,178	240,751	190,064
Net investment income									38,838	31,507	27,257
Interest expense									0	0	0
Underwriting profit of operating segments									5,103	(1,765)	(194)
Segment goodwill	0				0				0	0	0
Segment assets	1,453,754				1,379,866				1,453,754	1,379,866	1,195,230
Corporate and other segment
Segment Reporting Information [Line Items]
Gross written premiums									0	0	0
Net earned premiums									0	0	0
Segment revenues									4,919	12,655	9,661
Net investment income									4,700	12,066	4,788
Interest expense									11,553	8,974	8,448
Underwriting profit of operating segments									0	0	0
Segment goodwill	0				0				0	0	0
Segment assets	$ 77,222				$ 93,927				$ 77,222	$ 93,927	$ 110,640

Segment Information - Reconcili

Segment Information - Reconciliation of Underwriting Profit (Loss) of Insurance Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Underwriting profit of operating segments	$ 54,909	$ 31,094	$ 49,913
Other operating expenses	(26,903)	(25,330)	(20,433)
Underwriting profit	28,006	5,764	29,480
Net investment income	61,256	61,119	52,638
Net realized investment (losses) gains	(5,479)	(1,989)	7,565
Other income	505	361	295
Other expenses	(1,300)	(539)	(1,590)
Interest expense	(11,553)	(8,974)	(8,448)
Amortization of intangible assets	(597)	(597)	(597)
Income before income taxes	70,838	55,145	79,343
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Underwriting profit of operating segments	42,834	29,693	47,235
Net investment income	14,456	15,014	18,051
Interest expense	0	0	0
Amortization of intangible assets	(362)	(362)	(362)
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Underwriting profit of operating segments	6,972	3,166	2,872
Net investment income	3,262	2,532	2,542
Interest expense	0	0	0
Amortization of intangible assets	(235)	(235)	(235)
Casualty Reinsurance
Segment Reporting Information [Line Items]
Underwriting profit of operating segments	5,103	(1,765)	(194)
Net investment income	38,838	31,507	27,257
Interest expense	0	0	0
Corporate and other segment
Segment Reporting Information [Line Items]
Underwriting profit of operating segments	0	0	0
Net investment income	4,700	12,066	4,788
Interest expense	$ (11,553)	$ (8,974)	$ (8,448)

Segment Information - Gross Wri

Segment Information - Gross Written Premiums (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total gross written premiums	$ 295,310	$ 279,969	$ 293,378	$ 298,116	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 1,166,773	$ 1,081,905	$ 737,398
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Total gross written premiums									656,538	530,120	370,844
Excess and Surplus Lines | Commercial Auto
Segment Reporting Information [Line Items]
Total gross written premiums									322,126	247,960	110,050
Excess and Surplus Lines | Manufacturers and Contractors
Segment Reporting Information [Line Items]
Total gross written premiums									79,160	85,719	83,279
Excess and Surplus Lines | Excess Casualty
Segment Reporting Information [Line Items]
Total gross written premiums									66,452	51,160	43,574
Excess and Surplus Lines | General Casualty
Segment Reporting Information [Line Items]
Total gross written premiums									54,127	38,097	36,858
Excess and Surplus Lines | Energy
Segment Reporting Information [Line Items]
Total gross written premiums									33,942	29,704	29,709
Excess and Surplus Lines | Allied Health
Segment Reporting Information [Line Items]
Total gross written premiums									30,450	19,181	14,413
Excess and Surplus Lines | Excess Property
Segment Reporting Information [Line Items]
Total gross written premiums									16,963	14,447	14,083
Excess and Surplus Lines | Life Sciences
Segment Reporting Information [Line Items]
Total gross written premiums									16,636	12,981	11,132
Excess and Surplus Lines | Small Business
Segment Reporting Information [Line Items]
Total gross written premiums									14,808	11,307	9,104
Excess and Surplus Lines | Professional Liability
Segment Reporting Information [Line Items]
Total gross written premiums									5,916	6,326	8,361
Excess and Surplus Lines | Environmental
Segment Reporting Information [Line Items]
Total gross written premiums									10,499	7,920	5,321
Excess and Surplus Lines | Medical Professionals
Segment Reporting Information [Line Items]
Total gross written premiums									1,774	2,297	2,739
Excess and Surplus Lines | Sports and Entertainment
Segment Reporting Information [Line Items]
Total gross written premiums									3,685	3,021	2,221
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Total gross written premiums									374,346	316,430	182,221
Casualty Reinsurance
Segment Reporting Information [Line Items]
Total gross written premiums									$ 135,889	$ 235,355	$ 184,333

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	$ 1,184,202,000	$ 1,016,098,000
Equity securities:
Equity securities	78,385,000	82,522,000
Trading securities:
Fixed maturity securities	0	3,808,000
Short-term investments	81,966,000	36,804,000
Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	1,184,202,000	1,016,098,000
State and municipal
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	149,295,000	144,366,000
Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	204,109,000	158,661,000
Corporate
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	524,768,000	413,721,000
Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	197,025,000	182,611,000
Obligations of U.S. government corporations and agencies
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)		35,847,000
U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	107,193,000	78,874,000
Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	1,812,000	2,018,000
Recurring basis
Trading securities:
Fixed maturity securities		3,808,000
Short-term investments	81,966,000	36,804,000
Recurring basis | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	1,184,202,000	1,016,098,000
Recurring basis | State and municipal
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	149,295,000	144,366,000
Recurring basis | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	204,109,000	158,661,000
Recurring basis | Corporate
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	524,768,000	413,721,000
Recurring basis | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	197,025,000	182,611,000
Recurring basis | Obligations of U.S. government corporations and agencies
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)		35,847,000
Recurring basis | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	107,193,000	78,874,000
Recurring basis | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	1,812,000	2,018,000
Recurring basis | Equity securities
Equity securities:
Equity securities	78,385,000	82,522,000
Recurring basis | Preferred stock
Equity securities:
Equity securities	60,740,000	66,281,000
Recurring basis | Common stock
Equity securities:
Equity securities	17,645,000	16,241,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1
Trading securities:
Fixed maturity securities		0
Short-term investments	1,250,000	1,000,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	106,651,000	78,265,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | State and municipal
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Corporate
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Obligations of U.S. government corporations and agencies
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)		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
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	106,651,000	78,265,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Equity securities
Equity securities:
Equity securities	16,674,000	15,507,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Preferred stock
Equity securities:
Equity securities	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets Level 1 | Common stock
Equity securities:
Equity securities	16,674,000	15,507,000
Recurring basis | Significant Other Observable Inputs Level 2
Trading securities:
Fixed maturity securities		3,808,000
Short-term investments	80,716,000	35,804,000
Recurring basis | Significant Other Observable Inputs Level 2 | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	1,073,323,000	933,153,000
Recurring basis | Significant Other Observable Inputs Level 2 | State and municipal
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	149,295,000	144,366,000
Recurring basis | Significant Other Observable Inputs Level 2 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	204,109,000	158,661,000
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	524,768,000	413,721,000
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	192,797,000	177,931,000
Recurring basis | Significant Other Observable Inputs Level 2 | Obligations of U.S. government corporations and agencies
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)		35,847,000
Recurring basis | Significant Other Observable Inputs Level 2 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	542,000	609,000
Recurring basis | Significant Other Observable Inputs Level 2 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	1,812,000	2,018,000
Recurring basis | Significant Other Observable Inputs Level 2 | Equity securities
Equity securities:
Equity securities	61,497,000	67,015,000
Recurring basis | Significant Other Observable Inputs Level 2 | Preferred stock
Equity securities:
Equity securities	60,740,000	66,281,000
Recurring basis | Significant Other Observable Inputs Level 2 | Common stock
Equity securities:
Equity securities	757,000	734,000
Recurring basis | Significant Unobservable Inputs Level 3
Trading securities:
Fixed maturity securities		0
Short-term investments	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Fixed maturity securities:
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	4,228,000	4,680,000
Recurring basis | Significant Unobservable Inputs Level 3 | State and municipal
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Residential mortgage-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Corporate
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Commercial mortgage and asset-backed
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	4,228,000	4,680,000
Recurring basis | Significant Unobservable Inputs Level 3 | Obligations of U.S. government corporations and agencies
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)		0
Recurring basis | Significant Unobservable Inputs Level 3 | U.S. Treasury securities and obligations guaranteed by the U.S. government
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Redeemable preferred stock
Fixed maturity securities, available-for-sale:
Available-for-sale, at fair value (amortized cost: 2018 – $1,199,409; 2017 – $1,008,662)	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Equity securities
Equity securities:
Equity securities	214,000	0
Recurring basis | Significant Unobservable Inputs Level 3 | Preferred stock
Equity securities:
Equity securities	0	0
Recurring basis | Significant Unobservable Inputs Level 3 | Common stock
Equity securities:
Equity securities	$ 214,000	$ 0

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Available-for-Sale Fixed Maturity Securities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale fixed maturity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4,680	$ 5,000	$ 5,000
Transfers out of Level 3	0	0	0
Transfers in to Level 3	0	0	0
Sales	0	0	0
Maturities and calls	(452)	(320)	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
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases	214	0	0
Available-for-sale fixed maturity and equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	4,680	5,000
Ending balance	$ 4,442	$ 4,680	$ 5,000

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities, available-for-sale	$ 1,184,202,000	$ 1,016,098,000
Unpaid principal on bank loan participations for which external sources were unavailable to determine fair value		807,000
Carrying value of bank loan participations for which external sources were unavailable to determine fair value		0
Equity securities	78,385,000	82,522,000
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
Commercial mortgage and asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities, available-for-sale	197,025,000	182,611,000
Commercial mortgage and asset-backed | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities, available-for-sale	197,025,000	182,611,000
Commercial mortgage and asset-backed | Significant Unobservable Inputs Level 3 | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities, available-for-sale	4,228,000	4,680,000
Common stock | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	17,645,000	16,241,000
Common stock | Significant Unobservable Inputs Level 3 | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 214,000	$ 0

Fair Value Measurements - Sum_3

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Nonecurring Basis (Details) - Nonrecurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Bank loan participations held-for-investment	$ 0	$ 5,111
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	$ 0	$ 5,111

Fair Value Measurements - Sum_4

Fair Value Measurements - Summary of the Carrying Value and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed maturity securities, available-for-sale	$ 1,184,202	$ 1,016,098
Equity securities, at fair value (cost: 2018 – $77,152; 2017 – $75,318)	78,385	82,522
Bank loan participations held-for-investment, at amortized cost, net of allowance	260,972	238,214
Cash and cash equivalents	172,457	163,495	$ 109,784	$ 106,406
Short-term investments	81,966	36,804
Liabilities
Senior debt	118,300	98,300
Carrying Value
Assets
Fixed maturity securities, available-for-sale	1,184,202	1,016,098
Equity securities, at fair value (cost: 2018 – $77,152; 2017 – $75,318)	78,385	82,522
Fixed maturity securities, trading	0	3,808
Bank loan participations held-for-investment, at amortized cost, net of allowance	260,972	238,214
Cash and cash equivalents	172,457	163,495
Short-term investments	81,966	36,804
Other invested assets – notes receivable	13,250	11,778
Liabilities
Senior debt	118,300	98,300
Junior subordinated debt	104,055	104,055
Fair Value
Assets
Fixed maturity securities, available-for-sale	1,184,202	1,016,098
Equity securities, at fair value (cost: 2018 – $77,152; 2017 – $75,318)	78,385	82,522
Fixed maturity securities, trading	0	3,808
Bank loan participations held-for-investment, at amortized cost, net of allowance	250,697	236,532
Cash and cash equivalents	172,457	163,495
Short-term investments	81,966	36,804
Other invested assets – notes receivable	18,687	17,104
Liabilities
Senior debt	118,317	97,884
Junior subordinated debt	$ 117,057	$ 116,569

Statutory Matters - Schedule of

Statutory Matters - Schedule of Statutory Accounting (Details) - UNITED STATES - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Line Items]
Statutory net income	$ 6,770	$ 31,881	$ 13,468
Statutory capital and surplus	241,668	219,132	184,859
Minimum required statutory capital and surplus	$ 24,850	$ 25,000	$ 25,000

Statutory Matters - Narrative (

Statutory Matters - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
BERMUDA
Statutory Accounting Practices [Line Items]
Minimum liquidity ratio	75.00%
Casualty Reinsurance
Statutory Accounting Practices [Line Items]
Minimum statutory solvency margin		$ 275.4
Casualty Reinsurance | BERMUDA
Statutory Accounting Practices [Line Items]
Minimum statutory solvency margin	$ 161.3	109.9
Actual statutory capital and surplus	439.4	409.2
Statutory net income	$ 86.5	$ 20.1	$ 74.7

Dividend Restrictions (Details)

Dividend Restrictions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statutory Accounting Practices [Line Items]
Percentage of dividend withholding tax for distributions	5.00%
Percentage of dividend distribution tax	30.00%
Retained earnings	$ 79,753	$ 48,198
UNITED STATES
Statutory Accounting Practices [Line Items]
Dividend limitations percentage of statutory surplus	10.00%
Maximum amount of dividends available without regulatory approval	$ 24,200
BERMUDA
Statutory Accounting Practices [Line Items]
Maximum amount of dividends and return of capital available	$ 109,800

Other Related Party Transacti_2

Other Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Related Party Transaction [Line Items]
Lease term (in years)	10 years
Increase in fixed annual rent	2.00%
Assets	$ 3,136,776	$ 2,756,695	$ 2,346,533
Liabilities	2,427,535	2,061,996
Non-Executive Chairman
Related Party Transaction [Line Items]
Total fees paid for the use of plane	$ 123	314	246
Independent Directors | Consulting Agreement
Related Party Transaction [Line Items]
Investment and other business consulting services fees		$ 38	$ 150
Subsequent event | ASU 2016-02
Related Party Transaction [Line Items]
Assets				$ (22,600)
Liabilities				(30,900)
Cumulative effect of adoption of ASU				$ 8,300

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 20, 2019	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
RSUs
Subsequent Event [Line Items]
Vesting period (in years)			3 years
Number of awards granted			227,481	137,034	60,291
Fair value on grant date			$ 39.74	$ 42.20	$ 32.03
RSUs | Minimum
Subsequent Event [Line Items]
Vesting period (in years)			1 year
RSUs | Maximum
Subsequent Event [Line Items]
Vesting period (in years)			5 years
Subsequent event
Subsequent Event [Line Items]
Dividends payable, per common share	$ 0.30
Subsequent event | 2014 Long Term Incentive Plan | RSUs
Subsequent Event [Line Items]
Number of awards granted	167,295
Fair value on grant date	$ 42.07
Subsequent event | 2014 Long Term Incentive Plan | RSUs | Minimum
Subsequent Event [Line Items]
Vesting period (in years)	1 year
Subsequent event | 2014 Long Term Incentive Plan | RSUs | Maximum
Subsequent Event [Line Items]
Vesting period (in years)	3 years
Line of Credit | 2017 Facility | Subsequent event
Subsequent Event [Line Items]
Repayments of unsecured borrowings		$ 20
Unsecured loans outstanding		$ 10

Unaudited Selected Quarterly _3

Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 295,310	$ 279,969	$ 293,378	$ 298,116	$ 237,900	$ 338,351	$ 281,475	$ 224,179	$ 1,166,773	$ 1,081,905	$ 737,398
Total revenues	214,522	224,692	228,041	218,344	217,963	220,866	202,394	176,402	885,599	817,625	586,227
Net income	11,632	19,581	16,984	15,633	224	10,351	14,541	18,450	63,830	43,566	74,471
Comprehensive (loss) income	$ 18,923	$ 15,192	$ 10,426	$ (2,914)	$ (2,015)	$ 11,067	$ 21,249	$ 22,484	$ 41,627	$ 52,785	$ 72,412
Earnings per share:
Basic earnings per share (in dollars per share)	$ 0.39	$ 0.65	$ 0.57	$ 0.53	$ 0.01	$ 0.35	$ 0.49	$ 0.63	$ 2.14	$ 1.48	$ 2.56
Diluted earnings per share (in dollars per share)	$ 0.38	$ 0.64	$ 0.56	$ 0.52	$ 0.01	$ 0.34	$ 0.48	$ 0.61	$ 2.11	$ 1.44	$ 2.49
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									$ 135,889	$ 235,355	$ 184,333
Total revenues									$ 243,178	$ 240,751	$ 190,064
Commission expense | Casualty Reinsurance
Segment Reporting Information [Line Items]
Net income							$ (2,000)

SCHEDULE I - Summary of Inves_2

SCHEDULE I - Summary of Investments - Other than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2018USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amount at which shown on Balance Sheet	$ 1,634,801	[1]
Bank loan participations, held-for-investment, net of allowance
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	260,972
Fair Value	250,697
Amount at which shown on Balance Sheet	260,972	[1]
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	81,966
Fair Value	81,966
Amount at which shown on Balance Sheet	81,966	[1]
Other invested assets
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amount at which shown on Balance Sheet	29,276	[1]
Available-for-sale Securities | Fixed maturity securities:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	1,199,409
Fair Value	1,184,202
Amount at which shown on Balance Sheet	1,184,202	[1]
Available-for-sale Securities | State and municipal
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	147,160
Fair Value	149,295
Amount at which shown on Balance Sheet	149,295	[1]
Available-for-sale Securities | Residential mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	208,869
Fair Value	204,109
Amount at which shown on Balance Sheet	204,109	[1]
Available-for-sale Securities | Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	534,024
Fair Value	524,768
Amount at which shown on Balance Sheet	524,768	[1]
Available-for-sale Securities | Commercial mortgage and asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	199,528
Fair Value	197,025
Amount at which shown on Balance Sheet	197,025	[1]
Available-for-sale Securities | U.S. Treasury securities and obligations guaranteed by the U.S. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	107,803
Fair Value	107,193
Amount at which shown on Balance Sheet	107,193	[1]
Available-for-sale Securities | Redeemable preferred stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	2,025
Fair Value	1,812
Amount at which shown on Balance Sheet	1,812	[1]
Available-for-sale Securities | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	77,152
Fair Value	78,385
Amount at which shown on Balance Sheet	78,385	[1]
Available-for-sale Securities | Preferred stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	57,689
Fair Value	60,740
Amount at which shown on Balance Sheet	60,740	[1]
Available-for-sale Securities | Common stock
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or Amortized Cost	19,463
Fair Value	17,645
Amount at which shown on Balance Sheet	$ 17,645	[1]
[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 Finan_2

SCHEDULE II - Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets [Abstract]
Cash and cash equivalents	$ 172,457	$ 163,495	$ 109,784	$ 106,406
Other assets	94,379	83,598
Total assets	3,136,776	2,756,695	2,346,533
Liabilities:
Accrued expenses	51,792	39,295
Senior debt	118,300	98,300
Junior subordinated debt	104,055	104,055
Other liabilities	41,879	48,368
Total liabilities	2,427,535	2,061,996
Commitments and contingent liabilities	0	0
Shareholders’ equity:
Class A common shares	6	6
Additional paid-in capital	645,310	636,149
Retained earnings	79,753	48,198
Accumulated other comprehensive (loss) income	(15,828)	10,346
Total shareholders’ equity	709,241	694,699	693,221	681,038
Total liabilities and shareholders’ equity	3,136,776	2,756,695
Parent Company
Assets [Abstract]
Cash and cash equivalents	240	2,849	$ 2,969	$ 1,976
Investment in subsidiaries	882,407	910,610
Due from subsidiaries	5,775	0
Other assets	1,579	2,351
Total assets	890,001	915,810
Liabilities:
Accrued expenses	2,322	1,406
Senior debt	103,300	83,300
Junior subordinated debt	15,928	15,928
Notes payable to subsidiary	40,000	100,000
Due to subsidiaries	18,605	20,043
Other liabilities	605	434
Total liabilities	180,760	221,111
Commitments and contingent liabilities	0	0
Shareholders’ equity:
Class A common shares	6	6
Additional paid-in capital	645,310	636,149
Retained earnings	79,753	48,198
Accumulated other comprehensive (loss) income	(15,828)	10,346
Total shareholders’ equity	709,241	694,699
Total liabilities and shareholders’ equity	$ 890,001	$ 915,810

SCHEDULE II - Condensed Finan_3

SCHEDULE II - Condensed Financial Information of Registrant (Details 1) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Other income									$ 14,424	$ 17,386	$ 10,361
Total revenues	$ 214,522	$ 224,692	$ 228,041	$ 218,344	$ 217,963	$ 220,866	$ 202,394	$ 176,402	885,599	817,625	586,227
Expenses:
Other operating expenses									201,035	196,993	170,828
Other expenses									1,300	539	1,590
Interest expense									11,553	8,974	8,448
Total expenses									814,761	762,480	506,884
Loss before equity in net income of subsidiaries									70,838	55,145	79,343
Equity in net income of subsidiaries									3,540	13,209	8,743
Net income	11,632	19,581	16,984	15,633	224	10,351	14,541	18,450	63,830	43,566	74,471
Other comprehensive (loss) income:
Equity in other comprehensive (losses) earnings of subsidiaries									(22,203)	9,219	(2,059)
Total comprehensive income	$ 18,923	$ 15,192	$ 10,426	$ (2,914)	$ (2,015)	$ 11,067	$ 21,249	$ 22,484	41,627	52,785	72,412
Parent Company
Revenues:
Other income									58	49	44
Total revenues									58	49	44
Expenses:
Other operating expenses									13,768	10,375	9,467
Other expenses									337	640	293
Interest expense									5,122	4,130	3,974
Total expenses									19,227	15,145	13,734
Loss before equity in net income of subsidiaries									(19,169)	(15,096)	(13,690)
Equity in net income of subsidiaries									82,999	58,662	88,161
Net income									63,830	43,566	74,471
Other comprehensive (loss) income:
Equity in other comprehensive (losses) earnings of subsidiaries									(22,203)	9,219	(2,059)
Total comprehensive income									$ 41,627	$ 52,785	$ 72,412

SCHEDULE II - Condensed Finan_4

SCHEDULE II - Condensed Financial Information of Registrant (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net income	$ 11,632	$ 19,581	$ 16,984	$ 15,633	$ 224	$ 10,351	$ 14,541	$ 18,450	$ 63,830	$ 43,566	$ 74,471
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Provision for depreciation and amortization									4,215	2,123	2,414
Share based compensation expense									6,228	7,688	5,492
Net cash provided by operating activities									290,028	207,816	154,349
Investing activities
Net cash used in investing activities									(266,772)	(104,741)	(80,764)
Financing activities
Dividends paid									(36,123)	(50,832)	(65,988)
Senior debt issuance									20,000	10,000	0
Issuances of common shares under equity incentive plans									5,172	1,708	2,260
Common share repurchases									(2,239)	(9,448)	(4,907)
Net cash used in financing activities									(14,294)	(49,364)	(70,207)
Increase in cash and cash equivalents									8,962	53,711	3,378
Cash and cash equivalents at beginning of year				163,495				109,784	163,495	109,784	106,406
Cash and cash equivalents at end of year	172,457				163,495				172,457	163,495	109,784
Supplemental information
Interest paid									11,269	8,909	8,121
Parent Company
Operating activities
Net income									63,830	43,566	74,471
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Provision for depreciation and amortization									176	176	157
Share based compensation expense									6,228	7,688	5,492
Equity in undistributed earnings of subsidiaries									6,000	(23,662)	(88,161)
Changes in operating assets and liabilities:									(5,653)	684	(1,889)
Net cash provided by operating activities									70,581	28,452	(9,930)
Investing activities
Dividends from subsidiaries									0	20,000	80,000
Net cash used in investing activities									0	20,000	80,000
Financing activities
Dividends paid									(36,123)	(50,832)	(65,988)
Senior debt issuance									20,000	10,000	0
Senior debt repayments									0	0	0
Subsidiary note repayments									(60,000)	0	0
Debt issue costs paid									0	0	(442)
Issuances of common shares under equity incentive plans									5,172	1,708	2,260
Common share repurchases									(2,239)	(9,448)	(4,907)
Net cash used in financing activities									(73,190)	(48,572)	(69,077)
Increase in cash and cash equivalents									(2,609)	(120)	993
Cash and cash equivalents at beginning of year				$ 2,849				$ 2,969	2,849	2,969	1,976
Cash and cash equivalents at end of year	$ 240				$ 2,849				240	2,849	2,969
Supplemental information
Interest paid									$ 5,052	$ 4,612	$ 4,676

SCHEDULE II - Condensed Finan_5

SCHEDULE II - Condensed Financial Information of Registrant - Narrative (Details) - USD ($) $ in Thousands

3 Months Ended

12 Months Ended

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Jan. 01, 2018

Condensed Financial Statements, Captions [Line Items]

Net income

$ 11,632

$ 19,581

$ 16,984

$ 15,633

$ 224

$ 10,351

$ 14,541

$ 18,450

$ 63,830

$ 43,566

$ 74,471

ASU 2016-01

Condensed Financial Statements, Captions [Line Items]

Cumulative effect of adoption of ASU

$ 0

Net income

(4,700)

ASU 2018-02

Condensed Financial Statements, Captions [Line Items]

Cumulative effect of adoption of ASU

0

Retained Earnings

Condensed Financial Statements, Captions [Line Items]

Net income

$ 63,830

$ 43,566

$ 74,471

Retained Earnings | ASU 2016-01

Condensed Financial Statements, Captions [Line Items]

Cumulative effect of adoption of ASU

4,682

Retained Earnings | ASU 2018-02

Condensed Financial Statements, Captions [Line Items]

Cumulative effect of adoption of ASU

$ (711)

SCHEDULE III - Supplementary _2

SCHEDULE III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	$ 54,450	$ 72,365	$ 64,789
Reserve for Losses and Loss Adjustment Expenses	1,661,459	1,292,349	943,865
Unearned Premiums	386,473	418,114	390,563
Net Earned Premiums	815,398	741,109	515,663
Net Investment Income	61,256	61,119	52,638
Losses and Loss Adjustment Expenses	600,276	555,377	325,421
Amortization of Policy Acquisition Costs	111,103	116,001	101,624
Other Operating Expenses	201,035	196,993	170,828
Net Written Premiums	762,672	766,626	557,708
Excess and Surplus Lines
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	16,736	17,050	14,808
Reserve for Losses and Loss Adjustment Expenses	960,562	759,043	575,280
Unearned Premiums	175,231	149,047	137,290
Net Earned Premiums	555,684	463,521	301,404
Net Investment Income	14,456	15,014	18,051
Losses and Loss Adjustment Expenses	437,904	371,717	188,768
Amortization of Policy Acquisition Costs	48,299	45,158	46,669
Other Operating Expenses	88,865	79,136	75,467
Net Written Premiums	571,098	469,891	316,922
Specialty Admitted Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	(1,829)	(725)	1,767
Reserve for Losses and Loss Adjustment Expenses	426,315	271,446	128,795
Unearned Premiums	84,659	73,649	84,156
Net Earned Premiums	55,146	68,110	52,281
Net Investment Income	3,262	2,532	2,542
Losses and Loss Adjustment Expenses	32,623	44,863	30,897
Amortization of Policy Acquisition Costs	(1,792)	5,605	5,968
Other Operating Expenses	15,551	20,081	18,512
Net Written Premiums	55,840	60,957	55,803
Casualty Reinsurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	39,543	56,040	48,214
Reserve for Losses and Loss Adjustment Expenses	274,582	261,860	239,790
Unearned Premiums	126,583	195,418	169,117
Net Earned Premiums	204,568	209,478	161,978
Net Investment Income	38,838	31,507	27,257
Losses and Loss Adjustment Expenses	129,749	138,797	105,756
Amortization of Policy Acquisition Costs	64,596	65,238	48,987
Other Operating Expenses	69,716	72,446	56,416
Net Written Premiums	135,734	235,778	184,983
Corporate and other segment
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [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	4,700	12,066	4,788
Losses and Loss Adjustment Expenses	0	0	0
Amortization of Policy Acquisition Costs	0	0	0
Other Operating Expenses	26,903	25,330	20,433
Net Written Premiums	$ 0	$ 0	$ 0

SCHEDULE IV - Reinsurance (Deta

SCHEDULE IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 1,027,222	$ 843,719	$ 549,700
Ceded to Other Companies Written Premiums	404,101	315,279	179,690
Assumed from Other Companies Written Premiums	139,551	238,186	187,698
Net written premiums	$ 762,672	$ 766,626	$ 557,708
Percentage of Amount Assumed to Net	18.30%	31.10%	33.70%
Excess and Surplus Lines
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 656,538	$ 530,077	$ 370,802
Ceded to Other Companies Written Premiums	85,440	60,229	53,922
Assumed from Other Companies Written Premiums	0	43	42
Net written premiums	$ 571,098	$ 469,891	$ 316,922
Percentage of Amount Assumed to Net	0.00%	0.00%	0.00%
Specialty Admitted Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 370,684	$ 313,642	$ 178,898
Ceded to Other Companies Written Premiums	318,506	255,473	126,418
Assumed from Other Companies Written Premiums	3,662	2,788	3,323
Net written premiums	$ 55,840	$ 60,957	$ 55,803
Percentage of Amount Assumed to Net	6.60%	4.60%	6.00%
Casualty Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 0	$ 0	$ 0
Ceded to Other Companies Written Premiums	155	(423)	(650)
Assumed from Other Companies Written Premiums	135,889	235,355	184,333
Net written premiums	$ 135,734	$ 235,778	$ 184,983
Percentage of Amount Assumed to Net	100.10%	99.80%	99.60%

SCHEDULE V - Valuation and Qu_2

SCHEDULE V - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 5,976	$ 3,079	$ 7,088
Additions Amounts Charged to Expense	2,454	3,453	23
Deductions Amounts Written Off or Disposals	(4,482)	(556)	(4,032)
Balance at End of Period	3,948	5,976	3,079
Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	2,757	2,136	2,778
Additions Amounts Charged to Expense	1,504	1,029	814
Deductions Amounts Written Off or Disposals	(313)	(408)	(1,456)
Balance at End of Period	3,948	2,757	2,136
Allowance for Credit Losses on Bank Loans
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	3,219	943	4,310
Additions Amounts Charged to Expense	950	2,424	(791)
Deductions Amounts Written Off or Disposals	(4,169)	(148)	(2,576)
Balance at End of Period	$ 0	$ 3,219	$ 943

SCHEDULE VI - Supplementary I_2

SCHEDULE VI - Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred policy acquisition costs	$ 54,450	$ 72,365	$ 64,789
Reserve for losses and loss adjustment expenses	1,661,459	1,292,349	943,865
Unearned premiums	386,473	418,114	390,563
Net earned premiums	815,398	741,109	515,663
Net investment income	61,256	61,119	52,638
Current year	582,604	533,905	349,137
Prior year	17,672	21,472	(23,716)
Total incurred losses and loss and adjustment expenses	600,276	555,377	325,421
Amortization of policy acquisition costs	111,103	116,001	101,624
Paid losses and loss adjustment expenses, net of reinsurance	396,013	326,680	217,827
Net written premiums	$ 762,672	$ 766,626	$ 557,708

Uncategorized Items - jrvr-2018

Label	Element	Value
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (4,682,000)
Accounting Standards Update 2018-02 [Member] | AOCI Attributable to Parent [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 711,000