Investments (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Security
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Fair value						$ 723,616
Cost or amortized cost						651,768
Gross unrealized gains						78,904
Gross unrealized losses						(7,056)
Net unrealized gains (losses)						$ 71,848
Available-for-sale Securities Continuous Unrealized Loss Position [Abstract]
Number of securities, total | Security	[1]	643		643		636
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, total	[1]	$ 477,576		$ 477,576		$ 435,713
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, total	[1]	(9,743)		(9,743)		(7,056)
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains		3,691	$ 7,696	6,134	$ 10,190
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses		(3,088)	(5,131)	(5,796)	(6,617)
Other-than-temporary impairments		0	0	0	0
Change in value of limited partnership investments		(2,842)	731	(5,445)	6,017
Gains (losses) on equity securities [Abstract]
Realized gains on equity securities sold during the period	[2]	312	0	953	0
Unrealized losses on equity securities held at the end of the period		(1,508)	0	(3,814)	0
Realized and unrealized losses on equity securities held at the end of the period		(1,196)	0	(2,861)	0
Net realized and unrealized gains (losses) on investments		(3,435)	3,296	(7,968)	9,590
Proceeds from sales of equity securities		27,800		87,557	16,626
Realized gains on equity securities		9,990		45,128
Federal income taxes				35,635
Fixed Maturities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		576,388		576,388
Cost or amortized cost		582,068		582,068
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value		48,916		48,916
Excess of one year to five years, Fair Value		303,116		303,116
Excess of five years to ten years, Fair Value		87,208		87,208
Excess of ten years, Fair Value		2,531		2,531
Total contractual maturities, Fair Value		441,771		441,771
Asset-backed securities, Fair Value		134,617		134,617
Total, Fair Value		576,388		576,388
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost		49,514		49,514
Excess of one year to five years, Cost or Amortized Cost		307,337		307,337
Excess of five years to ten years, Cost or Amortized Cost		89,044		89,044
Excess of ten years, Cost or Amortized Cost		2,590		2,590
Total contractual maturities, Cost or Amortized Cost		448,485		448,485
Asset-backed securities, Cost or Amortized Cost		133,583		133,583
Cost or amortized cost		582,068		582,068
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains		3,691	4,804	6,134	5,692
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses		(3,088)	(4,776)	(5,796)	(6,149)
Agency Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		14,961		14,961		16,586
Cost or amortized cost		14,480		14,480		15,839
Gross unrealized gains		646		646		818
Gross unrealized losses		(165)		(165)		(71)
Net unrealized gains (losses)		481		481		747
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		14,961		14,961		16,586
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		14,480		14,480		15,839
Agency Mortgage-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		33,708		33,708		27,075
Cost or amortized cost		34,459		34,459		27,180
Gross unrealized gains		12		12		47
Gross unrealized losses		(763)		(763)		(152)
Net unrealized gains (losses)		(751)		(751)		(105)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		33,708		33,708		27,075
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		34,459		34,459		27,180
Asset-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		51,454		51,454		43,469
Cost or amortized cost		50,994		50,994		42,861
Gross unrealized gains		689		689		749
Gross unrealized losses		(229)		(229)		(141)
Net unrealized gains (losses)		460		460		608
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		51,454		51,454		43,469
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		50,994		50,994		42,861
Bank Loans [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		20,202		20,202		19,488
Cost or amortized cost		20,127		20,127		19,271
Gross unrealized gains		204		204		266
Gross unrealized losses		(129)		(129)		(49)
Net unrealized gains (losses)		75		75		217
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		20,202		20,202		19,488
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		20,127		20,127		19,271
Certificates of Deposit [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		3,122		3,122		3,135
Cost or amortized cost		3,124		3,124		3,124
Gross unrealized gains		0		0		11
Gross unrealized losses		(2)		(2)		0
Net unrealized gains (losses)		(2)		(2)		11
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		3,122		3,122		3,135
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		3,124		3,124		3,124
Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		5,135		5,135		6,492
Cost or amortized cost		4,789		4,789		6,079
Gross unrealized gains		369		369		451
Gross unrealized losses		(23)		(23)		(38)
Net unrealized gains (losses)		346		346		413
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		5,135		5,135		6,492
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		4,789		4,789		6,079
Corporate Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		212,059		212,059		198,349
Cost or amortized cost		216,496		216,496		198,419
Gross unrealized gains		716		716		1,602
Gross unrealized losses		(5,153)		(5,153)		(1,672)
Net unrealized gains (losses)		(4,437)		(4,437)		(70)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		212,059		212,059		198,349
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		216,496		216,496		198,419
Mortgage-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		30,954		30,954		24,204
Cost or amortized cost		30,475		30,475		23,656
Gross unrealized gains		940		940		933
Gross unrealized losses		(461)		(461)		(385)
Net unrealized gains (losses)		479		479		548
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		30,954		30,954		24,204
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		30,475		30,475		23,656
Municipal Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		48,327		48,327		96,650
Cost or amortized cost		48,468		48,468		97,059
Gross unrealized gains		296		296		322
Gross unrealized losses		(437)		(437)		(731)
Net unrealized gains (losses)		(141)		(141)		(409)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		48,327		48,327		96,650
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		48,468		48,468		97,059
Non-U.S. Government Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		38,602		38,602		37,394
Cost or amortized cost		39,681		39,681		37,971
Gross unrealized gains		191		191		475
Gross unrealized losses		(1,270)		(1,270)		(1,052)
Net unrealized gains (losses)		(1,079)		(1,079)		(577)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		38,602		38,602		37,394
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		39,681		39,681		37,971
U.S. Government Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		117,864		117,864		49,011
Cost or amortized cost		118,975		118,975		49,558
Gross unrealized gains		0		0		0
Gross unrealized losses		(1,111)		(1,111)		(547)
Net unrealized gains (losses)		(1,111)		(1,111)		(547)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		117,864		117,864		49,011
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		118,975		118,975		49,558
Total Fixed Maturities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		576,388		576,388		521,853
Cost or amortized cost		582,068		582,068		521,017
Gross unrealized gains		4,063		4,063		5,674
Gross unrealized losses		(9,743)		(9,743)		(4,838)
Net unrealized gains (losses)		$ (5,680)		$ (5,680)		$ 836
Available-for-sale Securities Continuous Unrealized Loss Position [Abstract]
Number of securities, 12 months or less | Security		542		542		459
Number of securities, Greater than 12 months | Security		101		101		112
Number of securities, total | Security		643		643		571
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less		$ 419,376		$ 419,376		$ 313,421
Fair value, Greater than 12 months		58,200		58,200		75,638
Fair value, total		477,576		477,576		389,059
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less		(7,862)		(7,862)		(2,683)
Gross unrealized loss, Greater than 12 months		(1,881)		(1,881)		(2,155)
Gross unrealized loss, total		(9,743)		(9,743)		(4,838)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		576,388		576,388		521,853
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		582,068		582,068		521,017
Equity Securities [Member]
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains	[1]	0	2,892	0	4,498
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses	[1]	$ 0	$ (355)	$ 0	$ (468)
Consumer [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					46,578
Cost or amortized cost						23,565
Gross unrealized gains						24,031
Gross unrealized losses						(1,018)
Net unrealized gains (losses)						23,013
Energy [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					10,278
Cost or amortized cost						6,763
Gross unrealized gains						3,602
Gross unrealized losses						(87)
Net unrealized gains (losses)						3,515
Financial [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					45,470
Cost or amortized cost						31,859
Gross unrealized gains						13,937
Gross unrealized losses						(326)
Net unrealized gains (losses)						13,611
Industrial [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					25,402
Cost or amortized cost						8,949
Gross unrealized gains						16,793
Gross unrealized losses						(340)
Net unrealized gains (losses)						16,453
Technology [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					13,061
Cost or amortized cost						5,768
Gross unrealized gains						7,401
Gross unrealized losses						(108)
Net unrealized gains (losses)						7,293
Funds (e.g. mutual funds, closed end funds, ETFs) [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					50,291
Cost or amortized cost						46,177
Gross unrealized gains						4,153
Gross unrealized losses						(39)
Net unrealized gains (losses)						4,114
Other [Member]
Available-for-sale Equity Securities [Abstract]
Fair value	[1]					10,683
Cost or amortized cost						7,670
Gross unrealized gains						3,313
Gross unrealized losses						(300)
Net unrealized gains (losses)						3,013
Total Equity Securities [Member]
Available-for-sale Equity Securities [Abstract]
Fair value						201,763
Cost or amortized cost						130,751
Gross unrealized gains						73,230
Gross unrealized losses						(2,218)
Net unrealized gains (losses)						$ 71,012
Available-for-sale Securities Continuous Unrealized Loss Position [Abstract]
Number of securities, 12 months or less | Security	[1]	0		0		65
Number of securities, Greater than 12 months | Security	[1]	0		0		0
Number of securities, total | Security	[1]	0		0		65
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	[1]	$ 0		$ 0		$ 46,654
Fair value, Greater than 12 months	[1]	0		0		0
Fair value, total	[1]	0		0		46,654
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	[1]	0		0		(2,218)
Gross unrealized loss, Greater than 12 months	[1]	0		0		0
Gross unrealized loss, total	[1]	$ 0		$ 0		$ (2,218)

[1] Effective January 1, 2018, the Company adopted ASU 2016-01 and equity securities are no longer classified as available-for-sale. Prior periods have not been restated to conform to the current presentation. See Note 1 - Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements for further discussion.

[2] During the three and six months ended June 30, 2018, the Company sold $27,800 and $87,557 in equity securities, resulting in a realized gain of $9,990 and $45,128, respectively. The majority of this gain was included in unrealized gains within other comprehensive income at December 31, 2017 and, as a result of the adoption of ASU 2016-01, was reclassified to retained earnings as of January 1, 2018 and was therefore not recognized in the condensed consolidated statements of operations for the three and six months ended June 30, 2018.