Investments (Details) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	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]	621		621		636
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, total	[1]	$ 512,692		$ 512,692		$ 435,713
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, total	[1]	(10,020)		(10,020)		(7,056)
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains		2,690	$ 7,955	8,824	$ 18,145
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses		(2,743)	(4,471)	(8,539)	(11,057)
Other-than-temporary impairments		0	(38)	0	(69)
Change in value of limited partnership investments		(1,073)	2,498	(6,518)	8,515
Gains (losses) on equity securities [Abstract]
Realized gains on equity securities sold during the period	[2]	502	0	1,455	0
Unrealized gains (losses) on equity securities held at the end of the period		2,997	0	(817)	0
Realized and unrealized losses on equity securities held at the end of the period		3,499	0	638	0
Net realized and unrealized gains (losses) on investments		2,373	5,944	(5,595)	15,534
Proceeds from sales of equity securities		30,135		117,692	32,233
Realized gains on equity securities		5,721		50,848
Federal income taxes				34,787
Fixed Maturities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		590,961		590,961
Cost or amortized cost		598,098		598,098
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value		47,012		47,012
Excess of one year to five years, Fair Value		298,807		298,807
Excess of five years to ten years, Fair Value		104,228		104,228
Excess of ten years, Fair Value		2,460		2,460
Total contractual maturities, Fair Value		452,507		452,507
Asset-backed securities, Fair Value		138,454		138,454
Total, Fair Value		590,961		590,961
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost		47,538		47,538
Excess of one year to five years, Cost or Amortized Cost		303,274		303,274
Excess of five years to ten years, Cost or Amortized Cost		106,384		106,384
Excess of ten years, Cost or Amortized Cost		2,506		2,506
Total contractual maturities, Cost or Amortized Cost		459,702		459,702
Asset-backed securities, Cost or Amortized Cost		138,396		138,396
Cost or amortized cost		598,098		598,098
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains		2,690	3,852	8,824	9,544
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses		(2,743)	(3,973)	(8,539)	(10,112)
Agency Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		10,471		10,471		16,586
Cost or amortized cost		10,499		10,499		15,839
Gross unrealized gains		183		183		818
Gross unrealized losses		(211)		(211)		(71)
Net unrealized gains (losses)		(28)		(28)		747
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		10,471		10,471		16,586
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		10,499		10,499		15,839
Agency Mortgage-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		36,181		36,181		27,075
Cost or amortized cost		37,179		37,179		27,180
Gross unrealized gains		6		6		47
Gross unrealized losses		(1,004)		(1,004)		(152)
Net unrealized gains (losses)		(998)		(998)		(105)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		36,181		36,181		27,075
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		37,179		37,179		27,180
Asset-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		53,881		53,881		43,469
Cost or amortized cost		53,603		53,603		42,861
Gross unrealized gains		533		533		749
Gross unrealized losses		(255)		(255)		(141)
Net unrealized gains (losses)		278		278		608
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		53,881		53,881		43,469
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		53,603		53,603		42,861
Bank Loans [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		17,067		17,067		19,488
Cost or amortized cost		16,923		16,923		19,271
Gross unrealized gains		166		166		266
Gross unrealized losses		(22)		(22)		(49)
Net unrealized gains (losses)		144		144		217
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		17,067		17,067		19,488
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		16,923		16,923		19,271
Certificates of Deposit [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		3,126		3,126		3,135
Cost or amortized cost		3,123		3,123		3,124
Gross unrealized gains		3		3		11
Gross unrealized losses		0		0		0
Net unrealized gains (losses)		3		3		11
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		3,126		3,126		3,135
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		3,123		3,123		3,124
Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		5,131		5,131		6,492
Cost or amortized cost		4,755		4,755		6,079
Gross unrealized gains		405		405		451
Gross unrealized losses		(29)		(29)		(38)
Net unrealized gains (losses)		376		376		413
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		5,131		5,131		6,492
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		4,755		4,755		6,079
Corporate Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		198,698		198,698		198,349
Cost or amortized cost		202,977		202,977		198,419
Gross unrealized gains		315		315		1,602
Gross unrealized losses		(4,594)		(4,594)		(1,672)
Net unrealized gains (losses)		(4,279)		(4,279)		(70)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		198,698		198,698		198,349
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		202,977		202,977		198,419
Mortgage-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		32,790		32,790		24,204
Cost or amortized cost		32,360		32,360		23,656
Gross unrealized gains		807		807		933
Gross unrealized losses		(377)		(377)		(385)
Net unrealized gains (losses)		430		430		548
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		32,790		32,790		24,204
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		32,360		32,360		23,656
Municipal Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		30,594		30,594		96,650
Cost or amortized cost		30,602		30,602		97,059
Gross unrealized gains		279		279		322
Gross unrealized losses		(287)		(287)		(731)
Net unrealized gains (losses)		(8)		(8)		(409)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		30,594		30,594		96,650
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		30,602		30,602		97,059
Non-U.S. Government Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		39,750		39,750		37,394
Cost or amortized cost		40,947		40,947		37,971
Gross unrealized gains		180		180		475
Gross unrealized losses		(1,377)		(1,377)		(1,052)
Net unrealized gains (losses)		(1,197)		(1,197)		(577)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		39,750		39,750		37,394
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		40,947		40,947		37,971
U.S. Government Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		163,272		163,272		49,011
Cost or amortized cost		165,130		165,130		49,558
Gross unrealized gains		6		6		0
Gross unrealized losses		(1,864)		(1,864)		(547)
Net unrealized gains (losses)		(1,858)		(1,858)		(547)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		163,272		163,272		49,011
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		165,130		165,130		49,558
Total Fixed Maturities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value		590,961		590,961		521,853
Cost or amortized cost		598,098		598,098		521,017
Gross unrealized gains		2,883		2,883		5,674
Gross unrealized losses		(10,020)		(10,020)		(4,838)
Net unrealized gains (losses)		$ (7,137)		$ (7,137)		$ 836
Available-for-sale Securities Continuous Unrealized Loss Position [Abstract]
Number of securities, 12 months or less | Security		486		486		459
Number of securities, Greater than 12 months | Security		135		135		112
Number of securities, total | Security		621		621		571
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less		$ 431,601		$ 431,601		$ 313,421
Fair value, Greater than 12 months		81,091		81,091		75,638
Fair value, total		512,692		512,692		389,059
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less		(7,330)		(7,330)		(2,683)
Gross unrealized loss, Greater than 12 months		(2,690)		(2,690)		(2,155)
Gross unrealized loss, total		(10,020)		(10,020)		(4,838)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		590,961		590,961		521,853
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		598,098		598,098		521,017
Equity Securities [Member]
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains	[1]	0	4,103	0	8,601
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses	[1]	$ 0	$ (498)	$ 0	$ (945)
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 nine months ended September 30, 2018, the Company sold $30,135 and $117,692 in equity securities, resulting in realized gains of $5,721 and $50,848, respectively. The majority of these gains were included in unrealized gains within other comprehensive income at December 31, 2017 and, as a result of the adoption of ASU 2016-01, were reclassified to retained earnings as of January 1, 2018 and were therefore not recognized in the condensed consolidated statements of operations for the three and nine months ended September 30, 2018.