Investments (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)	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]	596		636
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, total	[1]	$ 402,657		$ 435,713
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, total	[1]	(7,655)		(7,056)
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains		2,443	$ 2,494
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses		(2,708)	(1,486)
Other-than-temporary impairments		0	0
Change in value of limited partnership investments		(2,603)	5,286
Gains (losses) on equity securities [Abstract]
Realized gains on equity securities sold during the period	[2]	641	0
Unrealized losses on equity securities held at the end of the period		(2,306)	0
Realized and unrealized losses on equity securities held at the end of the period		(1,665)	0
Net realized and unrealized gains (losses) on investments		(4,533)	6,294
Proceeds from sales of equity securities		59,757	7,054
Realized gains on equity securities		35,141
Federal income taxes		31,166
Fixed Maturities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		544,116
Cost or amortized cost		547,231
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
One year or less, Fair Value		38,435
Excess of one year to five years, Fair Value		310,711
Excess of five years to ten years, Fair Value		78,116
Excess of ten years, Fair Value		2,747
Total contractual maturities, Fair Value		430,009
Asset-backed securities, Fair Value		114,107
Total, Fair Value		544,116
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
One year or less, Cost or Amortized Cost		38,652
Excess of one year to five years, Cost or Amortized Cost		314,263
Excess of five years to ten years, Cost or Amortized Cost		78,539
Excess of ten years, Cost or Amortized Cost		2,781
Total contractual maturities, Cost or Amortized Cost		434,235
Asset-backed securities, Cost or Amortized Cost		112,996
Cost or amortized cost		547,231
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains		2,443	888
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses		(2,708)	(1,373)
Agency Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		14,995		16,586
Cost or amortized cost		14,500		15,839
Gross unrealized gains		632		818
Gross unrealized losses		(137)		(71)
Net unrealized gains (losses)		495		747
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		14,995		16,586
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		14,500		15,839
Agency Mortgage-backed Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		30,740		27,075
Cost or amortized cost		31,312		27,180
Gross unrealized gains		32		47
Gross unrealized losses		(604)		(152)
Net unrealized gains (losses)		(572)		(105)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		30,740		27,075
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		31,312		27,180
Asset-backed Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		44,740		43,469
Cost or amortized cost		44,211		42,861
Gross unrealized gains		750		749
Gross unrealized losses		(221)		(141)
Net unrealized gains (losses)		529		608
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		44,740		43,469
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		44,211		42,861
Bank Loans [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		22,693		19,488
Cost or amortized cost		22,458		19,271
Gross unrealized gains		269		266
Gross unrealized losses		(34)		(49)
Net unrealized gains (losses)		235		217
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		22,693		19,488
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		22,458		19,271
Certificates of Deposit [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		3,127		3,135
Cost or amortized cost		3,124		3,124
Gross unrealized gains		3		11
Gross unrealized losses		0		0
Net unrealized gains (losses)		3		11
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		3,127		3,135
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		3,124		3,124
Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		5,727		6,492
Cost or amortized cost		5,382		6,079
Gross unrealized gains		370		451
Gross unrealized losses		(25)		(38)
Net unrealized gains (losses)		345		413
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		5,727		6,492
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		5,382		6,079
Corporate Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		213,462		198,349
Cost or amortized cost		216,105		198,419
Gross unrealized gains		1,180		1,602
Gross unrealized losses		(3,823)		(1,672)
Net unrealized gains (losses)		(2,643)		(70)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		213,462		198,349
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		216,105		198,419
Mortgage-backed Securities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		23,620		24,204
Cost or amortized cost		23,398		23,656
Gross unrealized gains		581		933
Gross unrealized losses		(359)		(385)
Net unrealized gains (losses)		222		548
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		23,620		24,204
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		23,398		23,656
Municipal Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		48,764		96,650
Cost or amortized cost		48,971		97,059
Gross unrealized gains		290		322
Gross unrealized losses		(497)		(731)
Net unrealized gains (losses)		(207)		(409)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		48,764		96,650
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		48,971		97,059
Non-U.S. Government Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		38,848		37,394
Cost or amortized cost		39,539		37,971
Gross unrealized gains		413		475
Gross unrealized losses		(1,104)		(1,052)
Net unrealized gains (losses)		(691)		(577)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		38,848		37,394
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		39,539		37,971
U.S. Government Obligations [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		97,400		49,011
Cost or amortized cost		98,231		49,558
Gross unrealized gains		20		0
Gross unrealized losses		(851)		(547)
Net unrealized gains (losses)		(831)		(547)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		97,400		49,011
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		98,231		49,558
Total Fixed Maturities [Member]
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Fair value		544,116		521,853
Cost or amortized cost		547,231		521,017
Gross unrealized gains		4,540		5,674
Gross unrealized losses		(7,655)		(4,838)
Net unrealized gains (losses)		$ (3,115)		$ 836
Available-for-sale Securities Continuous Unrealized Loss Position [Abstract]
Number of securities, 12 months or less | Security		491		459
Number of securities, Greater than 12 months | Security		105		112
Number of securities, total | Security		596		571
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less		$ 336,083		$ 313,421
Fair value, Greater than 12 months		66,574		75,638
Fair value, total		402,657		389,059
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less		(5,580)		(2,683)
Gross unrealized loss, Greater than 12 months		(2,075)		(2,155)
Gross unrealized loss, total		(7,655)		(4,838)
Available-for-sale Securities Debt Maturities Fair Value [Abstract]
Total, Fair Value		544,116		521,853
Available-for-sale Securities Debt Maturities Amortized Cost Basis [Abstract]
Cost or amortized cost		547,231		521,017
Equity Securities [Member]
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains	[1]	0	1,606
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses	[1]	$ 0	$ (113)
Consumer [Member]
Available-for-sale Equity Securities, Amortized Cost Basis [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, Amortized Cost Basis [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, Amortized Cost Basis [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, Amortized Cost Basis [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, Amortized Cost Basis [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, Amortized Cost Basis [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, Amortized Cost Basis [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, Amortized Cost Basis [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		65
Number of securities, Greater than 12 months | Security	[1]	0		0
Number of securities, total | Security	[1]	0		65
Available-for-sale Securities Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair value, 12 months or less	[1]	$ 0		$ 46,654
Fair value, Greater than 12 months	[1]	0		0
Fair value, total	[1]	0		46,654
Available-for-sale Securities Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross unrealized loss, 12 months or less	[1]	0		(2,218)
Gross unrealized loss, Greater than 12 months	[1]	0		0
Gross unrealized loss, total	[1]	$ 0		$ (2,218)

[1] Effective January 1, 2018, the Company adopted ASU No. 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 - Recently Adopted Accounting Pronouncements for further discussion.

[2] During the three months ended March 31, 2018, the Company sold $59,757 in equity securities resulting in a realized gain of $35,141. 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 statement of income for the three months ended March 31, 2018.