Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016		Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 1,640,210	[1]	$ 1,690,043	[2]
Available-for-sale debt securities gross unrealized gain	85,609	[1]	68,467	[2]
Available-for-sale debt securities, gross unrealized loss	(20,114)	[1]	(27,051)	[2]
Available-for-sale debt securities, at fair value	1,705,705	[1]	1,731,459	[2]
Available-for-sale equity securities, cost	40,159		39,969
Available-for-sale equity securities, gross unrealized gain	10,034		8,252
Available-for-sale equity securities, gross unrealized loss	(639)		(382)
Available-for-sale equity securities, at fair value	49,554		47,839
Available-for-sale securities, amortized cost basis	1,680,369		1,730,012
Available-for-sale securities, gross unrealized gain	95,643		76,719
Available-for-sale securities, gross unrealized loss	(20,753)		(27,433)
Available-for-sale securities, fair value	1,755,259		1,779,298
U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	16,675		20,233
Available-for-sale debt securities gross unrealized gain	565		448
Available-for-sale debt securities, gross unrealized loss			(22)
Available-for-sale debt securities, at fair value	17,240		20,659
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	111,706		114,656
Available-for-sale debt securities gross unrealized gain	7,937		7,082
Available-for-sale debt securities, gross unrealized loss	(858)		(1,522)
Available-for-sale debt securities, at fair value	118,785		120,216
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	41,093		38,995
Available-for-sale debt securities gross unrealized gain	2,892		2,111
Available-for-sale debt securities, gross unrealized loss	(565)		(541)
Available-for-sale debt securities, at fair value	43,420		40,565
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,243,942		1,276,965
Available-for-sale debt securities gross unrealized gain	62,343		49,008
Available-for-sale debt securities, gross unrealized loss	(18,400)		(24,211)
Available-for-sale debt securities, at fair value	1,287,885		1,301,762
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	226,794		239,194
Available-for-sale debt securities gross unrealized gain	11,872		9,818
Available-for-sale debt securities, gross unrealized loss	(291)		(755)
Available-for-sale debt securities, at fair value	$ 238,375		$ 248,257

[1] Includes approximately $0.1 million of other-than-temporary impairment (“OTTI”) losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income.

[2] Includes approximately $0.1 million of OTTI related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income.