Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016		Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 1,660,347	[1]	$ 1,690,043	[2]
Available-for-sale debt securities gross unrealized gain	104,051	[1]	68,467	[2]
Available-for-sale debt securities, gross unrealized loss	(3,224)	[1]	(27,051)	[2]
Available-for-sale debt securities, at fair value	1,761,174	[1]	1,731,459	[2]
Available-for-sale equity securities, cost	37,966		39,969
Available-for-sale equity securities, gross unrealized gain	10,147		8,252
Available-for-sale equity securities, gross unrealized loss	(574)		(382)
Available-for-sale equity securities, at fair value	47,539		47,839
Available-for-sale securities, amortized cost basis	1,698,313		1,730,012
Available-for-sale securities, gross unrealized gain	114,198		76,719
Available-for-sale securities, gross unrealized loss	(3,798)		(27,433)
Available-for-sale securities, fair value	1,808,713		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	15,515		20,233
Available-for-sale debt securities gross unrealized gain	547		448
Available-for-sale debt securities, gross unrealized loss	(1)		(22)
Available-for-sale debt securities, at fair value	16,061		20,659
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	121,993		114,656
Available-for-sale debt securities gross unrealized gain	9,843		7,082
Available-for-sale debt securities, gross unrealized loss	(106)		(1,522)
Available-for-sale debt securities, at fair value	131,730		120,216
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	43,326		38,995
Available-for-sale debt securities gross unrealized gain	3,167		2,111
Available-for-sale debt securities, gross unrealized loss	(34)		(541)
Available-for-sale debt securities, at fair value	46,459		40,565
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,246,969		1,276,965
Available-for-sale debt securities gross unrealized gain	78,951		49,008
Available-for-sale debt securities, gross unrealized loss	(2,840)		(24,211)
Available-for-sale debt securities, at fair value	1,323,080		1,301,762
Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	78,549		94,532
Available-for-sale debt securities gross unrealized gain	6,836		6,814
Available-for-sale debt securities, gross unrealized loss	(107)		(121)
Available-for-sale debt securities, at fair value	85,278		101,225
Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	49,524		46,996
Available-for-sale debt securities gross unrealized gain	352		129
Available-for-sale debt securities, gross unrealized loss	(85)		(79)
Available-for-sale debt securities, at fair value	49,791		47,046
Commercial Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	104,471		97,666
Available-for-sale debt securities gross unrealized gain	4,355		2,875
Available-for-sale debt securities, gross unrealized loss	(51)		(555)
Available-for-sale debt securities, at fair value	$ 108,775		$ 99,986

[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.