Investments - Schedule of AFS Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018		Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 2,078,822	[1]	$ 1,877,326	[2]
Available-for-sale debt securities gross unrealized gain	20,997	[1]	56,438	[2]
Available-for-sale debt securities, gross unrealized loss	(30,184)	[1]	(5,922)	[2]
Available-for-sale debt securities, at fair value	2,069,635	[1]	1,927,842	[2]
Available-for-sale securities, amortized cost basis	2,086,993		1,908,657
Available-for-sale securities, gross unrealized gain	20,997		66,234
Available-for-sale securities, gross unrealized loss	(30,184)		(5,942)
Available-for-sale securities, fair value	2,077,806		1,968,949
Available-for-sale equity securities, cost	0		31,331
Available-for-sale equity securities, gross unrealized gain			9,796
Available-for-sale equity securities, gross unrealized loss			(20)
Equity securities available-for-sale, at fair value (amortized cost: $0 in 2018 and $31,331 in 2017)			41,107
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	8,171
Available-for-sale debt securities, at fair value	8,171
U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	12,115		11,577
Available-for-sale debt securities gross unrealized gain	176		283
Available-for-sale debt securities, gross unrealized loss	(58)		(47)
Available-for-sale debt securities, at fair value	12,233		11,813
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	155,723		139,486
Available-for-sale debt securities gross unrealized gain	3,347		5,651
Available-for-sale debt securities, gross unrealized loss	(958)		(648)
Available-for-sale debt securities, at fair value	158,112		144,489
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	59,075		54,714
Available-for-sale debt securities gross unrealized gain	1,202		1,554
Available-for-sale debt securities, gross unrealized loss	(271)		(141)
Available-for-sale debt securities, at fair value	60,006		56,127
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,447,075		1,337,321
Available-for-sale debt securities gross unrealized gain	11,916		42,616
Available-for-sale debt securities, gross unrealized loss	(24,773)		(3,655)
Available-for-sale debt securities, at fair value	1,434,218		1,376,282
Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	191,245		119,672
Available-for-sale debt securities gross unrealized gain	2,439		3,583
Available-for-sale debt securities, gross unrealized loss	(1,733)		(297)
Available-for-sale debt securities, at fair value	191,951		122,958
Commercial Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	131,279		134,003
Available-for-sale debt securities gross unrealized gain	1,712		2,299
Available-for-sale debt securities, gross unrealized loss	(1,636)		(910)
Available-for-sale debt securities, at fair value	131,355		135,392
Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	82,310		80,553
Available-for-sale debt securities gross unrealized gain	205		452
Available-for-sale debt securities, gross unrealized loss	(755)		(224)
Available-for-sale debt securities, at fair value	$ 81,760		$ 80,781

[1] ^{Includes $0.1 million of OTTI losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income (loss).}

[2] Includes $0.2 million of OTTI losses related to corporates and mortgage- and asset-backed securities recognized in accumulated other comprehensive income (loss).