Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 2,237,474	$ 2,274,770
Available-for-sale debt securities gross unrealized gain	132,295	84,838
Available-for-sale debt securities, gross unrealized loss	(12,615)	(2,612)
Available-for-sale debt securities, at fair value	2,357,154	2,356,996
U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	9,635	10,197
Available-for-sale debt securities gross unrealized gain	516	287
Available-for-sale debt securities, at fair value	10,151	10,484
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	157,685	154,945
Available-for-sale debt securities gross unrealized gain	12,402	6,362
Available-for-sale debt securities, gross unrealized loss	(19)	(235)
Available-for-sale debt securities, at fair value	170,068	161,072
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	119,584	120,000
Available-for-sale debt securities gross unrealized gain	7,251	3,288
Available-for-sale debt securities, gross unrealized loss	(81)	(695)
Available-for-sale debt securities, at fair value	126,754	122,593
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,432,856	1,436,877
Available-for-sale debt securities gross unrealized gain	92,812	63,892
Available-for-sale debt securities, gross unrealized loss	(9,781)	(1,118)
Available-for-sale debt securities, at fair value	1,515,887	1,499,651
Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	305,131	305,897
Available-for-sale debt securities gross unrealized gain	12,561	6,848
Available-for-sale debt securities, gross unrealized loss	(335)	(222)
Available-for-sale debt securities, at fair value	317,357	312,523
Commercial Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	117,355	128,913
Available-for-sale debt securities gross unrealized gain	5,516	3,191
Available-for-sale debt securities, gross unrealized loss	(827)	(99)
Available-for-sale debt securities, at fair value	122,044	132,005
Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	95,228	117,941
Available-for-sale debt securities gross unrealized gain	1,237	970
Available-for-sale debt securities, gross unrealized loss	(1,572)	(243)
Available-for-sale debt securities, at fair value	$ 94,893	$ 118,668