Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	$ 2,318,098	$ 2,274,770
Available-for-sale debt securities gross unrealized gain	145,546	84,838
Available-for-sale debt securities, gross unrealized loss	(10,568)	(2,612)
Available-for-sale debt securities, at fair value	2,453,076	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	9,759	10,197
Available-for-sale debt securities gross unrealized gain	466	287
Available-for-sale debt securities, gross unrealized loss	(1)
Available-for-sale debt securities, at fair value	10,224	10,484
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	160,379	154,945
Available-for-sale debt securities gross unrealized gain	12,615	6,362
Available-for-sale debt securities, gross unrealized loss	(44)	(235)
Available-for-sale debt securities, at fair value	172,950	161,072
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	154,666	120,000
Available-for-sale debt securities gross unrealized gain	7,927	3,288
Available-for-sale debt securities, gross unrealized loss	(452)	(695)
Available-for-sale debt securities, at fair value	162,141	122,593
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,507,770	1,436,877
Available-for-sale debt securities gross unrealized gain	104,860	63,892
Available-for-sale debt securities, gross unrealized loss	(8,555)	(1,118)
Available-for-sale debt securities, at fair value	1,604,075	1,499,651
Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	282,899	305,897
Available-for-sale debt securities gross unrealized gain	12,276	6,848
Available-for-sale debt securities, gross unrealized loss	(53)	(222)
Available-for-sale debt securities, at fair value	295,122	312,523
Commercial Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	115,155	128,913
Available-for-sale debt securities gross unrealized gain	5,762	3,191
Available-for-sale debt securities, gross unrealized loss	(391)	(99)
Available-for-sale debt securities, at fair value	120,526	132,005
Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	87,470	117,941
Available-for-sale debt securities gross unrealized gain	1,640	970
Available-for-sale debt securities, gross unrealized loss	(1,072)	(243)
Available-for-sale debt securities, at fair value	$ 88,038	$ 118,668