Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, at fair value	$ 2,595,504	$ 2,464,611
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	2,487,846	2,301,238
Available-for-sale debt securities gross unrealized gain	115,802	166,656
Available-for-sale debt securities, gross unrealized loss	(8,144)	(3,283)
Available-for-sale debt securities, at fair value	2,595,504	2,464,611
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	40,043	0
Available-for-sale debt securities, gross unrealized loss	(6)
Available-for-sale debt securities, at fair value	40,037
Fixed-Maturity Securities and Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	2,527,889
Available-for-sale debt securities gross unrealized gain	115,802
Available-for-sale debt securities, gross unrealized loss	(8,150)
Available-for-sale debt securities, at fair value	2,635,541
U.S. Government and Agencies [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	11,154	9,747
Available-for-sale debt securities gross unrealized gain	242	400
Available-for-sale debt securities, gross unrealized loss	(33)	(3)
Available-for-sale debt securities, at fair value	11,363	10,144
Foreign Government [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	136,069	169,967
Available-for-sale debt securities gross unrealized gain	6,766	13,324
Available-for-sale debt securities, gross unrealized loss	(757)	(39)
Available-for-sale debt securities, at fair value	142,078	183,252
States and Political Subdivisions [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	152,065	161,058
Available-for-sale debt securities gross unrealized gain	6,658	9,632
Available-for-sale debt securities, gross unrealized loss	(203)	(1)
Available-for-sale debt securities, at fair value	158,520	170,689
Corporates [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,612,500	1,506,549
Available-for-sale debt securities gross unrealized gain	88,384	124,164
Available-for-sale debt securities, gross unrealized loss	(5,749)	(2,545)
Available-for-sale debt securities, at fair value	1,695,135	1,628,168
Residential Mortgage-backed Securities [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	311,622	261,376
Available-for-sale debt securities gross unrealized gain	6,927	11,419
Available-for-sale debt securities, gross unrealized loss	(1,031)	(54)
Available-for-sale debt securities, at fair value	317,518	272,741
Commercial Mortgage-backed Securities [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	146,278	107,020
Available-for-sale debt securities gross unrealized gain	4,869	5,901
Available-for-sale debt securities, gross unrealized loss	(100)	(56)
Available-for-sale debt securities, at fair value	151,047	112,865
Other Asset-backed Securities [Member] \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	118,158	85,521
Available-for-sale debt securities gross unrealized gain	1,956	1,816
Available-for-sale debt securities, gross unrealized loss	(271)	(585)
Available-for-sale debt securities, at fair value	$ 119,843	$ 86,752

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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