Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		$ 2,301,238
Available-for-sale debt securities gross unrealized gain		166,656
Available-for-sale debt securities, gross unrealized loss		(3,283)
Available-for-sale debt securities, at fair value	$ 2,538,197	2,464,611
Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	2,440,181
Available-for-sale debt securities gross unrealized gain	114,687
Available-for-sale debt securities, gross unrealized loss	(16,671)
Available-for-sale debt securities, at fair value	2,538,197
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	50,759
Available-for-sale debt securities, gross unrealized loss	(1)
Available-for-sale debt securities, at fair value	50,758
Fixed-Maturity Securities and Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	2,490,940
Available-for-sale debt securities gross unrealized gain	114,687
Available-for-sale debt securities, gross unrealized loss	(16,672)
Available-for-sale debt securities, at fair value	2,588,955
U.S. Government and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		9,747
Available-for-sale debt securities gross unrealized gain		400
Available-for-sale debt securities, gross unrealized loss		(3)
Available-for-sale debt securities, at fair value		10,144
U.S. Government and Agencies [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	9,756
Available-for-sale debt securities gross unrealized gain	325
Available-for-sale debt securities, gross unrealized loss	(20)
Available-for-sale debt securities, at fair value	10,061
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		169,967
Available-for-sale debt securities gross unrealized gain		13,324
Available-for-sale debt securities, gross unrealized loss		(39)
Available-for-sale debt securities, at fair value		183,252
Foreign Government [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	176,333
Available-for-sale debt securities gross unrealized gain	8,211
Available-for-sale debt securities, gross unrealized loss	(2,267)
Available-for-sale debt securities, at fair value	182,277
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		161,058
Available-for-sale debt securities gross unrealized gain		9,632
Available-for-sale debt securities, gross unrealized loss		(1)
Available-for-sale debt securities, at fair value		170,689
States and Political Subdivisions [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	161,227
Available-for-sale debt securities gross unrealized gain	5,746
Available-for-sale debt securities, gross unrealized loss	(777)
Available-for-sale debt securities, at fair value	166,196
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		1,506,549
Available-for-sale debt securities gross unrealized gain		124,164
Available-for-sale debt securities, gross unrealized loss		(2,545)
Available-for-sale debt securities, at fair value		1,628,168
Corporates [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	1,619,734
Available-for-sale debt securities gross unrealized gain	84,794
Available-for-sale debt securities, gross unrealized loss	(12,329)
Available-for-sale debt securities, at fair value	1,692,199
Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		261,376
Available-for-sale debt securities gross unrealized gain		11,419
Available-for-sale debt securities, gross unrealized loss		(54)
Available-for-sale debt securities, at fair value		272,741
Residential Mortgage-backed Securities [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	247,497
Available-for-sale debt securities gross unrealized gain	9,190
Available-for-sale debt securities, gross unrealized loss	(361)
Available-for-sale debt securities, at fair value	256,326
Commercial Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		107,020
Available-for-sale debt securities gross unrealized gain		5,901
Available-for-sale debt securities, gross unrealized loss		(56)
Available-for-sale debt securities, at fair value		112,865
Commercial Mortgage-backed Securities [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	140,293
Available-for-sale debt securities gross unrealized gain	4,616
Available-for-sale debt securities, gross unrealized loss	(644)
Available-for-sale debt securities, at fair value	144,265
Other Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis		85,521
Available-for-sale debt securities gross unrealized gain		1,816
Available-for-sale debt securities, gross unrealized loss		(585)
Available-for-sale debt securities, at fair value		$ 86,752
Other Asset-backed Securities [Member] | Fixed-Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale debt securities, amortized cost basis	85,341
Available-for-sale debt securities gross unrealized gain	1,805
Available-for-sale debt securities, gross unrealized loss	(273)
Available-for-sale debt securities, at fair value	$ 86,873