Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in 1 year or less	$ 255,416
Due after 1 year through 5 years	1,712,114
Due after 5 years through 10 years	1,608,614
Due after 10 years through 20 years	1,358,865
Due after 20 years	853,542
Amortized Cost	5,788,551	$ 5,456,980
Fair Value
Due in 1 year or less	261,387
Due after 1 year through 5 years	1,793,111
Due after 5 years through 10 years	1,775,766
Due after 10 years through 20 years	1,559,501
Due after 20 years	955,525
Total	$ 6,345,290	$ 5,791,676
Percent of Total Fair Value
Due in 1 year or less	4.00%
Due after 1 year through 5 years	28.30%
Due after 5 years through 10 years	28.00%
Due after 10 years through 20 years	24.60%
Due after 20 years	15.10%
Total	100.00%
Average option-adjusted duration, in years	6 years 4 months 24 days