Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in 1 year or less	$ 205,798
Due after 1 year through 5 years	1,541,749
Due after 5 years through 10 years	1,613,539
Due after 10 years through 20 years	1,393,503
Due after 20 years	702,391
Amortized Cost	5,456,980	$ 7,373,911
Fair Value
Due in 1 year or less	211,420
Due after 1 year through 5 years	1,587,300
Due after 5 years through 10 years	1,712,236
Due after 10 years through 20 years	1,512,769
Due after 20 years	767,951
Total	$ 5,791,676	$ 7,515,318
Percent of Total Fair Value
Due in 1 year or less	3.60%
Due after 1 year through 5 years	27.40%
Due after 5 years through 10 years	29.60%
Due after 10 years through 20 years	26.10%
Due after 20 years	13.30%
Total	100.00%
Average option-adjusted duration, in years	6 years