Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in 1 year or less	$ 358,797
Due after 1 year through 5 years	1,690,400
Due after 5 years through 10 years	2,453,572
Due after 10 years through 20 years	1,931,599
Due after 20 years	939,543
Amortized Cost/Cost	7,373,911	$ 7,302,950
Fair Value
Due in 1 year or less	363,049
Due after 1 year through 5 years	1,713,593
Due after 5 years through 10 years	2,465,337
Due after 10 years through 20 years	1,991,726
Due after 20 years	981,613
Total	$ 7,515,318	$ 7,724,075
Percent of Total Fair Value
Due in 1 year or less	4.80%
Due after 1 year through 5 years	22.80%
Due after 5 years through 10 years	32.80%
Due after 10 years through 20 years	26.50%
Due after 20 years	13.10%
Total	100.00%
Average option-adjusted duration, in years	5 years 10 months 24 days