Investments - Maturities of Fixed Maturities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in 1 year or less	$ 243,998
Due after 1 year through 5 years	1,985,554
Due after 5 years through 10 years	2,428,868
Due after 10 years through 20 years	1,731,226
Due after 20 years	913,304
Amortized Cost/Cost	7,302,950	$ 7,152,127
Fair Value
Due in 1 year or less	248,959
Due after 1 year through 5 years	2,059,625
Due after 5 years through 10 years	2,522,414
Due after 10 years through 20 years	1,867,055
Due after 20 years	1,026,022
Total	$ 7,724,075	$ 7,456,708
Percent of Total Fair Value
Due in 1 year or less	3.20%
Due after 1 year through 5 years	26.70%
Due after 5 years through 10 years	32.60%
Due after 10 years through 20 years	24.20%
Due after 20 years	13.30%
Total	100.00%
Average option-adjusted duration, in years	5 years 10 months 24 days