Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	3.40%	3.90%
Due after 1 year through 5 years, Percent of Total Fair Value	27.50%	28.70%
Due after 5 years through 10 years, Percent of Total Fair Value	33.30%	35.20%
Due after 10 years through 20 years, Percent of Total Fair Value	23.30%	19.50%
Due after 20 years, Percent of Total Fair Value	12.50%	12.70%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 256,527
Due after 1 year through 5 years, Fair Value	2,097,243
Due after 5 years through 10 years, Fair Value	2,540,303
Due after 10 years through 20 years, Fair Value	1,780,760
Due after 20 years, Fair Value	955,801
Total, Fair Value	7,630,634	$ 7,456,708
Amortized Cost
Due in 1 year or less, Amortized Cost	250,803
Due after 1 year through 5 years, Amortized Cost	1,998,498
Due after 5 years through 10 years, Amortized Cost	2,431,895
Due after 10 years through 20 years, Amortized Cost	1,654,259
Due after 20 years, Amortized Cost	858,942
Total, Amortized Cost	$ 7,194,397	$ 7,152,127
Average option-adjusted duration, in years	6 years	5 years 10 months 25 days