Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	4.50%	3.20%
Due after 1 year through 5 years, Percent of Total Fair Value	23.70%	26.70%
Due after 5 years through 10 years, Percent of Total Fair Value	32.80%	32.60%
Due after 10 years through 20 years, Percent of Total Fair Value	26.10%	24.20%
Due after 20 years, Percent of Total Fair Value	12.90%	13.30%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 342,221
Due after 1 year through 5 years, Fair Value	1,783,259
Due after 5 years through 10 years, Fair Value	2,469,126
Due after 10 years through 20 years, Fair Value	1,962,671
Due after 20 years, Fair Value	971,862
Total, Fair Value	7,529,139	$ 7,724,075
Amortized Cost
Due in 1 year or less, Amortized Cost	339,321
Due after 1 year through 5 years, Amortized Cost	1,758,061
Due after 5 years through 10 years, Amortized Cost	2,464,959
Due after 10 years through 20 years, Amortized Cost	1,929,967
Due after 20 years, Amortized Cost	927,183
Total, Amortized Cost	$ 7,419,491	$ 7,302,950
Average option-adjusted duration, in years	6 years	5 years 10 months 24 days