Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	4.20%	3.20%
Due after 1 year through 5 years, Percent of Total Fair Value	24.50%	26.70%
Due after 5 years through 10 years, Percent of Total Fair Value	32.40%	32.60%
Due after 10 years through 20 years, Percent of Total Fair Value	25.20%	24.20%
Due after 20 years, Percent of Total Fair Value	13.70%	13.30%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 312,584
Due after 1 year through 5 years, Fair Value	1,829,489
Due after 5 years through 10 years, Fair Value	2,423,204
Due after 10 years through 20 years, Fair Value	1,885,539
Due after 20 years, Fair Value	1,024,749
Total, Fair Value	7,475,565	$ 7,724,075
Amortized Cost
Due in 1 year or less, Amortized Cost	308,703
Due after 1 year through 5 years, Amortized Cost	1,794,597
Due after 5 years through 10 years, Amortized Cost	2,405,751
Due after 10 years through 20 years, Amortized Cost	1,828,415
Due after 20 years, Amortized Cost	954,557
Total, Amortized Cost	$ 7,292,023	$ 7,302,950
Average option-adjusted duration, in years	6 years	5 years 10 months 24 days