Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	4.00%	3.20%
Due after 1 year through 5 years, Percent of Total Fair Value	25.40%	26.70%
Due after 5 years through 10 years, Percent of Total Fair Value	32.70%	32.60%
Due after 10 years through 20 years, Percent of Total Fair Value	24.70%	24.20%
Due after 20 years, Percent of Total Fair Value	13.20%	13.30%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 303,914
Due after 1 year through 5 years, Fair Value	1,936,362
Due after 5 years through 10 years, Fair Value	2,498,529
Due after 10 years through 20 years, Fair Value	1,884,116
Due after 20 years, Fair Value	1,011,761
Total, Fair Value	7,634,682	$ 7,724,075
Amortized Cost
Due in 1 year or less, Amortized Cost	299,275
Due after 1 year through 5 years, Amortized Cost	1,887,648
Due after 5 years through 10 years, Amortized Cost	2,457,018
Due after 10 years through 20 years, Amortized Cost	1,800,969
Due after 20 years, Amortized Cost	928,642
Total, Amortized Cost	$ 7,373,552	$ 7,302,950
Average option-adjusted duration, in years	6 years	5 years 10 months 24 days