Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	4.80%	4.80%
Due after 1 year through 5 years, Percent of Total Fair Value	23.80%	22.80%
Due after 5 years through 10 years, Percent of Total Fair Value	32.30%	32.80%
Due after 10 years through 20 years, Percent of Total Fair Value	26.50%	26.50%
Due after 20 years, Percent of Total Fair Value	12.60%	13.10%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 371,816
Due after 1 year through 5 years, Fair Value	1,854,784
Due after 5 years through 10 years, Fair Value	2,522,113
Due after 10 years through 20 years, Fair Value	2,068,846
Due after 20 years, Fair Value	980,445
Fair Value	7,798,004	$ 7,515,318
Amortized Cost
Due in 1 year or less, Amortized Cost	366,071
Due after 1 year through 5 years, Amortized Cost	1,813,271
Due after 5 years through 10 years, Amortized Cost	2,450,544
Due after 10 years through 20 years, Amortized Cost	1,947,733
Due after 20 years, Amortized Cost	909,854
Total, Amortized Cost	$ 7,487,473	$ 7,373,911
Average option-adjusted duration, in years	5 years 8 months 12 days	5 years 10 months 24 days