Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	4.70%	4.80%
Due after 1 year through 5 years, Percent of Total Fair Value	26.70%	22.80%
Due after 5 years through 10 years, Percent of Total Fair Value	29.60%	32.80%
Due after 10 years through 20 years, Percent of Total Fair Value	25.60%	26.50%
Due after 20 years, Percent of Total Fair Value	13.40%	13.10%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 258,331
Due after 1 year through 5 years, Fair Value	1,478,291
Due after 5 years through 10 years, Fair Value	1,638,887
Due after 10 years through 20 years, Fair Value	1,414,857
Due after 20 years, Fair Value	743,904
Fair Value	5,534,270	$ 7,515,318
Amortized Cost
Due in 1 year or less, Amortized Cost	252,586
Due after 1 year through 5 years, Amortized Cost	1,439,296
Due after 5 years through 10 years, Amortized Cost	1,555,825
Due after 10 years through 20 years, Amortized Cost	1,316,715
Due after 20 years, Amortized Cost	677,333
Total, Amortized Cost	$ 5,241,755	$ 7,373,911
Average option-adjusted duration, in years	5 years 7 months 6 days	5 years 10 months 24 days