Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Percent of Total Fair Value
Due in 1 year or less, Percent of Total Fair Value	4.00%	4.80%
Due after 1 year through 5 years, Percent of Total Fair Value	26.50%	22.80%
Due after 5 years through 10 years, Percent of Total Fair Value	29.30%	32.80%
Due after 10 years through 20 years, Percent of Total Fair Value	24.90%	26.50%
Due after 20 years, Percent of Total Fair Value	15.30%	13.10%
Total, Percent of Total Fair Value	100.00%	100.00%
Fair Value
Due in 1 year or less, Fair Value	$ 234,818
Due after 1 year through 5 years, Fair Value	1,546,779
Due after 5 years through 10 years, Fair Value	1,709,053
Due after 10 years through 20 years, Fair Value	1,454,033
Due after 20 years, Fair Value	894,877
Fair Value	5,839,560	$ 7,515,318
Amortized Cost
Due in 1 year or less, Amortized Cost	228,604
Due after 1 year through 5 years, Amortized Cost	1,499,348
Due after 5 years through 10 years, Amortized Cost	1,601,491
Due after 10 years through 20 years, Amortized Cost	1,318,362
Due after 20 years, Amortized Cost	805,628
Total, Amortized Cost	$ 5,453,433	$ 7,373,911
Average option-adjusted duration, in years	6 years	5 years 10 months 24 days