Investments - Distribution of Fixed Maturity Securities by Expected Maturity (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Percent of Total Fair Value
Due in 1 year or less	4.00%	3.60%
Due after 1 year through 5 years	27.90%	27.40%
Due after 5 years through 10 years	30.10%	29.60%
Due after 10 years through 20 years	24.70%	26.10%
Due after 20 years	13.30%	13.30%
Total	100.00%	100.00%
Fair Value
Due in 1 year or less	$ 240,942
Due after 1 year through 5 years	1,680,037
Due after 5 years through 10 years	1,814,204
Due after 10 years through 20 years	1,487,294
Due after 20 years	799,506
Total	6,021,983	$ 5,791,676
Amortized Cost
Due in 1 year or less	240,701
Due after 1 year through 5 years	1,650,193
Due after 5 years through 10 years	1,680,118
Due after 10 years through 20 years	1,314,841
Due after 20 years	718,552
Amortized Cost	$ 5,604,405	$ 5,456,980
Average option-adjusted duration, in years	6 years 2 months 12 days	6 years