Investments - Amortized Cost and Fair Value of Available-for-sale Fixed Income Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 57,625
Due after one year through five years	626,953
Due after five years through 10 years	537,777
Due after 10 years	496,778
ABS/CMBS/MBS*	627,134
Total available-for-sale	2,346,267	$ 2,061,467
Fair Value
Due in one year or less	58,134
Due after one year through five years	647,068
Due after five years through 10 years	560,653
Due after 10 years	512,791
ABS/CMBS/MBS*	631,241
Total available-for-sale	$ 2,409,887	$ 2,196,626