Investments - Amortized Cost and Fair Value of Available-for-sale Fixed Income Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 88,847
Due after one year through five years	560,060
Due after five years through 10 years	562,132
Due after 10 years	435,468
ABS/CMBS/MBS	572,995
Total available-for-sale	2,219,502	$ 2,061,467
Fair Value
Due in one year or less	89,750
Due after one year through five years	588,237
Due after five years through 10 years	597,118
Due after 10 years	457,202
ABS/CMBS/MBS	586,937
Total available-for-sale	$ 2,319,244	$ 2,196,626