Investments - Amortized Cost and Fair Value of Available-for-sale Fixed Income Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 105,159
Due after one year through five years	512,686
Due after five years through 10 years	522,850
Due after 10 years	382,756
ABS/CMBS/MBS	573,401
Total available-for-sale	2,096,852	$ 2,061,467
Fair Value
Due in one year or less	106,340
Due after one year through five years	539,027
Due after five years through 10 years	552,085
Due after 10 years	390,973
ABS/CMBS/MBS	587,444
Total available-for-sale	$ 2,175,869	$ 2,196,626