Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Amortized cost
Due in one year or less	$ 319,247
Due after one year through five years	1,575,075
Due after five years through ten years	1,747,456
Due after ten years	1,439,400
Total debt securities with single maturity date, amortized cost	5,081,178
Amortized Cost	6,088,948	$ 6,397,658
Fair Value
Due in one year or less	320,108
Due after one year through five years	1,557,833
Due after five years through ten years	1,703,148
Due after ten years	1,372,806
Total debt securities with single maturity date, fair value	4,953,895
Total at end of period	5,935,916	6,587,581
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	117,517
Amortized Cost	117,517	150,888
Fair Value
Total debt securities without single maturity date, fair value	114,144
Total at end of period	114,144	150,710
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	259,849
Amortized Cost	259,849	309,991
Fair Value
Total debt securities without single maturity date, fair value	248,311
Total at end of period	248,311	309,110
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	298,490
Amortized Cost	298,490	315,330
Fair Value
Total debt securities without single maturity date, fair value	289,317
Total at end of period	289,317	319,130
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	331,914
Amortized Cost	331,914	360,436
Fair Value
Total debt securities without single maturity date, fair value	330,249
Total at end of period	$ 330,249	$ 360,939