Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Amortized cost
Due in one year or less	$ 422,546
Due after one year through five years	1,413,885
Due after five years through ten years	1,814,408
Due after ten years	1,349,776
Total debt securities with single maturity date, amortized cost	5,000,615
Amortized Cost	5,970,459	$ 6,397,658
Fair Value
Due in one year or less	416,536
Due after one year through five years	1,334,284
Due after five years through ten years	1,614,687
Due after ten years	1,133,197
Total debt securities with single maturity date, fair value	4,498,704
Total at end of period	5,400,982	6,587,581
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	134,309
Amortized Cost	134,309	150,888
Fair Value
Total debt securities without single maturity date, fair value	128,495
Total at end of period	128,495	150,710
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	230,997
Amortized Cost	230,997	309,991
Fair Value
Total debt securities without single maturity date, fair value	202,640
Total at end of period	202,640	309,110
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	266,711
Amortized Cost	266,711	315,330
Fair Value
Total debt securities without single maturity date, fair value	245,645
Total at end of period	245,645	319,130
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	337,827
Amortized Cost	337,827	360,436
Fair Value
Total debt securities without single maturity date, fair value	325,498
Total at end of period	$ 325,498	$ 360,939