Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Amortized cost
Due in one year or less	$ 667,113
Due after one year through five years	1,362,173
Due after five years through ten years	1,831,636
Due after ten years	1,188,575
Total debt securities with single maturity date, amortized cost	5,049,497
Amortized Cost	5,979,045	$ 5,926,785
Fair Value
Due in one year or less	657,832
Due after one year through five years	1,300,280
Due after five years through ten years	1,693,166
Due after ten years	1,035,420
Total debt securities with single maturity date, fair value	4,686,698
Total at end of period	5,564,053	5,409,698
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	124,175
Amortized Cost	124,175	126,723
Fair Value
Total debt securities without single maturity date, fair value	119,371
Total at end of period	119,371	120,687
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	217,751
Amortized Cost	217,751	223,743
Fair Value
Total debt securities without single maturity date, fair value	195,505
Total at end of period	195,505	198,009
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	250,589
Amortized Cost	250,589	257,785
Fair Value
Total debt securities without single maturity date, fair value	231,905
Total at end of period	231,905	237,216
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	337,033
Amortized Cost	337,033	337,656
Fair Value
Total debt securities without single maturity date, fair value	330,574
Total at end of period	$ 330,574	$ 329,832