Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Amortized Cost
Due in one year or less	$ 452,188
Due after one year through five years	1,358,606
Due after five years through ten years	1,890,875
Due after ten years	1,279,209
Total debt securities with single maturity date	4,980,878
Amortized Cost	5,926,785	$ 6,397,658
Fair Value
Due in one year or less	445,210
Due after one year through five years	1,288,152
Due after five years through ten years	1,713,608
Due after ten years	1,076,984
Total debt securities with single maturity date	4,523,954
Total at end of period	5,409,698	6,587,581
ABS
Amortized Cost
Total debt securities without single maturity date, amortized cost	126,723
Amortized Cost	126,723	150,888
Fair Value
Total debt securities without single maturity date, fair value	120,687
Total at end of period	120,687	150,710
RMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	223,743
Amortized Cost	223,743	309,991
Fair Value
Total debt securities without single maturity date, fair value	198,009
Total at end of period	198,009	309,110
CMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	257,785
Amortized Cost	257,785	315,330
Fair Value
Total debt securities without single maturity date, fair value	237,216
Total at end of period	237,216	319,130
CLOs
Amortized Cost
Total debt securities without single maturity date, amortized cost	337,656
Amortized Cost	337,656	360,436
Fair Value
Total debt securities without single maturity date, fair value	329,832
Total at end of period	329,832	360,939
Total fixed income securities
Amortized Cost
Amortized Cost	5,926,785	6,397,658
Fair Value
Total at end of period	$ 5,409,698	$ 6,587,581