Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Amortized Cost
Due in one year or less	$ 645,017
Due after one year through five years	1,530,186
Due after five years through ten years	1,824,666
Due after ten years	920,094
Total debt securities with single maturity date	4,919,963
Fair Value
Due in one year or less	639,245
Due after one year through five years	1,487,270
Due after five years through ten years	1,708,591
Due after ten years	782,408
Total debt securities with single maturity date	4,617,514
ABS
Amortized Cost
Total debt securities without single maturity date, amortized cost	173,256
Amortized Cost	173,256	$ 126,723
Fair Value
Total debt securities without single maturity date, fair value	171,273
Total at end of period	171,273	120,687
RMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	347,132
Amortized Cost	347,132	223,743
Fair Value
Total debt securities without single maturity date, fair value	330,773
Total at end of period	330,773	198,009
CMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	293,204
Amortized Cost	293,204	257,785
Fair Value
Total debt securities without single maturity date, fair value	277,457
Total at end of period	277,457	237,216
CLOs
Amortized Cost
Total debt securities without single maturity date, amortized cost	327,467
Amortized Cost	327,467	337,656
Fair Value
Total debt securities without single maturity date, fair value	326,096
Total at end of period	326,096	329,832
Total fixed income securities
Amortized Cost
Amortized Cost	6,061,022	5,926,785
Fair Value
Total at end of period	$ 5,723,113	$ 5,409,698