Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Amortized cost
Due in one year or less	$ 614,805
Due after one year through five years	1,557,863
Due after five years through ten years	1,800,564
Due after ten years	926,197
Total debt securities with single maturity date, amortized cost	4,899,429
Fair Value
Due in one year or less	608,478
Due after one year through five years	1,510,968
Due after five years through ten years	1,676,467
Due after ten years	789,465
Total debt securities with single maturity date, fair value	4,585,378
Total fixed income securities
Amortized cost
Amortized Cost	6,023,778	$ 6,061,022
Fair Value
Total at end of period	5,672,714	5,723,113
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	173,761
Amortized Cost	173,761	173,256
Fair Value
Total debt securities without single maturity date, fair value	172,516
Total at end of period	172,516	171,273
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	345,029
Amortized Cost	345,029	347,132
Fair Value
Total debt securities without single maturity date, fair value	323,826
Total at end of period	323,826	330,773
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	286,876
Amortized Cost	286,876	293,204
Fair Value
Total debt securities without single maturity date, fair value	272,378
Total at end of period	272,378	277,457
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	318,683
Amortized Cost	318,683	327,467
Fair Value
Total debt securities without single maturity date, fair value	318,616
Total at end of period	$ 318,616	$ 326,096