Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 449,359
Due after one year through five years	1,870,031
Due after five years through ten years	1,586,713
Due after ten years	1,530,063
Total debt securities with single maturity date, amortized cost	5,436,166
Amortized Cost	6,682,052	$ 6,317,164
Fair Value
Due in one year or less	453,780
Due after one year through five years	1,939,373
Due after five years through ten years	1,690,066
Due after ten years	1,621,862
Total debt securities with single maturity date, fair value	5,705,081
Total at end of period	6,964,409	6,661,596
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	154,031
Amortized Cost	154,031	203,807
Fair Value
Total debt securities without single maturity date, fair value	155,733
Total at end of period	155,733	206,686
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	377,315
Amortized Cost	377,315	425,532
Fair Value
Total debt securities without single maturity date, fair value	379,203
Total at end of period	379,203	431,166
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	318,470
Amortized Cost	318,470	312,572
Fair Value
Total debt securities without single maturity date, fair value	327,435
Total at end of period	327,435	327,502
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	396,070
Amortized Cost	396,070	310,616
Fair Value
Total debt securities without single maturity date, fair value	396,957
Total at end of period	$ 396,957	$ 310,490