Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 415,090
Due after one year through five years	1,856,136
Due after five years through ten years	1,666,042
Due after ten years	1,489,749
Total debt securities with single maturity date, amortized cost	5,427,017
Amortized Cost	6,650,315	$ 6,317,164
Fair Value
Due in one year or less	418,673
Due after one year through five years	1,919,980
Due after five years through ten years	1,757,537
Due after ten years	1,565,338
Total debt securities with single maturity date, fair value	5,661,528
Total at end of period	6,897,734	6,661,596
ABS
Amortized cost
Total debt securities without single maturity date, amortized cost	158,637
Amortized Cost	158,637	203,807
Fair Value
Total debt securities without single maturity date, fair value	159,775
Total at end of period	159,775	206,686
RMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	337,353
Amortized Cost	337,353	425,532
Fair Value
Total debt securities without single maturity date, fair value	339,268
Total at end of period	339,268	431,166
CMBS
Amortized cost
Total debt securities without single maturity date, amortized cost	319,187
Amortized Cost	319,187	312,572
Fair Value
Total debt securities without single maturity date, fair value	328,193
Total at end of period	328,193	327,502
CLOs
Amortized cost
Total debt securities without single maturity date, amortized cost	408,121
Amortized Cost	408,121	310,616
Fair Value
Total debt securities without single maturity date, fair value	408,970
Total at end of period	$ 408,970	$ 310,490