Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 437,605
Due after one year through five years	1,993,330
Due after five years through ten years	1,152,214
Due after ten years	1,283,240
Total debt securities with single maturity date, amortized cost	4,866,389
Amortized Cost	6,037,701	$ 5,562,550
Fair Value
Due in one year or less	440,557
Due after one year through five years	2,077,983
Due after five years through ten years	1,256,504
Due after ten years	1,380,468
Total debt securities with single maturity date, fair value	5,155,512
Total at end of period	6,344,718	5,737,892
Asset backed securities (“ABS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	201,731
Amortized Cost	201,731	227,376
Fair Value
Total debt securities without single maturity date, fair value	204,696
Total at end of period	204,696	229,664
Residential mortgage backed securities (“RMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	398,934
Amortized Cost	398,934	271,384
Fair Value
Total debt securities without single maturity date, fair value	403,893
Total at end of period	403,893	268,586
Commercial mortgage backed securities (“CMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	258,579
Amortized Cost	258,579	274,234
Fair Value
Total debt securities without single maturity date, fair value	271,524
Total at end of period	271,524	278,986
Collateralized loan obligations (“CLOs”)
Amortized cost
Total debt securities without single maturity date, amortized cost	312,068
Amortized Cost	312,068	327,076
Fair Value
Total debt securities without single maturity date, fair value	309,093
Total at end of period	$ 309,093	$ 325,466