Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 465,716
Due after one year through five years	1,854,619
Due after five years through ten years	1,009,452
Due after ten years	1,197,637
Total debt securities with single maturity date, amortized cost	4,527,424
Amortized Cost	5,597,554	$ 5,562,550
Fair Value
Due in one year or less	469,024
Due after one year through five years	1,935,032
Due after five years through ten years	1,096,842
Due after ten years	1,283,527
Total debt securities with single maturity date, fair value	4,784,425
Total at end of period	5,862,262	5,737,892
Asset backed securities (“ABS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	198,238
Amortized Cost	198,238	227,376
Fair Value
Total debt securities without single maturity date, fair value	199,556
Total at end of period	199,556	229,664
Residential mortgage backed securities (“RMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	293,790
Amortized Cost	293,790	271,384
Fair Value
Total debt securities without single maturity date, fair value	299,492
Total at end of period	299,492	268,586
Commercial mortgage backed securities (“CMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	264,857
Amortized Cost	264,857	274,234
Fair Value
Total debt securities without single maturity date, fair value	273,899
Total at end of period	273,899	278,986
Collateralized loan obligations (“CLOs”)
Amortized cost
Total debt securities without single maturity date, amortized cost	313,245
Amortized Cost	313,245	327,076
Fair Value
Total debt securities without single maturity date, fair value	304,890
Total at end of period	$ 304,890	$ 325,466