Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 421,097
Due after one year through five years	1,774,678
Due after five years through ten years	994,134
Due after ten years	1,130,034
Total debt securities with single maturity date, amortized cost	4,319,943
Amortized Cost	5,375,382	$ 5,562,550
Fair Value
Due in one year or less	421,290
Due after one year through five years	1,782,206
Due after five years through ten years	1,021,561
Due after ten years	1,196,598
Total debt securities with single maturity date, fair value	4,421,655
Total at end of period	5,458,846	5,737,892
Asset backed securities (“ABS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	210,930
Amortized Cost	210,930	227,376
Fair Value
Total debt securities without single maturity date, fair value	206,650
Total at end of period	206,650	229,664
Residential mortgage backed securities (“RMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	259,641
Amortized Cost	259,641	271,384
Fair Value
Total debt securities without single maturity date, fair value	264,915
Total at end of period	264,915	268,586
Commercial mortgage backed securities (“CMBS”)
Amortized cost
Total debt securities without single maturity date, amortized cost	268,598
Amortized Cost	268,598	274,234
Fair Value
Total debt securities without single maturity date, fair value	268,587
Total at end of period	268,587	278,986
Collateralized loan obligations (“CLOs”)
Amortized cost
Total debt securities without single maturity date, amortized cost	316,270
Amortized Cost	316,270	327,076
Fair Value
Total debt securities without single maturity date, fair value	297,039
Total at end of period	$ 297,039	$ 325,466