Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 511,087
Due after one year through five years	1,552,123
Due after five years through ten years	940,185
Due after ten years	1,210,635
Total debt securities with single maturity date, amortized cost	4,214,030
Amortized Cost	5,042,011	$ 4,946,278
Fair Value
Due in one year or less	509,952
Due after one year through five years	1,526,802
Due after five years through ten years	918,757
Due after ten years	1,210,376
Total debt securities with single maturity date, fair value	4,165,887
Total at end of period	4,969,612	4,983,315
Asset backed securities (“ABS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	71,481
Amortized Cost	71,481	4,925
Fair Value
Total debt securities without single maturity date, fair value	70,935
Total at end of period	70,935	4,923
Residential mortgage backed securities (“RMBS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	166,522
Amortized Cost	166,522	189,153
Fair Value
Total debt securities without single maturity date, fair value	154,963
Total at end of period	154,963	181,849
Commercial mortgage backed securities (“CMBS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	281,980
Amortized Cost	281,980	301,014
Fair Value
Total debt securities without single maturity date, fair value	270,220
Total at end of period	270,220	297,312
Collateralized loan obligations (“CLO”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	307,998
Amortized Cost	307,998	100,798
Fair Value
Total debt securities without single maturity date, fair value	307,607
Total at end of period	$ 307,607	$ 101,023