Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 650,415
Due after one year through five years	1,507,245
Due after five years through ten years	909,711
Due after ten years	1,305,525
Total debt securities with single maturity date, amortized cost	4,372,896
Amortized Cost	4,974,616	$ 4,946,278
Fair Value
Due in one year or less	649,088
Due after one year through five years	1,487,678
Due after five years through ten years	893,329
Due after ten years	1,317,743
Total debt securities with single maturity date, fair value	4,347,838
Total at end of period	4,930,063	$ 4,983,315
Asset backed securities (“ABS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	9,451
Fair Value
Total debt securities without single maturity date, fair value	9,422
Residential mortgage backed securities (“RMBS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	182,050
Fair Value
Total debt securities without single maturity date, fair value	171,540
Commercial mortgage backed securities (“CMBS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	302,434
Fair Value
Total debt securities without single maturity date, fair value	293,356
Collateralized loan obligations (“CLO”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	107,785
Fair Value
Total debt securities without single maturity date, fair value	$ 107,907