Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 493,653
Due after one year through five years	1,560,626
Due after five years through ten years	984,376
Due after ten years	1,290,860
Total debt securities with single maturity date, amortized cost	4,329,515
Amortized Cost	4,983,355	$ 4,946,278
Fair Value
Due in one year or less	492,697
Due after one year through five years	1,538,297
Due after five years through ten years	961,805
Due after ten years	1,302,066
Total debt securities with single maturity date, fair value	4,294,865
Total at end of period	4,926,247	$ 4,983,315
Asset backed securities (“ABS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	71,625
Fair Value
Total debt securities without single maturity date, fair value	71,292
Residential mortgage backed securities (“RMBS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	174,255
Fair Value
Total debt securities without single maturity date, fair value	163,726
Commercial mortgage backed securities (“CMBS”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	294,839
Fair Value
Total debt securities without single maturity date, fair value	283,343
Collateralized loan obligations (“CLO”)
Amortized Cost
Total debt securities without single maturity date, amortized cost	113,121
Fair Value
Total debt securities without single maturity date, fair value	$ 113,021