Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 341,831
Due after one year through five years	1,356,023
Due after five years through ten years	1,026,851
Due after ten years	1,294,632
Total debt securities with single maturity date, amortized cost	4,019,337
Total at end of period	4,674,965	$ 4,717,211
Fair Value
Due in one year or less	342,028
Due after one year through five years	1,362,799
Due after five years through ten years	1,030,223
Due after ten years	1,321,793
Total debt securities with single maturity date, fair value	4,056,843
Total at end of period	4,701,211	$ 4,685,222
ABS
Amortized Cost
Total debt securities without single maturity date, amortized cost	21,988
Fair Value
Total debt securities without single maturity date, fair value	21,987
RMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	209,874
Fair Value
Total debt securities without single maturity date, fair value	202,340
CMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	310,997
Fair Value
Total debt securities without single maturity date, fair value	307,078
CLOs
Amortized Cost
Total debt securities without single maturity date, amortized cost	112,769
Fair Value
Total debt securities without single maturity date, fair value	$ 112,963