Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 439,771
Due after one year through five years	1,333,112
Due after five years through ten years	972,932
Due after ten years	1,293,585
Total debt securities with single maturity date, amortized cost	4,039,400
Total at end of period	4,666,165	$ 4,717,211
Fair Value
Due in one year or less	440,227
Due after one year through five years	1,342,110
Due after five years through ten years	980,813
Due after ten years	1,329,088
Total debt securities with single maturity date, fair value	4,092,238
Total at end of period	4,710,153	$ 4,685,222
ABS
Amortized Cost
Total debt securities without single maturity date, amortized cost	9,857
Fair Value
Total debt securities without single maturity date, fair value	9,860
RMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	198,756
Fair Value
Total debt securities without single maturity date, fair value	192,896
CMBS
Amortized Cost
Total debt securities without single maturity date, amortized cost	308,735
Fair Value
Total debt securities without single maturity date, fair value	305,425
CLOs
Amortized Cost
Total debt securities without single maturity date, amortized cost	109,417
Fair Value
Total debt securities without single maturity date, fair value	$ 109,734