Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 338,124
Due after one year through five years	1,318,577
Due after five years through ten years	1,091,184
Due after ten years	1,228,385
Total debt securities with single maturity date, amortized cost	3,976,270
Total at end of period	4,655,457	$ 4,717,211
Maturities, Fair Value [Abstract]
Due in one year or less	338,309
Due after one year through five years	1,323,558
Due after five years through ten years	1,082,035
Due after ten years	1,232,706
Total debt securities with single maturity date, fair value	3,976,608
Total at end of period	4,642,119	$ 4,685,222
ABS
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	37,107
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	37,117
RMBS
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	220,739
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	212,909
CMBS
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	300,185
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	294,117
CLOs
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	121,156
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	$ 121,368