Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 310,484
Due after one year through five years	1,265,465
Due after five years through ten years	1,120,853
Due after ten years	1,161,722
Total debt securities with sngle maturity date, amortized cost	3,858,524
Total at end of period	4,506,178	$ 4,684,148
Maturities, Fair Value [Abstract]
Due in one year or less	311,202
Due after one year through five years	1,278,045
Due after five years through ten years	1,122,243
Due after ten years	1,204,375
Total debt securities with single maturity date, fair value	3,915,865
Total at end of period	4,557,914	$ 4,657,561
Asset-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	110,241
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	110,328
Residential mortgage-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	255,344
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	250,641
Commercial mortgage-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	220,719
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	220,696
Collateralized loan obligations
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	61,350
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	$ 60,384