Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 191,371
Due after one year through five years	1,611,819
Due after five years through ten years	1,130,625
Due after ten years	939,039
Total debt securities with single maturity date, amortized cost	3,872,854
Total at end of period	4,598,347	$ 4,602,514
Maturities, Fair Value [Abstract]
Due in one year or less	192,007
Due after one year through five years	1,622,178
Due after five years through ten years	1,113,217
Due after ten years	943,948
Total debt securities with single maturity date, fair value	3,871,350
Total at end of period	4,588,331	$ 4,609,614
Asset-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities with single maturity date, amortized cost	132,271
Maturities, Fair Value [Abstract]
Total debt securities with single maturity date, fair value	132,537
Residential mortgage-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities with single maturity date, amortized cost	278,737
Maturities, Fair Value [Abstract]
Total debt securities with single maturity date, fair value	271,471
Commercial mortgage-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities with single maturity date, amortized cost	253,143
Maturities, Fair Value [Abstract]
Total debt securities with single maturity date, fair value	252,687
Collateralized loan obligations
Maturities, Amortized Cost [Abstract]
Total debt securities with single maturity date, amortized cost	61,342
Maturities, Fair Value [Abstract]
Total debt securities with single maturity date, fair value	$ 60,286