Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Maturities, Amortized Cost [Abstract]
Due in one year or less	$ 272,706
Due after one year through five years	1,143,174
Due after five years through ten years	1,128,998
Due after ten years	1,188,093
Total debt securities with single maturity date, amortized cost	3,732,971
Total at end of period	4,421,363	$ 4,684,148
Maturities, Fair Value [Abstract]
Due in one year or less	273,365
Due after one year through five years	1,163,590
Due after five years through ten years	1,156,648
Due after ten years	1,275,144
Total debt securities with single maturity date, fair value	3,868,747
Total at end of period	4,556,202	$ 4,657,561
Asset-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	104,221
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	104,407
Residential mortgage-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	241,314
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	238,340
Commercial mortgage-backed securities
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	281,502
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	284,158
Collateralized loan obligations
Maturities, Amortized Cost [Abstract]
Total debt securities without single maturity date, amortized cost	61,355
Maturities, Fair Value [Abstract]
Total debt securities without single maturity date, fair value	$ 60,550