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Investments - Amortized Cost and Fair Values of Debt Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Maturities, Amortized Cost [Abstract]    
Due in one year or less $ 350,683  
Due after one year through five years 1,166,094  
Due after five years through ten years 1,148,505  
Due after ten years 1,143,491  
Total debt securities with single maturity date, amortized cost 3,808,773  
Total at end of period 4,602,457 $ 4,684,148
Maturities, Fair Value [Abstract]    
Due in one year or less 351,117  
Due after one year through five years 1,184,500  
Due after five years through ten years 1,177,837  
Due after ten years 1,211,340  
Total debt securities with single maturity date, fair value 3,924,794  
Total at end of period 4,718,625 $ 4,657,561
Asset-backed securities    
Maturities, Amortized Cost [Abstract]    
Total debt securities without single maturity date, amortized cost 79,032  
Maturities, Fair Value [Abstract]    
Total debt securities without single maturity date, fair value 79,182  
Residential mortgage-backed securities    
Maturities, Amortized Cost [Abstract]    
Total debt securities without single maturity date, amortized cost 245,876  
Maturities, Fair Value [Abstract]    
Total debt securities without single maturity date, fair value 243,436  
Commercial mortgage-backed securities    
Maturities, Amortized Cost [Abstract]    
Total debt securities without single maturity date, amortized cost 347,630  
Maturities, Fair Value [Abstract]    
Total debt securities without single maturity date, fair value 350,213  
Collateralized loan obligations    
Maturities, Amortized Cost [Abstract]    
Total debt securities without single maturity date, amortized cost 121,146  
Maturities, Fair Value [Abstract]    
Total debt securities without single maturity date, fair value $ 121,000