v2.4.0.8
Investments - Contractual Maturities (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Schedule of Investments [Line Items]    
Due in one year or less, Amortized cost $ 95,601  
Due after one year through five years, Amortized cost 398,953  
Due after five years through ten years, Amortized cost 888,970  
Due after ten years, Amortized cost 75,099  
Total before securities without single maturities, Amortized cost 1,458,623  
Total Amortized cost 2,620,107  
Due in one year or less, Fair value 97,385  
Due after one year through five years, Fair value 428,246  
Due after five years through ten years, Fair value 917,690  
Due after ten years, Fair value 77,609  
Total before securities without single maturities, Fair value 1,520,930  
Total Investments, Fair Value 2,674,305 2,618,697
Due in one year or less, % of Total fair value 3.70%  
Due after one year through five years, % of Total fair value 16.00%  
Due after five years through ten years, % of Total fair value 34.30%  
Due after ten years, % of Total fair value 2.90%  
Total before securities without single maturities, % of Total fair value 56.90%  
% of Total fair value 100.00% 100.00%
U.S. agency bonds – mortgage-backed [Member]
    
Schedule of Investments [Line Items]    
Amortized cost of securities without single maturities 1,138,860  
Fair value of securities without single maturities 1,130,713  
Total Investments, Fair Value 1,130,713 992,174
% of Total fair value for securities without single maturities 42.30%  
Other mortgage-backed securities [Member]
    
Schedule of Investments [Line Items]    
Amortized cost of securities without single maturities 22,624  
Fair value of securities without single maturities 22,662  
Total Investments, Fair Value $ 22,662 $ 24,068
% of Total fair value for securities without single maturities 0.80%