v2.4.0.8
Investments - Contractual Maturities (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Schedule of Investments [Line Items]    
Due in one year or less, Amortized cost $ 89,450  
Due after one year through five years, Amortized cost 397,591  
Due after five years through ten years, Amortized cost 880,828  
Due after ten years, Amortized cost 48,766  
Total before securities without single maturities, Amortized cost 1,416,635  
Total Amortized cost 2,665,870  
Due in one year or less, Fair value 90,778  
Due after one year through five years, Fair value 419,897  
Due after five years through ten years, Fair value 907,750  
Due after ten years, Fair value 50,493  
Total before securities without single maturities, Fair value 1,468,918  
Total Investments, Fair Value 2,714,867 2,618,697
Due in one year or less, % of Total fair value 3.30%  
Due after one year through five years, % of Total fair value 15.50%  
Due after five years through ten years, % of Total fair value 33.40%  
Due after ten years, % of Total fair value 1.90%  
Total before securities without single maturities, % of Total fair value 54.10%  
% 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 1,221,606  
Fair value of securities without single maturities 1,218,486  
Total Investments, Fair Value 1,218,486 992,174
% of Total fair value for securities without single maturities 44.90%  
Other mortgage-backed securities [Member]
    
Schedule of Investments [Line Items]    
Amortized cost of securities without single maturities 27,629  
Fair value of securities without single maturities 27,463  
Total Investments, Fair Value $ 27,463 $ 24,068
% of Total fair value for securities without single maturities 1.00%