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Investments (Details) (USD $)
In Thousands, unless otherwise specified
 6 Months Ended 12 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Estimated expected maturity:    
Due in 1 year or less, Percent of Total Fair Value 4.40% 4.80%
Due in 1 year or less, Fair Value $ 251,582  
Due in 1 year or less, Amortized Cost 227,379  
Due after 1 year through 5 years, Percent of Total Fair Value 18.40% 19.40%
Due after 1 year through 5 years, Fair Value 1,049,930  
Due after 1 year through 5 years, Amortized Cost 948,923  
Due after 5 years through 10 years, Percent of Total Fair Value 36.30% 33.20%
Due after 5 years through 10 years, Fair Value 2,062,870  
Due after 5 years through 10 years, Amortized Cost 1,864,414  
Due after 10 years through 20 years, Percent of Total Fair Value 21.10% 22.10%
Due after 10 years through 20 years, Fair Value 1,201,837  
Due after 10 years through 20 years, Amortized Cost 1,086,216  
Due after 20 years, Percent of Total Fair Value 19.80% 20.50%
Due after 20 years, Fair Value 1,128,507  
Due after 20 years, Amortized Cost 1,019,941  
Total, Percent of Total Fair Value 100.00% 100.00%
Fair Value 5,694,726 5,421,832
Total, Amortized Cost $ 5,146,873 $ 4,984,389
Average option-adjusted duration, in years 6 years 3 months 18 days 6 years 4 months 24 days