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Investments (Details) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Estimated expected maturity:    
Due in 1 year or less, Percent of Total Fair Value 4.10% 4.80%
Due in 1 year or less, Fair Value $ 234,365  
Due in 1 year or less, Amortized Cost 208,189  
Due after 1 year through 5 years, Percent of Total Fair Value 18.30% 19.40%
Due after 1 year through 5 years, Fair Value 1,059,693  
Due after 1 year through 5 years, Amortized Cost 941,341  
Due after 5 years through 10 years, Percent of Total Fair Value 37.70% 33.20%
Due after 5 years through 10 years, Fair Value 2,180,310  
Due after 5 years through 10 years, Amortized Cost 1,936,801  
Due after 10 years through 20 years, Percent of Total Fair Value 20.70% 22.10%
Due after 10 years through 20 years, Fair Value 1,194,868  
Due after 10 years through 20 years, Amortized Cost 1,061,418  
Due after 20 years, Percent of Total Fair Value 19.20% 20.50%
Due after 20 years, Fair Value 1,112,816  
Due after 20 years, Amortized Cost 988,531  
Total, Percent of Total Fair Value 100.00% 100.00%
Fair Value 5,782,052 5,421,832
Total, Amortized Cost $ 5,136,280 $ 4,984,389
Average option-adjusted duration, in years 6 years 3 months 18 days 6 years 4 months 24 days