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Investments (Details 3) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Estimated expected maturity:    
Due in 1 year or less, Amortized Cost $ 232,974  
Due in 1 year or less, Fair Value 259,774  
Due in 1 year or less, Percent of Total Fair Value 4.20% 4.30%
Due after 1 year through 5 years, Amortized Cost 1,117,496  
Due after 1 year through 5 years, Fair Value 1,246,044  
Due after 1 year through 5 years, Percent of Total Fair Value 20.30% 20.80%
Due after 5 years through 10 years, Amortized Cost 2,118,549  
Due after 5 years through 10 years, Fair Value 2,362,252  
Due after 5 years through 10 years, Percent of Total Fair Value 38.40% 38.40%
Due after 10 years through 20 years, Amortized Cost 1,066,515  
Due after 10 years through 20 years, Fair Value 1,189,199  
Due after 10 years through 20 years, Percent of Total Fair Value 19.40% 18.70%
Due after 20 years, Amortized Cost 974,325  
Due after 20 years, Fair Value 1,086,404  
Due after 20 years, Percent of Total Fair Value 17.70% 17.80%
Total, Amortized Cost 5,509,859 5,311,457
Fair Value $ 6,143,673 $ 5,962,232
Total, Percent of Total Fair Value 100.00% 100.00%
Average option-adjusted duration, in years 6 years 2 months 12 days 6 years 3 months 18 days