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Investments (Details 5) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Estimated expected maturity:    
Due in 1 year or less, Amortized Cost $ 228,109  
Due in 1 year or less, Fair Value 256,057  
Due in 1 year or less, Percent of Total Fair Value 4.30%  
Due after 1 year through 5 years, Amortized Cost 1,104,146  
Due after 1 year through 5 years, Fair Value 1,239,429  
Due after 1 year through 5 years, Percent of Total Fair Value 20.80%  
Due after 5 years through 10 years, Amortized Cost 2,037,645  
Due after 5 years through 10 years, Fair Value 2,287,304  
Due after 5 years through 10 years, Percent of Total Fair Value 38.40%  
Due after 10 years through 20 years, Amortized Cost 993,533  
Due after 10 years through 20 years, Fair Value 1,115,264  
Due after 10 years through 20 years, Percent of Total Fair Value 18.70%  
Due after 20 years, Amortized Cost 948,024  
Due after 20 years, Fair Value 1,064,178  
Due after 20 years, Percent of Total Fair Value 17.80%  
Total, Amortized Cost 5,311,457 4,984,389
Fair Value $ 5,962,232 $ 5,421,832
Total, Percent of Total Fair Value 100.00%  
Average option-adjusted duration, in years 6 years 3 months 18 days