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Investments (Details 3) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Estimated expected maturity:    
Due in 1 year or less, Percent of Total Fair Value 3.10% 4.00%
Due in 1 year or less, Amortized Cost $ 209,133  
Due in 1 year or less, Fair Value $ 221,731  
Due after 1 year through 5 years, Percent of Total Fair Value 24.50% 23.40%
Due after 1 year through 5 years, Amortized Cost $ 1,635,135  
Due after 1 year through 5 years, Fair Value $ 1,733,636  
Due after 5 years through 10 years, Percent of Total Fair Value 39.00% 40.10%
Due after 5 years through 10 years, Amortized Cost $ 2,600,082  
Due after 5 years through 10 years, Fair Value $ 2,756,711  
Due after 10 years through 20 years, Percent of Total Fair Value 21.30% 20.10%
Due after 10 years through 20 years, Amortized Cost $ 1,423,418  
Due after 10 years through 20 years, Fair Value $ 1,509,166  
Due after 20 years, Percent of Total Fair Value 12.10% 12.40%
Due after 20 years, Amortized Cost $ 808,777  
Due after 20 years, Fair Value $ 857,498  
Total, Percent of Total Fair Value 100.00% 100.00%
Total, Amortized Cost $ 6,676,545 $ 6,375,237
Fair Value $ 7,078,742 $ 6,893,090
Average option-adjusted duration, in years 5 years 9 months 18 days 5 years 9 months 18 days