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Investments (Details 5) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Estimated expected maturity:    
Due in 1 year or less, Percent of Total Fair Value 3.90%  
Due in 1 year or less, Amortized Cost $ 276,403  
Due in 1 year or less, Fair Value $ 290,811  
Due after 1 year through 5 years, Percent of Total Fair Value 28.70%  
Due after 1 year through 5 years, Amortized Cost $ 2,051,674  
Due after 1 year through 5 years, Fair Value $ 2,140,074  
Due after 5 years through 10 years, Percent of Total Fair Value 35.20%  
Due after 5 years through 10 years, Amortized Cost $ 2,518,896  
Due after 5 years through 10 years, Fair Value $ 2,624,759  
Due after 10 years through 20 years, Percent of Total Fair Value 19.50%  
Due after 10 years through 20 years, Amortized Cost $ 1,397,499  
Due after 10 years through 20 years, Fair Value $ 1,454,057  
Due after 20 years, Percent of Total Fair Value 12.70%  
Due after 20 years, Amortized Cost $ 907,655  
Due after 20 years, Fair Value $ 947,007  
Total, Percent of Total Fair Value 100.00%  
Total, Amortized Cost $ 7,152,127 $ 6,785,626
Fair Value $ 7,456,708 $ 7,091,340
Average option-adjusted duration, in years 5 years 10 months 24 days