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Investments (Details 3) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Estimated expected maturity:    
Due in 1 year or less, Percent of Total Fair Value 3.60% 3.10%
Due in 1 year or less, Amortized Cost $ 251,555  
Due in 1 year or less, Fair Value $ 269,865  
Due after 1 year through 5 years, Percent of Total Fair Value 26.30% 24.20%
Due after 1 year through 5 years, Amortized Cost $ 1,885,946  
Due after 1 year through 5 years, Fair Value $ 1,970,758  
Due after 5 years through 10 years, Percent of Total Fair Value 37.90% 39.60%
Due after 5 years through 10 years, Amortized Cost $ 2,609,256  
Due after 5 years through 10 years, Fair Value $ 2,828,911  
Due after 10 years through 20 years, Percent of Total Fair Value 20.40% 20.90%
Due after 10 years through 20 years, Amortized Cost $ 1,346,414  
Due after 10 years through 20 years, Fair Value $ 1,529,037  
Due after 20 years, Percent of Total Fair Value 11.80% 12.20%
Due after 20 years, Amortized Cost $ 817,070  
Due after 20 years, Fair Value $ 883,141  
Total, Percent of Total Fair Value 100.00% 100.00%
Total, Amortized Cost $ 6,910,241 $ 6,785,626
Fair Value $ 7,481,712 $ 7,091,340
Average option-adjusted duration, in years 5 years 9 months 18 days 5 years 9 months 18 days