Investments (Details 5) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Estimated expected maturity:
Due in 1 year or less, Percent of Total Fair Value	3.10%
Due in 1 year or less, Amortized Cost	$ 210,171
Due in 1 year or less, Fair Value	$ 219,639
Due after 1 year through 5 years, Percent of Total Fair Value	24.20%
Due after 1 year through 5 years, Amortized Cost	$ 1,641,139
Due after 1 year through 5 years, Fair Value	$ 1,715,078
Due after 5 years through 10 years, Percent of Total Fair Value	39.60%
Due after 5 years through 10 years, Amortized Cost	$ 2,688,583
Due after 5 years through 10 years, Fair Value	$ 2,809,712
Due after 10 years through 20 years, Percent of Total Fair Value	20.90%
Due after 10 years through 20 years, Amortized Cost	$ 1,420,578
Due after 10 years through 20 years, Fair Value	$ 1,484,580
Due after 20 years, Percent of Total Fair Value	12.20%
Due after 20 years, Amortized Cost	$ 825,155
Due after 20 years, Fair Value	$ 862,331
Total, Percent of Total Fair Value	100.00%
Total, Amortized Cost	$ 6,785,626	$ 6,375,237
Fair Value	$ 7,091,340	$ 6,893,090
Average option-adjusted duration, in years	5 years 9 months 18 days