Investments (Details 3) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 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	$ 244,935
Due in 1 year or less, Fair Value	$ 260,299
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	$ 1,788,903
Due after 1 year through 5 years, Fair Value	$ 1,900,899
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	$ 2,567,871
Due after 5 years through 10 years, Fair Value	$ 2,728,632
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	$ 1,387,922
Due after 10 years through 20 years, Fair Value	$ 1,474,836
Due after 20 years, Percent of Total Fair Value	11.80%	12.20%
Due after 20 years, Amortized Cost	$ 801,670
Due after 20 years, Fair Value	$ 851,836
Total, Percent of Total Fair Value	100.00%	100.00%
Total, Amortized Cost	$ 6,791,301	$ 6,785,626
Fair Value	$ 7,216,502	$ 7,091,340
Average option-adjusted duration, in years	5 years 8 months 12 days	5 years 9 months 18 days