Investments (Details 3) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Estimated expected maturity:
Due in 1 year or less, Percent of Total Fair Value	3.80%	3.90%
Due in 1 year or less, Fair Value	$ 284,164
Due in 1 year or less, Amortized Cost	$ 271,394
Due after 1 year through 5 years, Percent of Total Fair Value	28.10%	28.70%
Due after 1 year through 5 years, Fair Value	$ 2,109,683
Due after 1 year through 5 years, Amortized Cost	$ 2,014,879
Due after 5 years through 10 years, Percent of Total Fair Value	34.10%	35.20%
Due after 5 years through 10 years, Fair Value	$ 2,562,229
Due after 5 years through 10 years, Amortized Cost	$ 2,447,089
Due after 10 years through 20 years, Percent of Total Fair Value	21.10%	19.50%
Due after 10 years through 20 years, Fair Value	$ 1,582,732
Due after 10 years through 20 years, Amortized Cost	$ 1,511,608
Due after 20 years, Percent of Total Fair Value	12.90%	12.70%
Due after 20 years, Fair Value	$ 971,942
Due after 20 years, Amortized Cost	$ 928,265
Total, Percent of Total Fair Value	100.00%	100.00%
Total, Fair Value	$ 7,510,750	$ 7,456,708
Total, Amortized Cost	$ 7,173,235	$ 7,152,127
Average option-adjusted duration, in years	6 years	5 years 10 months 24 days