Investments (Details 5) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Estimated expected maturity:
Due in 1 year or less, Amortized Cost	$ 251,888
Due in 1 year or less, Fair Value	272,348
Due in 1 year or less, Percent of Total Fair Value	4.00%
Due after 1 year through 5 years, Amortized Cost	1,494,345
Due after 1 year through 5 years, Fair Value	1,615,729
Due after 1 year through 5 years, Percent of Total Fair Value	23.40%
Due after 5 years through 10 years, Amortized Cost	2,557,026
Due after 5 years through 10 years, Fair Value	2,764,730
Due after 5 years through 10 years, Percent of Total Fair Value	40.10%
Due after 10 years through 20 years, Amortized Cost	1,278,903
Due after 10 years through 20 years, Fair Value	1,382,787
Due after 10 years through 20 years, Percent of Total Fair Value	20.10%
Due after 20 years, Amortized Cost	793,075
Due after 20 years, Fair Value	857,496
Due after 20 years, Percent of Total Fair Value	12.40%
Total, Amortized Cost	6,375,237	5,784,205
Total, Fair Value	$ 6,893,090	$ 6,009,573
Total, Percent of Total Fair Value	100.00%
Average option-adjusted duration, in years	5 years 9 months 18 days