Investments (Details 5) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Estimated expected maturity:
Due in 1 year or less, Amortized Cost	$ 234,952
Due in 1 year or less, Fair Value	244,106
Due in 1 year or less, Percent of Total Fair Value	4.10%
Due after 1 year through 5 years, Amortized Cost	1,206,390
Due after 1 year through 5 years, Fair Value	1,253,394
Due after 1 year through 5 years, Percent of Total Fair Value	20.90%
Due after 5 years through 10 years, Amortized Cost	2,227,741
Due after 5 years through 10 years, Fair Value	2,314,540
Due after 5 years through 10 years, Percent of Total Fair Value	38.40%
Due after 10 years through 20 years, Amortized Cost	1,202,062
Due after 10 years through 20 years, Fair Value	1,248,897
Due after 10 years through 20 years, Percent of Total Fair Value	20.80%
Due after 20 years, Amortized Cost	913,060
Due after 20 years, Fair Value	948,636
Due after 20 years, Percent of Total Fair Value	15.80%
Total, Amortized Cost	5,784,205	5,311,457
Total, Fair Value	$ 6,009,573	$ 5,962,232
Total, Percent of Total Fair Value	100.00%
Average option-adjusted duration, in years	6 years 3 months 18 days