Investments (Details 3) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Estimated expected maturity:
Due in 1 year or less, Amortized Cost	$ 263,813
Due in 1 year or less, Fair Value	279,815
Due in 1 year or less, Percent of Total Fair Value	4.70%	4.30%
Due after 1 year through 5 years, Amortized Cost	1,155,912
Due after 1 year through 5 years, Fair Value	1,226,101
Due after 1 year through 5 years, Percent of Total Fair Value	20.70%	20.80%
Due after 5 years through 10 years, Amortized Cost	2,136,503
Due after 5 years through 10 years, Fair Value	2,266,237
Due after 5 years through 10 years, Percent of Total Fair Value	38.20%	38.40%
Due after 10 years through 20 years, Amortized Cost	1,080,363
Due after 10 years through 20 years, Fair Value	1,145,965
Due after 10 years through 20 years, Percent of Total Fair Value	19.30%	18.70%
Due after 20 years, Amortized Cost	954,534
Due after 20 years, Fair Value	1,012,496
Due after 20 years, Percent of Total Fair Value	17.10%	17.80%
Total, Amortized Cost	5,591,125	5,311,457
Fair Value	$ 5,930,614	$ 5,962,232
Total, Percent of Total Fair Value	100.00%	100.00%
Average option-adjusted duration, in years	6 years 3 months 18 days	6 years 3 months 18 days