Investments (Details 3) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Estimated expected maturity:
Due in 1 year or less, Amortized Cost	$ 259,224
Due in 1 year or less, Fair Value	271,941
Due in 1 year or less, Percent of Total Fair Value	4.60%	4.30%
Due after 1 year through 5 years, Amortized Cost	1,152,678
Due after 1 year through 5 years, Fair Value	1,209,226
Due after 1 year through 5 years, Percent of Total Fair Value	20.30%	20.80%
Due after 5 years through 10 years, Amortized Cost	2,182,994
Due after 5 years through 10 years, Fair Value	2,290,089
Due after 5 years through 10 years, Percent of Total Fair Value	38.50%	38.40%
Due after 10 years through 20 years, Amortized Cost	1,124,817
Due after 10 years through 20 years, Fair Value	1,179,999
Due after 10 years through 20 years, Percent of Total Fair Value	19.90%	18.70%
Due after 20 years, Amortized Cost	943,005
Due after 20 years, Fair Value	989,267
Due after 20 years, Percent of Total Fair Value	16.70%	17.80%
Total, Amortized Cost	5,662,718	5,311,457
Fair Value	$ 5,940,522	$ 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