Commercial Paper (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Commercial paper
Carrying value	97,987	97,536
Commercial paper
Commercial paper
Maximum commercial paper support available under the corporate credit facility	50,000
Face value	97,987	97,536
Carrying value	97,987	97,536
Commercial paper | Minimum
Commercial paper
Effective interest rate (as a percent)	0.30%	0.20%
Maturity range (days)	3 days	4 days
Commercial paper | Maximum
Commercial paper
Effective interest rate (as a percent)	0.40%	0.40%
Maturity range (days)	74 days	75 days