Fair Value Measurements and Derivative Instruments (Details 13) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Standard & Poor's, BBB- Rating Lower Limit	Dec. 31, 2012 Moody's, Baa3 Rating	Dec. 31, 2012 Standard & Poor's, BB Rating	Dec. 31, 2012 Moody's, Ba1 Rating	Dec. 31, 2012 Maturity of at least five years Lower Limit	Dec. 31, 2012 Interest rate contracts item	Dec. 31, 2011 Interest rate contracts
Derivative instruments disclosure
Credit ratings for senior debt	BBB-	Baa3	BB	Ba1
Number of derivative instruments						4
Interest rate instrument term					5 years
Aggregate fair value of all derivative instruments with credit-related contingent features in net liability positions						$ 55.5	$ 11.4