Derivative Instruments (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Collateral in margin accounts	$ 0	$ 2.9
Derivative Asset	11.8	19.2
Derivative Liability	0.3	1.9
Reclassification in treasury lock agreement settlement payments deferred in Accumulated Other Comprehensive Income as increase to interest Expense	252.1	248.2	235.8
Derviative Instrument by Counterparty, Gross Amount Recognized on Balance Sheet [Member]
Derivative [Line Items]
Derivative Asset	11.8	19.2
Derivative Liability	0.3	1.9
Derviative Instrument by Counterparty, Gross Amount Not Offset on Balance Sheet [Member] [Member]
Derivative [Line Items]
Collateral in margin accounts	0	1.3
Derivative Asset	0	(0.5)
Derivative Liability	0	(1.8)
Derviative Instrument by Counterparty, Net Amount [Member]
Derivative [Line Items]
Derivative Asset	11.8	18.7
Derivative Liability	$ 0.3	$ 0.1