CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other comprehensive income (loss), tax
Tax expense (benefit) on gains (losses) on changes in unrealized fair value of derivatives designated as cash flow hedges	$ 101.3	$ (14.7)	$ (81.0)
Pension and other postretirement plans adjustments:
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net Transition Asset (Obligation), Recognized in Net Periodic Benefit Cost, Tax			20.9
Tax expense (benefit) on prior service cost incurred	(2.2)	(2.1)	(1.0)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	(0.8)
Current year net actuarial gain loss [Member]
Pension and other postretirement plans adjustments:
Tax expense (benefit) on net unamortized gain (loss) incurred	6.3	9.2	(1.6)
Amortization net actuarial gain loss [Member] [Member]
Pension and other postretirement plans adjustments:
Tax expense (benefit) on net unamortized gain (loss) incurred	$ (0.9)