CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other comprehensive income (loss), tax
Tax benefit (expense) on gains (losses) on changes in unrealized fair value of derivatives designated as cash flow hedges		$ 45.9	$ 101.3
Pension and other postretirement plans adjustments:
Tax expense on prior service cost incurred	(6.0)	(2.1)	(2.2)
Tax expense on net curtailment and settlements cost incurred	(3.5)		(0.8)
Current year net actuarial gain loss [Member]
Pension and other postretirement plans adjustments:
Tax benefit (expense) on net unamortized gain (loss) incurred	8.5	(8.3)	6.3
Amortization net actuarial gain loss [Member] [Member]
Pension and other postretirement plans adjustments:
Tax benefit (expense) on net unamortized gain (loss) incurred	$ (0.3)	$ (0.9)	$ (0.9)