Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative assets:
Derivative asset, gross	$ 63.3	$ 59.8	[1]
Derivative liabilities:
Derivative liability, gross	(314.9)	(49.2)	[1]
Designated as Hedging Instrument
Derivative assets:
Derivative asset, gross	23.1	41.9	[1]
Derivative liabilities:
Derivative liability, gross	(17.0)	(39.1)	[1]
Designated as Hedging Instrument | Commodity Contracts
Derivative assets:
Derivative asset, gross	0	41.5	[1]
Derivative liabilities:
Derivative liability, gross	0	(0.6)	[1]
Designated as Hedging Instrument | Foreign Currency Contracts
Derivative assets:
Derivative asset, gross	18.7	0.4	[1]
Derivative liabilities:
Derivative liability, gross	0	(7.2)	[1]
Designated as Hedging Instrument | Cross Currency Contracts
Derivative assets:
Derivative asset, gross	4.3	0	[1]
Derivative liabilities:
Derivative liability, gross	0	(2.1)	[1]
Designated as Hedging Instrument | Interest rate contracts
Derivative assets:
Derivative asset, gross	0.1	0	[1]
Derivative liabilities:
Derivative liability, gross	(17.0)	(29.2)	[1]
Derivatives Subject to Utility Rate Regulation | Commodity Contracts
Derivative assets:
Derivative asset, gross	0.2	2.0	[1]
Derivative liabilities:
Derivative liability, gross	(9.4)	(3.4)	[1]
Derivatives Not Designated as Hedging Instruments | Commodity Contracts
Derivative assets:
Derivative asset, gross	40.0	15.9	[1]
Derivative liabilities:
Derivative liability, gross	$ (288.5)	$ (6.7)	[1]

[1] Certain immaterial amounts have been revised to correct the classification of derivatives.