Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative assets:
Derivative asset, gross	$ 67.8	$ 31.6	[1]
Derivative liabilities:
Derivative liability, gross	(180.8)	(65.3)	[1]
Designated as Hedging Instrument
Derivative assets:
Derivative asset, gross	45.6	11.0	[1]
Derivative liabilities:
Derivative liability, gross	(12.5)	(36.7)	[1]
Designated as Hedging Instrument | Commodity Contracts
Derivative assets:
Derivative asset, gross	0	10.9	[1]
Derivative liabilities:
Derivative liability, gross	0	(1.1)	[1]
Designated as Hedging Instrument | Foreign Currency Contracts
Derivative assets:
Derivative asset, gross	35.8	0.1	[1]
Derivative liabilities:
Derivative liability, gross	0	(5.6)	[1]
Designated as Hedging Instrument | Cross Currency Contracts
Derivative assets:
Derivative asset, gross	9.7	0	[1]
Derivative liabilities:
Derivative liability, gross	0	(2.2)	[1]
Designated as Hedging Instrument | Interest Rate Contracts
Derivative assets:
Derivative asset, gross	0.1	0	[1]
Derivative liabilities:
Derivative liability, gross	(12.5)	(27.8)	[1]
Derivatives Subject to Utility Rate Regulation | Commodity Contracts
Derivative assets:
Derivative asset, gross	0	3.5	[1]
Derivative liabilities:
Derivative liability, gross	(5.2)	(0.7)	[1]
Derivatives Not Designated as Hedging Instruments | Commodity Contracts
Derivative assets:
Derivative asset, gross	22.2	17.1	[1]
Derivative liabilities:
Derivative liability, gross	$ (163.1)	$ (27.9)	[1]

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