Disclosures About Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	$ 23.0	$ 19.3
Derivative Liability, Fair Value, Net	(56.7)	(76.3)
Designated as Hedging Instrument
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	10.3	11.1
Derivative Liability, Fair Value, Net	(36.0)	(68.0)
Designated as Hedging Instrument | Cross Currency Interest Rate Contract
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	0	0
Derivative Financial Instruments and Other Assets | Designated as Hedging Instrument | Commodity Contracts
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	10.2	5.4
Derivative Financial Instruments and Other Assets | Designated as Hedging Instrument | Foreign Currency Contracts
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	0.1	1.5
Derivative Financial Instruments and Other Assets | Derivatives Not Designated as Hedging Instruments | Commodity Contracts
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	9.5	4.1
Derivative Financial Instruments, Assets | Designated as Hedging Instrument | Interest Rate Contracts
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	0	4.2
Derivative Financial Instruments, Assets | Derivatives Subject to Utility Rate Regulation | Commodity Contracts
Derivatives, Fair Value
Derivative Asset, Fair Value, Net	3.2	4.1
Derivative Financial Instruments and Other Noncurrent Liabilities | Designated as Hedging Instrument | Commodity Contracts
Derivatives, Fair Value
Derivative Liability, Fair Value, Net	(0.4)	(7.0)
Derivative Financial Instruments and Other Noncurrent Liabilities | Designated as Hedging Instrument | Foreign Currency Contracts
Derivatives, Fair Value
Derivative Liability, Fair Value, Net	(5.6)	(0.5)
Derivative Financial Instruments and Other Noncurrent Liabilities | Designated as Hedging Instrument | Cross Currency Interest Rate Contract
Derivatives, Fair Value
Derivative Liability, Fair Value, Net	(2.2)	0
Derivative Financial Instruments and Other Noncurrent Liabilities | Designated as Hedging Instrument | Interest Rate Contracts
Derivatives, Fair Value
Derivative Liability, Fair Value, Net	(27.8)	(60.5)
Derivative Financial Instruments and Other Noncurrent Liabilities | Derivatives Subject to Utility Rate Regulation | Commodity Contracts
Derivatives, Fair Value
Derivative Liability, Fair Value, Net	(0.4)	(5.5)
Derivative Financial Instruments and Other Noncurrent Liabilities | Derivatives Not Designated as Hedging Instruments | Commodity Contracts
Derivatives, Fair Value
Derivative Liability, Fair Value, Net	$ (20.3)	$ (2.8)