Impact of Rights of Offset Associated with Recognized Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Current Asset
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Assets	$ 18.7
Long-term assets
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Assets	6.8
Current liabilities
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Liabilities	(5.4)
Long-term liabilities
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Liabilities	(3.0)
Effects of Netting | Current Asset
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Assets	(5.7)
Effects of Netting | Long-term assets
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Assets	(2.0)
Effects of Netting | Current liabilities
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Liabilities	4.7
Effects of Netting | Long-term liabilities
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Liabilities	3.0
With Effects Of Netting | Current Asset
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Assets	13.0
With Effects Of Netting | Long-term assets
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Assets	4.8
With Effects Of Netting | Current liabilities
Derivative Instruments And Hedging Activities [Line Items]
Fair Value of Derivative Instruments, Liabilities	$ (0.7)