Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	$ 8.8	$ 8.4
Assets, Fair Value, Total	91.4	161.9
Liabilities, Fair Value, Total	0.0	0.0
Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	82.6	153.5
Derivative Liabilities, Fair Value	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	8.8	8.4
Assets, Fair Value, Total	8.8	8.4
Liabilities, Fair Value, Total	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, Fair Value	0.0	0.0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value, Total	82.6	153.5
Liabilities, Fair Value, Total	0.0	0.0
Significant Other Observable Inputs (Level 2) [Member] | Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Assets, Fair Value	82.6	153.5
Derivative Liabilities, Fair Value	0.0	0.0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value, Total	0.0	0.0
Significant Unobservable Inputs (Level 3) [Member] | Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liabilities, Fair Value	$ 0.0	$ 0.0