Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	$ 8.3	$ 8.4
Assets, fair value, total	79.7	161.9
Liabilities, fair value, total	0.2	0.0
Derivative Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, contracts	71.4	153.5
Derivative liabilities, contracts	0.2	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)	8.3	8.4
Assets, fair value, total	8.3	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 assets, contracts	0.0	0.0
Derivative liabilities, contracts	0.0	0.0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	0.0	0.0
Assets, fair value, total	71.4	153.5
Liabilities, fair value, total	0.2	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, contracts	71.4	153.5
Derivative liabilities, contracts	0.2	0.0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities (Other assets)	0.0	0.0
Assets, fair value, total	0.0	0.0
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 assets, contracts	0.0	0.0
Derivative liabilities, contracts	$ 0.0	$ 0.0