Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	$ (21,303)	$ (11,426)
Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	6,088	3,043
Derivative liabilities	(4,552)	(102)
Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(11,203)	(10,293)
Foreign exchange derivative contract
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(5,551)	(1,031)
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(11,203)	(10,293)
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(11,203)	(10,293)
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign exchange derivative contract
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(10,103)	(1,133)
Significant Other Observable Inputs (Level 2) | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	6,088	3,043
Derivative liabilities	(4,552)	(102)
Significant Other Observable Inputs (Level 2) | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2) | Foreign exchange derivative contract
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(5,551)	(1,031)
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Commodity derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Nonqualified employee savings plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Foreign exchange derivative contract
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	$ 0	$ 0