Fair Value Measurements - Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans	$ 67,429	$ 50,237
Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	33,282	69,054
Collars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	109,629	211,621
Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	(3,732)	(29,348)
Put options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	77
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans	67,429	50,237
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Collars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Put options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans
Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	33,282	69,054
Significant Other Observable Inputs (Level 2) [Member] | Collars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	109,629	211,621
Significant Other Observable Inputs (Level 2) [Member] | Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	(3,732)	(29,348)
Significant Other Observable Inputs (Level 2) [Member] | Put options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net	77
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in the deferred compensation plans
Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Significant Unobservable Inputs (Level 3) [Member] | Collars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Significant Unobservable Inputs (Level 3) [Member] | Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net
Significant Unobservable Inputs (Level 3) [Member] | Put options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets liabilities at fair value net