Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	$ 343,300
Fair Value, Measurements, Recurring [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Trading securities held in the deferred compensation plans	56,345	50,237
Fair Value, Measurements, Recurring [Member] | Collars [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	212,320	211,621
Fair Value, Measurements, Recurring [Member] | Swap [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	134,629	69,054
Fair Value, Measurements, Recurring [Member] | Call Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	(4,722)	(29,348)
Fair Value, Measurements, Recurring [Member] | Put Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	1,071
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Trading securities held in the deferred compensation plans	56,345	50,237
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Collars [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Swap [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Call Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets [Member] | Put Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Trading securities held in the deferred compensation plans
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs [Member] | Collars [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	212,320	211,621
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs [Member] | Swap [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	134,629	69,054
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs [Member] | Call Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	(4,722)	(29,348)
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs [Member] | Put Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net	1,071
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Trading securities held in the deferred compensation plans
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs [Member] | Collars [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs [Member] | Swap [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs [Member] | Call Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs [Member] | Put Options [Member]
Fair Value Hierarchy Table for Assets and Liabilities Measured at Fair Value
Derivative assets liabilities at fair value net