Derivative Financial Instruments (Details 2) (Derivatives not designated as hedging instruments, USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Current Assets
Fair value of derivative assets and liabilities
Derivative Instruments, Assets	$ 67,457	$ 55,813
Noncurrent Assets
Fair value of derivative assets and liabilities
Derivative Instruments, Assets	30,595	31,062
Current Liabilities
Fair value of derivative assets and liabilities
Derivative Instruments, Liabilities	50,764	42,806
Noncurrent liabilities
Fair value of derivative assets and liabilities
Derivative Instruments, Liabilities	25,397	12,875
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair value of derivative assets and liabilities
Fair value: Derivative Assets	98,052	86,875	[1]
Fair value: Derivative Liabilities	$ 76,161	$ 55,681	[1]

[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.