Fair Value Measurements on a Recurring Basis (Details) (USD $) In Thousands	Sep. 30, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments, assets	$ 776,413
Fair value assets measured on recurring basis, derivative financial instruments, liabilities	6,880
Fair Value, Inputs, Level 2 [Member] | Commodity Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments, assets	980,187
Fair value assets measured on recurring basis, derivative financial instruments, liabilities	210,654
Counterparty netting under agreements governing derivatives [Member] | Commodity Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Financial Statement Captions [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments, assets	(203,774)	[1]
Fair value assets measured on recurring basis, derivative financial instruments, liabilities	$ (203,774)	[1]

[1] Represents counterparty netting under agreements governing such derivatives.