Fair Value Measurements on a Recurring Basis (Details) (USD $) In Thousands	Jun. 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	$ 170,616
Fair value assets measured on recurring basis, derivative financial instruments, liabilities	200,155
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	440,485
Fair value assets measured on recurring basis, derivative financial instruments, liabilities	470,024
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	(269,869)	[1]
Fair value assets measured on recurring basis, derivative financial instruments, liabilities	$ (269,869)	[1]

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