Derivative Financial Instruments (Derivative Instruments) (Details) (Not Designated As Hedging Instruments [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Derivative [Line Items]
Total derivatives, Asset	$ 506,177	[1]	$ 347,802	[1]
Total derivatives, Liability	132,078	[1]	162,683	[1]
Commodity Price Derivatives [Member] | Derivatives - Current [Member]
Derivative [Line Items]
Total derivatives, Asset	248,809	[1]	167,406	[1]
Total derivatives, Liability	68,735	[1]	87,741	[1]
Commodity Price Derivatives [Member] | Derivatives - Noncurrent [Member]
Derivative [Line Items]
Total derivatives, Asset	257,368	[1]	152,731	[1]
Total derivatives, Liability	47,689	[1]	64,829	[1]
Interest Rate Derivatives [Member] | Derivatives - Current [Member]
Derivative [Line Items]
Total derivatives, Asset		[1]	11,903	[1]
Total derivatives, Liability	15,654	[1]	886	[1]
Interest Rate Derivatives [Member] | Derivatives - Noncurrent [Member]
Derivative [Line Items]
Total derivatives, Asset		[1]	15,762	[1]
Total derivatives, Liability		[1]	$ 9,227	[1]

[1] Derivative assets and liabilities shown in the tables above are presented as gross assets and liabilities, without regard to master netting arrangements which are considered in the presentations of derivative assets and liabilities in the accompanying consolidated balance sheets.