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Basis of Presentation Basis of Presentation - Fair Value of Financial Instruments (Details) (Designated as hedging instrument [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Designated as hedging instrument [Member]
Derivatives, Fair Value [Line Items]
Gain or loss recognized in oil and gas contracts	$ 9,100	$ 2,400	$ 17,500

The amount of net gains and losses recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments.

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