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Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Other Assets [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Derivative assets, Estimated Fair Value	$ 3,521	$ 7,082
Other Liabilities [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Derivative liabilities, Estimated Fair Value	1,801	5,420
Oil [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Notional Units	Barrels
Derivative assets, Notional Amount	738	454
Derivative assets, Estimated Fair Value	2,568	4,693
Derivative liabilities, Notional Amount	(738)	(454)
Derivative liabilities, Estimated Fair Value	(1,591)	(3,693)
Natural Gas [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Notional Units	MMBTUs
Derivative assets, Notional Amount	4,431	3,307
Derivative assets, Estimated Fair Value	953	2,389
Derivative liabilities, Notional Amount	(4,431)	(3,307)
Derivative liabilities, Estimated Fair Value	$ (210)	$ (1,727)