Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012
Other Assets [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Derivative assets, Estimated Fair Value	$ 7,652
Other Liabilities [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Derivative liabilities, Estimated Fair Value	5,648
Oil [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Notional Units	Barrels
Derivative assets, Notional Amount	633
Derivative liabilities, Notional Amount	(633)
Derivative assets, Estimated Fair Value	5,062
Derivative liabilities, Estimated Fair Value	(3,838)
Natural Gas [Member]
Notional amounts and estimated fair values of outstanding derivative positions
Notional Units	MMBTUs
Derivative assets, Notional Amount	4,627
Derivative liabilities, Notional Amount	(4,627)
Derivative assets, Estimated Fair Value	2,590
Derivative liabilities, Estimated Fair Value	$ (1,810)