Derivative Instruments (Details 3) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative assets, current	$ 10,573,000	$ 6,117,000
Derivative liabilities, current	1,309,000	20,365,000
Derivative assets, noncurrent	11,122,000	7,027,000
Derivative liabilities, noncurrent	1,358,000	15,505,000
Total derivative assets	21,700,000	13,100,000
Total derivative liabilities	2,700,000	35,900,000
Commodity contract
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative assets, current	10,572,000	6,117,000
Derivative liabilities, current	1,309,000	20,365,000
Derivative assets, noncurrent	11,122,000	7,027,000
Derivative liabilities, noncurrent	$ 1,358,000	$ 15,505,000