Derivative Instruments (Details 3) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative Assets, Current	$ 28,650,000	$ 6,117,000
Derivative Liabilities, Current	2,000	20,365,000
Derivative Assets, Noncurrent	20,092,000	7,027,000
Derivative Liabilities, Noncurrent	0	15,505,000
Total Derivative Assets	48,700,000	13,100,000
Total Derivative Liabilities	0	35,900,000
Commodity contract
Fair value derivative assets and liabilities classified in condensed balance sheets
Derivative Assets, Current	28,649,000	6,117,000
Derivative Liabilities, Current	2,000	20,365,000
Derivative Assets, Noncurrent	20,092,000	7,027,000
Derivative Liabilities, Noncurrent	$ 0	$ 15,505,000