Fair Value Measurements (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Interest rate derivatives
Liabilities:
Decrease in fair value of derivative liabilities attributable to nonperformance risk	$ 200,000
Commodity derivatives
Liabilities:
Decrease in fair value of derivative liabilities attributable to nonperformance risk	300,000
Recurring basis | Level 2
Liabilities:
Total fair value, net	(18,585,000)	(22,542,000)
Recurring basis | Level 2 | Interest rate derivatives
Liabilities:
Derivative liability, fair value	(5,912,000)	(5,939,000)
Recurring basis | Level 2 | Commodity derivatives
Assets:
Derivative asset, fair value	3,204,000	1,061,000
Liabilities:
Derivative liability, fair value	(15,877,000)	(17,664,000)
Recurring basis | Total
Liabilities:
Total fair value, net	(18,585,000)	(22,542,000)
Recurring basis | Total | Interest rate derivatives
Liabilities:
Derivative liability, fair value	(5,912,000)	(5,939,000)
Recurring basis | Total | Commodity derivatives
Assets:
Derivative asset, fair value	3,204,000	1,061,000
Liabilities:
Derivative liability, fair value	$ (15,877,000)	$ (17,664,000)