Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets:
Derivative asset, fair value	$ 3,389
Recurring basis | Level 2
Liabilities:
Total fair value, net	(4,813)	(22,542)
Recurring basis | Level 2 | Interest rate derivatives
Liabilities:
Derivative liability, fair value	(3,872)	(5,939)
Recurring basis | Level 2 | Commodity derivatives
Assets:
Derivative asset, fair value	3,389	1,061
Liabilities:
Derivative liability, fair value	(4,330)	(17,664)
Recurring basis | Total
Liabilities:
Total fair value, net	(4,813)	(22,542)
Recurring basis | Total | Interest rate derivatives
Liabilities:
Derivative liability, fair value	(3,872)	(5,939)
Recurring basis | Total | Commodity derivatives
Assets:
Derivative asset, fair value	3,389	1,061
Liabilities:
Derivative liability, fair value	$ (4,330)	$ (17,664)