Fair Value Measurements of Financial Instruments (Details) (Recurring, Commodity Derivatives, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 item	Dec. 31, 2012 item
Significant Other Observable Inputs (Level 2)
Fair Value Measurements of Financial Instruments
Number of bank counterparties for company's commodity derivative contracts	9	5
Assets:
Total assets	$ 22,733	$ 23,601
Liabilities:
Total liabilities	18,474	27,714
Significant Other Observable Inputs (Level 2) | Swaps | Oil
Assets:
Total assets	11,428	16,133
Liabilities:
Total liabilities	17,430	15,091
Significant Other Observable Inputs (Level 2) | Swaps | NGL
Assets:
Total assets	3,461	2,353
Liabilities:
Total liabilities		458
Significant Other Observable Inputs (Level 2) | Swaps | Gas
Assets:
Total assets	5,670
Significant Other Observable Inputs (Level 2) | Collars | Oil
Assets:
Total assets	488	428
Liabilities:
Total liabilities	118	287
Significant Other Observable Inputs (Level 2) | Collars | Gas
Assets:
Total assets	1,539	2,026
Liabilities:
Total liabilities	48	185
Significant Other Observable Inputs (Level 2) | Basis Differential Swaps
Assets:
Total assets	147	2,661
Liabilities:
Total liabilities	878	11,693
Total
Assets:
Total assets	22,733	23,601
Liabilities:
Total liabilities	18,474	27,714
Total | Swaps | Oil
Assets:
Total assets	11,428	16,133
Liabilities:
Total liabilities	17,430	15,091
Total | Swaps | NGL
Assets:
Total assets	3,461	2,353
Liabilities:
Total liabilities		458
Total | Swaps | Gas
Assets:
Total assets	5,670
Total | Collars | Oil
Assets:
Total assets	488	428
Liabilities:
Total liabilities	118	287
Total | Collars | Gas
Assets:
Total assets	1,539	2,026
Liabilities:
Total liabilities	48	185
Total | Basis Differential Swaps
Assets:
Total assets	147	2,661
Liabilities:
Total liabilities	$ 878	$ 11,693