Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2012 item
Subsidiary Guarantee
Number of wholly owned subsidiaries that are inactive	1
Restricted assets of wholly owned subsidiaries as a maximum percentage of net assets	25.00%
Impairment of Oil and Gas Properties
Maximum amortization period for unproved properties whose acquisition costs are not relatively significant	3 years
Trade Accounts Receivable
Period for which accrued revenue is accounted for operated properties	2 months
Period for which accrued revenue is accounted for non-operated properties	3 months
Minimum
Depreciation, Depletion and Amortization
Estimated useful lives	3 years
Maximum
Depreciation, Depletion and Amortization
Estimated useful lives	13 years