Property and Equipment (AEC) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Acquisition costs	$ 135,723
Exploration costs	188,481
Capitalized interest	25,229
Total oil and gas properties not subject to amortization	349,433	258,334
[CurrentYearMember]
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Acquisition costs	47,995
Exploration costs	72,490
Capitalized interest	21,352
Total oil and gas properties not subject to amortization	141,837
[OneYearPriorMember]
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Acquisition costs	48,904
Exploration costs	54,729
Capitalized interest	3,877
Total oil and gas properties not subject to amortization	107,510
[TwoYearPriorMember]
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Acquisition costs	23,648
Exploration costs	23,500
Capitalized interest	0
Total oil and gas properties not subject to amortization	47,148
[ThreeYearsAndPriorMember]
Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost [Abstract]
Acquisition costs	15,176
Exploration costs	37,762
Capitalized interest	0
Total oil and gas properties not subject to amortization	$ 52,938

- Definition

The capitalized costs incurred (excluded from amortization), as of the date of the balance sheet, in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. Exploration costs also include costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, the maintenance of land and lease records, dry hole contributions and bottom hole contributions, costs of drilling and equipping exploratory wells and costs of drilling exploratory-type stratigraphic test wells.

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