Property, Plant and Equipment - Summary of Oil and Natural Gas Property Costs Not Being Amortized (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Acquisition	$ 23,871
Exploration	48,131
Total unproved properties, not subject to amortization	72,002
2017
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Exploration	27,137
Total unproved properties, not subject to amortization	27,137
2016
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Acquisition	3,845
Exploration	7,174
Total unproved properties, not subject to amortization	11,019
2015
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Acquisition	4,089
Exploration	2,621
Total unproved properties, not subject to amortization	6,710
2014 and Prior
Capitalized Costs Of Unproved Properties Excluded From Amortization [Line Items]
Acquisition	15,937
Exploration	11,199
Total unproved properties, not subject to amortization	$ 27,136

- Definition

The capitalized costs incurred during the period (excluded from amortization) 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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