Summary of Significant Accounting Policies - Non-Cash Impairment Charges, Included within Impairment, Dry Hole Costs and Abandonment Expense in Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Non-cash impairment of oil and gas properties	$ 0
Dry hole costs	0	$ 0	$ 97
Abandonment expense	719	455	3,969
Total impairment, dry hole costs and abandonment expense	719	49,553	4,249
Proved Oil And Gas Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-cash impairment of oil and gas properties	0	37,945	0
Unproved Oil And Gas Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-cash impairment of oil and gas properties	$ 0	$ 11,153	$ 183