Summary of Significant Accounting Policies - Schedule of Fair Value Impairments, Average Prices of Oil and Gas Properties And Applicable Discount Rates (Detail) $ in Thousands	3 Months Ended							12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / bbl $ / Mcf	Sep. 30, 2015 USD ($) $ / bbl $ / Mcf	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value And Impairments Oil And Gas Properties [Line Items]
Impairment of oil and gas properties	$ 43,990	$ 2,561	$ 113	$ 1,742	$ 22,718			$ 43,990	$ 27,134	$ 801,347
Oil Properties
Fair Value And Impairments Oil And Gas Properties [Line Items]
Fair Value						$ 3,030	$ 330,257			3,030
Impairment of oil and gas properties						$ 16,036	$ 405,308
Management's Oil Price Outlook | $ / bbl						73.48	73.70
Management's Gas Price Outlook | $ / Mcf							4.04
Oil Properties | Minimum
Fair Value And Impairments Oil And Gas Properties [Line Items]
Annual Discount Rate						10.00%	10.00%
Oil Properties | Maximum
Fair Value And Impairments Oil And Gas Properties [Line Items]
Annual Discount Rate						20.00%	20.00%
Natural Gas Properties
Fair Value And Impairments Oil And Gas Properties [Line Items]
Fair Value						$ 123,926	$ 61,625			$ 123,926
Impairment of oil and gas properties						$ 238,210	$ 139,406
Management's Oil Price Outlook | $ / bbl						70.76	75.91
Management's Gas Price Outlook | $ / Mcf						3.74	3.91
Natural Gas Properties | Minimum
Fair Value And Impairments Oil And Gas Properties [Line Items]
Annual Discount Rate						10.00%	10.00%
Natural Gas Properties | Maximum
Fair Value And Impairments Oil And Gas Properties [Line Items]
Annual Discount Rate						20.00%	20.00%