FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($) shares in Millions	1 Months Ended	3 Months Ended	5 Months Ended	7 Months Ended	12 Months Ended
	Jan. 31, 2019	Jun. 30, 2019	May 31, 2018	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 04, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Designation, Predecessor and Successor [Fixed List]			Predecessor	Successor	Successor	Successor	Predecessor	Predecessor
Impairment of oil and natural gas properties		$ 129,100,000	$ 3,000	$ 3,065,000	$ 129,082,000		$ 93,607,000
Senior notes, fair value									$ 297,000,000
Magnolia Oil Gas Parent LLC And Magnolia Oil Gas Corporation [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business acquisition equity interests issued or issuable number of shares originally acquired then sold	4.2
Proceeds from sale of available-for-sale securities, equity	$ (51,700,000)
Long Lived Assets Held and Used [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties			$ 0	$ 0	8,800,000		$ 66,900,000
New Credit Facility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility outstanding					0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock of Magnolia traded in public exchange						4.2
Mid Continent Area Oil and Gas Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties					6,700,000
Permian Basin Property [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties					$ 8,000,000