FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($) shares in Millions	1 Months Ended	2 Months Ended	3 Months Ended		4 Months Ended	5 Months Ended	9 Months Ended	12 Months Ended
	Jan. 31, 2019	May 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2018	May 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties			$ 16,325,000	$ 2,565,000	$ 2,565,000		$ 115,604,000
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
Nonrecurring basis | Proved property in Barnett Shale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties							75,300,000
Nonrecurring basis | Proved property in San Juan Basin And Mid Continent Area [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties							27,000,000
Nonrecurring basis | Proved property in Monroe Field [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties							1,600,000
Nonrecurring basis | Proved property in Central Texas and Karnes County [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties					2,600,000
Long Lived Assets Held and Used [Member] | Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties					$ 0		11,700,000
Quoted Prices in Active Markets for Identical Assets (Level 1) | Magnolia Oil Gas Parent LLC And Magnolia Oil Gas Corporation [Member]
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] | Long Lived Assets Held and Used [Member] | Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties							3,600,000
Permian Basin Property [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties							8,100,000
Permian Basin Property [Member] | Long Lived Assets Held and Used [Member] | Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties							$ 8,100,000
Predecessor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties						$ 3,000
Predecessor [Member] | Nonrecurring basis | Proved properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties		$ 0				$ 0