Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands, shares in Millions	5 Months Ended	6 Months Ended	7 Months Ended	12 Months Ended
	May 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties			$ 3,065
Senior notes, fair value	$ 297,000
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
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
Predecessor
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of oil and natural gas properties	3	$ 93,600			$ 93,607	$ 131,260
Predecessor | 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				66,900	$ 0
Predecessor | Significant Other Observable Inputs (Level 2) | 8.0% Senior Notes due 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes, fair value		176,800			176,800
Senior notes, carrying amount		$ 342,500			$ 342,500