Nature of Operations and Recent Events - Operations & Oil and Natural Gas Prices and Drilling Activity (Details)		3 Months Ended			12 Months Ended
	Oct. 15, 2018 $ / MMBTU	Jun. 30, 2018 $ / bbl	Mar. 31, 2016 $ / bbl	Jun. 30, 2014 $ / bbl	Dec. 31, 2016 $ / MMBTU	Dec. 31, 2014 $ / MMBTU	Sep. 30, 2018 drilling_rig
Property, Plant and Equipment [Line Items]
Number of rigs | drilling_rig							15
Closing price of oil (in dollars per barrel) | $ / bbl				107.95
Market price of oil (in dollars per barrel) | $ / bbl		77.41	26.19
Natural gas price (in dollars per MMBtu) | $ / MMBTU					2.52	4.37
Subsequent Event
Property, Plant and Equipment [Line Items]
Natural gas price (in dollars per MMBtu) | $ / MMBTU	2.97