Significant Acquisitions and Divestitures (Narrative) (Details) $ in Thousands		1 Months Ended			11 Months Ended	12 Months Ended
	Feb. 09, 2018 USD ($)	Oct. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jul. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($) well	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Contribution of interests in oil and gas properties					$ 0	$ 65,700
Cash contributions						11,300
Cash collected						7,900
Alta Mesa Holdings GP, LLC
Business Acquisition [Line Items]
Contribution to affiliates	$ 554,000
Fenter Energy, LLC
Business Acquisition [Line Items]
Cash		$ 22,300
B&B Acquisition
Business Acquisition [Line Items]
Cash			$ 8,200
Total fair value assets acquired			9,900
Bargain purchase gain			$ 1,700
Undeveloped Leasehold In Oklahoma
Business Acquisition [Line Items]
Cash consideration for undeveloped leasehold						$ 10,600
SRII Opco, LP | Alta Mesa RBL
Business Acquisition [Line Items]
Economic interests	100.00%
Voting interests	90.00%
Weeks Island Field, Louisiana | Disposed of by Sale
Business Acquisition [Line Items]
Initial net proceeds							$ 22,500
Oklahoma | Unproved Leasehold
Business Acquisition [Line Items]
Adjusted cost of business acquisition				$ 45,600
High Mesa
Business Acquisition [Line Items]
Number of wells | well						24