Acquisitions (Details) a in Thousands, Boe in Thousands, $ in Millions			3 Months Ended
	Jun. 01, 2017 USD ($) a	May 01, 2017 USD ($) a Boe	Mar. 31, 2018 USD ($)	Jun. 30, 2017 a	Mar. 31, 2017 USD ($)
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired | $			$ 4		$ 0
Permian Basin [Member]
Business Acquisition [Line Items]
Gas and Oil Acres, Undeveloped and Developed, Net | a				91
Permian Basin [Member] | Black Mountain Oil and Gas Transaction [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired | $	$ 700
Gas and Oil Acres, Undeveloped and Developed, Net | a	21
Permian Basin [Member] | BC Operating Transaction [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired | $		$ 1,100
Gas and Oil Acres, Undeveloped and Developed, Net | a		70
Production, Barrels of Oil Equivalents | Boe		5