Acquisitions (Details) a in Thousands, $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2017 USD ($) a acquisition	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | acquisition	2
Payments to acquire businesses		$ 25	$ 1,828
Permian Basin
Business Acquisition [Line Items]
Gas and oil acres acquired (in acres) | a	91
Permian Basin | BC Operating, Inc
Business Acquisition [Line Items]
Payments to acquire businesses	$ 1,100
Permian Basin | Black Mountain Oil & Gas
Business Acquisition [Line Items]
Payments to acquire businesses	$ 700