Acquisitions Acquisitions Asset (Details) a in Thousands, $ in Millions		3 Months Ended		12 Months Ended
	Aug. 01, 2016 USD ($) a	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) a	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired				$ 25	$ 1,891	$ 902
Permian Basin [Member]
Business Acquisition [Line Items]
Gas and Oil Acres, Undeveloped and Developed, Net | a			91
Payments to Acquire Businesses, Net of Cash Acquired		$ 63
Permian Basin [Member] | BC Operating Transaction [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired			$ 1,100
Permian Basin [Member] | Black Mountain Oil and Gas Transaction [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Net of Cash Acquired			$ 700
Anadarko Basin STACK Play [Member]
Business Acquisition [Line Items]
Gas and Oil Acres, Undeveloped and Developed, Net | a	61
Business Combination, Consideration Transferred	$ 904