Asset Impairments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Impaired Long Lived Assets Held And Used [Line Items]
Asset impairments	$ 154	$ 154
Unproved Impairments [Member]
Impaired Long Lived Assets Held And Used [Line Items]
Asset impairments		$ 61	$ 62
Proved Asset Impairments [Member]
Impaired Long Lived Assets Held And Used [Line Items]
Asset impairments	109
Non-oil and Gas Asset Impairments [Member]
Impaired Long Lived Assets Held And Used [Line Items]
Asset impairments	$ 45