Presentation LIFO restatement Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Inventory, Net	$ 12,810	$ 11,176
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	10,553	10,762
Retained Earnings (Accumulated Deficit)	$ 23,384	21,906
Scenario, Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Inventory, Net		10,277
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent		10,973
Retained Earnings (Accumulated Deficit)		21,218
Scenario, Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Inventory, Net		899
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent		(211)
Retained Earnings (Accumulated Deficit)		$ 688	$ 559