Fair Value Measurements (Details 3-Reported)	Dec. 31, 2017 USD ($)	May 31, 2017	Mar. 31, 2018 USD ($)
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Assets, Fair Value Disclosure	$ 762,000,000		$ 15,000,000
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Other noncurrent financial assets	135,000,000		129,000,000
Total financial assets	897,000,000		144,000,000
Financial liabilities
Other current financial liabilities	32,000,000		31,000,000
Long-term debt, including current portion	5,976,000,000		5,841,000,000
Deferred credits and other financial liabilities	110,000,000		105,000,000
Financial Liabilities Fair Value Disclosure	6,118,000,000		5,977,000,000
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Assets, Fair Value Disclosure	761,000,000		15,000,000
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Other noncurrent financial assets	137,000,000		132,000,000
Total financial assets	898,000,000		147,000,000
Financial liabilities
Other current financial liabilities	43,000,000		43,000,000
Long-term debt, including current portion	5,526,000,000		5,527,000,000
Deferred credits and other financial liabilities	103,000,000		103,000,000
Financial Liabilities Fair Value Disclosure	$ 5,672,000,000		$ 5,673,000,000
Oil Sands Mining Segment [Member] | Discontinued Operations, Disposed of by Sale [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Number of notes receivable	2	2