Fair Value Measurements - Fair Values - Financial Instruments (Details) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) notes_receivable	May 31, 2017 notes_receivable
Canadian Business | Discontinued Operations, Disposed of by Sale
Financial liabilities
Number of notes receivable | notes_receivable		2	2
Fair Value
Financial assets
Current assets	$ 15	$ 762
Other noncurrent assets	82	135
Total financial assets	97	897
Financial liabilities
Other current liabilities	32	32
Long-term debt, including current portion	5,777	5,976
Deferred credits and other liabilities	97	110
Total financial liabilities	5,906	6,118
Carrying Amount
Financial assets
Current assets	15	761
Other noncurrent assets	86	137
Total financial assets	101	898
Financial liabilities
Other current liabilities	46	43
Long-term debt, including current portion	5,528	5,526
Deferred credits and other liabilities	92	103
Total financial liabilities	$ 5,666	$ 5,672