Financial Instruments - Long-Term Debt (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Net fair value adjustments related to hedging and purchase accounting		$ (5)	$ 14
Net unamortized discounts, premiums and debt issuance costs		(7)	(12)
Total long-term debt, carried at historical proceeds, as adjusted	[1]	33,652	33,538
Current portion of long-term debt, carried at historical proceeds, as adjusted	[1]	4,255	3,546
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total long-term debt, principal amount		33,658	32,783
Net fair value adjustments related to hedging and purchase accounting		129	872
Net unamortized discounts, premiums and debt issuance costs		(142)	(125)
Other long-term debt		7	8
Total long-term debt, carried at historical proceeds, as adjusted		33,652	33,538
Current portion of long-term debt, carried at historical proceeds, as adjusted		$ 4,255	$ 3,546

[1] Amounts may not add due to rounding.