Financial Instruments - Cumulative Basis Adjustments for Fair Value Hedges (Details) $ in Millions	Mar. 31, 2018 USD ($)
Short-term debt
Amounts recorded in the Consolidated Statement of Financial Position related to cumulative basis adjustments for fair value hedges
Carrying Amount of the Hedged Item Assets/(Liabilities)	$ (744)
Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of Assets/(Liabilities)	5
Long-term debt
Amounts recorded in the Consolidated Statement of Financial Position related to cumulative basis adjustments for fair value hedges
Carrying Amount of the Hedged Item Assets/(Liabilities)	(9,503)
Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of Assets/(Liabilities)	(348)
Hedging adjustments on discontinued hedging relationships	$ (190)