Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt	$ 484	$ 377	$ 951	$ 736
Interest income on derivatives	(18)	(5)	(34)	(7)
Interest expense on debt and derivatives, net	466	372	917	729
Total interest expense	452	297	900	604
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
Loss (gain) on non-hedge accounting debt denominated in foreign currencies	373	156	1,023	(62)
(Gain) loss on non-hedge accounting foreign currency swaps	(383)	(156)	(1,033)	32
Gain on U.S. dollar non-hedge accounting interest rate swaps	(4)	(75)	(7)	(95)
Hedge accounting derivatives [Member] | Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
Loss (gain) on hedge accounting derivatives	3	(4)	10	(37)
Less hedged item: change in fair value of fixed rate debt denominated in a foreign currency	(3)	4	(10)	37
Ineffectiveness related to hedge accounting derivatives	0	0	0	0
Hedge accounting derivatives [Member] | Interest expense [Member] | Foreign currency swaps [Member]
Derivative Instruments Gain Loss [Line Items]
Loss (gain) on hedge accounting derivatives	$ 3	$ (4)	$ 10	$ (37)