Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Income Statement) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivatives, Hedging Activities and Interest Expense [Abstract]
Interest expense on debt	$ 320	$ 347	$ 638	$ 684
Interest expense on hedge accounting derivatives	(25)	(27)	(49)	(54)
Interest expense on non-hedge accounting foreign currency swaps	(54)	(67)	(105)	(134)
Interest expense on non-hedge accounting interest rate swaps	61	95	127	198
Interest expense on debt and derivatives	302	348	611	694
Derivative Instruments, Loss (Gain) [Line Items]
Total interest expense	314	283	850	341
Relative fair value allocation of derivative credit valuation adjustments within interest expense
(Gain) loss related to hedge accounting derivatives	0	(2)	0	(1)
(Gain) Loss on non-hedge accounting foreign currency swaps	0	(4)	0	0
(Gain) loss on non-hedge accounting interest rate swaps	0	(1)	0	0
Total credit valuation adjustment allocated to interest expense	0	(7)	0	(1)
Interest expense [Member]
Derivative Instruments, Loss (Gain) [Line Items]
(Gain) loss on hedge accounting derivatives	(56)	(6)	(6)	116
Less hedged item: change in fair value of fixed rate debt	55	3	4	(122)
Ineffectiveness related to hedge accounting derivatives	(1)	(3)	(2)	(6)
(Gain) loss on foreign currency transactions	352	196	(98)	152
Total interest expense	314	283	850	341
Interest expense [Member] | Interest rate swap [Member]
Derivative Instruments, Loss (Gain) [Line Items]
(Gain) loss on hedge accounting derivatives	4	2	10	5
(Gain) loss on interest rate swaps	(4)	(52)	108	(261)
Interest expense [Member] | Foreign currency swaps [Member]
Derivative Instruments, Loss (Gain) [Line Items]
(Gain) loss on hedge accounting derivatives	(60)	(8)	(16)	111
(Gain) loss on foreign currency swaps	$ (335)	$ (206)	$ 231	$ (238)