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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivatives, Hedging Activities and Interest Expense [Abstract]
Interest expense on debt	$ 347	$ 405	$ 684	$ 886
Interest expense on hedge accounting derivatives	(27)	(54)	(54)	(135)
Interest expense on non-hedge accounting foreign currency swaps	(67)	(87)	(134)	(219)
Interest expense on non-hedge accounting interest rate swaps	95	173	198	353
Interest expense on debt and derivatives	348	437	694	885
Derivative Instruments, Gain (Loss) [Line Items]
Total interest expense	283	149	341	606
Relative fair value allocation of derivative credit valuation adjustments within interest expense
(Gain) loss related to hedge accounting derivatives	(2)	3	(1)	5
(Gain) Loss on non-hedge accounting foreign currency swaps	(4)	2	0	7
(Gain) Loss on non-hedge accounting interest rate swaps	(1)	1	0	2
Total credit valuation adjustment allocated to interest expense	(7)	6	(1)	14
Interest expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss on hedge accounting derivatives	(6)	299	116	(108)
Less hedged item: change in fair value of fixed rate debt	3	(302)	(122)	101
Ineffectiveness related to hedge accounting derivatives	(3)	(3)	(6)	(7)
Loss (gain) on foreign currency transactions	196	(999)	152	(295)
Total interest expense	283	149	341	606
Interest expense [Member] | Interest rate swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss on hedge accounting derivatives	2	(7)	5	(14)
Loss (gain) on other non-hedge accounting derivatives	(52)	(66)	(261)	52
Interest expense [Member] | Foreign currency swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss on hedge accounting derivatives	(8)	306	111	(94)
Loss (gain) on other non-hedge accounting derivatives	$ (206)	$ 780	$ (238)	$ (29)