Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Income Statement) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Derivatives, Hedging Activities and Interest Expense [Abstract]
Interest expense on debt	$ 318	$ 337
Interest expense on hedge accounting derivatives	(24)	(27)
Interest expense on non-hedge accounting foreign currency swaps	(51)	(67)
Interest expense on non-hedge accounting interest rate swaps	66	103
Interest expense on debt and derivatives	309	346
Derivative Instruments, Loss (Gain) [Line Items]
Total interest expense	536	58
Relative fair value allocation of derivative credit valuation adjustments within interest expense
(Gain) loss related to hedge accounting derivatives	0	1
(Gain) Loss on non-hedge accounting foreign currency swaps	0	4
Loss on non-hedge accounting interest rate swaps	0	1
Total credit valuation adjustment allocated to interest expense	0	6
Interest expense [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss (gain) on hedge accounting derivatives	50	122
Less hedged item: change in fair value of fixed rate debt	(51)	(125)
Ineffectiveness related to hedge accounting derivatives	(1)	(3)
(Gain) loss on foreign currency transactions	(450)	(44)
Total interest expense	536	58
Interest expense [Member] | Interest rate swap [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss (gain) on hedge accounting derivatives	6	3
(Gain) on interest rate swaps	112	(209)
Interest expense [Member] | Foreign currency swaps [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss (gain) on hedge accounting derivatives	44	119
Loss (gain) on foreign currency swaps	$ 566	$ (32)