Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Income Statement) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Hedging Activities and Interest Expense [Abstract]
Interest expense on debt	$ 304	$ 325	$ 923	$ 963
Interest expense on hedge accounting derivatives	(10)	(18)	(35)	(67)
Interest expense on non-hedge accounting foreign currency swaps	(37)	(54)	(129)	(159)
Interest expense on non-hedge accounting interest rate swaps	30	39	94	166
Interest expense on debt and derivatives	287	292	853	903
Derivative Instruments, Loss (Gain) [Line Items]
Total interest expense	161	386	506	1,236
Interest expense [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss on hedge accounting derivatives	55	37	128	31
Less hedged item: change in fair value of fixed rate debt	(56)	(38)	(129)	(34)
Ineffectiveness related to hedge accounting derivatives	(1)	(1)	(1)	(3)
Gain on non-hedge accounting foreign currency transactions	(567)	(87)	(1,354)	(185)
Total interest expense	161	386	506	1,236
Interest expense [Member] | Interest rate swap [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss on hedge accounting derivatives	5	5	15	15
(Gain) loss on non-hedge accounting interest rate swaps	(100)	29	(245)	137
Interest expense [Member] | Foreign currency swaps [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss on hedge accounting derivatives	50	32	113	16
Loss (gain) on non-hedge accounting foreign currency swaps	$ 542	$ 153	$ 1,253	$ 384