Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Income Statement) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Derivatives, Hedging Activities and Interest Expense [Abstract]
Interest expense on debt									$ 1,330	$ 1,677	$ 1,943
Interest expense on hedge accounting derivatives									(103)	(221)	(449)
Interest expense on non-hedge accounting foreign currency swaps									(258)	(386)	(379)
Interest expense on non-hedge accounting interest rate swaps									359	606	807
Interest expense on debt and derivatives									1,328	1,676	1,922
Derivative Instruments, Loss (Gain) [Line Items]
Total interest expense	315	284	283	58	531	163	149	457	940	1,300	1,614
Relative fair value allocation of derivative credit valuation adjustments within interest expense
(Gain) loss related to hedge accounting derivatives									(2)	(3)	(2)
(Gain) Loss on non-hedge accounting foreign currency swaps									0	(6)	5
Loss on non-hedge accounting interest rate swaps									0	0	2
Total credit valuation adjustment allocated to interest expense									(2)	(9)	5
Interest expense [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss (gain) on hedge accounting derivatives									289	17	(834)
Less hedged item: change in fair value of fixed rate debt									(299)	(38)	801
Ineffectiveness related to hedge accounting derivatives									(10)	(21)	(33)
(Gain) loss on foreign currency transactions									(430)	(182)	1,494
Total interest expense									940	1,300	1,614
Interest expense [Member] | Interest rate swap [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss (gain) on hedge accounting derivatives									15	(6)	(2)
(Gain) on interest rate swaps									(379)	(89)	(174)
Interest expense [Member] | Foreign currency swaps [Member]
Derivative Instruments, Loss (Gain) [Line Items]
Loss (gain) on hedge accounting derivatives									274	23	(832)
Loss (gain) on foreign currency swaps									$ 431	$ (84)	$ (1,595)