Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Other assets:
Notional	$ 26,646
Fair value	2,635
Carrying value of derivative contracts - Other assets	199	70
Collateral held	1,607	1,748
Collateral posted in excess of the fair value of derivative liabilities	58	23
Other liabilities:
Carrying value of derivative contracts - Other liabilities	50	67
Collateral posted	(198)	(196)
Collateral held in excess of the fair value of derivative assets	(6)	(7)
Other assets [Member]
Other assets:
Notional		29,426
Fair value		2,660
Counterparty netting and collateral	(2,436)	(2,590)
Carrying value of derivative contracts - Other assets	199	70
Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	16,721	16,269
Fair value	660	439
Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	9,925	13,157
Fair value	1,975	2,221
Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Other liabilities [Member]
Other liabilities:
Notional	55,649	52,741
Fair value	1,077	1,105
Counterparty netting and collateral	(1,027)	(1,038)
Carrying value of derivative contracts - Other liabilities	50	67
Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	52,926	51,175
Fair value	980	1,008
Other liabilities [Member] | Interest rate cap [Member]
Other liabilities:
Notional	50	50
Fair value	0	0
Other liabilities [Member] | Foreign currency swaps [Member]
Other liabilities:
Notional	2,581	1,424
Fair value	70	73
Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	92	92
Fair value	27	24
Hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	2,041	3,756
Fair value	745	831
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	465	465
Fair value	58	59
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	1,576	3,291
Fair value	687	772
Hedge accounting derivatives [Member] | Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	977	437
Fair value	26	29
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [Member]
Other liabilities:
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Other liabilities:
Notional	977	437
Fair value	26	29
Hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	0	0
Fair value	0	0
Non-hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	24,605	25,670
Fair value	1,890	1,829
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	16,256	15,804
Fair value	602	380
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	8,349	9,866
Fair value	1,288	1,449
Non-hedge accounting derivatives [Member] | Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	54,672	52,304
Fair value	1,051	1,076
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	52,926	51,175
Fair value	980	1,008
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [Member]
Other liabilities:
Notional	50	50
Fair value	0	0
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Other liabilities:
Notional	1,604	987
Fair value	44	44
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	92	92
Fair value	$ 27	$ 24