Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Other assets:
Carrying value of derivative contracts - Other assets	$ 49	$ 70
Collateral held	1,640	1,748
Other liabilities:
Carrying value of derivative contracts - Other liabilities	42	67
Collateral posted	(96)	(196)
Collateral held in excess of the fair value of derivative assets	130
Other assets [Member]
Other assets:
Notional	28,155	29,426
Fair value	2,643	2,660
Counterparty netting and collateral	(2,594)	(2,590)
Carrying value of derivative contracts - Other assets	49	70
Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	18,558	16,269
Fair value	715	439
Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	9,597	13,157
Fair value	1,928	2,221
Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Other liabilities [Member]
Other liabilities:
Notional	51,795	52,741
Fair value	1,092	1,105
Counterparty netting and collateral	(1,050)	(1,038)
Carrying value of derivative contracts - Other liabilities	42	67
Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	50,535	51,175
Fair value	1,018	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	1,131	1,424
Fair value	51	73
Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	79	92
Fair value	23	24
Hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	1,724	3,756
Fair value	693	831
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	465	465
Fair value	54	59
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	1,259	3,291
Fair value	639	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	900	437
Fair value	39	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	900	437
Fair value	39	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	26,431	25,670
Fair value	1,950	1,829
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	18,093	15,804
Fair value	661	380
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	8,338	9,866
Fair value	1,289	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	50,895	52,304
Fair value	1,053	1,076
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	50,535	51,175
Fair value	1,018	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	231	987
Fair value	12	44
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	79	92
Fair value	$ 23	$ 24