Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Mar. 31, 2013
Other assets:
Carrying value of derivative contracts - Other assets	$ 62	$ 58
Collateral held	781	953
Collateral posted in excess of the fair value of derivative liabilities	5	6
Other liabilities:
Carrying value of derivative contracts - Other liabilities	10	17
Collateral posted	337	184
Collateral held in excess of the fair value of derivative assets	1	3
Other assets [Member]
Other assets:
Notional	29,114	31,545
Fair value	1,357	1,719
Counterparty netting and collateral	(1,295)	(1,661)
Carrying value of derivative contracts - Other assets	62	58
Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	20,501	22,801
Fair value	406	580
Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	8,613	8,744
Fair value	951	1,139
Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Other liabilities [Member]
Other liabilities:
Notional	62,005	55,349
Fair value	861	909
Counterparty netting and collateral	(851)	(892)
Carrying value of derivative contracts - Other liabilities	10	17
Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	57,888	51,342
Fair value	688	766
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	4,027	3,893
Fair value	164	131
Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	40	64
Fair value	9	12
Hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	1,647	1,711
Fair value	526	535
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	465	465
Fair value	35	44
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	1,182	1,246
Fair value	491	491
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	157	790
Fair value	11	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	157	790
Fair value	11	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	27,467	29,834
Fair value	831	1,184
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	20,036	22,336
Fair value	371	536
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	7,431	7,498
Fair value	460	648
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	61,848	54,559
Fair value	850	880
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	57,888	51,342
Fair value	688	766
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	3,870	3,103
Fair value	153	102
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	40	64
Fair value	9	12
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate forwards [Member]
Other liabilities:
Fair value	$ 0