Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Other assets:
Carrying value of derivative contracts - Other assets	$ 47	$ 58
Collateral held	579	953
Collateral posted in excess of the fair value of derivative liabilities	0	6
Other liabilities:
Carrying value of derivative contracts - Other liabilities	65	17
Collateral posted	371	184
Collateral held in excess of the fair value of derivative assets	22	3
Other assets [Member]
Other assets:
Notional	31,459	31,545
Fair value	1,198	1,719
Counterparty netting and collateral	(1,151)	(1,661)
Carrying value of derivative contracts - Other assets	47	58
Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	25,426	22,801
Fair value	453	580
Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	6,021	8,744
Fair value	745	1,139
Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Other assets [Member] | Interest rate forwards [Member]
Other assets:
Notional	12
Fair value	0
Other liabilities [Member]
Other liabilities:
Notional	58,297	55,349
Fair value	1,008	909
Counterparty netting and collateral	(943)	(892)
Carrying value of derivative contracts - Other liabilities	65	17
Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	52,022	51,342
Fair value	702	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	6,173	3,893
Fair value	299	131
Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	40	64
Fair value	7	12
Other liabilities [Member] | Interest rate forwards [Member]
Other liabilities:
Notional	12
Fair value	0
Hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	1,647	1,711
Fair value	501	535
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	465	465
Fair value	40	44
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	1,182	1,246
Fair value	461	491
Hedge accounting derivatives [Member] | Other assets [Member] | Embedded derivatives
Other assets:
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate forwards [Member]
Other assets:
Notional	0
Fair value	0
Hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	854	790
Fair value	23	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	854	790
Fair value	23	29
Hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate forwards [Member]
Other liabilities:
Notional	0
Fair value	0
Non-hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	29,812	29,834
Fair value	697	1,184
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	24,961	22,336
Fair value	413	536
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	4,839	7,498
Fair value	284	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 assets [Member] | Interest rate forwards [Member]
Other assets:
Notional	12
Fair value	0
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	57,443	54,559
Fair value	985	880
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	52,022	51,342
Fair value	702	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	5,319	3,103
Fair value	276	102
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives
Other liabilities:
Notional	40	64
Fair value	7	12
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate forwards [Member]
Other liabilities:
Notional	12
Fair value	$ 0