Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Mar. 31, 2014
Other assets:
Carrying value of derivative contracts - Other assets	$ 49	$ 49
Collateral held	343	718
Collateral posted in excess of the fair value of derivative liabilities	7	3
Other liabilities:
Carrying value of derivative contracts - Other liabilities	22	6
Collateral posted	497	331
Collateral held in excess of the fair value of derivative assets	16	5
Other assets [Member]
Other assets:
Notional	40,304	34,633
Fair value	756	1,235
Counterparty netting and collateral	(707)	(1,186)
Carrying value of derivative contracts - Other assets	49	49
Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	35,736	26,407
Fair value	336	361
Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	4,568	8,226
Fair value	420	874
Other liabilities [Member]
Other liabilities:
Notional	62,011	61,718
Fair value	883	805
Counterparty netting and collateral	(861)	(799)
Carrying value of derivative contracts - Other liabilities	22	6
Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	53,863	57,689
Fair value	366	553
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	8,098	3,979
Fair value	517	252
Hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	829	1,317
Fair value	76	367
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	465	465
Fair value	15	25
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	364	852
Fair value	61	342
Hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	157	157
Fair value	20	14
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	157
Fair value	20	14
Non-hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	39,475	33,316
Fair value	680	868
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	35,271	25,942
Fair value	321	336
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	4,204	7,374
Fair value	359	532
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	61,854	61,561
Fair value	863	791
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	53,863	57,689
Fair value	366	553
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	7,941	3,822
Fair value	$ 497	$ 238