Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Mar. 31, 2014
Other assets:
Carrying value of derivative contracts - Other assets	$ 53	$ 49
Collateral held	242	718
Collateral posted in excess of the fair value of derivative liabilities	2	3
Other liabilities:
Carrying value of derivative contracts - Other liabilities	26	6
Collateral posted	751	331
Collateral held in excess of the fair value of derivative assets	12	5
Other assets [Member]
Other assets:
Notional	36,427	34,633
Fair value	644	1,235
Counterparty netting and collateral	(591)	(1,186)
Carrying value of derivative contracts - Other assets	53	49
Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	34,721	26,407
Fair value	402	361
Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	1,706	8,226
Fair value	242	874
Other liabilities [Member]
Other liabilities:
Notional	67,035	61,718
Fair value	1,126	805
Counterparty netting and collateral	(1,100)	(799)
Carrying value of derivative contracts - Other liabilities	26	6
Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	56,343	57,689
Fair value	324	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	10,642	3,979
Fair value	802	252
Hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	736	1,317
Fair value	39	367
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	465	465
Fair value	11	25
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	271	852
Fair value	28	342
Hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	251	157
Fair value	33	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	251	157
Fair value	33	14
Non-hedge accounting derivatives [Member] | Other assets [Member]
Other assets:
Notional	35,691	33,316
Fair value	605	868
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Other assets:
Notional	34,256	25,942
Fair value	391	336
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Other assets:
Notional	1,435	7,374
Fair value	214	532
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Other liabilities:
Notional	66,784	61,561
Fair value	1,093	791
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Other liabilities:
Notional	56,343	57,689
Fair value	324	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	10,391	3,822
Fair value	$ 769	$ 238