Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2016	Mar. 31, 2015
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 68	$ 53
Carrying value of derivative contracts – Other liabilities	7	90
Counterparty netting and collateral held	(320)	(145)
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	35,849	27,923
Fair value	973	688
Counterparty netting and collateral held	(905)	(635)
Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	29,469	26,739
Fair value	640	471
Other assets [Member] | Interest Rate Floor [Member]
Derivatives Fair Value [Line Items]
Notional	1,679
Fair value	4
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	4,701	1,184
Fair value	329	217
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	77,753	78,124
Fair value	1,310	2,274
Counterparty netting and collateral posted	(1,303)	(2,184)
Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	68,383	64,852
Fair value	475	386
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	9,340	13,222
Fair value	835	1,888
Other liabilities [Member] | Interest rate cap [Member]
Derivatives Fair Value [Line Items]
Notional	30	50
Fair value	0	0
Hedge accounting derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	364	461
Fair value	39	28
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	0	190
Fair value	0	4
Hedge accounting derivatives [Member] | Other assets [Member] | Interest Rate Floor [Member]
Derivatives Fair Value [Line Items]
Notional	0
Fair value	0
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	364	271
Fair value	39	24
Hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	0	251
Fair value	0	43
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	0	251
Fair value	0	43
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [Member]
Derivatives Fair Value [Line Items]
Notional	0	0
Fair value	0	0
Non-hedge accounting derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	35,485	27,462
Fair value	934	660
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	29,469	26,549
Fair value	640	467
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest Rate Floor [Member]
Derivatives Fair Value [Line Items]
Notional	1,679
Fair value	4
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	4,337	913
Fair value	290	193
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	77,753	77,873
Fair value	1,310	2,231
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	68,383	64,852
Fair value	475	386
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	9,340	12,971
Fair value	835	1,845
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [Member]
Derivatives Fair Value [Line Items]
Notional	30	50
Fair value	0	0
Carrying value [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	68	53
Carrying value [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other liabilities	$ 7	$ 90