Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 61	$ 51
Carrying value of derivative contracts – Other liabilities	6	46
Counterparty netting and collateral held	(737)	(154)
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	79,678	66,117
Fair value	1,358	599
Counterparty netting and collateral held	(1,297)	(548)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	71,464	62,525
Fair value	872	475
Other assets [Member] | Interest rate floors [Member]
Derivatives Fair Value [Line Items]
Notional	847	1,673
Fair value	1	2
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	7,367	1,919
Fair value	485	122
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	47,376	57,990
Fair value	828	1,453
Counterparty netting and collateral posted	(822)	(1,407)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	41,513	45,297
Fair value	628	277
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,863	12,663
Fair value	200	1,176
Other liabilities [Member] | Interest rate caps [Member]
Derivatives Fair Value [Line Items]
Notional		30
Fair value		0
Hedge accounting derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	119	271
Fair value	8	28
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate floors [Member]
Derivatives Fair Value [Line Items]
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	119	271
Fair value	8	28
Hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	0	93
Fair value	0	1
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swaps [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	93
Fair value	0	1
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate caps [Member]
Derivatives Fair Value [Line Items]
Notional		0
Fair value		0
Non-hedge accounting derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	79,559	65,846
Fair value	1,350	571
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	71,464	62,525
Fair value	872	475
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate floors [Member]
Derivatives Fair Value [Line Items]
Notional	847	1,673
Fair value	1	2
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	7,248	1,648
Fair value	477	94
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	47,376	57,897
Fair value	828	1,452
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	41,513	45,297
Fair value	628	277
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,863	12,570
Fair value	200	1,175
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate caps [Member]
Derivatives Fair Value [Line Items]
Notional		30
Fair value		0
Carrying value [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	61	51
Carrying value [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other liabilities	$ 6	$ 46