Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 78	$ 51
Carrying value of derivative contracts – Other liabilities	0	46
Counterparty netting and collateral held	(465)	(154)
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	82,174	66,117
Fair value	940	599
Counterparty netting and collateral held	(862)	(548)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	73,683	62,525
Fair value	563	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	7,644	1,919
Fair value	376	122
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	43,675	57,990
Fair value	575	1,453
Counterparty netting and collateral posted	(575)	(1,407)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	37,809	45,297
Fair value	290	277
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,866	12,663
Fair value	285	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	271	271
Fair value	19	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	271	271
Fair value	19	28
Hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	93	93
Fair value	3	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	93	93
Fair value	3	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	81,903	65,846
Fair value	921	571
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	73,683	62,525
Fair value	563	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	7,373	1,648
Fair value	357	94
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	43,582	57,897
Fair value	572	1,452
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	37,809	45,297
Fair value	290	277
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,773	12,570
Fair value	282	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	78	51
Carrying value [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other liabilities	$ 0	$ 46