Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 61	$ 51
Carrying value of derivative contracts – Other liabilities	14	46
Counterparty netting and collateral held	(258)	(154)
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	73,442	66,117
Fair value	781	599
Counterparty netting and collateral held	(720)	(548)
Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	66,469	62,525
Fair value	503	475
Other assets [Member] | Interest rate floors [Member]
Derivatives Fair Value [Line Items]
Notional	1,673	1,673
Fair value	2	2
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,300	1,919
Fair value	276	122
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	51,732	57,990
Fair value	869	1,453
Counterparty netting and collateral posted	(855)	(1,407)
Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	42,817	45,297
Fair value	271	277
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	8,885	12,663
Fair value	598	1,176
Other liabilities [Member] | Interest rate caps [Member]
Derivatives Fair Value [Line Items]
Notional	30	30
Fair value	0	0
Hedge accounting derivatives [Member] | Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	271	271
Fair value	25	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	25	28
Hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	93	93
Fair value	2	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	2	1
Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate caps [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	73,171	65,846
Fair value	756	571
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	66,469	62,525
Fair value	503	475
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate floors [Member]
Derivatives Fair Value [Line Items]
Notional	1,673	1,673
Fair value	2	2
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	5,029	1,648
Fair value	251	94
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	51,639	57,897
Fair value	867	1,452
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swaps [Member]
Derivatives Fair Value [Line Items]
Notional	42,817	45,297
Fair value	271	277
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	8,792	12,570
Fair value	596	1,175
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate caps [Member]
Derivatives Fair Value [Line Items]
Notional	30	30
Fair value	0	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	$ 14	$ 46