Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2016	Mar. 31, 2016
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 51	$ 68
Counterparty netting and collateral held	(111)	(320)
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	61,740	35,849
Fair value	567	973
Counterparty netting and collateral held	(516)	(905)
Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	59,033	29,469
Fair value	508	640
Other assets [Member] | Interest rate floor [Member]
Derivatives Fair Value [Line Items]
Notional	1,673	1,679
Fair value	2	4
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	1,034	4,701
Fair value	57	329
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	59,258	77,753
Fair value	1,735	1,310
Counterparty netting and collateral posted	(1,734)	(1,303)
Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	45,235	68,383
Fair value	305	475
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	13,993	9,340
Fair value	1,430	835
Other liabilities [Member] | Interest rate cap [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	364
Fair value	19	39
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	0	0
Fair value	0	0
Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate floor [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	364
Fair value	19	39
Hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	93	0
Fair value	4	0
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	93	0
Fair value	4	0
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	61,469	35,485
Fair value	548	934
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	59,033	29,469
Fair value	508	640
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate floor [Member]
Derivatives Fair Value [Line Items]
Notional	1,673	1,679
Fair value	2	4
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	763	4,337
Fair value	38	290
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	59,165	77,753
Fair value	1,731	1,310
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	45,235	68,383
Fair value	305	475
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	13,900	9,340
Fair value	1,426	835
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [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	51	68
Carrying value [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other liabilities	$ 1	$ 7