Derivatives, Hedging Activities and Interest Expense - Derivative Activity Impact on Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2015	Mar. 31, 2015
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other assets	$ 53	$ 53
Carrying value of derivative contracts – Other liabilities	75	90
Counterparty netting and collateral held	(120)	(145)
Other assets [Member]
Derivatives Fair Value [Line Items]
Notional	45,348	27,923
Fair value	586	688
Counterparty netting and collateral held	(533)	(635)
Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	44,447	26,739
Fair value	423	471
Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	901	1,184
Fair value	163	217
Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	65,927	78,124
Fair value	2,106	2,274
Counterparty netting and collateral posted	(2,031)	(2,184)
Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	50,970	64,852
Fair value	235	386
Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	14,927	13,222
Fair value	1,871	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	271	461
Fair value	20	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] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	271	271
Fair value	20	24
Hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	251	251
Fair value	59	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	251	251
Fair value	59	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	45,077	27,462
Fair value	566	660
Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	44,447	26,549
Fair value	423	467
Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	630	913
Fair value	143	193
Non-hedge accounting derivatives [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Notional	65,676	77,873
Fair value	2,047	2,231
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]
Derivatives Fair Value [Line Items]
Notional	50,970	64,852
Fair value	235	386
Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]
Derivatives Fair Value [Line Items]
Notional	14,676	12,971
Fair value	1,812	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	53	53
Carrying value [Member] | Other liabilities [Member]
Derivatives Fair Value [Line Items]
Carrying value of derivative contracts – Other liabilities	$ 75	$ 90