Intangible Assets - Summary of Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 9,338	$ 8,979
Accumulated Amortization	3,399	2,603
Net Carrying Value	5,939	6,376
Software
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	3,864	3,484
Accumulated Amortization	2,235	1,918
Net Carrying Value	1,629	1,566
Customer related intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	5,389	5,405
Accumulated Amortization	1,139	666
Net Carrying Value	4,250	4,739
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	85	90
Accumulated Amortization	25	19
Net Carrying Value	$ 60	$ 71