Intangible Assets - Summary of Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 8,564	$ 10,078
Accumulated Amortization	4,545	4,347
Net Carrying Value	4,019	5,731
Software
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	4,001	4,048
Accumulated Amortization	2,891	2,614
Net Carrying Value	1,110	1,434
Customer related intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	4,296	5,795
Accumulated Amortization	1,606	1,697
Net Carrying Value	2,690	4,098
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	267	235
Accumulated Amortization	48	36
Net Carrying Value	$ 219	$ 199