Intangible Assets - Summary of Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 10,820	$ 10,548
Accumulated Amortization	4,050	3,369
Net Carrying Value	6,770	7,179
Software
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	3,695	3,484
Accumulated Amortization	2,070	1,918
Net Carrying Value	1,625	1,566
Transition and transformation contract costs
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	1,721	1,569
Accumulated Amortization	941	766
Net Carrying Value	780	803
Customer related intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	5,333	5,405
Accumulated Amortization	1,016	666
Net Carrying Value	4,317	4,739
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	71	90
Accumulated Amortization	23	19
Net Carrying Value	$ 48	$ 71