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Revenue from Contracts with Customers Revenue from Contracts with Customers - Narrative (Details)
$ in Millions
12 Months Ended
Mar. 31, 2019
USD ($)
segments
Mar. 31, 2021
USD ($)
Mar. 31, 2020
USD ($)
Disaggregation of Revenue [Line Items]      
Number of Reportable Segments | segments 2    
Revenue from Contract with Customer, Excluding Assessed Tax $ 33,600    
Revenue, Remaining Performance Obligation, Amount $ 9,168    
Scenario, Forecast [Member]      
Disaggregation of Revenue [Line Items]      
Revenue, Remaining Performance Obligation, Amount   $ 340 $ 3,044