Summary of Significant Accounting Policies (Property Plant and Equipment and Internal Use Software Policy) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property, Plant and Equipment [Line Items]
Capitalized internal-use software	$ 56	$ 37
Minimum [Member]
Property, Plant and Equipment [Line Items]
Finite-Lived Intangible Asset, Useful Life	1 year
Maximum [Member]
Property, Plant and Equipment [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Buildings | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Buildings | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	25 years
Computer equipment and software | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Computer equipment and software | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	6 years
Equipment, Furniture and Fixtures, and Other [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Equipment, Furniture and Fixtures, and Other [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Leasehold Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	1 year
Leasehold Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Computer Software, Intangible Asset [Member]
Property, Plant and Equipment [Line Items]
Capitalized Internal-use Software, Estimated Useful Life	3 years