Description Of Business And Summary Of Significant Accounting Policies Accounting Standard Cash Flow Reclassification (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payments Related to Tax Withholding for Share-based Compensation	$ 120	$ 130	$ 156
Increase to net cash provided by Operating Activities and net cash used in Financing Activities	163	195	242
Increase to net cash provided by Operating Activities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities		65	86
Increase to net cash provided by Operating Activities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payments Related to Tax Withholding for Share-based Compensation	120	$ 130	$ 156
Provision for income taxes [Member] | Increase to net cash provided by Operating Activities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities	$ 43