Additional Financial Information (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash and Cash Equivalents	$ 5,726	$ 6,610	$ 4,871	$ 2,942
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	5,773	6,659	$ 4,871	$ 2,942
Checks Issued In Excess Of Associated Bank Balances But Not Yet Presented For Collection	77	66
Prepaid Expenses and Other Current Assets [Member]
Cash and Cash Equivalents [Line Items]
Restricted Cash, Current	1	0
Other Assets [Member]
Cash and Cash Equivalents [Line Items]
Restricted Cash, Current	$ 46	$ 49