Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019		Jun. 30, 2018	Mar. 31, 2018
Financial Instruments [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 3,533	$ 4,708	[1]	$ 3,876	$ 4,258

[1] Derived from audited Consolidated Financial Statements.