Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018		Sep. 30, 2017	Mar. 31, 2017
Financial Instruments
Cash and Cash Equivalents, at Carrying Value	$ 2,881	$ 4,258	[1]	$ 2,067	$ 2,565

[1] Derived from audited Consolidated Financial Statements.