Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017		Dec. 31, 2016	Mar. 31, 2016
Financial Instruments
Cash and Cash Equivalents, at Carrying Value	$ 2,566	$ 2,565	[1]	$ 2,483	$ 2,493

[1] Derived from audited Consolidated Financial Statements.