Income Taxes (Summary Of Activity In Unrecognized Tax Benefits) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
January 1 balance	$ 19	$ 7	$ 12
Additions for tax positions of prior years	0	13	6
Reductions for tax positions of prior years	(5)	0	(10)
Settlements, decrease	0	(1)	(1)
Statute of limitations	(12)	0	0
Acquired from Andeavor	209	0	0
December 31 balance	$ 211	$ 19	$ 7