Cash And Cash Equivalents (Summary Of Cash And Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash	$ 259	$ 254
Marketable securities	680	1,169
Other cash equivalents	50
Total cash and cash equivalents	$ 989	$ 1,423	$ 1,474	$ 15