Cash And Cash Equivalents (Summary Of Cash And Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash	$ 265	$ 254
Marketable securities	1,117	1,169
Total	$ 1,382	$ 1,423	$ 1,597	$ 15