Capital Stock (Summary of Cash Dividend Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Capital [Abstract]
Declaration date	Jul. 24, 2018	Apr. 24, 2018	Jan. 30, 2018	Jul. 25, 2017	Apr. 27, 2017	Feb. 22, 2017
Record date	Sep. 10, 2018	Jun. 08, 2018	Mar. 09, 2018	Sep. 08, 2017	Jun. 09, 2017	Mar. 10, 2017
Payment date	Sep. 25, 2018	Jun. 25, 2018	Mar. 23, 2018	Sep. 25, 2017	Jun. 23, 2017	Mar. 24, 2017
Cash dividends per share	$ 0.75	$ 0.75	$ 0.75	$ 0.70	$ 0.65	$ 0.65
Total payment	$ 195	$ 196	$ 192	$ 181	$ 172	$ 172	$ 583	$ 525