Capital Stock (Summary of Share Repurchases) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	3 Months Ended				6 Months Ended
	Jul. 17, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Equity, Class of Treasury Stock [Line Items]
Shares repurchased						2.8	3.4
Average price per share						$ 273.84	$ 231.61
Aggregate cost		$ 458	$ 294	$ 401	$ 394	$ 752	$ 795
Authorization remaining at the end of the period		$ 4,741		$ 6,383		$ 4,741	$ 6,383
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased	0.3
Average price per share	$ 286.03
Aggregate cost	$ 72
Authorization remaining at the end of the period	$ 4,669