Capital Stock (Summary of Share Repurchases) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	9 Months Ended
	Oct. 18, 2018	Sep. 30, 2018	Sep. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Shares repurchased		5.0	8.7
Average price per share		$ 240.15	$ 186.80
Aggregate cost		$ 1,192	$ 1,635
Authorization remaining at the end of the period		$ 5,986	$ 2,541
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased	0.3
Average price per share	$ 274.32
Aggregate cost	$ 95
Authorization remaining at the end of the period	$ 5,891