Earnings Per Share (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share
Number of common shares outstanding at year-end	433.3	439.3	447.2
Effect of using weighted average common shares outstanding	5.5	4.2	5.5
Weighted average basic common shares outstanding	438.8	443.5	452.7
Dilutive effect of equity-based compensation awards and other contingently issuable shares	3.1	3.0	3.2
Weighted average diluted common shares outstanding	441.9	446.5	455.9
Potentially issuable shares	8.1	9.8	10.2
Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding	1.9	1.0	2.0