Earnings Per Share (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share
Number of common shares outstanding at end of period	434.2	442.0	434.2	442.0
Effect of using weighted average common shares outstanding	3.6	0.9	6.1	2.3
Weighted average basic common shares outstanding	437.8	442.9	440.3	444.3
Dilutive effect of equity-based compensation awards and other contingently issuable shares	3.0	2.8	3.0	2.8
Weighted average diluted common shares outstanding	440.8	445.7	443.3	447.1
Potentially issuable shares	8.2	9.2	8.2	9.2
Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding	1.9	0.5	2.3	0.5