Common stock (Detail) - shares	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Class A [Member]
Class of Stock [Line Items]
Shares issued	745,905	762,755
Treasury shares	(11,905)	(11,680)
Shares outstanding	734,000	751,075
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(16,850)
Treasury stock acquired	(225)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,367,952,812	1,342,066,749
Treasury shares	(5,548,954)	(1,409,762)
Shares outstanding	1,362,403,858	1,340,656,987
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	25,886,063
Treasury stock acquired	(4,139,192)