Common stock (Detail) - shares	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Class A [Member]
Class of Stock [Line Items]
Shares issued	766,364	788,058
Treasury shares	(11,680)	(11,680)
Shares outstanding	754,684	776,378
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(21,694)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,336,458,340	1,303,323,927
Treasury shares	(1,409,762)	(1,409,762)
Shares outstanding	1,335,048,578	1,301,914,165
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	33,134,413