Common stock (Detail) - shares	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Class A [Member]
Class of Stock [Line Items]
Shares issued	797,474	820,102
Treasury shares	(11,680)	(11,680)
Shares outstanding	785,794	808,422
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(22,628)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,288,765,809	1,253,866,598
Treasury shares	(1,409,762)	(1,409,762)
Shares outstanding	1,287,356,047	1,252,456,836
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	34,899,211