Common stock (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class A [Member]
Class of Stock [Line Items]
Shares issued	742,213	762,755	788,058	820,102
Treasury shares	(12,897)	(11,680)	(11,680)	(11,680)
Shares outstanding	729,316	751,075	776,378	808,422
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(20,542)	(25,303)	(32,044)
Treasury stock acquired	(1,217)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,373,558,983	1,342,066,749	1,303,323,927	1,253,866,598
Treasury shares	(6,138,909)	(1,409,762)	(1,409,762)	(1,409,762)
Shares outstanding	1,367,420,074	1,340,656,987	1,301,914,165	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	31,492,234	38,742,822	49,457,329
Treasury stock acquired	(4,729,147)