Common stock (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class A [Member]
Class of Stock [Line Items]
Shares issued	762,755	788,058	820,102	838,019
Treasury shares	(11,680)	(11,680)	(11,680)	(11,680)
Shares outstanding	751,075	776,378	808,422	826,339
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(25,303)	(32,044)	(17,917)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,342,066,749	1,303,323,927	1,253,866,598	1,226,265,250
Treasury shares	(1,409,762)	(1,409,762)	(1,409,762)	(1,409,762)
Shares outstanding	1,340,656,987	1,301,914,165	1,252,456,836	1,224,855,488
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	38,742,822	49,457,329	27,601,348