Common stock (Detail) - shares	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Class A [Member]
Class of Stock [Line Items]
Shares issued	818,493	820,102
Treasury shares	(11,680)	(11,680)
Shares outstanding	806,813	808,422
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(1,609)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,256,678,655	1,253,866,598
Treasury shares	(1,409,762)	(1,409,762)
Shares outstanding	1,255,268,893	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	2,812,057