Common stock (Detail) - shares	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Class A [Member]
Class of Stock [Line Items]
Shares issued	779,934	788,058
Treasury shares	(11,680)	(11,680)
Shares outstanding	768,254	776,378
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(8,124)
Class B [Member]
Class of Stock [Line Items]
Shares issued	1,315,933,675	1,303,323,927
Treasury shares	(1,409,762)	(1,409,762)
Shares outstanding	1,314,523,913	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	12,609,748