Common stock (Detail)	9 Months Ended
	Sep. 30, 2020 shares
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	719,307
Treasury shares, beginning balance	(17,337)
Shares outstanding, beginning balance	701,970
Conversions of Class A common stock to Class B common stock	(39,694)
Treasury stock acquired	(11,330)
Shares issued, ending balance	679,613
Treasury shares, ending balance	(28,667)
Shares outstanding, ending balance	650,946
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	1,408,183,852
Treasury shares, beginning balance	(23,702,319)
Shares outstanding, beginning balance	1,384,481,533
Conversions of Class A common stock to Class B common stock	59,541,000
Treasury stock acquired	(64,487,462)
Shares issued, ending balance	1,467,724,852
Treasury shares, ending balance	(88,189,781)
Shares outstanding, ending balance	1,379,535,071