Common stock (Detail)	9 Months Ended
	Sep. 30, 2021 shares
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	678,523
Treasury shares, beginning balance	(34,592)
Shares outstanding, beginning balance	643,931
Conversions of Class A to Class B common stock	(11,849)
Treasury stock acquired	(11,285)
Shares issued, ending balance	666,674
Treasury shares, ending balance	(45,877)
Shares outstanding, ending balance	620,797
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	1,469,359,852
Treasury shares, beginning balance	(119,316,381)
Shares outstanding, beginning balance	1,350,043,471
Conversions of Class A to Class B common stock	17,773,500
Treasury stock acquired	(59,366,771)
Shares issued, ending balance	1,487,133,352
Treasury shares, ending balance	(178,683,152)
Shares outstanding, ending balance	1,308,450,200