Common stock (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	719,307	742,213	762,755
Treasury shares, beginning balance	(17,337)	(12,897)	(11,680)
Shares outstanding, beginning balance	701,970	729,316	751,075
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options	(40,784)	(22,906)	(20,542)
Treasury stock acquired	(17,255)	(4,440)	(1,217)
Shares issued, ending balance	678,523	719,307	742,213
Treasury shares, ending balance	(34,592)	(17,337)	(12,897)
Shares outstanding, ending balance	643,931	701,970	729,316
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	1,408,183,852	1,373,558,983	1,342,066,749
Treasury shares, beginning balance	(23,702,319)	(6,138,909)	(1,409,762)
Shares outstanding, beginning balance	1,384,481,533	1,367,420,074	1,340,656,987
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options	61,176,000	34,624,869	31,492,234
Treasury stock acquired	(95,614,062)	(17,563,410)	(4,729,147)
Shares issued, ending balance	1,469,359,852	1,408,183,852	1,373,558,983
Treasury shares, ending balance	(119,316,381)	(23,702,319)	(6,138,909)
Shares outstanding, ending balance	1,350,043,471	1,384,481,533	1,367,420,074