Common stock (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	742,213	762,755	788,058
Treasury shares, beginning balance	(12,897)	(11,680)	(11,680)
Shares outstanding, beginning balance	729,316	751,075	776,378
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options	(22,906)	(20,542)	(25,303)
Treasury stock acquired	(4,440)	(1,217)
Shares issued, ending balance	719,307	742,213	762,755
Treasury shares, ending balance	(17,337)	(12,897)	(11,680)
Shares outstanding, ending balance	701,970	729,316	751,075
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	1,373,558,983	1,342,066,749	1,303,323,927
Treasury shares, beginning balance	(6,138,909)	(1,409,762)	(1,409,762)
Shares outstanding, beginning balance	1,367,420,074	1,340,656,987	1,301,914,165
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options	34,624,869	31,492,234	38,742,822
Treasury stock acquired	(17,563,410)	(4,729,147)
Shares issued, ending balance	1,408,183,852	1,373,558,983	1,342,066,749
Treasury shares, ending balance	(23,702,319)	(6,138,909)	(1,409,762)
Shares outstanding, ending balance	1,384,481,533	1,367,420,074	1,340,656,987