Common stock (Detail)	6 Months Ended
	Jun. 30, 2019 shares
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	742,213
Treasury shares, beginning balance	(12,897)
Shares outstanding, beginning balance	729,316
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options	(5,919)
Treasury stock acquired	(1,539)
Shares issued, ending balance	736,294
Treasury shares, ending balance	(14,436)
Shares outstanding, ending balance	721,858
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning balance	1,373,558,983
Treasury shares, beginning balance	(6,138,909)
Shares outstanding, beginning balance	1,367,420,074
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options	9,144,369
Treasury stock acquired	(8,285,970)
Shares issued, ending balance	1,382,703,352
Treasury shares, ending balance	(14,424,879)
Shares outstanding, ending balance	1,368,278,473