Common stock (Detail)	9 Months Ended
	Sep. 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	(18,581)
Treasury stock acquired	(1,752)
Shares issued, ending balance	723,632
Treasury shares, ending balance	(14,649)
Shares outstanding, ending balance	708,983
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	28,137,369
Treasury stock acquired	(11,454,833)
Shares issued, ending balance	1,401,696,352
Treasury shares, ending balance	(17,593,742)
Shares outstanding, ending balance	1,384,102,610