Common stock (Detail) - shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Class A [Member]        
Class of Stock [Line Items]        
Shares issued 742,213 762,755 788,058 820,102
Treasury shares (12,897) (11,680) (11,680) (11,680)
Shares outstanding 729,316 751,075 776,378 808,422
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition (20,542) (25,303) (32,044)  
Treasury stock acquired (1,217)      
Class B [Member]        
Class of Stock [Line Items]        
Shares issued 1,373,558,983 1,342,066,749 1,303,323,927 1,253,866,598
Treasury shares (6,138,909) (1,409,762) (1,409,762) (1,409,762)
Shares outstanding 1,367,420,074 1,340,656,987 1,301,914,165 1,252,456,836
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition 31,492,234 38,742,822 49,457,329  
Treasury stock acquired (4,729,147)