Common stock (Detail)	9 Months Ended
	Sep. 30, 2015 shares
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	838,019
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(15,058)
Shares issued, end of period (in shares)	822,961
Treasury shares, beginning of year	(11,680)
Treasury shares, end of period	(11,680)
Outstanding, beginning of year (in shares)	826,339
Outstanding, end of period (in shares)	811,281
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	1,226,265,250
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	23,255,846
Shares issued, end of period (in shares)	1,249,521,096
Treasury shares, beginning of year	(1,409,762)
Treasury shares, end of period	(1,409,762)
Outstanding, beginning of year (in shares)	1,224,855,488
Outstanding, end of period (in shares)	1,248,111,334