Common stock (Detail)	9 Months Ended
	Sep. 30, 2013
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	904,528
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(30,363)
Shares issued, end of period (in shares)	874,165
Treasury shares, beginning of year	(9,573)
Treasury shares, end of period	(9,573)
Outstanding, beginning of year (in shares)	894,955
Outstanding, end of period (in shares)	864,592
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	1,123,393,956
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	46,882,626
Shares issued, end of period (in shares)	1,170,276,582
Treasury shares, beginning of year	(1,408,484)
Treasury shares, end of period	(1,408,484)
Outstanding, beginning of year (in shares)	1,121,985,472
Outstanding, end of period (in shares)	1,168,868,098