Common stock (Detail)	3 Months Ended
	Mar. 31, 2014
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	868,616
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(1,443)
Shares issued, end of period (in shares)	867,173
Treasury shares, beginning of year	(9,573)
Treasury shares, end of period	(9,573)
Outstanding, beginning of year (in shares)	859,043
Outstanding, end of period (in shares)	857,600
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	1,178,775,092
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	2,666,267
Shares issued, end of period (in shares)	1,181,441,359
Treasury shares, beginning of year	(1,408,484)
Treasury shares, end of period	(1,408,484)
Outstanding, beginning of year (in shares)	1,177,366,608
Outstanding, end of period (in shares)	1,180,032,875