Common stock (Detail)	3 Months Ended
	Mar. 31, 2012
Class A [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	938,342
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	(5,630)
Shares issued, end of period (in shares)	932,712
Treasury shares, beginning of year	(98)
Treasury shares, end of period	(98)
Outstanding, beginning of year (in shares)	938,244
Outstanding, end of period (in shares)	932,614
Class B [Member]
Class of Stock [Line Items]
Shares issued, beginning of year (in shares)	1,069,645,361
Conversions of Class A common stock to Class B common stock and exercises of replacement stock options issued in a business acquisition	9,029,871
Shares issued, end of period (in shares)	1,078,675,232
Treasury shares, beginning of year	(801,985)
Treasury shares, end of period	(801,985)
Outstanding, beginning of year (in shares)	1,068,843,376
Outstanding, end of period (in shares)	1,077,873,247