Shareholders' equity - Additional Information (Detail) (USD $)	1 Months Ended
	May 31, 2011	Sep. 30, 2012	Dec. 31, 2011
Class of Stock [Line Items]
Preferred stock, shares authorized		1,000,000	1,000,000
Preferred stock, par value		$ 1.00	$ 1.00
Preferred stock, shares redeemed	330,000,000
Ten Year Warrant Series D Preferred [Member]
Class of Stock [Line Items]
Exercise price of each class of warrants or rights outstanding		518.96	518.96
Ten-year warrant to purchase common stock		95,383	95,383