Shareholders' equity - Additional Information (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, par value	$ 1.00	$ 1.00
Exercise price of each class of warrants or rights outstanding	26.50
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