Shareholders' equity - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended
	May 31, 2011	Dec. 31, 2011	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
Preferred stock, value of shares redeem	$ 330,000	$ 330,000
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