Shareholders' equity - Additional Information (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, par value	$ 1.00	$ 1.00
Warrant [Member] | Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Exercise price of each class of warrants or rights outstanding	$ 518.96	$ 518.96
Warrant to purchase common stock	95,383	95,383