Subsequent events - Additional Information (Detail) (Subsequent events [Member], USD $)	1 Months Ended
	Apr. 25, 2013
Subsequent events [Member]
Dividends Payable [Line Items]
Quarterly dividend payable amount per share	$ 0.11
Date of payment of dividend	May 24, 2013
Date of record of dividend	May 10, 2013
Date of announcement of dividend	Apr. 26, 2013