Subsequent events - Additional Information (Detail) (USD $)	3 Months Ended						0 Months Ended
	Jul. 31, 2014	Apr. 30, 2014	Jan. 31, 2014	Jul. 31, 2013	Apr. 30, 2013	Jan. 31, 2013	Oct. 15, 2014 Subsequent Event
Subsequent Event [Line Items]
Dividends paid, per share	$ 0.07	$ 0.07	$ 0.07	$ 0.07	$ 0.07	$ 0.10	$ 0.07
Dividends payable, record date							Sep. 30, 2014