Treasury stock (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Mar. 31, 2015	Feb. 28, 2015	Jan. 31, 2017	Jan. 31, 2017	Jan. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares	17,000	28,000	27,000	27,000	44,570
Treasury Stock Acquired, Average Cost Per Share	$ 6.27	$ 6.64	$ 0.00
Treasury stock released (purchased)			0
Investment Repurchase Agreement, Date of Agreement				Feb. 05, 2015
Stock Repurchase Program, Authorized Amount			$ 2	$ 2
Stock Repurchase Program Expiration Date				Dec. 31, 2015