Inventories - Summary of inventories (Details) - USD ($) $ in Thousands	Apr. 30, 2016	Jan. 31, 2016	Apr. 30, 2015
Inventory Disclosure [Abstract]
Finished goods	$ 405,658	$ 484,805	$ 363,458
Raw materials and work-in-process	1,062	506	7,766
Inventories	$ 406,720	$ 485,311	$ 371,224