Inventories - Summary of inventories (Details) - USD ($) $ in Thousands	Jul. 31, 2016	Jan. 31, 2016	Jul. 31, 2015
Inventory Disclosure [Abstract]
Finished goods	$ 567,864	$ 484,805	$ 600,529
Raw materials and work-in-process	2,132	506	4,685
Inventories	$ 569,996	$ 485,311	$ 605,214