Notes Payable - Principal Amount, Unamortized Debt Discount, Net Carrying Amount of the Liability Component, and Carrying Amount of the Equity Component of the 4.00% Debentures (Detail) - USD ($) $ in Thousands	Jul. 31, 2015	Jan. 31, 2015
Debt Instrument
Net carrying amount of the liability component	$ 236,825	$ 230,400
4.00% Debentures due 2031
Debt Instrument
Principal amount	253,000	253,000
Unamortized debt discount	(19,363)	(22,600)
Net carrying amount of the liability component	233,637	230,400
Equity component, net of debt issuance costs	$ 42,531	$ 42,531