Notes Payable - Principal Amount, Unamortized Debt Discount, Unamortized Debt Issuance Costs, Net Carrying Amount of the Liability Component, and Carrying Amount of the Equity Component of the 4.00% Debentures (Detail) - USD ($) $ in Thousands	Apr. 30, 2016	Jan. 31, 2016
Debt Instrument
Net carrying amount of liability component	$ 242,037	$ 240,076
4.00% Debentures due 2031
Debt Instrument
Principal amount	252,957	252,957
Unamortized debt discount	(14,284)	(16,007)
Unamortized debt issuance costs	(1,824)	(2,062)
Net carrying amount of liability component	236,849	234,888
Equity component, net of debt issuance costs	$ 42,518	$ 42,518