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, unless otherwise specified	Apr. 30, 2015	Jan. 31, 2015
Debt Instrument
Net carrying amount of the liability component	$ 235,604	$ 230,400
4.00% Debentures due 2031
Debt Instrument
Principal amount	253,000	253,000
Unamortized debt discount	(20,996)	(22,600)
Net carrying amount of the liability component	232,004	230,400
Equity component	$ 43,930	$ 43,930