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 (Details) - USD ($) $ in Thousands	Jan. 31, 2017	Jan. 31, 2016
Debt Instrument
Principal amount	$ 258,145
Net carrying amount of the liability component	248,109	$ 240,076
4.00% Debentures due 2031
Debt Instrument
Principal amount	252,957	252,957
Unamortized debt discount	(8,926)	(16,007)
Unamortized debt issuance costs	(1,110)	(2,062)
Net carrying amount of the liability component	242,921	234,888
Equity component	$ 42,518	$ 42,518