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, 2016	Jan. 31, 2015
Debt Instrument
Net carrying amount of the liability component	$ 240,076	$ 227,386
4.00% Debentures due 2031
Debt Instrument
Principal amount	252,957	253,000
Unamortized debt discount	(16,007)	(22,600)
Unamortized debt issuance costs	(2,062)	(3,014)
Net carrying amount of the liability component	234,888	227,386
Equity component	$ 42,518	$ 42,531