Notes Payable - Recognized Amounts in Interest Expense in the Condensed Consolidated Statement of Operations Related to the 4.00% Debentures (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2013
Debt Instrument
Interest expense at the contractual interest rate	$ 19,276	$ 19,452	$ 18,866
4.00% Debentures due 2031
Debt Instrument
Interest expense at the contractual interest rate	10,120	10,120	10,120
Amortization of debt discount	$ 6,139	$ 5,715	$ 5,322