Credit Facility (Details) (USD $)	Jan. 31, 2014
Line of Credit Facility [Line Items]
Credit facility, current borrowing capacity	$ 150,000,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Letters of credit collateralized by the credit facility	51,400,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Borrowings outstanding	$ 0
4.0% Convertible Senior Notes due March 15, 2015 [Member]
Line of Credit Facility [Line Items]
Interest rate on convertible notes	4.00%
4.0% Convertible Senior Notes due March 15, 2015 [Member] | Convertible Notes Payable [Member]
Line of Credit Facility [Line Items]
Interest rate on convertible notes	4.00%