LONG-TERM DEBT - 1.50% CONVERTIBLE SENIOR NOTES (Details)	Jun. 18, 2014 USD ($) shares	Jan. 31, 2018 USD ($) $ / shares	Jan. 31, 2017 USD ($)	Jan. 31, 2016
Debt Instrument [Line Items]
Convertible Notes Payable, Noncurrent		$ 354,000,000
Unamortized debt discount		42,100,000
Unamortized Debt Issuance Expense		4,000,000
Common Stock, Number of Shares and Other Disclosures
Common stock issued in public offering (in shares) | shares	5,750,000
1.50% Convertible Senior Notes
Debt Instrument [Line Items]
Principal Amount - 1.50% Convertible Senior Notes	$ 400,000,000	$ 400,000,000	$ 400,000,000
Coupon Interest Rate	1.50%
Proceeds from issuance of convertible notes, net of issuance costs	$ 391,900,000
1.50% Convertible Notes - Conversion Ratio	15.5129
1.50% Convertible Notes - Base Principal Amount For Conversion Rate	$ 1,000
1.50% Convertible Notes - Conversion Price (in dollars per share) | $ / shares		$ 64.46
1.50% Convertible Notes - Number of Common Shares (in shares)	6,205,000
1.50% Convertible Notes - Carrying Value	$ 319,900,000
1.50% Convertible Notes - Carrying Value of Equity Component	$ 80,100,000		$ 78,200,000
Assumed Noncovertible Debt Interest Rate	5.00%
Debt Issuance Costs	$ 7,600,000
Common stock issuance costs	1,900,000
Effective Interest Rate		5.29%	5.29%	5.29%
Common Stock, Number of Shares and Other Disclosures
Common stock issuance costs	$ 1,900,000
Conversion Scenario One | 1.50% Convertible Senior Notes
Debt Instrument [Line Items]
1.50% Convertible Notes - Threshold Trading Days	20
1.50% Convertible Notes - Window of Consecutive Trading Days	30
1.50% Convertible Notes - Threshold Percentage for Conversion Trigger	130.00%
Conversion Scenario Two | 1.50% Convertible Senior Notes
Debt Instrument [Line Items]
1.50% Convertible Notes - Window of Consecutive Trading Days	5
1.50% Convertible Notes - Threshold Percentage for Conversion Trigger	98.00%