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FAIR VALUE MEASUREMENTS FAIR VALUE MEASUREMENTS - Convertible Senior Notes (Details) - USD ($) | Jul. 31, 2018 | Jan. 31, 2018 | Jul. 31, 2017
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Reported Value Measurement | Fair Value, Measurements, Nonrecurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Instrument, Fair Value Disclosure | $ 429,598,000 |  | $ 0
Estimate of Fair Value Measurement | Fair Value, Measurements, Nonrecurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Instrument, Fair Value Disclosure | 685,527,000 |  | $ 0
Convertible Debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 0.00% |
Debt Instrument, Unamortized Discount | (137,719,000) |  |
Unamortized Debt Issuance Expense | $ (7,683,000) |  |

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