Convertible Senior Notes - Carrying Value of Liability Component (Details) - Convertible Senior Notes Due 2023 - Convertible Debt - USD ($) $ in Thousands	Jan. 31, 2020	Jan. 31, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Principal		$ 575,000	$ 575,000
Less: unamortized debt discount	$ (101,461)	(125,872)
Less: unamortized transaction costs	(8,218)	(10,196)
Net carrying amount	$ 465,321	$ 438,932

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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