Convertible Senior Notes - Convertible Debt (Details) - USD ($) $ in Thousands		6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Apr. 30, 2018
Liability:
Less: debt issuance costs, net of amortization			$ (12,900)
Stockholders' equity:
Allocated value of the conversion feature		$ 133,265
Less: debt issuance costs			(12,900)
Convertible Senior Notes
Liability:
Principal	$ 575,000
Less: debt discount, net of amortization	(104,329)
Less: debt issuance costs, net of amortization	(7,553)		(9,800)
Net carrying amount of the Notes	463,118
Stockholders' equity:
Less: debt issuance costs	(7,553)		(9,800)
Additional Paid-In Capital
Liability:
Less: debt issuance costs, net of amortization	(3,068)		(3,100)
Stockholders' equity:
Allocated value of the conversion feature	136,333
Less: debt issuance costs	(3,068)		$ (3,100)
Additional paid-in capital	$ 133,265