Convertible Senior Notes - Allocation of Notes (Details) - USD ($) $ in Thousands	Oct. 31, 2018	Apr. 30, 2018
Liability:
Less: debt issuance costs, net of amortization		$ (12,900)
Stockholders' equity:
Less: debt issuance costs		(12,900)
Convertible Senior Notes
Liability:
Principal	$ 575,000
Less: debt discount, net of amortization	(122,891)
Less: debt issuance costs, net of amortization	(8,897)	(9,800)
Net carrying amount of the Notes	443,212
Stockholders' equity:
Less: debt issuance costs	(8,897)	(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