Convertible Senior Notes - Allocation of Notes (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 31, 2019	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	(116,722)
Less: debt issuance costs, net of amortization	(8,450)	(9,800)
Net carrying amount of the Notes	449,828
Stockholders' equity:
Less: debt issuance costs	(8,450)	(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