Summary of Significant Accounting Policies - Going concern thru intangibles (Detail) - USD ($)	3 Months Ended						9 Months Ended
	Feb. 28, 2021	Nov. 30, 2020	Aug. 31, 2020	Feb. 29, 2020	Nov. 30, 2019	Aug. 31, 2019	Feb. 28, 2021	Feb. 29, 2020	May 31, 2020
Accounting Policies [Abstract]
Net loss	$ 43,985,000	$ 34,966,000	$ 30,832,000	$ 35,768,000	$ 14,860,000	$ 16,164,000	$ 109,783,000	$ 66,792,000
Accumulated deficit	465,983,000						465,983,000		$ 354,711,000
Balance in excess of federally insured limits	14,000,000.0						14,000,000.0		$ 14,000,000.0
Intangible asset impairment charges	$ 10.0			$ 0			$ 10.0	$ 0