Note 10 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2022

Notes Tables

Share-Based Payment Arrangement, Option, Activity [Table Text Block]

	Number of Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)
Balance at December 31, 2021		4,640,660		$	6.64		5.66
Granted		2,173,832			0.82
Forfeited		(4,155	)		1.71
Cancelled		(13,816	)		38.80
Exercised		(30,001	)		0.41
Balance at March 31, 2022		6,766,520		$	4.74		5.91

Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Exercisable [Table Text Block]

	Number of Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (Millions)
Exercisable at March 31, 2022		2,392,065	$	10.06		5.41	$	0.2
Vested or expected to vest at March 31, 2022		6,421,165	$	4.91		5.88	$	0.4

Share-Based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]

	Number of Restricted Stock Units Outstanding			Weighted- Average Grant Date Fair Value
Unvested December 31, 2021		3,839,030		$	2.36
Granted		5,133,307			0.80
Vested		(1,603,449	)		2.53
Forfeited		(69,777	)		1.49
Unvested March 31, 2022		7,299,111		$	1.24

Share-Based Payment Arrangement, Cost by Plan [Table Text Block]

	Three Months Ended March 31,
	2022		2021
	(in thousands)
Stock options	$	978	$	1,119
Restricted stock units		883		629
Performance restricted stock units		384		38
	$	2,245	$	1,786

Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2022			2021
Expected dividend yield		0%			0%
Expected volatility	131%	-	133%	118%	-	136%
Risk-free interest rate	1.25%	-	1.94%	0.33%	-	0.58%
Expected life (in years)	4.3	-	4.5	3.8	-	4.5%