Note 8 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Restricted Stock [Member]
Outstanding, Shares (in shares)	[1]	449,284	407,786
Granted, Shares (in shares)	[1]	166,814	199,810
Vested, Shares (in shares)		(136,056)	(158,312)
Forfeited, Shares (in shares)		0	0
Outstanding, Shares (in shares)	[1]	480,042	449,284
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 13.55	$ 11.82
Granted, Weighted average grant date fair value (in dollars per share)	[1]	19.87	15.36
Vested, Weighted average grant date fair value (in dollars per share)		12.32	11.38
Forfeited, Weighted average grant date fair value (in dollars per share)		0	0
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 16.10	$ 13.55
Restricted Stock [Member] | Common Class A [Member]
Outstanding, Shares (in shares)	[1]	977,547	578,894
Granted, Shares (in shares)	[1]	359,481	718,783
Vested, Shares (in shares)		(333,865)	0
Forfeited, Shares (in shares)		(85,630)	(352,810)
Outstanding, Shares (in shares)	[1]	917,533	944,867
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 15.45	$ 13.14
Granted, Weighted average grant date fair value (in dollars per share)	[1]	18.92	16.08
Vested, Weighted average grant date fair value (in dollars per share)		15.35	0
Forfeited, Weighted average grant date fair value (in dollars per share)		15.53	12.98
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 16.84	$ 15.44
Restricted Stock Units (RSUs) [Member]
Outstanding, Shares (in shares)		398,000	0
Granted, Shares (in shares)		93,184	398,000
Vested, Shares (in shares)		(374,500)	0
Forfeited, Shares (in shares)		(26,500)	0
Outstanding, Shares (in shares)		90,184	398,000
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 18.21	$ 0
Granted, Weighted average grant date fair value (in dollars per share)		18.77	18.21
Vested, Weighted average grant date fair value (in dollars per share)		18.18	0
Forfeited, Weighted average grant date fair value (in dollars per share)		18.21	0
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 18.92	$ 18.21

[1] For awards subject to future performance conditions, amounts assume target performance.