Note 8 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Stock [Member]
Outstanding, Shares (in shares)	578,894	977,547	578,894	503,685
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 13.14	$ 15.45	$ 13.14	$ 11.14
Granted, Shares (in shares)		359,481	751,463	391,836
Granted, Weighted average grant date fair value (in dollars per share)		$ 18.92	$ 16.07	$ 14.63
Vested, Shares (in shares)		(333,865)	(352,810)	(225,570)
Vested, Weighted average grant date fair value (in dollars per share)		$ 15.35	$ 12.98	$ 11.21
Forfeited, Shares (in shares)		(85,630)	0	(91,057)
Forfeited, Weighted average grant date fair value (in dollars per share)		$ 15.53	$ 0	$ 13.27
Outstanding, Shares (in shares)		917,533	977,547	578,894
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 16.84	$ 15.45	$ 13.14
Restricted Stock [Member] | Common Class A [Member]
Outstanding, Shares (in shares)	407,786	449,284	407,786	462,632
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 11.82	$ 13.55	$ 11.82	$ 10.63
Granted, Shares (in shares)		166,814	199,810	220,080
Granted, Weighted average grant date fair value (in dollars per share)		$ 19.87	$ 15.36	$ 12.65
Vested, Shares (in shares)		(136,056)	(158,312)	(274,926)
Vested, Weighted average grant date fair value (in dollars per share)		$ 12.32	$ 11.38	$ 10.48
Outstanding, Shares (in shares)		480,042	449,284	407,786
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 16.10	$ 13.55	$ 11.82
Restricted Stock Units (RSUs) [Member]
Outstanding, Shares (in shares)	0	398,000	0	209,500
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 0	$ 18.21	$ 0	$ 15.70
Granted, Shares (in shares)	398,000	93,184	398,000	0
Granted, Weighted average grant date fair value (in dollars per share)	$ 18.21	$ 18.77	$ 18.21	$ 0
Vested, Shares (in shares)		(374,500)	0	(209,500)
Vested, Weighted average grant date fair value (in dollars per share)		$ 18.18	$ 0	$ 15.70
Forfeited, Shares (in shares)		(26,500)	0	0
Forfeited, Weighted average grant date fair value (in dollars per share)		$ 18.21	$ 0	$ 0
Outstanding, Shares (in shares)		90,184	398,000	0
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 18.92	$ 18.21	$ 0