Note 7 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019
Restricted Stock [Member]
Outstanding, Shares (in shares)	[1]	977,547	578,894
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 15.45	$ 13.14
Granted, Shares (in shares)	[1]	327,265	718,783
Granted, Weighted average grant date fair value (in dollars per share)	[1]	$ 19.25	$ 16.08
Vested, Shares (in shares)		(301,185)	(352,810)
Vested, Weighted average grant date fair value (in dollars per share)		$ 15.28	$ 12.98
Forfeited, Shares (in shares)		(85,630)	0
Forfeited, Weighted average grant date fair value (in dollars per share)		$ 15.53	$ 0.00
Outstanding, Shares (in shares)	[1]	917,997	944,867
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 16.86	$ 15.44
Restricted Stock [Member] | Common Class A [Member]
Outstanding, Shares (in shares)	[1]	449,284	407,786
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 13.55	$ 11.82
Granted, Shares (in shares)	[1]	166,814	199,810
Granted, Weighted average grant date fair value (in dollars per share)	[1]	$ 19.87	$ 15.36
Vested, Shares (in shares)		(136,056)	(158,312)
Vested, Weighted average grant date fair value (in dollars per share)		$ 12.32	$ 11.38
Outstanding, Shares (in shares)	[1]	480,042	449,284
Outstanding, Weighted average grant date fair value (in dollars per share)	[1]	$ 16.10	$ 13.55
Restricted Stock Units (RSUs) [Member]
Outstanding, Shares (in shares)		398,000	0
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 18.21	$ 0.00
Granted, Shares (in shares)		93,184	398,000
Granted, Weighted average grant date fair value (in dollars per share)		$ 18.77	$ 18.21
Vested, Shares (in shares)		(374,500)	0
Vested, Weighted average grant date fair value (in dollars per share)		$ 18.18	$ 0.00
Forfeited, Shares (in shares)		(26,500)	0
Forfeited, Weighted average grant date fair value (in dollars per share)		$ 18.21	$ 0.00
Outstanding, Shares (in shares)		90,184	398,000
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.