Note 7 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Restricted Stock [Member]
Outstanding, Shares (in shares)		977,547	578,894
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 15.45	$ 13.14
Granted, Shares (in shares)	[1]	248,543	677,602
Granted, Weighted average grant date fair value (in dollars per share)	[1]	$ 21.69	$ 15.72
Forfeited, Shares (in shares)		(85,630)	0
Forfeited, Weighted average grant date fair value (in dollars per share)		$ 15.53	$ 0
Vested, Shares (in shares)		(260,004)	(279,170)
Vested, Weighted average grant date fair value (in dollars per share)		$ 14.20	$ 13.25
Outstanding, Shares (in shares)		880,456	977,326
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 17.57	$ 14.89
Restricted Stock [Member] | Common Class A [Member]
Outstanding, Shares (in shares)		449,284	407,786
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 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)		480,042	449,284
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 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
Granted, Shares (in shares)		90,184	0
Granted, Weighted average grant date fair value (in dollars per share)		$ 18.92	$ 0
Vested, Shares (in shares)		0	0
Vested, Weighted average grant date fair value (in dollars per share)		$ 0	$ 0
Outstanding, Shares (in shares)		488,184	0
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 18.34	$ 0

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