Note 8 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock [Member]
Outstanding, Shares (in shares)	578,894	503,685	396,033
Granted, Shares (in shares)	751,463	391,836	307,943
Vested, Shares (in shares)	(352,810)	(225,570)	(200,291)
Forfeited, Shares (in shares)	0	(91,057)	0
Outstanding, Shares (in shares)	977,547	578,894	503,685
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 13.14	$ 11.14	$ 12.06
Granted, Weighted average grant date fair value (in dollars per share)	16.07	14.63	10.40
Vested, Weighted average grant date fair value (in dollars per share)	12.98	11.21	11.82
Forfeited, Weighted average grant date fair value (in dollars per share)	0	13.27	0
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 15.45	$ 13.14	$ 11.14
Restricted Stock [Member] \| Common Class A [Member]
Outstanding, Shares (in shares)	407,786	462,632	415,082
Granted, Shares (in shares)	199,810	220,080	275,076
Vested, Shares (in shares)	(158,312)	(274,926)	(227,526)
Outstanding, Shares (in shares)	449,284	407,786	462,632
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 11.82	$ 10.63	$ 10.15
Granted, Weighted average grant date fair value (in dollars per share)	15.36	12.65	10.84
Vested, Weighted average grant date fair value (in dollars per share)	11.38	10.48	10.00
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 13.55	$ 11.82	$ 10.63
Restricted Stock Units (RSUs) [Member]
Outstanding, Shares (in shares)	0	209,500	0
Granted, Shares (in shares)	398,000	0	215,500
Vested, Shares (in shares)	0	(209,500)	0
Forfeited, Shares (in shares)	0	0	(6,000)
Outstanding, Shares (in shares)	398,000	0	209,500
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 0	$ 15.70	$ 0
Granted, Weighted average grant date fair value (in dollars per share)	18.21	0	15.70
Vested, Weighted average grant date fair value (in dollars per share)	0	15.70	0
Forfeited, Weighted average grant date fair value (in dollars per share)	0	0	15.70
Outstanding, Weighted average grant date fair value (in dollars per share)	$ 18.21	$ 0	$ 15.70

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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