Note 7 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Restricted Stock [Member]
Outstanding, Shares (in shares)		503,685	396,033
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 11.14	$ 12.06
Granted, Shares (in shares)		391,836	307,943
Granted, Weighted average grant date fair value (in dollars per share)		$ 14.63	$ 10.40
Vested, Shares (in shares)		(225,570)	(200,291)
Vested, Weighted average grant date fair value (in dollars per share)		$ 11.21	$ 11.82
Forfeited, Shares (in shares)		(91,057)	0
Forfeited, Weighted average grant date fair value (in dollars per share)		$ 13.27	$ 0
Outstanding, Shares (in shares)		578,894	503,685
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 13.14	$ 11.14
Restricted Stock [Member] | Common Class A [Member]
Outstanding, Shares (in shares)		462,632	415,082
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 10.63	$ 10.15
Granted, Shares (in shares)	[1]	220,080	275,076
Granted, Weighted average grant date fair value (in dollars per share)	[1]	$ 12.65	$ 10.84
Vested, Shares (in shares)		(274,926)	(227,526)
Vested, Weighted average grant date fair value (in dollars per share)		$ 10.48	$ 10
Outstanding, Shares (in shares)		407,786	462,632
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 11.82	$ 10.63
Restricted Stock Units (RSUs) [Member]
Outstanding, Shares (in shares)		209,500	0
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 15.70	$ 0
Vested, Shares (in shares)		(209,500)	0
Vested, Weighted average grant date fair value (in dollars per share)		$ 15.70	$ 0
Outstanding, Shares (in shares)
Outstanding, Weighted average grant date fair value (in dollars per share)

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