Note 9 - Stock-based Compensation - Summary of Restricted Common Stock Activity (Details) - $ / shares		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Restricted Stock [Member]
Outstanding, Shares (in shares)		578,894	503,685
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 13.14	$ 11.14
Granted, Shares (in shares)	[1]	677,602	318,196
Granted, Weighted average grant date fair value (in dollars per share)	[1]	$ 15.72	$ 15.25
Vested, Shares (in shares)		(279,170)	(225,570)
Vested, Weighted average grant date fair value (in dollars per share)		$ 13.25	$ 11.21
Outstanding, Shares (in shares)		977,326	596,311
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 14.89	$ 13.31
Restricted Stock [Member] | Common Class A [Member]
Outstanding, Shares (in shares)		407,786	462,632
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 11.82	$ 10.63
Granted, Shares (in shares)	[1]	199,810	220,080
Granted, Weighted average grant date fair value (in dollars per share)	[1]	$ 15.36	$ 12.65
Vested, Shares (in shares)		(158,312)	(274,926)
Vested, Weighted average grant date fair value (in dollars per share)		$ 11.38	$ 10.48
Outstanding, Shares (in shares)		449,284	407,786
Outstanding, Weighted average grant date fair value (in dollars per share)		$ 13.55	$ 11.82
Restricted Stock Units (RSUs) [Member]
Outstanding, Shares (in shares)			209,500
Outstanding, Weighted average grant date fair value (in dollars per share)			$ 15.70
Vested, Shares (in shares)			(209,500)
Vested, Weighted average grant date fair value (in dollars per share)			$ 15.70
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.