Note 6 - Stock-based Compensation (Details) - Summary of Restricted Common Stock Activity - Restricted Stock [Member] - $ / shares	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Restricted stock - common:
Number of Shares, Beginning of Period	385,056	274,838
Weighted- Average Grant Date Fair Value Per Share, Beginning of Period	$ 9.09	$ 4.43
Number of Shares, Granted	150,308	312,961
Weighted- Average Grant Date Fair Value Per Share, Granted	$ 10.27	$ 11.78
Number of Shares, Vested	(197,858)	(202,743)
Weighted- Average Grant Date Fair Value Per Share, Vested	$ 9.16	$ 6.93
Number of Shares, End of Period	337,506	385,056
Weighted- Average Grant Date Fair Value Per Share, End of Period	$ 9.57	$ 9.09
Common Class A [Member]
Restricted stock - common:
Number of Shares, Beginning of Period	204,473	0
Weighted- Average Grant Date Fair Value Per Share, Beginning of Period	$ 9.81	$ 0
Number of Shares, Granted	287,513	236,294
Weighted- Average Grant Date Fair Value Per Share, Granted	$ 9.37	$ 9.80
Number of Shares, Vested	(117,293)	(31,821)
Weighted- Average Grant Date Fair Value Per Share, Vested	$ 9.85	$ 9.75
Number of Shares, End of Period	374,693	204,473
Weighted- Average Grant Date Fair Value Per Share, End of Period	$ 9.46	$ 9.81