Note 11 - Stock Based Compensation - Summary of Stock Option Awards and Changes (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Outstanding, Number (in Shares) (in shares)	2,932,663	2,139,822	1,744,755
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 4.31	$ 5.64	$ 7.20
Options Exercisable, Number (in Shares) (in shares)	2,198,907
Exercisable at December 31, 2016 (Year)	6 years 328 days
Range 1 [Member]
Range of Exercise Prices, Lower Range (in dollars per share)	$ 1
Range of Exercise Prices, Upper Range (in dollars per share)	$ 2.99
Options Outstanding, Number (in Shares) (in shares)	1,560,699
Options Outstanding, Weighted Average Remaining Contractual Term (in years) (Year)	8 years 328 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 1.77
Options Exercisable, Number (in Shares) (in shares)	857,897
Exercisable at December 31, 2016 (Year)	8 years 328 days
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 1.78
Range 2 [Member]
Range of Exercise Prices, Lower Range (in dollars per share)	3
Range of Exercise Prices, Upper Range (in dollars per share)	$ 5.99
Options Outstanding, Number (in Shares) (in shares)	890,857
Options Outstanding, Weighted Average Remaining Contractual Term (in years) (Year)	7 years 73 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 3.81
Options Exercisable, Number (in Shares) (in shares)	859,903
Exercisable at December 31, 2016 (Year)	7 years 73 days
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 3.69
Range 3 [Member]
Range of Exercise Prices, Lower Range (in dollars per share)	6
Range of Exercise Prices, Upper Range (in dollars per share)	$ 12.99
Options Outstanding, Number (in Shares) (in shares)	322,387
Options Outstanding, Weighted Average Remaining Contractual Term (in years) (Year)	3 years
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 10.82
Options Exercisable, Number (in Shares) (in shares)	322,387
Exercisable at December 31, 2016 (Year)	3 years
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 10.82
Range 4 [Member]
Range of Exercise Prices, Lower Range (in dollars per share)	13
Range of Exercise Prices, Upper Range (in dollars per share)	$ 19.99
Options Outstanding, Number (in Shares) (in shares)	88,856
Options Outstanding, Weighted Average Remaining Contractual Term (in years) (Year)	3 years 146 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 14.19
Options Exercisable, Number (in Shares) (in shares)	88,856
Exercisable at December 31, 2016 (Year)	3 years 146 days
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 14.19
Range 5 [Member]
Range of Exercise Prices, Lower Range (in dollars per share)	$ 20
Options Outstanding, Number (in Shares) (in shares)	69,864
Options Outstanding, Weighted Average Remaining Contractual Term (in years) (Year)	1 year 73 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 24.85
Options Exercisable, Number (in Shares) (in shares)	69,864
Exercisable at December 31, 2016 (Year)	1 year 73 days
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 24.85