Note 10 - Stock-based Compensation - Stock Options Outstanding and Exercisable (Details) - $ / shares	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Outstanding options, exercise price range, upper limit (in dollars per share)
Stock options outstanding (in shares)	214,184	216,859	274,844	298,740
Outstanding options, weighted-average exercise price (in dollars per share)	$ 8.60	$ 8.80	$ 9.76	$ 9.97
Outstanding options, weighted-average remaining contractual life (Year)	1 year 295 days
Stock options exercisable (in shares)	171,076
Exercisable options, weighted-average exercise price (in dollars per share)	$ 7.49
Range 1 [Member]
Outstanding options, exercise price range, lower limit (in dollars per share)	5.76
Outstanding options, exercise price range, upper limit (in dollars per share)	$ 10.72
Stock options outstanding (in shares)	154,535
Outstanding options, weighted-average exercise price (in dollars per share)	$ 5.76
Outstanding options, weighted-average remaining contractual life (Year)	1 year 204 days
Stock options exercisable (in shares)	119,837
Exercisable options, weighted-average exercise price (in dollars per share)	$ 5.76
Range 2 [Member]
Outstanding options, exercise price range, lower limit (in dollars per share)	10.73
Outstanding options, exercise price range, upper limit (in dollars per share)	$ 15.45
Stock options outstanding (in shares)	47,886
Outstanding options, weighted-average exercise price (in dollars per share)	$ 10.83
Outstanding options, weighted-average remaining contractual life (Year)	1 year 65 days
Stock options exercisable (in shares)	47,886
Exercisable options, weighted-average exercise price (in dollars per share)	$ 10.83
Range 3 [Member]
Outstanding options, exercise price range, lower limit (in dollars per share)	15.46
Outstanding options, exercise price range, upper limit (in dollars per share)	$ 20.01
Stock options outstanding (in shares)	447
Outstanding options, weighted-average exercise price (in dollars per share)	$ 16.66
Outstanding options, weighted-average remaining contractual life (Year)	5 years 354 days
Stock options exercisable (in shares)	447
Exercisable options, weighted-average exercise price (in dollars per share)	$ 16.66
Range 4 [Member]
Outstanding options, exercise price range, lower limit (in dollars per share)	20.02
Outstanding options, exercise price range, upper limit (in dollars per share)	$ 49.91
Stock options outstanding (in shares)	11,316
Outstanding options, weighted-average exercise price (in dollars per share)	$ 37.59
Outstanding options, weighted-average remaining contractual life (Year)	7 years 281 days
Stock options exercisable (in shares)	2,906
Exercisable options, weighted-average exercise price (in dollars per share)	$ 22.20