Note 8 - Stockholders' Equity - Options Outstanding Separated Into Range of Exercise Price (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Upper range of exercise price (in dollars per share)
Options outstanding (in shares)	1,456,285	2,388,348
Weighted average remaining contractual life (Year)	4 years 330 days	5 years 314 days
Weighted average exercise price (in dollars per share)	$ 16.172	$ 15.243
Options exercisable (in shares)	1,262,206	1,752,416
Weighted average remaining contractual life, exercisable (Year)	4 years 217 days	5 years 43 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 15.052	$ 12.583
Range One [Member]
Upper range of exercise price (in dollars per share)	1.576	1.576
Lower range of exercise price (in dollars per share)	$ 1.039	$ 0.901
Options outstanding (in shares)	401,476	710,562
Weighted average remaining contractual life (Year)	1 year 139 days	2 years 175 days
Weighted average exercise price (in dollars per share)	$ 1.295	$ 1.317
Options exercisable (in shares)	401,476	710,562
Weighted average remaining contractual life, exercisable (Year)	1 year 139 days	2 years 175 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 1.295	$ 1.317
Range Two [Member]
Upper range of exercise price (in dollars per share)	8.80	8.80
Lower range of exercise price (in dollars per share)	$ 6.23	$ 6.23
Options outstanding (in shares)	55,929	89,373
Weighted average remaining contractual life (Year)	3 years 331 days	4 years 351 days
Weighted average exercise price (in dollars per share)	$ 7.125	$ 6.953
Options exercisable (in shares)	55,929	89,373
Weighted average remaining contractual life, exercisable (Year)	3 years 331 days	4 years 351 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 7.125	$ 6.953
Range Three [Member]
Upper range of exercise price (in dollars per share)	16.87	16.87
Lower range of exercise price (in dollars per share)	$ 12.47	$ 12.47
Options outstanding (in shares)	228,001	442,507
Weighted average remaining contractual life (Year)	5 years 299 days	7 years 28 days
Weighted average exercise price (in dollars per share)	$ 13.435	$ 13.812
Options exercisable (in shares)	200,917	279,863
Weighted average remaining contractual life, exercisable (Year)	5 years 185 days	6 years 59 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 12.972	$ 12.47
Range Four [Member]
Upper range of exercise price (in dollars per share)	21.66	21.66
Lower range of exercise price (in dollars per share)	$ 19.51	$ 19.51
Options outstanding (in shares)	374,257	546,959
Weighted average remaining contractual life (Year)	6 years 221 days	7 years 227 days
Weighted average exercise price (in dollars per share)	$ 21.207	$ 21.205
Options exercisable (in shares)	284,671	307,302
Weighted average remaining contractual life, exercisable (Year)	6 years 206 days	7 years 203 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 21.201	$ 21.20
Range Five [Member]
Upper range of exercise price (in dollars per share)	24.23	24.23
Lower range of exercise price (in dollars per share)	$ 22.01	$ 19.51
Options outstanding (in shares)	170,815	295,513
Weighted average remaining contractual life (Year)	6 years 70 days	7 years 109 days
Weighted average exercise price (in dollars per share)	$ 22.292	$ 22.278
Options exercisable (in shares)	144,967	191,891
Weighted average remaining contractual life, exercisable (Year)	6 years 66 days	7 years 105 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 22.342	$ 22.316
Range Six [Member]
Upper range of exercise price (in dollars per share)
Lower range of exercise price (in dollars per share)		$ 22.01
Options outstanding (in shares)	117,829	154,200
Weighted average remaining contractual life (Year)	7 years 52 days	8 years 52 days
Weighted average exercise price (in dollars per share)	$ 29.88	$ 29.88
Options exercisable (in shares)	74,758	70,684
Weighted average remaining contractual life, exercisable (Year)	7 years 52 days	8 years 52 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 29.88	$ 29.88
Range Seven [Member]
Upper range of exercise price (in dollars per share)
Options outstanding (in shares)	107,978	149,234
Weighted average remaining contractual life (Year)	6 years 82 days	7 years 82 days
Weighted average exercise price (in dollars per share)	$ 39.86	$ 39.86
Options exercisable (in shares)	99,488	102,741
Weighted average remaining contractual life, exercisable (Year)	6 years 82 days	7 years 82 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 39.86	$ 39.86