Note 8 - Stockholders' Equity - Outstanding Options Granted to Consultants for Sales and Pre-marketing Services (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Granted, weighted average exercise price (in dollars per share)		$ 16.87	$ 28.253
Options exercisable at the end of the year (in shares)	1,262,206	1,752,416	1,794,249
Consultants [Member]
Options for shares of common stock (in shares)	31,323	44,384
Granted, weighted average exercise price (in dollars per share)
Options exercisable at the end of the year (in shares)	25,397	23,000
Exercisable through
February 2013 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	1,500	1,500
Granted, weighted average exercise price (in dollars per share)	$ 12.47	$ 12.47
Options exercisable at the end of the year (in shares)	1,500	1,344
Exercisable through	February2023	February 2023
August 2013 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	4,188	4,188
Granted, weighted average exercise price (in dollars per share)	$ 21.14	$ 21.14
Options exercisable at the end of the year (in shares)	4,083	3,104
Exercisable through	August2023	August 2023
March 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	7,953	13,100
Granted, weighted average exercise price (in dollars per share)	$ 39.86	$ 39.86
Options exercisable at the end of the year (in shares)	7,163	8,187
Exercisable through	March2024	March 2024
October 2013 [Member] | Consultants [Member]
Options for shares of common stock (in shares)		750
Granted, weighted average exercise price (in dollars per share)		$ 24.23
Options exercisable at the end of the year (in shares)		547
Exercisable through		October 2023
May 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	6,075	6,850
Granted, weighted average exercise price (in dollars per share)	$ 22.01	$ 22.01
Options exercisable at the end of the year (in shares)	5,317	3,704
Exercisable through	May2024	May 2024
November 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	7,107	10,246
Granted, weighted average exercise price (in dollars per share)	$ 21.66	$ 21.66
Options exercisable at the end of the year (in shares)	4,897	4,364
Exercisable through	November2024	November2024
May 2015 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	2,000	5,250
Granted, weighted average exercise price (in dollars per share)	$ 19.51	$ 19.51
Options exercisable at the end of the year (in shares)	1,292	1,750
Exercisable through	May2025	May2025
February 2016 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	2,500	2,500
Granted, weighted average exercise price (in dollars per share)	$ 16.87	$ 16.87
Options exercisable at the end of the year (in shares)	1,145
Exercisable through	February2026	February2026