Note 4 - Stockholders' Equity - Outstanding Options Granted to Consultants for Sales and Pre-marketing Services (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Options exercisable at the end of the year (in shares)	1,122,468
Consultants [Member]
Options for shares of common stock (in shares)	30,117
Granted, weighted average exercise price (in dollars per share) | $ / shares
Options exercisable at the end of the year (in shares)	26,430
Exercisable through
February 2013 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	1,500
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 12.47
Options exercisable at the end of the year (in shares)	1,500
Exercisable through	February 2023
August 2013 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	4,188
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 21.14
Options exercisable at the end of the year (in shares)	4,188
Exercisable through	August 2023
March 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	7,844
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 39.86
Options exercisable at the end of the year (in shares)	7,844
Exercisable through	March 2024
May 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	5,991
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 22.01
Options exercisable at the end of the year (in shares)	5,690
Exercisable through	May 2024
November 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	7,032
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 21.66
Options exercisable at the end of the year (in shares)	5,428
Exercisable through	November 2024
May 2015 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	1,312
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 19.51
Options exercisable at the end of the year (in shares)	730
Exercisable through	May 2025
February 2016 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	2,250
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 16.87
Options exercisable at the end of the year (in shares)	1,050
Exercisable through	February 2026