Note 4 - Stockholders' Equity - Outstanding Options Granted to Consultants for Sales and Pre-marketing Services (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Granted (in dollars per share) | $ / shares
Options exercisable at the end of the year (in shares)	1,633,271
Consultants [Member]
Options exercisable at the end of the year (in shares)	26,785
Options for shares of common stock (in shares)	39,469
February 2013 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 12.47
Options exercisable at the end of the year (in shares)	1,500
Exercisable through	February 2023
Options for shares of common stock (in shares)	1,500
August 2013 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 21.14
Options exercisable at the end of the year (in shares)	3,559
Exercisable through	August 2023
Options for shares of common stock (in shares)	4,188
March 2014 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 39.86
Options exercisable at the end of the year (in shares)	10,546
Exercisable through	March 2024
Options for shares of common stock (in shares)	13,100
May 2014 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 22.01
Options exercisable at the end of the year (in shares)	4,580
Exercisable through	May 2024
Options for shares of common stock (in shares)	6,250
November 2014 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 21.66
Options exercisable at the end of the year (in shares)	4,663
Exercisable through	November 2024
Options for shares of common stock (in shares)	8,431
May 2015 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 19.51
Options exercisable at the end of the year (in shares)	1,105
Exercisable through	May 2025
Options for shares of common stock (in shares)	3,500
February 2016 [Member] | Consultants [Member]
Granted (in dollars per share) | $ / shares	$ 16.87
Options exercisable at the end of the year (in shares)	832
Exercisable through	February 2026
Options for shares of common stock (in shares)	2,500