Note 4 - Stockholders' Equity - Outstanding Options Granted to Consultants for Sales and Pre-marketing Services (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Granted (in dollars per share) | $ / shares
Options exercisable at the end of the year (in shares)	1,467,826
Consultants [Member]
Options for shares of common stock (in shares)	38,580
Options exercisable at the end of the year (in shares)	28,985
February 2013 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	1,500
Granted (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 (in dollars per share) | $ / shares	$ 21.14
Options exercisable at the end of the year (in shares)	3,821
Exercisable through	August 2023
March 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	13,100
Granted (in dollars per share) | $ / shares	$ 39.86
Options exercisable at the end of the year (in shares)	11,365
Exercisable through	March 2024
May 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	6,187
Granted (in dollars per share) | $ / shares	$ 22.01
Options exercisable at the end of the year (in shares)	4,974
Exercisable through	May 2024
November 2014 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	8,272
Granted (in dollars per share) | $ / shares	$ 21.66
Options exercisable at the end of the year (in shares)	5,170
Exercisable through	November 2024
May 2015 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	2,833
Granted (in dollars per share) | $ / shares	$ 19.51
Options exercisable at the end of the year (in shares)	1,168
Exercisable through	May 2025
February 2016 [Member] | Consultants [Member]
Options for shares of common stock (in shares)	2,500
Granted (in dollars per share) | $ / shares	$ 16.87
Options exercisable at the end of the year (in shares)	987
Exercisable through	February 2026