Note 10 - Summary of Stock Options - Summary of Options Outstanding and Exercisable (Details)	9 Months Ended
	Sep. 30, 2019 $ / shares shares
Options outstanding, number (in shares) | shares	50,471
Options outstanding, weighted average exercise price (in dollars per share)	$ 298.99
Options outstanding, weighted average remaining contractual term (Year)	8 years 109 days
Options exercisable, number exercisable (in shares) | shares	21,169
Options exercisable, weighted average exercise price (in dollars per share)	$ 397.45
Range One [Member]
Exercise price range, lower limit (in dollars per share)	38
Exercise price range, upper limit (in dollars per share)	$ 58
Options outstanding, number (in shares) | shares	1,650
Options outstanding, weighted average exercise price (in dollars per share)	$ 55.42
Options outstanding, weighted average remaining contractual term (Year)	9 years 255 days
Options exercisable, number exercisable (in shares) | shares
Range Two [Member]
Exercise price range, lower limit (in dollars per share)	$ 100
Exercise price range, upper limit (in dollars per share)	$ 197
Options outstanding, number (in shares) | shares	25,496
Options outstanding, weighted average exercise price (in dollars per share)	$ 138.25
Options outstanding, weighted average remaining contractual term (Year)	9 years 36 days
Options exercisable, number exercisable (in shares) | shares	6,839
Options exercisable, weighted average exercise price (in dollars per share)	$ 148.08
Range Three [Member]
Exercise price range, lower limit (in dollars per share)	202
Exercise price range, upper limit (in dollars per share)	$ 283
Options outstanding, number (in shares) | shares	775
Options outstanding, weighted average exercise price (in dollars per share)	$ 252.06
Options outstanding, weighted average remaining contractual term (Year)	8 years 109 days
Options exercisable, number exercisable (in shares) | shares	305
Options exercisable, weighted average exercise price (in dollars per share)	$ 258.36
Range Four [Member]
Exercise price range, lower limit (in dollars per share)	311
Exercise price range, upper limit (in dollars per share)	$ 382
Options outstanding, number (in shares) | shares	4,377
Options outstanding, weighted average exercise price (in dollars per share)	$ 357.96
Options outstanding, weighted average remaining contractual term (Year)	8 years 36 days
Options exercisable, number exercisable (in shares) | shares	2,318
Options exercisable, weighted average exercise price (in dollars per share)	$ 360.44
Range Five [Member]
Exercise price range, lower limit (in dollars per share)	430
Exercise price range, upper limit (in dollars per share)	$ 497
Options outstanding, number (in shares) | shares	8,908
Options outstanding, weighted average exercise price (in dollars per share)	$ 452.58
Options outstanding, weighted average remaining contractual term (Year)	7 years 255 days
Options exercisable, number exercisable (in shares) | shares	4,576
Options exercisable, weighted average exercise price (in dollars per share)	$ 454.70
Range Six [Member]
Exercise price range, lower limit (in dollars per share)	501
Exercise price range, upper limit (in dollars per share)	$ 567
Options outstanding, number (in shares) | shares	4,594
Options outstanding, weighted average exercise price (in dollars per share)	$ 536.63
Options outstanding, weighted average remaining contractual term (Year)	6 years 328 days
Options exercisable, number exercisable (in shares) | shares	3,126
Options exercisable, weighted average exercise price (in dollars per share)	$ 536.17
Range Seven [Member]
Exercise price range, lower limit (in dollars per share)	600
Exercise price range, upper limit (in dollars per share)	$ 661
Options outstanding, number (in shares) | shares	2,397
Options outstanding, weighted average exercise price (in dollars per share)	$ 601.91
Options outstanding, weighted average remaining contractual term (Year)	5 years 328 days
Options exercisable, number exercisable (in shares) | shares	2,236
Options exercisable, weighted average exercise price (in dollars per share)	$ 601.42
Range Eight [Member]
Exercise price range, lower limit (in dollars per share)	700
Exercise price range, upper limit (in dollars per share)	$ 792
Options outstanding, number (in shares) | shares	2,170
Options outstanding, weighted average exercise price (in dollars per share)	$ 769.23
Options outstanding, weighted average remaining contractual term (Year)	6 years 109 days
Options exercisable, number exercisable (in shares) | shares	1,665
Options exercisable, weighted average exercise price (in dollars per share)	$ 769.86
Range Nine [Member]
Exercise price range, lower limit (in dollars per share)	992
Exercise price range, upper limit (in dollars per share)	$ 992
Options outstanding, number (in shares) | shares	104
Options outstanding, weighted average exercise price (in dollars per share)	$ 992
Options outstanding, weighted average remaining contractual term (Year)	3 years 109 days
Options exercisable, number exercisable (in shares) | shares	104
Options exercisable, weighted average exercise price (in dollars per share)	$ 992