Note 10 - Stock-based Compensation - Stock Option and Restricted Stock (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Equity awards outstanding, options (in shares) | shares	210,023
Equity awards outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 59.77
Equity awards granted, options (in shares) | shares	513,464
Equity awards granted, weighted average exercise price (in dollars per share) | $ / shares	$ 2.69
Equity awards outstanding, contractual term (Year)	9 years 138 days
Equity awards exercisable, options (in shares) | shares	405,075
Equity awards exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 14.97
Equity awards exercisable, contractual term (Year)	9 years 36 days
Aggregate intrinsic value of awards exercisable, options | $
Equity awards forfeited, cancelled or expired, options (in shares) | shares	(42,545)
Equity awards forfeited, cancelled or expired, weighted average exercise price (in dollars per share) | $ / shares	$ 157.97
Equity awards outstanding, options (in shares) | shares	680,942
Equity awards outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 10.59
Restricted Stock [Member]
Equity awards outstanding, non-vested restricted stock (in shares) | shares	4,785
Equity awards outstanding, non-vested restricted stock weighted average grant date fair value (in dollars per share) | $ / shares	$ 37.42
Equity awards granted, non-vested restricted stock (in shares) | shares
Equity awards granted, non-vested restricted stock weighted average grant date fair value (in dollars per share) | $ / shares
Equity awards outstanding, non-vested restricted stock weighted average grant date fair value (in dollars per share) | $ / shares	$ 42.20
Aggregate intrinsic value of outstanding awards, non-vested restricted stock | $	$ 6,882
Equity awards forfeited, cancelled or expired, non-vested restricted stock (in shares) | shares	(1,428)
Equity awards forfeited, cancelled or expired, non-vested restricted stock weighted average grant date fair value (in dollars per share) | $ / shares	$ 26.18
Equity awards outstanding, non-vested restricted stock (in shares) | shares	3,357