Note 7 - Share-based Compensation - Fair Value Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Expected volatility		52.30%
Expected dividends	0.00%	0.00%
Performance Shares [Member]
Expected dividends	0.00%
Expected term (in years) (Year)	3 years
Minimum [Member]
Expected volatility	50.10%
Exercise price (in dollars per share)	$ 30.83	$ 7.93
Expected term (in years) (Year)	5 years 255 days	109 days
Risk-free rate	2.03%	0.03%
Minimum [Member] | Performance Shares [Member]
Expected volatility	30.80%
Risk-free rate	1.94%
30 calendar day volume weighted average stock price, including peer group (in dollars per share)	$ 2.13
Maximum [Member]
Expected volatility	53.20%
Exercise price (in dollars per share)	$ 42.65	$ 27.12
Expected term (in years) (Year)	6 years	4 years 109 days
Risk-free rate	2.24%	1.30%
Maximum [Member] | Performance Shares [Member]
Expected volatility	81.60%
Risk-free rate	1.95%
30 calendar day volume weighted average stock price, including peer group (in dollars per share)	$ 133.20