Note 10 - Fair Value Measurements - Discounted Cash Flow Assumptions (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Minimum [Member]
Time-to-liquidity (years) (Year)	2 years	2 years
Discount rate	5.30%	4.50%
Maximum [Member]
Time-to-liquidity (years) (Year)	3 years	3 years
Discount rate	10.30%	9.60%