Note 4 - Fair Value Measurements - Discounted Cash Flow Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Time-to-liquidity (years) (Year)		2 years
Minimum [Member]
Time-to-liquidity (years) (Year)	2 years
Discount rate	4.50%	4.30%
Maximum [Member]
Time-to-liquidity (years) (Year)	3 years
Discount rate	9.60%	9.30%