Note 8 - Stock Repurchase (Details Textual) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 29, 2016	Jul. 31, 2013
Stock Repurchase Program, Authorized Amount				$ 50.0	$ 100.0
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 50.0		$ 5.9
Stock Repurchased and Retired During Period, Shares	0	0	600
Shares Repurchased, Value			$ 32.3
Repurchase Stock Average Price Per Share			$ 50.05