Note 4 - Fair Value Measurements - Summary of Activities in Financial Instruments With Level 3 Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Balance	$ 14.2	$ 18.8
Fair value change included in net income (loss)	2.6	(4.6)
Balance	16.8	14.2
Changes in unrealized gains or losses for the period included in earnings (or changes in net assets) for liabilities held at the end of the reporting period	$ 2.6	$ (4.6)