Note 15 - Commitments and Contingent Liabilities (Details Textual) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Letters of Credit Outstanding, Amount		$ 32.9	$ 27.2
Revenue Equipment [Member]
Purchase Commitment, Remaining Minimum Amount Committed		$ 51.7	$ 86.5
Cargo Claim [Member] | Judicial Ruling [Member]
Increase (Decrease) in Estimated Litigation Liability	$ 0.9