Note 9 - Legal Proceedings (Details Textual) - Arbitration Case with BNSF [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Sep. 30, 2019
Loss Contingency, Loss in Period	$ 8.2
Final Award for Period Of January 1, 2018 Through December 31, 2018 [Member]
Loss Contingency, Loss in Period		$ 44.2
Final Award for Period Of January 1, 2018 Through December 31, 2018 [Member] | Certain Charges Claimed [Member]
Loss Contingency, Loss in Period		26.8
Final Award for Period Of January 1, 2018 Through December 31, 2018 [Member] | Legal Fees, Cost and Interest [Member]
Loss Contingency, Loss in Period		$ 17.4