Note 9 - Legal Proceedings (Details Textual) - USD ($) $ in Millions		1 Months Ended		3 Months Ended
	Oct. 24, 2019	Oct. 30, 2019	Jun. 30, 2019	Mar. 31, 2020
Arbitration Case with BNSF [Member]
Loss Contingency, Loss in Period				$ 8.2
Arbitration Case with BNSF [Member] | Final Award for Period Of January 1, 2018 Through December 31, 2018 [Member]
Loss Contingency, Loss in Period	$ 44.2	$ 26.8
Arbitration Case with BNSF [Member] | 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
Casualty Claim [Member] | Settled Litigation [Member] | DCS [Member]
Loss Contingency, Loss in Period			$ 20.0