Note 10 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jun. 30, 2019	Mar. 31, 2020	Sep. 30, 2019	Dec. 31, 2020
Purchase Obligation, to be Paid, Year One				$ 1,120.0
Loss Contingency, Estimate of Possible Loss				$ 3.8
Arbitration Case with BNSF [Member] | Interim Award, Contingent Liabilities for Period of January 1, 2018 through December 31, 2018 [Member]
Loss Contingency, Loss in Period			$ 26.8
Arbitration Case with BNSF [Member] | Interim Award, Contingent Liabilities for Period of January 1, 2018 through December 31, 2018 [Member] | Legal Fees, Cost and Interest [Member]
Loss Contingency, Loss in Period			17.4
Arbitration Case with BNSF [Member] | Settled Litigation [Member]
Loss Contingency, Loss in Period		$ 8.2	$ 44.2
Casualty Claim [Member] | Settled Litigation [Member] | DCS [Member]
Loss Contingency, Loss in Period	$ 20.0