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Note 10 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions	1 Months Ended				3 Months Ended
	Jun. 30, 2019	Jan. 31, 2019	Jan. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2019
Purchase Obligation, Due in Next Twelve Months								$ 938.0
Loss Contingency, Estimate of Possible Loss								$ 2.7
Operating Leases, Future Minimum Payments Due, Total						$ 117.8
Operating Leases, Future Minimum Payments Due, Next Twelve Months						34.9
Operating Leases, Future Minimum Payments, Due in Two Years						29.9
Operating Leases, Future Minimum Payments, Due in Three Years						20.7
Operating Leases, Future Minimum Payments, Due in Four Years						13.7
Operating Leases, Future Minimum Payments, Due Thereafter						18.6
Lawsuit for Claims of Unpaid Wages, Failure to Provide Meal and Rest Periods, and Other Items [Member]
Loss Contingency, Damages Sought, Value				$ 15.0
Arbitration Case with BNSF [Member]
Loss Contingency, Loss in Period							$ 18.3
Arbitration Case with BNSF [Member] \| Second Interim Award by Abritrators [Member]
Loss Contingency, Damages Awarded, Value		$ 89.4
Arbitration Case with BNSF [Member] \| Second Interim Award by Abritrators for 2016 Period at Issue [Member]
Loss Contingency, Damages Awarded, Value			$ 52.1
Arbitration Case with BNSF [Member] \| Second Interim Award by Abritrators for 2017 [Member]
Loss Contingency, Damages Awarded, Value			$ 37.3
Arbitration Case with BNSF [Member] \| Interim Award, Contingent Liabilities for Period of May 1, 2016 through December 31, 2017 [Member]
Loss Contingency, Loss in Period						89.4
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	44.6
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] \| Interim Award, Contingent Liabilities for Period of May 1, 2016 through December 31, 2018 [Member]
Loss Contingency, Loss in Period						$ 134.0
Arbitration Case with BNSF [Member] \| Settled Litigation [Member]
Loss Contingency, Loss in Period					$ 44.2
Casualty Claim [Member] \| Settled Litigation [Member] \| DCS [Member]
Loss Contingency, Loss in Period	$ 20.0