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Note 10 - Commitments and Contingencies (Details Textual) - USD ($) $ in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Jan. 11, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leases, Future Minimum Payments Due, Total			$ 117.8		$ 117.8
Operating Leases, Future Minimum Payments Due, Next Twelve Months			34.9		34.9
Operating Leases, Future Minimum Payments, Due in Two Years			29.9		29.9
Operating Leases, Future Minimum Payments, Due in Three Years			20.7		20.7
Operating Leases, Future Minimum Payments, Due in Four Years			13.7		13.7
Operating Leases, Future Minimum Payments, Due Thereafter			18.6		18.6
Operating Leases, Rent Expense, Net, Total					98.7	$ 64.3	$ 44.1
Purchase Obligation, Due in Next Twelve Months			381.6		381.6
Loss Contingency, Estimate of Possible Loss			1.3		$ 1.3
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] \| Subsequent Event [Member]
Loss Contingency, Damages Awarded, Value	$ 89.4
Arbitration Case with BNSF [Member] \| Second Interim Award by Abritrators for 2016 Period at Issue [Member] \| Subsequent Event [Member]
Loss Contingency, Damages Awarded, Value	52.1
Arbitration Case with BNSF [Member] \| Second Interim Award by Abritrators for 2017 [Member] \| Subsequent Event [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			44.6
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