Note 9 - Legal Proceedings (Details Textual) - USD ($) $ in Millions		1 Months Ended		3 Months Ended
	Jan. 11, 2019	Jun. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2018
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				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
Casualty Claim [Member] | Settled Litigation [Member] | DCS [Member]
Loss Contingency, Loss in Period		$ 20.0