Note 9 - Legal Proceedings (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
Jun. 30, 2019	Mar. 31, 2020
Arbitration Case with BNSF [Member]
Litigation Settlement, Charge Resulting from Adjustment to Amount Owed		$ 8.2
Casualty Claim [Member] \| DCS [Member] \| Settled Litigation [Member]
Litigation Settlement, Charge Resulting from Adjustment to Amount Owed	$ 20.0

The amount of loss pertaining to the specified contingency that was charged against earnings in the period, including the effects of revisions in previously reported estimates.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

Name:	us-gaap_LossContingencyLossInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration