Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Income Tax Contingency
Income tax expense	$ 790	$ 639	$ 1,137	$ 1,335
Effective income tax rate (percentage)	(472.00%)	26.30%	98.32%	25.49%
Taxes related to 2019 tax return	$ 341		$ 341
Effect of previous taxes on effective tax rate (percenage)	272.80%		35.78%
Current federal income tax expense	$ (307)	$ (78)	$ (174)	$ 218
State income tax expense (benefit)	164	20	93	(198)
Deferred income tax expense	733	487	1,018	856
Deferred tax asset	22,404		22,404		$ 23,422
Martin Transport Inc
Income Tax Contingency
Income tax expense	$ 590	$ 429	$ 937	$ 877