Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Tax Contingency
Income tax expense	$ 373	$ 237	$ 1,510	$ 1,572
Effective income tax rate (percentage)	45.72%	27.37%	80.42%	25.88%
Taxes related to 2019 tax return	$ 341		$ 341
Effect of previous taxes on effective tax rate (percentage)			23.60%
Current federal income tax expense	0	$ (51)	$ (174)	$ 167
State income tax expense (benefit)	41	45	134	(153)
Deferred income tax expense	183	244	1,202	1,100
Deferred tax asset	22,220		22,220		$ 23,422
Martin Transport Inc
Income Tax Contingency
Income tax expense	$ 225	$ 237	$ 1,162	$ 1,114