Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Income Tax Contingency
Income tax expense	$ 347	$ 696
Effective income tax rate (percentage)	32.17%	24.79%
Current federal income tax expense	$ 133	$ 297
State income tax expense (benefit)	(72)	(218)
Deferred income tax expense	286	369
Deferred tax asset	23,136		$ 23,422
Martin Transport Inc
Income Tax Contingency
Income tax expense	$ 347	$ 448