Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Income Tax Contingency
Income tax expense	$ 222	$ 347
Effective income tax rate (percentage)	29.32%	32.17%
Current federal income tax expense (benefit)	$ 15	$ 133
State income tax expense (benefit)	12	(72)
Deferred income tax expense	75	286
Deferred tax asset	22,178		$ 22,253
Martin Transport Inc
Income Tax Contingency
Income tax expense	$ 102	$ 347