Note 14 - Employee Benefits - Non-vested Performance Unit Award Activity (Details) - Performance Shares [Member]	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Nonvested (in shares) | shares	322,662
Nonvested (in dollars per share) | $ / shares	$ 13.32
Granted (in shares) | shares	77,700
Granted (in dollars per share) | $ / shares	$ 20.31
Vested (in shares) | shares	(182,481)	[1]
Vested (in dollars per share) | $ / shares	$ 14.55
Forfeited (in shares) | shares	(31,973)
Forfeited (in dollars per share) | $ / shares	$ 13.81
Nonvested (in shares) | shares	185,909
Nonvested (in dollars per share) | $ / shares	$ 14.89

[1] This number of performance unit award shares vested based on our financial performance in 2018 and was distributed or credited to the Marten Transport, Ltd. Deferred Compensation Plan in February 2019. As permitted in the performance unit award agreements granted in 2014 and 2015, the Compensation Committee of our Board of Directors adjusted the calculation of the performance vesting component for 2018 to be based on our increase in net income instead of our increase in diluted net income per share. Additionally, the $56.5 million deferred income taxes benefit in 2017 related to the federal Tax Cuts and Jobs Act of 2017 was excluded from the calculation of the increase in net income from 2017 to 2018. The calculation of the increase in net income also included an adjustment of 2017 net income to reflect a federal income tax rate of 21% instead of 35%, to be consistent with the federal rate in 2018. The fair value of unit award shares that vested in 2018 was $2.7 million.