Note 12 - Employee Benefits - Non-vested Performance Unit Award Activity (Details) - Performance Shares [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Nonvested (in shares) | shares	253,028
Nonvested (in dollars per share) | $ / shares	$ 11.66
Granted (in shares) | shares	154,511
Granted (in dollars per share) | $ / shares	$ 14.48
Vested (in shares) | shares	(37,648)	[1]
Vested (in dollars per share) | $ / shares	$ 12.86
Forfeited (in shares) | shares	(47,229)
Forfeited (in dollars per share) | $ / shares	$ 8.58
Nonvested (in shares) | shares	322,662
Nonvested (in dollars per share) | $ / shares	$ 13.32

[1] This number of performance unit award shares vested based on our financial performance in 2017 and was distributed or credited to the Marten Transport, Ltd. Deferred Compensation Plan in March 2018. As permitted in the performance unit award agreements granted in 2013 through 2015, the Compensation Committee of our Board of Directors adjusted the calculation of the performance vesting component for 2017 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 related to the federal Tax Cuts and Jobs Act of 2017 was excluded from the calculation of the increase in net income from 2016 to 2017. The fair value of unit award shares that vested in 2017 was $484,000.