Note 14 - Employee Benefits - Non-vested Performance Unit Award Activity (Details) - Performance Shares [Member]	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Nonvested, shares (in shares) | shares	185,909
Nonvested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 14.89
Granted, shares (in shares) | shares	70,000
Granted, weighted average grant date fair value (in dollars per share) | $ / shares	$ 19.89
Vested, shares (in shares) | shares	(78,305)	[1]
Vested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 15.88
Forfeited, shares (in shares) | shares	(39,487)
Forfeited, weighted average grant date fair value (in dollars per share) | $ / shares	$ 14.33
Nonvested, shares (in shares) | shares	138,117
Nonvested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 17.16

[1] This number of performance unit award shares vested based on our financial performance in 2019 and was distributed or credited to the Marten Transport, Ltd. Deferred Compensation Plan in March 2020. As permitted in the performance unit award agreements granted in 2015, the Compensation Committee of our Board of Directors adjusted the calculation of the performance vesting component for 2019 to be based on our increase in net income instead of our increase in diluted net income per share. The fair value of unit award shares that vested in 2019 was $1.2 million.