Note 13 - Employee Benefits - Non-vested Performance Unit Award Activity (Details) - Performance Shares [Member]	12 Months Ended
	Dec. 31, 2020 $ / shares shares
Nonvested, shares (in shares) | shares	207,179
Nonvested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 11.44
Granted, shares (in shares) | shares	73,205
Granted, weighted average grant date fair value (in dollars per share) | $ / shares	$ 16.44
Vested, shares (in shares) | shares	(109,304)	[1]
Vested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 11.79
Forfeited, shares (in shares) | shares	(16,719)
Forfeited, weighted average grant date fair value (in dollars per share) | $ / shares	$ 7.49
Nonvested, shares (in shares) | shares	154,361
Nonvested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 13.99

[1] This number of performance unit award shares vested based on our financial performance in 2020 and was distributed or credited to the Marten Transport, Ltd. Deferred Compensation Plan in March 2021. The fair value of unit award shares that vested in 2020 was $1.3 million.