Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 15, 2021	Jun. 30, 2020
Document Information [Line Items]
Entity Central Index Key	0000799167
Entity Registrant Name	MARTEN TRANSPORT LTD
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2020
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	0-15010
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	39-1140809
Entity Address, Address Line One	129 Marten Street
Entity Address, City or Town	Mondov
Entity Address, State or Province	WI
Entity Address, Postal Zip Code	54755
City Area Code	715
Local Phone Number	926-4216
Title of 12(b) Security	COMMON STOCK, PAR VALUE
Trading Symbol	MRTN
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Public Float			$ 1,080,744,000
Entity Common Stock, Shares Outstanding		82,705,005