Related Party Transactions (Schedule Of Services Received And Provided By Company) (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current		$ 135	$ 2,889
Accounts Payable, Related Parties, Current		51	2
Central Freight Lines, Inc. [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current		133	2,872
Accounts Payable, Related Parties, Current		0	0
SME Industries [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current		0	17
Accounts Payable, Related Parties, Current		0	0
DPF Mobile [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current		0	0
Accounts Payable, Related Parties, Current		41	2
Other Affiliated Entities [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current		2	0
Accounts Payable, Related Parties, Current		10	0
Freight Services [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift		7,893	19,996	$ 1,379
Received by Knight-Swift		0	0	0
Freight Services [Member] | Central Freight Lines, Inc. [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	7,837	19,651	681
Received by Knight-Swift	[1]	0	0	0
Freight Services [Member] | SME Industries [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	56	345	698
Received by Knight-Swift	[1]	0	0	0
Facility and Equipment Leases [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift		59	340	935
Received by Knight-Swift		506	369	370
Facility and Equipment Leases [Member] | Central Freight Lines, Inc. [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	48	322	916
Received by Knight-Swift	[1]	277	369	370
Facility and Equipment Leases [Member] | Other Affiliated Entities [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	11	18	19
Received by Knight-Swift	[1]	229	0	0
Other Services [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift		442	1,873	589
Received by Knight-Swift		68	2,652	2,590
Other Services [Member] | Central Freight Lines, Inc. [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	427	1,834	0
Received by Knight-Swift	[1]	0	0	0
Other Services [Member] | DPF Mobile [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	0	0	0
Received by Knight-Swift	[1]	33	220	308
Other Services [Member] | Other Affiliated Entities [Member]
Related Party Transaction [Line Items]
Provided by Knight-Swift	[1]	15	39	589
Received by Knight-Swift	[1]	$ 35	$ 2,432	$ 2,282

[1] Entities affiliated with former Board member Jerry Moyes include Central Freight Lines, SME Industries, Compensi Services, and DPF Mobile. "Other affiliates" includes entities that are associated with various board members and executives and require approval by the Board prior to completing transactions. Transactions with these entities generally include freight services, facility and equipment leases, equipment sales, and other services. •Freight Services Provided by Knight-Swift — The Company charges each of these companies for transportation services. •Freight Services Received by Knight-Swift — Transportation services received from Central Freight represent less-than-truckload freight services rendered to haul parts and equipment to Company shop locations. •Other Services Provided by Knight-Swift — Other services provided by the Company to the identified related parties include equipment sales and miscellaneous services. •Other Services Received by Knight-Swift — Consulting fees, diesel particulate filter cleaning, sales of various parts and tractor accessories, and certain third-party payroll and employee benefits administration services from the identified related parties are included in other services received by the Company. During the quarter ended September 30, 2020, the ownership percentage of Jerry Moyes and related affiliates fell below the threshold requiring related party disclosure. The amounts included in this Note 24 pertain to transactions that occurred prior to the date that the ownership percentage changed.