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Claims Accruals (Self Insurance) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Loss Contingencies [Line Items]    
Self Insurance Retention $ 10.0  
Excess coverage layer   $ 5.0
Excess of excess coverage layer   5.0
Cargo Insurance per truck or trailer 2.0  
Cargo insurance per occurrence 15.0  
Knight Transportation Company [Member]    
Loss Contingencies [Line Items]    
Insurance Aggregate Deductible Amount   2.5
Self Retention For Employee Medical Health 0.4  
Swift Transportation Company [Member]    
Loss Contingencies [Line Items]    
Self Insurance Retention   10.0
Self Insurance Retention Workers Compensation Claims Per Occurrence 5.0  
Self Retention For Employee Medical Health 0.5  
ACT    
Loss Contingencies [Line Items]    
Self Insurance Retention 2.0  
Insurance Aggregate Deductible Amount 5.0  
Self Insurance Retention Workers Compensation Claims Per Occurrence 1.0  
Self Retention For Employee Medical Health 1.0  
Policy Period November 1, 2021 to October 31, 2022    
Loss Contingencies [Line Items]    
Self Insurance Aggregate Coverage 100.0  
Policy Period November 1, 2020 to October 31, 2021    
Loss Contingencies [Line Items]    
Self Insurance Aggregate Coverage   100.0
Policy Period November 1, 2019 to October 31, 2020 [Member]    
Loss Contingencies [Line Items]    
Self Insurance Aggregate Coverage   130.0
Policy Period, March 1 2019 to March 1, 2020 [Member] | Knight Transportation Company [Member]    
Loss Contingencies [Line Items]    
Excess Personal Injury And Property Damage Liability Insurance 2.0  
Policy Period, March 1, 2018 to March 1, 2019 [Member] | Knight Transportation Company [Member]    
Loss Contingencies [Line Items]    
Excess Personal Injury And Property Damage Liability Insurance 1.0  
Minimum [Member] | Knight Transportation Company [Member]    
Loss Contingencies [Line Items]    
Self Insurance Retention   1.0
Maximum [Member] | Knight Transportation Company [Member]    
Loss Contingencies [Line Items]    
Self Insurance Retention   $ 3.0
Maximum [Member] | ACT    
Loss Contingencies [Line Items]    
Insurance Aggregate Deductible Amount $ 10.0