| Pay vs Performance Disclosure - USD ($) | 12 Months Ended |
| Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 |
| Pay vs Performance Disclosure [Table] | | | |
| Pay vs Performance [Table Text Block] | The following table presents information about our Named Executive Officers’ pay and Company performance for each of the Company’s three most recently completed fiscal years. PAY VERSUS PERFORMANCE TABLE Value of Initial Fixed $100 Investment Based On: Year Summary Compensation Table Total for CEO ($) (1) Compensation Actually Paid to CEO($) (1)(2) Average Summary Compensation Table Total for Non-CEO NEOs ($) (3) Average Compensation Actually Paid to Non-CEO NEOs ($) (2)(3) Total Shareholder Return ($) Peer Group Total Shareholder Return ($) (4) Net Income ($) (in 000’s) Operating Income ($) (in 000’s) 2022 5,460,198 2,465,428 1,094,807 701,308 115 176 245,580 323,076 2021 5,314,351 8,798,833 1,133,218 1,587,300 134 215 261,478 309,146 2020 4,810,029 6,822,876 1,102,850 1,423,523 109 132 169,078 227,438 (1) Represents compensation of Mr. Leathers for all periods presented. (2) SEC rules require certain adjustments be made to the Summary Compensation Table totals to determine “compensation actually paid.” The Reconciliation of Compensation Actually Paid Adjustments table below details the applicable adjustments that were made to determine “compensation actually paid” (all amounts are averages for the non-CEO NEOs). (3) Compensation of Messrs. Steele, Nordlund, Schelble, and Johnson are included in the average compensation calculations for all periods presented. (4) Our Peer Group includes companies similar to us in the transportation industry and has the following companies: ArcBest; Covenant Logistics Group; Forward Air; Heartland Express; Hub Group; JB Hunt; Knight-Swift Transportation; Landstar System; Marten Transport; Old Dominion Freight Line; Saia; Schneider National; US Xpress; and Yellow Corporation. | | |
| Named Executive Officers, Footnote [Text Block] | Represents compensation of Mr. Leathers for all periods presented.Compensation of Messrs. Steele, Nordlund, Schelble, and Johnson are included in the average compensation calculations for all periods presented. | | |
| Peer Group Issuers, Footnote [Text Block] | Our Peer Group includes companies similar to us in the transportation industry and has the following companies: ArcBest; Covenant Logistics Group; Forward Air; Heartland Express; Hub Group; JB Hunt; Knight-Swift Transportation; Landstar System; Marten Transport; Old Dominion Freight Line; Saia; Schneider National; US Xpress; and Yellow Corporation. | | |
| PEO Total Compensation Amount | $ 5,460,198 | $ 5,314,351 | $ 4,810,029 |
| PEO Actually Paid Compensation Amount | $ 2,465,428 | 8,798,833 | 6,822,876 |
| Adjustment To PEO Compensation, Footnote [Text Block] | SEC rules require certain adjustments be made to the Summary Compensation Table totals to determine “compensation actually paid.” The Reconciliation of Compensation Actually Paid Adjustments table below details the applicable adjustments that were made to determine “compensation actually paid” (all amounts are averages for the non-CEO NEOs). The following table reconciles total compensation reported in the Summary Compensation Table to “compensation actually paid” for the CEO and non-CEO NEOs (averaged) over the last three years. RECONCILIATION OF COMPENSATION ACTUALLY PAID ADJUSTMENTS Year Summary Compensation Table “Total” Column ($) Subtract Summary Compensation Table “Stock Awards” Column ($) Add Year End Fair Value of Unvested Equity Awards Granted in Year ($) Add/ (Subtract) Year Over Year Change in Fair Value of Unvested Equity Awards Granted in Prior Years ($) Add/ (Subtract) Year Over Year Change in Fair Value of Equity Awards Granted in Prior Years that Vested in Year ($) Compensation Actually Paid ($) CEO 2022 5,460,198 (3,200,000) 2,507,777 (2,102,923) (199,624) 2,465,428 2021 5,314,351 (2,800,000) 4,741,960 1,288,523 253,999 8,798,833 2020 4,810,029 (2,800,000) 4,252,764 302,174 257,909 6,822,876 Non-CEO NEOs (Average) 2022 1,094,807 (350,000) 274,289 (287,911) (29,877) 701,308 2021 1,133,218 (312,500) 529,187 195,957 41,438 1,587,300 2020 1,102,850 (412,500) 626,539 49,962 56,672 1,423,523 | | |
| Non-PEO NEO Average Total Compensation Amount | $ 1,094,807 | 1,133,218 | 1,102,850 |
| Non-PEO NEO Average Compensation Actually Paid Amount | $ 701,308 | 1,587,300 | 1,423,523 |
| Adjustment to Non-PEO NEO Compensation Footnote [Text Block] | SEC rules require certain adjustments be made to the Summary Compensation Table totals to determine “compensation actually paid.” The Reconciliation of Compensation Actually Paid Adjustments table below details the applicable adjustments that were made to determine “compensation actually paid” (all amounts are averages for the non-CEO NEOs). The following table reconciles total compensation reported in the Summary Compensation Table to “compensation actually paid” for the CEO and non-CEO NEOs (averaged) over the last three years. RECONCILIATION OF COMPENSATION ACTUALLY PAID ADJUSTMENTS Year Summary Compensation Table “Total” Column ($) Subtract Summary Compensation Table “Stock Awards” Column ($) Add Year End Fair Value of Unvested Equity Awards Granted in Year ($) Add/ (Subtract) Year Over Year Change in Fair Value of Unvested Equity Awards Granted in Prior Years ($) Add/ (Subtract) Year Over Year Change in Fair Value of Equity Awards Granted in Prior Years that Vested in Year ($) Compensation Actually Paid ($) CEO 2022 5,460,198 (3,200,000) 2,507,777 (2,102,923) (199,624) 2,465,428 2021 5,314,351 (2,800,000) 4,741,960 1,288,523 253,999 8,798,833 2020 4,810,029 (2,800,000) 4,252,764 302,174 257,909 6,822,876 Non-CEO NEOs (Average) 2022 1,094,807 (350,000) 274,289 (287,911) (29,877) 701,308 2021 1,133,218 (312,500) 529,187 195,957 41,438 1,587,300 2020 1,102,850 (412,500) 626,539 49,962 56,672 1,423,523 | | |
| Compensation Actually Paid vs. Total Shareholder Return [Text Block] | The following graphs illustrate the relationships between each of the financial performance measures included in the Pay Versus Performance Table above and the compensation actually paid to the CEO and, on average, to non-CEO NEOs over the Company’s three most recently completed fiscal years. | | |
| Compensation Actually Paid vs. Net Income [Text Block] | | | |
| Compensation Actually Paid vs. Company Selected Measure [Text Block] | | | |
| Total Shareholder Return Vs Peer Group [Text Block] | The following graphs illustrate the relationships between each of the financial performance measures included in the Pay Versus Performance Table above and the compensation actually paid to the CEO and, on average, to non-CEO NEOs over the Company’s three most recently completed fiscal years. | | |
| Total Shareholder Return Amount | $ 115 | 134 | 109 |
| Peer Group Total Shareholder Return Amount | 176 | 215 | 132 |
| Net Income (Loss) | $ 245,580,000 | $ 261,478,000 | $ 169,078,000 |
| Company Selected Measure Amount | 323,076,000 | 309,146,000 | 227,438,000 |
| PEO Name | Mr. Leathers | | |
| Measure [Axis]: 1 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | •Revenues, less fuel surcharge revenues | | |
| Measure [Axis]: 2 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | •Operating income | | |
| Measure [Axis]: 3 | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Measure Name | Diluted earnings per share | | |
| PEO [Member] | Stock Awards Adjustment [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | $ (3,200,000) | $ (2,800,000) | $ (2,800,000) |
| PEO [Member] | Equity Awards Granted in Current Year, Fair Value Adjustment [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 2,507,777 | 4,741,960 | 4,252,764 |
| PEO [Member] | Equity Awards Adjustment, Unvested, Granted in Prior Years [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (2,102,923) | 1,288,523 | 302,174 |
| PEO [Member] | Equity Awards Adjustment, Vested, Granted in Prior Years [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (199,624) | 253,999 | 257,909 |
| Non-PEO NEO [Member] | Stock Awards Adjustment [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (350,000) | (312,500) | (412,500) |
| Non-PEO NEO [Member] | Equity Awards Granted in Current Year, Fair Value Adjustment [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | 274,289 | 529,187 | 626,539 |
| Non-PEO NEO [Member] | Equity Awards Adjustment, Unvested, Granted in Prior Years [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | (287,911) | 195,957 | 49,962 |
| Non-PEO NEO [Member] | Equity Awards Adjustment, Vested, Granted in Prior Years [Member] | | | |
| Pay vs Performance Disclosure [Table] | | | |
| Adjustment to Compensation Amount | $ (29,877) | $ 41,438 | $ 56,672 |
