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Summary of Significant Accounting Policies (Schedule of Effects of Change in Accounting Estimate) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Change in Accounting Estimate [Line Items] |  |  |
Depreciation | $ 263,286 | $ 249,527 | $ 230,151
Change in accounting method accounted for as change in estimate [Member] |  |  |
Change in Accounting Estimate [Line Items] |  |  |
Change in Accounting Estimate, Description | During first quarter 2020, we changed the estimated life of certain trucks expected to be sold in 2020 to more rapidly depreciate the trucks to their estimated residual values due to the weak used truck market. |  |
Depreciation | $ 9,600 |  |

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