Summary of Significant Accounting Policies (Schedule of Effects of Change in Accounting Estimate) (Details) - USD ($) $ in Thousands	12 Months Ended
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

Describes the specific accounting estimate that was revised, including the nature of and justification for the revision.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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