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Summary of Significant Accounting Policies (Schedule of Effects of Change in Accounting Estimate) (Details) - Change in accounting method accounted for as change in estimate [Member] $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($)
Change in Accounting Estimate [Line Items] |
Change in Accounting Estimate, Description | During fourth quarter 2016, due to the weak used truck market, we reduced the estimated life of certain trucks to more rapidly depreciate the trucks to their residual values.
Depreciation | $ 3.4

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