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Significant Accounting Policies Self-Insurance Accruals (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Significant Accounting Policies [Abstract] |  |
Health insurance reserves | $ 4.5 | $ 6.0

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- Definition Self insurance health insurance reserves for estimated cost of health related claims, including estimated expenses incurred but not reported + References No definition available. + Details
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Namespace Prefix: | htld_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Significant Accounting Policies [Abstract] + References No definition available. + Details
Name: | htld_SignificantAccountingPoliciesAbstract
Namespace Prefix: | htld_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration