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Note I - Income Taxes - Reconciliation of Deferred Tax Valuation Allowance (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Balance | $ 38,379 | $ 31,170
Deferred Income Tax Expense | (7,534) | 6,086
Return to Provision Impact | 12 | (364)
Other Comprehensive Income | (16) | 1,487
Balance | $ 30,841 | $ 38,379

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- Definition Represents valuation allowance of deferred tax assets for deferred income tax expense (benefit). + References No definition available. + Details
Name: | hrth_ValuationAllowanceDeferredTaxAssetDeferredIncomeTaxExpenseBenefit
Namespace Prefix: | hrth_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents other comprehensive income for deferred tax asset valuation. + References No definition available. + Details
Name: | hrth_ValuationAllowanceDeferredTaxAssetOtherComprehensiveIncome
Namespace Prefix: | hrth_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of valuation allowance for deferred tax asset return to provision impact. + References No definition available. + Details
Name: | hrth_ValuationAllowanceDeferredTaxAssetReturnToProvisionImpact
Namespace Prefix: | hrth_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
Name: | us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant