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Note 4 - Investments (Details Textual) - CRFH Joint Venture [Member] - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Jun. 30, 2026 | Mar. 18, 2026
Amount Committed to Invest |  | $ 87,500
Payments to Acquire Investments | $ 0 |
CR Financial Holdings [Member] |  |
Amount Committed to Invest |  | $ 12,500

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- Definition Represents the amount committed to invest by the entity. + References No definition available. + Details
Name: | hrzn_AmountCommittedToInvest
Namespace Prefix: | hrzn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The cash outflow associated with the purchase of all investments (debt, security, other) during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-13 + Details
Name: | us-gaap_PaymentsToAcquireInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=hrzn_CrfhJointVentureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=hrzn_CrFinancialHoldingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |