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Note 5 - Transactions With Affiliated Companies (Details Textual)	Dec. 31, 2023
Cadrenal Therapeutics, Inc. [Member] | Investment, Affiliated Issuer, Noncontrolled [Member]
Equity Ownership, Excluding Consolidated Entity and Equity Method Investee, Percentage	5.00%

X

- Definition

Percentage of ownership of equity interest excluding interest in entity that is consolidated and equity method investee.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 10
-Topic 321
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479536/321-10-50-1

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_InvestmentIssuerNameAxis=hrzn_CadrenalTherapeuticsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentIssuerAffiliationAxis=us-gaap_InvestmentAffiliatedIssuerNoncontrolledMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: