Note 8 - Federal Income Tax - Tax Characters of Distributions Paid (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Ordinary income	$ 42,576	$ 31,490	$ 25,099
Total	$ 42,576	$ 31,490	$ 25,099

Amount of dividend distribution from ordinary income and capital gain paid during the period. Excludes distribution for tax return of capital.

No definition available.

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Namespace Prefix:	hrzn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration