| Document and Entity Information | Feb. 15, 2023 |
| Cover [Abstract] | |
| Entity Registrant Name | Fidus Investment Corporation |
| Amendment Flag | false |
| Entity Central Index Key | 0001513363 |
| Document Type | 8-K |
| Document Period End Date | Feb. 15, 2023 |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 814-00861 |
| Entity Tax Identification Number | 27-5017321 |
| Entity Address, Address Line One | 1603 Orrington Avenue |
| Entity Address, Address Line Two | Suite 1005 |
| Entity Address, City or Town | Evanston |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60201 |
| City Area Code | 847 |
| Local Phone Number | 859-3940 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.001 per share |
| Trading Symbol | FDUS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
