| Document and Entity Information | Aug. 09, 2022 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001464963 |
| Document Type | 8-K |
| Document Period End Date | Aug. 09, 2022 |
| Entity Registrant Name | FIRST EAGLE ALTERNATIVE CAPITAL BDC, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 814-00789 |
| Entity Tax Identification Number | 27-0344947 |
| Entity Address, Address Line One | 500 Boylston St. |
| Entity Address, Address Line Two | Suite 1200 |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02116 |
| City Area Code | (800) |
| Local Phone Number | 450-4424 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock, par value $0.001 per share |
| Trading Symbol | FCRD |
| Security Exchange Name | NASDAQ |
| Senior Notes [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.0% Senior Notes due 2026 |
| Trading Symbol | FCRX |
| Security Exchange Name | NYSE |
