| Cover Page | Jan. 28, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 28, 2021 |
| Entity Registrant Name | First Business Financial Services, Inc. |
| Entity Incorporation, State or Country Code | WI |
| Entity File Number | 1-34095 |
| Entity Tax Identification Number | 39-1576570 |
| Entity Address, Address Line One | 401 Charmany Drive |
| Entity Address, City or Town | Madison |
| Entity Address, State or Province | WI |
| Entity Address, Postal Zip Code | 53719 |
| City Area Code | 608 |
| Local Phone Number | 238-8008 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock, $0.01 par value |
| Trading Symbol | FBIZ |
| Security Exchange Name | NASDAQ |
| Entity Central Index Key | 0001521951 |
| Amendment Flag | false |
