| Cover | Oct. 20, 2021 |
| Class of Stock [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 18, 2021 |
| Entity Registrant Name | First Internet Bancorp |
| Entity Incorporation, State or Country Code | IN |
| Entity File Number | 001-35750 |
| Entity Tax Identification Number | 20-3489991 |
| Entity Address, Address Line One | 11201 USA Parkway |
| Entity Address, City or Town | Fishers |
| Entity Address, State or Province | IN |
| Entity Address, Postal Zip Code | 46037 |
| City Area Code | 317 |
| Local Phone Number | 532-7900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001562463 |
| Amendment Flag | false |
| Common Stock | |
| Class of Stock [Line Items] | |
| Title of 12(b) Security | Common Stock, without par value |
| Trading Symbol | INBK |
| Security Exchange Name | NASDAQ |
| 6.0% Fixed to Floating Subordinated Notes due 2029 | |
| Class of Stock [Line Items] | |
| Title of 12(b) Security | 6.0% Fixed to Floating Subordinated Notes due 2029 |
| Trading Symbol | INBKZ |
| Security Exchange Name | NASDAQ |
