Cover - shares	6 Months Ended
	Jun. 30, 2021	Aug. 06, 2021
Class of Stock [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Transition Report	false
Entity File Number	001-35750
Entity Registrant Name	First Internet Bancorp
Entity Incorporation, State or Country Code	IN
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		9,854,153
Document Fiscal Period Focus	Q2
Document Fiscal Year Focus	2021
Amendment Flag	false
Entity Central Index Key	0001562463
Current Fiscal Year End Date	--12-31
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 2026
Class of Stock [Line Items]
Title of 12(b) Security	6.0% Fixed to Floating Subordinated Notes due 2026
Trading Symbol	INBKL
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