Loans - Change in the Balances of the ALLL (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Allowance for loan losses:
Beginning balance	$ 36,058	$ 29,153	$ 31,737	$ 27,841
Provision for credit losses 1	1,850	892	11,976	2,868
Losses Charged Off	(1,517)	(236)	(10,639)	(2,467)
Recoveries	61	57	416	1,624
Ending balance	36,452	29,866	36,452	29,866
Commercial and industrial
Allowance for loan losses:
Beginning balance	1,849	2,026	1,711	1,891
Provision for credit losses 1	260	(301)	7,265	(166)
Losses Charged Off	0	0	(6,965)	0
Recoveries	1	2	219	2
Ending balance	2,110	1,727	2,110	1,727
Owner-occupied commercial real estate
Allowance for loan losses:
Beginning balance	789	703	651	742
Provision for credit losses 1	69	(87)	145	(126)
Losses Charged Off	0	0	0	0
Recoveries	0	0	0	0
Ending balance	858	616	858	616
Investor commercial real estate
Allowance for loan losses:
Beginning balance	1,416	621	1,099	328
Provision for credit losses 1	488	453	996	746
Losses Charged Off	(591)	0	(591)	0
Recoveries	0	0	0	0
Ending balance	1,313	1,074	1,313	1,074
Construction
Allowance for loan losses:
Beginning balance	1,940	1,707	2,074	1,612
Provision for credit losses 1	163	(117)	464	(22)
Losses Charged Off	0	0	0	0
Recoveries	0	0	0	0
Ending balance	2,103	1,590	2,103	1,590
Single tenant lease financing
Allowance for loan losses:
Beginning balance	9,970	9,712	10,519	10,385
Provision for credit losses 1	(1,605)	315	(1,808)	(1,589)
Losses Charged Off	0	0	0	0
Recoveries	0	0	0	1,231
Ending balance	8,365	10,027	8,365	10,027
Public finance
Allowance for loan losses:
Beginning balance	1,509	1,850	1,753	1,776
Provision for credit losses 1	(98)	(61)	(207)	13
Losses Charged Off	0	0	0	0
Recoveries	0	0	0	0
Ending balance	1,411	1,789	1,411	1,789
Healthcare finance
Allowance for loan losses:
Beginning balance	2,421	4,762	2,997	5,940
Provision for credit losses 1	(194)	(1,150)	(1,779)	(2,328)
Losses Charged Off	0	0	(25)	0
Recoveries	0	0	0	0
Ending balance	2,227	3,612	2,227	3,612
Small business lending
Allowance for loan losses:
Beginning balance	2,618	1,956	2,168	1,387
Provision for credit losses 1	2,341	217	3,834	847
Losses Charged Off	(751)	(130)	(2,169)	(210)
Recoveries	13	3	54	22
Ending balance	4,221	2,046	4,221	2,046
Franchise finance
Allowance for loan losses:
Beginning balance	4,484	2,281	3,988	1,083
Provision for credit losses 1	763	734	1,903	1,932
Losses Charged Off	0	0	(331)	0
Recoveries	0	0	0	0
Ending balance	5,247	3,015	5,247	3,015
Residential mortgage
Allowance for loan losses:
Beginning balance	2,550	1,138	1,559	643
Provision for credit losses 1	(215)	231	367	724
Losses Charged Off	(56)	0	(56)	0
Recoveries	1	1	4	3
Ending balance	2,280	1,370	2,280	1,370
Home equity
Allowance for loan losses:
Beginning balance	224	54	69	64
Provision for credit losses 1	(34)	7	(15)	(139)
Losses Charged Off	0	0	0	0
Recoveries	2	1	5	137
Ending balance	192	62	192	62
Other consumer loans
Allowance for loan losses:
Beginning balance	6,288	2,343	3,149	1,990
Provision for credit losses 1	(88)	651	811	1,116
Losses Charged Off	(119)	(106)	(502)	(397)
Recoveries	44	50	134	229
Ending balance	$ 6,125	2,938	$ 6,125	2,938
Tax Refund Advance Loans
Allowance for loan losses:
Beginning balance				0
Provision for credit losses 1				1,860
Losses Charged Off				(1,860)
Recoveries				0
Ending balance		$ 0		$ 0

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	us-gaap_AllowanceForLoanAndLeaseLossesRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoff
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestCreditLossExpenseReversal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_OwnerOccupiedCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_SingleTenantLeaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_PublicFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_HealthcareFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_SmallBusinessLendingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_FranchiseFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_OtherConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=inbk_TaxRefundAdvanceLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: