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Servicing Asset - Unpaid principal balance (Details) - Loan servicing - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Servicing Assets at Fair Value [Line Items] |  |
Loans serviced for others | $ 103,878 | $ 103,981
SBA guaranteed loans |  |
Servicing Assets at Fair Value [Line Items] |  |
Loans serviced for others | $ 103,878 | $ 103,981

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- Definition Amount, before allowance for credit loss, of financing receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447 + Details
Name: | us-gaap_NotesReceivableGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=inbk_LoanServicingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=inbk_SBAGuaranteedLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |