Fair Value of Financial Instruments - Quantitative Information About Unobservable Inputs (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset	$ 23,614	$ 22,793
Other real estate owned	1,945	2,631
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset	23,614	22,793
Other real estate owned	1,945	2,631
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset	0	0
Other real estate owned	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset	0	0
Other real estate owned	0	0
Collateral dependent loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	282	336
Collateral dependent loans \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	282	336
Collateral dependent loans \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	0	0
Collateral dependent loans \| Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans	0	0
Servicing asset \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset	23,614	22,793
Other Real Estate Owned \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	$ 1,945	$ 2,631
Discount for type of property and current market conditions \| Fair value of collateral \| Collateral dependent loans \| Significant Unobservable Inputs (Level 3) \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans, measurement input	0.00%	0.00%
Discount for type of property and current market conditions \| Fair value of collateral \| Collateral dependent loans \| Significant Unobservable Inputs (Level 3) \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans, measurement input	40.00%	40.00%
Discount for type of property and current market conditions \| Fair value of collateral \| Collateral dependent loans \| Significant Unobservable Inputs (Level 3) \| Weighted-Average Range
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Collateral dependent loans, measurement input	30.80%	30.80%
Prepayment speeds \| Discounted cash flow \| Servicing asset \| Significant Unobservable Inputs (Level 3) \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0	0
Prepayment speeds \| Discounted cash flow \| Servicing asset \| Significant Unobservable Inputs (Level 3) \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.25	0.25
Prepayment speeds \| Discounted cash flow \| Servicing asset \| Significant Unobservable Inputs (Level 3) \| Weighted-Average Range
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.121	0.119
Discount rate \| Discounted cash flow \| Servicing asset \| Significant Unobservable Inputs (Level 3) \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.13	0.13
Discount rate \| Discounted cash flow \| Servicing asset \| Significant Unobservable Inputs (Level 3) \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.14	0.15
Discount rate \| Discounted cash flow \| Servicing asset \| Significant Unobservable Inputs (Level 3) \| Weighted-Average Range
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing asset, measurement input	0.130	0.130
Discount to reflect current market conditions \| Fair value of collateral \| Other Real Estate Owned \| Significant Unobservable Inputs (Level 3) \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned, measurement input	0.30	0.30
Discount to reflect current market conditions \| Fair value of collateral \| Other Real Estate Owned \| Significant Unobservable Inputs (Level 3) \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned, measurement input	0.35	0.35
Discount to reflect current market conditions \| Fair value of collateral \| Other Real Estate Owned \| Significant Unobservable Inputs (Level 3) \| Weighted-Average Range
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned, measurement input	0.330	0.320

First Internet Bancorp 10-Q