Long-Term Debt and Other Borrowed Funds (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt Instrument
Stated interest rate		4.77%
Long-term debt and capital lease obligations	$ 146.3	$ 132.2
Fiscal Year Maturities of Long-term Debt
2018	0.1
2019	0.2
2020	0.2
2021	4.1
2022	0.0
Thereafter	141.7
Long-term debt and capital lease obligations	146.3	132.2
Other borrowings	0.0	1,567.5
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(2.7)
Other Borrowings	$ 0.0	0.0
FIB
Fiscal Year Maturities of Long-term Debt
Ownership percentage in subsidiary	99.90%
Parent Company
Debt Instrument
Long-term debt	$ 122.3	99.1
Fixed to floating subordinated notes, 5.25%
Fiscal Year Maturities of Long-term Debt
Debt Instrument, Face Amount	$ 100.0
Fixed to floating subordinated notes, 5.25% | Parent
Debt Instrument
Stated interest rate	5.25%
Long-term debt	$ 0.0	99.1
Fixed to floating subordinated notes, 7.625%
Fiscal Year Maturities of Long-term Debt
Debt Instrument, Face Amount	$ 125.0
Fixed to floating subordinated notes, 7.625% | Parent
Debt Instrument
Stated interest rate	7.625%
Long-term debt	$ 122.3	0.0
Capital lease obligations | Capital Lease Obligation, 8.00% | Subsidiaries
Debt Instrument
Stated interest rate	8.00%
Finance Lease, Liability	$ 0.6	0.8
Capital lease obligations | Capital Lease Obligation, 3.90% | Subsidiaries
Debt Instrument
Stated interest rate	3.90%
Finance Lease, Liability	$ 0.1	0.0
Notes payable | Subsidiaries
Fiscal Year Maturities of Long-term Debt
Long Term Debt, Acquired or Assumed	$ 19.4	28.4
Notes payable | Note Payable, 1.30%, maturing March 31, 2038 [Member] | Subsidiaries
Debt Instrument
Stated interest rate	1.30%
Long-term debt	$ 0.0	2.0
Notes payable | Note Payable, 1.30%, Maturing June 1, 2034 [Member] | Subsidiaries
Debt Instrument
Stated interest rate	1.30%
Long-term debt	$ 0.0	0.6
Notes payable | Note Payable, Three Point Two Five Percent | Subsidiaries
Debt Instrument
Stated interest rate	3.25%
Notes payable | Note Payable, 1.12%, Maturing December 31, 2045 | Subsidiaries
Debt Instrument
Stated interest rate	1.12%
Long-term debt	$ 6.8	6.8
Notes payable | Note Payable, 1.35%, Maturing December 31, 2046 | Subsidiaries
Debt Instrument
Stated interest rate	1.35%
Long-term debt	$ 0.0	6.4
Notes payable | Note Payable, 1.26%, Maturing December 31, 2051 | Subsidiaries
Debt Instrument
Stated interest rate	1.26%
Long-term debt	$ 12.6	12.6
Notes payable | FHLB Borrowing maturing August 2025 | Subsidiaries
Debt Instrument
Stated interest rate	0.00%
Long-term debt	$ 3.9	$ 3.9
Federal Home Loan Bank | Notes Payable to FHLB | Subsidiaries
Fiscal Year Maturities of Long-term Debt
FHLB, advances, maximum amount available	5,406.6
Federal Home Loan Bank, Advances, General Debt Obligations, Amount of Available, Unused Funds	5,402.7
Secured Debt
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	3,585.5
Federal Funds Purchased
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	$ 235.0
Minimum | Note Payable, 1.00% | Subsidiaries
Debt Instrument
Stated interest rate	1.00%
Maximum [Member] | Note Payable, Three Point Two Five Percent | Subsidiaries
Debt Instrument
Stated interest rate	3.25%