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Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | $ 871.4 | $ 896.6
Investment debt securities held-to-maturity | 2,361.2 | 2,358.6
Accrued interest receivable | 107.9 | 116.8
Mortgage servicing rights, net | 36.0 | 35.4
Net loans held for investment | 16,697.9 | 17,154.7
Total financial assets | 20,074.4 | 20,562.1
Financial liabilities: |  |
Time deposits | 2,811.5 | 2,875.5
Securities sold under repurchase agreements | 528.0 | 523.9
Other Borrowings | 960.0 | 1,567.5
Accrued interest payable | 40.8 | 47.0
Long-term debt | 130.5 | 131.9
Subordinated debentures held by subsidiary trusts | 152.1 | 152.3
Total financial liabilities | 24,528.7 | 25,423.2
Deposits, Excluding Time Deposits, Fair Value Disclosure | 19,905.8 | 20,125.1
Carrying Amount |  |
Financial assets: |  |
Cash and cash equivalents | 871.4 | 896.6
Investment debt securities held-to-maturity | 2,639.5 | 2,687.5
Accrued interest receivable | 107.9 | 116.8
Mortgage servicing rights, net | 24.9 | 25.7
Net loans held for investment | 17,162.0 | 17,640.8
Total financial assets | 20,805.7 | 21,367.4
Financial liabilities: |  |
Time deposits | 2,827.0 | 2,890.5
Securities sold under repurchase agreements | 528.0 | 523.9
Other Borrowings | 960.0 | 1,567.5
Accrued interest payable | 40.8 | 47.0
Long-term debt | 130.2 | 132.2
Subordinated debentures held by subsidiary trusts | 163.1 | 163.1
Total financial liabilities | 24,554.9 | 25,449.3
Deposits, Excluding Time Deposits, Fair Value Disclosure | 19,905.8 | 20,125.1
Investment debt securities held-to-maturity | 2,361.2 | 2,358.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 871.4 | 896.6
Investment debt securities held-to-maturity | 0.0 | 0.0
Accrued interest receivable | 0.0 | 0.0
Mortgage servicing rights, net | 0.0 | 0.0
Net loans held for investment | 0.0 | 0.0
Total financial assets | 871.4 | 896.6
Financial liabilities: |  |
Time deposits | 0.0 | 0.0
Securities sold under repurchase agreements | 0.0 | 0.0
Other Borrowings | 0.0 | 0.0
Accrued interest payable | 0.0 | 0.0
Long-term debt | 0.0 | 0.0
Subordinated debentures held by subsidiary trusts | 0.0 | 0.0
Total financial liabilities | 19,905.8 | 20,125.1
Deposits, Excluding Time Deposits, Fair Value Disclosure | 19,905.8 | 20,125.1
Significant Other Observable Inputs (Level 2) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0.0 | 0.0
Investment debt securities held-to-maturity | 2,361.2 | 2,358.6
Accrued interest receivable | 107.9 | 116.8
Mortgage servicing rights, net | 36.0 | 35.4
Net loans held for investment | 0.0 | 0.0
Total financial assets | 2,505.1 | 2,510.8
Financial liabilities: |  |
Time deposits | 2,811.5 | 2,875.5
Securities sold under repurchase agreements | 528.0 | 523.9
Other Borrowings | 960.0 | 1,567.5
Accrued interest payable | 40.8 | 47.0
Long-term debt | 130.5 | 131.9
Subordinated debentures held by subsidiary trusts | 152.1 | 152.3
Total financial liabilities | 4,622.9 | 5,298.1
Deposits, Excluding Time Deposits, Fair Value Disclosure | 0.0 | 0.0
Significant Unobservable Inputs (Level 3) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0.0 | 0.0
Investment debt securities held-to-maturity | 0.0 | 0.0
Accrued interest receivable | 0.0 | 0.0
Mortgage servicing rights, net | 0.0 | 0.0
Net loans held for investment | 16,697.9 | 17,154.7
Total financial assets | 16,697.9 | 17,154.7
Financial liabilities: |  |
Time deposits | 0.0 | 0.0
Securities sold under repurchase agreements | 0.0 | 0.0
Other Borrowings | 0.0 | 0.0
Accrued interest payable | 0.0 | 0.0
Long-term debt | 0.0 | 0.0
Subordinated debentures held by subsidiary trusts | 0.0 | 0.0
Total financial liabilities | 0.0 | 0.0
Deposits, Excluding Time Deposits, Fair Value Disclosure | $ 0.0 | $ 0.0

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