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