Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Estimated Fair Value
Financial assets:
Cash and cash equivalents	$ 1,208.7	$ 1,309.7
Investment debt securities held-to-maturity	2,082.7	2,136.6
Accrued interest receivable	101.3	102.6
Mortgage servicing rights, net	33.1	32.3
Net loans held for investment	14,345.3	14,769.1
Total financial assets	17,771.1	18,350.3
Financial liabilities:
Time deposits	2,422.4	2,629.2
Securities sold under repurchase agreements	476.1	479.6
Accrued interest payable	33.1	36.9
Long-term debt	153.3	153.4
Subordinated debentures held by subsidiary trusts	139.1	140.0
Total financial liabilities	22,672.0	22,889.1
Deposits, Excluding Time Deposits, Fair Value Disclosure	19,448.0	19,450.0
Carrying Amount
Financial assets:
Cash and cash equivalents	1,208.7	1,309.7
Investment debt securities held-to-maturity	2,293.4	2,342.1
Accrued interest receivable	101.3	102.6
Mortgage servicing rights, net	22.5	23.1
Net loans held for investment	14,532.6	15,010.2
Total financial assets	18,158.5	18,787.7
Financial liabilities:
Time deposits	2,435.0	2,638.3
Securities sold under repurchase agreements	476.1	479.6
Accrued interest payable	33.1	36.9
Long-term debt	146.7	146.3
Subordinated debentures held by subsidiary trusts	149.9	149.8
Total financial liabilities	22,688.8	22,900.9
Deposits, Excluding Time Deposits, Fair Value Disclosure	19,448.0	19,450.0
Investment debt securities held-to-maturity	2,082.7	2,136.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,208.7	1,309.7
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	1,208.7	1,309.7
Financial liabilities:
Time deposits	0.0	0.0
Securities sold under repurchase agreements	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	19,448.0	19,450.0
Deposits, Excluding Time Deposits, Fair Value Disclosure	19,448.0	19,450.0
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	2,082.7	2,136.6
Accrued interest receivable	101.3	102.6
Mortgage servicing rights, net	0.0	0.0
Net loans held for investment	0.0	0.0
Total financial assets	2,184.0	2,239.2
Financial liabilities:
Time deposits	2,422.4	2,629.2
Securities sold under repurchase agreements	476.1	479.6
Accrued interest payable	33.1	36.9
Long-term debt	153.3	153.4
Subordinated debentures held by subsidiary trusts	139.1	140.0
Total financial liabilities	3,224.0	3,439.1
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	33.1	32.3
Net loans held for investment	14,345.3	14,769.1
Total financial assets	14,378.4	14,801.4
Financial liabilities:
Time deposits	0.0	0.0
Securities sold under repurchase agreements	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