Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financial assets:
Investment debt securities available-for-sale	$ 4,403.6	$ 4,008.7
Investment debt securities held-to-maturity	1,632.5	55.0
Derivative assets	34.1	55.3
Financial liabilities:
Derivative liabilities	21.1	17.9
Carrying Amount
Financial assets:
Cash and cash equivalents	2,233.5	2,276.8
Investment debt securities available-for-sale	4,403.6	4,008.7
Investment debt securities held-to-maturity	1,618.1	51.6
Accrued interest receivable	52.9	51.1
Mortgage servicing rights, net	27.0	24.0
Loans held for sale	42.5	74.0
Net loans held for investment	9,487.4	9,663.2
Derivative assets	34.1	55.3
Deferred compensation plan assets	21.6	19.1
Total financial assets	17,920.7	16,223.8
Financial liabilities:
Total deposits, excluding time deposits	15,019.3	13,158.3
Time deposits	988.0	1,058.7
Securities sold under repurchase agreements	1,007.5	1,091.4
Accrued interest payable	6.6	5.8
Long-term debt	112.4	112.4
Subordinated debentures held by subsidiary trusts	87.0	87.0
Derivative liabilities	21.1	17.9
Deferred compensation plan liabilities	21.6	19.1
Total financial liabilities	17,263.5	15,550.6
Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,233.5	2,276.8
Investment debt securities available-for-sale	4,403.6	4,008.7
Investment debt securities held-to-maturity	1,632.5	55.0
Accrued interest receivable	52.9	51.1
Mortgage servicing rights, net	27.4	24.0
Loans held for sale	42.5	74.0
Net loans held for investment	9,587.1	9,785.6
Derivative assets	34.1	55.3
Deferred compensation plan assets	21.6	19.1
Total financial assets	18,035.2	16,349.6
Financial liabilities:
Total deposits, excluding time deposits	15,019.3	13,158.3
Time deposits	987.3	1,061.1
Securities sold under repurchase agreements	1,007.5	1,091.4
Accrued interest payable	6.6	5.8
Long-term debt	121.5	116.5
Subordinated debentures held by subsidiary trusts	83.9	81.3
Derivative liabilities	21.1	17.9
Deferred compensation plan liabilities	21.6	19.1
Total financial liabilities	17,268.8	15,551.4
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,233.5	2,276.8
Investment debt securities available-for-sale	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
Loans held for sale	0.0	0.0
Net loans held for investment	0.0	0.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	2,233.5	2,276.8
Financial liabilities:
Total deposits, excluding time deposits	15,019.3	13,158.3
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
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	15,019.3	13,158.3
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	4,403.6	4,008.7
Investment debt securities held-to-maturity	1,632.5	55.0
Accrued interest receivable	52.9	51.1
Mortgage servicing rights, net	27.4	24.0
Loans held for sale	42.5	74.0
Net loans held for investment	9,574.6	9,770.9
Derivative assets	34.1	55.3
Deferred compensation plan assets	21.6	19.1
Total financial assets	15,789.2	14,058.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	987.3	1,061.1
Securities sold under repurchase agreements	1,007.5	1,091.4
Accrued interest payable	6.6	5.8
Long-term debt	121.5	116.5
Subordinated debentures held by subsidiary trusts	83.9	81.3
Derivative liabilities	21.1	17.9
Deferred compensation plan liabilities	21.6	19.1
Total financial liabilities	2,249.5	2,393.1
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Investment debt securities available-for-sale	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
Loans held for sale	0.0	0.0
Net loans held for investment	12.5	14.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	12.5	14.7
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
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
Derivative liabilities	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Total financial liabilities	$ 0.0	$ 0.0