Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financial assets:
Investment debt securities available-for-sale	$ 3,957.8	$ 4,008.7
Investment debt securities held-to-maturity	1,701.9	55.0
Derivative assets	36.7	55.3
Financial liabilities:
Derivative liabilities	19.1	17.9
Carrying Amount
Financial assets:
Cash and cash equivalents	1,948.4	2,276.8
Investment debt securities available-for-sale	3,957.8	4,008.7
Investment debt securities held-to-maturity	1,685.5	51.6
Accrued interest receivable	50.9	51.1
Mortgage servicing rights, net	27.4	24.0
Loans held for sale	48.8	74.0
Net loans held for investment	9,699.2	9,663.2
Derivative assets	45.1	55.3
Deferred compensation plan assets	20.8	19.1
Total financial assets	17,483.9	16,223.8
Financial liabilities:
Total deposits, excluding time deposits	14,554.2	13,158.3
Time deposits	1,011.5	1,058.7
Securities sold under repurchase agreements	1,038.7	1,091.4
Accrued interest payable	5.0	5.8
Long-term debt	112.4	112.4
Subordinated debentures held by subsidiary trusts	87.0	87.0
Derivative liabilities	43.3	53.3
Deferred compensation plan liabilities	20.8	19.1
Total financial liabilities	16,872.9	15,586.0
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,948.4	2,276.8
Investment debt securities available-for-sale	3,957.8	4,008.7
Investment debt securities held-to-maturity	1,701.9	55.0
Accrued interest receivable	50.9	51.1
Mortgage servicing rights, net	27.4	24.0
Loans held for sale	48.8	74.0
Net loans held for investment	9,749.0	9,785.6
Derivative assets	45.1	55.3
Deferred compensation plan assets	20.8	19.1
Total financial assets	17,550.1	16,349.6
Financial liabilities:
Total deposits, excluding time deposits	14,554.2	13,158.3
Time deposits	1,011.7	1,061.1
Securities sold under repurchase agreements	1,038.7	1,091.4
Accrued interest payable	5.0	5.8
Long-term debt	121.1	116.5
Subordinated debentures held by subsidiary trusts	83.0	81.3
Derivative liabilities	43.3	53.3
Deferred compensation plan liabilities	20.8	19.1
Total financial liabilities	16,877.8	15,586.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,948.4	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	1,948.4	2,276.8
Financial liabilities:
Total deposits, excluding time deposits	14,554.2	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	14,554.2	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	3,957.8	4,008.7
Investment debt securities held-to-maturity	1,701.9	55.0
Accrued interest receivable	50.9	51.1
Mortgage servicing rights, net	27.4	24.0
Loans held for sale	48.8	74.0
Net loans held for investment	9,736.2	9,770.9
Derivative assets	45.1	55.3
Deferred compensation plan assets	20.8	19.1
Total financial assets	15,588.9	14,058.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,011.7	1,061.1
Securities sold under repurchase agreements	1,038.7	1,091.4
Accrued interest payable	5.0	5.8
Long-term debt	121.1	116.5
Subordinated debentures held by subsidiary trusts	83.0	81.3
Derivative liabilities	43.3	53.3
Deferred compensation plan liabilities	20.8	19.1
Total financial liabilities	2,323.6	2,428.5
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	12.8	14.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	12.8	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