Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financial assets:
Investment debt securities available-for-sale	$ 4,020.1	$ 4,008.7
Investment debt securities held-to-maturity	855.6	55.0
Derivative assets	49.4	55.3
Financial liabilities:
Derivative liabilities	47.0	53.3
Carrying Amount
Financial assets:
Cash and cash equivalents	2,196.0	2,276.8
Investment debt securities available-for-sale	4,020.1	4,008.7
Investment debt securities held-to-maturity	866.3	51.6
Accrued interest receivable	50.0	51.1
Mortgage servicing rights, net	28.0	24.0
Loans held for sale	57.2	74.0
Net loans held for investment	9,726.6	9,663.2
Derivative assets	49.4	55.3
Deferred compensation plan assets	18.9	19.1
Total financial assets	17,012.5	16,223.8
Financial liabilities:
Total deposits, excluding time deposits	14,057.7	13,158.3
Time deposits	1,036.3	1,058.7
Securities sold under repurchase agreements	1,052.6	1,091.4
Accrued interest payable	6.9	5.8
Long-term debt	112.4	112.4
Subordinated debentures held by subsidiary trusts	87.0	87.0
Derivative liabilities	47.0	53.3
Deferred compensation plan liabilities	18.9	19.1
Total financial liabilities	16,418.8	15,586.0
Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,196.0	2,276.8
Investment debt securities available-for-sale	4,020.1	4,008.7
Investment debt securities held-to-maturity	855.6	55.0
Accrued interest receivable	50.0	51.1
Mortgage servicing rights, net	29.0	24.0
Loans held for sale	57.2	74.0
Net loans held for investment	9,836.9	9,785.6
Derivative assets	49.4	55.3
Deferred compensation plan assets	18.9	19.1
Total financial assets	17,113.1	16,349.6
Financial liabilities:
Total deposits, excluding time deposits	14,057.7	13,158.3
Time deposits	1,037.2	1,061.1
Securities sold under repurchase agreements	1,052.6	1,091.4
Accrued interest payable	6.9	5.8
Long-term debt	120.1	116.5
Subordinated debentures held by subsidiary trusts	78.3	81.3
Derivative liabilities	47.0	53.3
Deferred compensation plan liabilities	18.9	19.1
Total financial liabilities	16,418.7	15,586.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,196.0	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	2,196.0	2,276.8
Financial liabilities:
Total deposits, excluding time deposits	14,057.7	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	14,057.7	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	4,020.1	4,008.7
Investment debt securities held-to-maturity	855.6	55.0
Accrued interest receivable	50.0	51.1
Mortgage servicing rights, net	29.0	24.0
Loans held for sale	57.2	74.0
Net loans held for investment	9,819.7	9,770.9
Derivative assets	49.4	55.3
Deferred compensation plan assets	18.9	19.1
Total financial assets	14,899.9	14,058.1
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,037.2	1,061.1
Securities sold under repurchase agreements	1,052.6	1,091.4
Accrued interest payable	6.9	5.8
Long-term debt	120.1	116.5
Subordinated debentures held by subsidiary trusts	78.3	81.3
Derivative liabilities	47.0	53.3
Deferred compensation plan liabilities	18.9	19.1
Total financial liabilities	2,361.0	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	17.2	14.7
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	17.2	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