Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Estimated Fair Value	$ 4,008.7	$ 2,960.0
Held-to-maturity investment securities, estimated fair value	55.0	94.5
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Financial liabilities:
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Carrying Amount
Financial assets:
Cash and cash equivalents	2,276.8	1,076.8
Estimated Fair Value	4,008.7	2,960.0
Held-to-maturity investment securities, estimated fair value	51.6	92.3
Accrued interest receivable	51.1	46.7
Mortgage servicing rights, net	24.0	30.2
Loans Held-for-sale, Fair Value Disclosure	74.0	100.9
Net loans held for investment	9,663.2	8,857.7
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Total financial assets	16,223.8	13,206.0
Financial liabilities:
Total deposits, excluding time deposits	13,158.3	10,213.5
Time deposits	1,058.7	1,450.0
Securities sold under repurchase agreements	1,091.4	697.6
Accrued interest payable	5.8	12.1
Long-term debt	112.4	13.9
Subordinated debentures held by subsidiary trusts	87.0	86.9
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Total financial liabilities	15,586.0	12,514.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,276.8	1,076.8
Estimated Fair Value	4,008.7	2,960.0
Held-to-maturity investment securities, estimated fair value	55.0	94.5
Accrued interest receivable	51.1	46.7
Mortgage servicing rights, net	24.0	34.8
Loans Held-for-sale, Fair Value Disclosure	74.0	100.9
Net loans held for investment	9,785.6	8,930.7
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Total financial assets	16,349.6	13,285.8
Financial liabilities:
Total deposits, excluding time deposits	13,158.3	10,213.5
Time deposits	1,061.1	1,446.6
Securities sold under repurchase agreements	1,091.4	697.6
Accrued interest payable	5.8	12.1
Long-term debt	116.5	10.4
Subordinated debentures held by subsidiary trusts	81.3	81.3
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Total financial liabilities	15,586.8	12,501.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,276.8	1,076.8
Estimated Fair Value	0.0	0.0
Held-to-maturity investment securities, estimated fair value	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans Held-for-sale, Fair Value Disclosure	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	2,276.8	1,076.8
Financial liabilities:
Total deposits, excluding time deposits	13,158.3	10,213.5
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	13,158.3	10,213.5
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Estimated Fair Value	4,008.7	2,960.0
Held-to-maturity investment securities, estimated fair value	55.0	94.5
Accrued interest receivable	51.1	46.7
Mortgage servicing rights, net	24.0	34.8
Loans Held-for-sale, Fair Value Disclosure	74.0	100.9
Net loans held for investment	9,770.9	8,906.7
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Total financial assets	14,058.1	12,185.0
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,061.1	1,446.6
Securities sold under repurchase agreements	1,091.4	697.6
Accrued interest payable	5.8	12.1
Long-term debt	116.5	10.4
Subordinated debentures held by subsidiary trusts	81.3	81.3
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Total financial liabilities	2,428.5	2,288.4
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0	0.0
Estimated Fair Value	0.0	0.0
Held-to-maturity investment securities, estimated fair value	0.0	0.0
Accrued interest receivable	0.0	0.0
Mortgage servicing rights, net	0.0	0.0
Loans Held-for-sale, Fair Value Disclosure	0.0	0.0
Net loans held for investment	14.7	24.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	14.7	24.0
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