Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financial assets:
Investment debt securities available-for-sale	$ 3,453.5	$ 2,960.0
Investment debt securities held-to-maturity	58.4	94.5
Derivative assets	68.3	23.2
Financial liabilities:
Derivative liabilities	63.1	22.2
Carrying Amount
Financial assets:
Cash and cash equivalents	1,860.6	1,076.8
Investment debt securities available-for-sale	3,453.5	2,960.0
Investment debt securities held-to-maturity	55.0	92.3
Accrued interest receivable	56.7	46.7
Mortgage servicing rights, net	24.1	30.2
Loans held for sale	102.0	100.9
Net loans held for investment	10,006.7	8,857.7
Derivative assets	68.3	23.2
Deferred compensation plan assets	18.9	18.2
Total financial assets	15,645.8	13,206.0
Financial liabilities:
Total deposits, excluding time deposits	12,770.3	10,213.5
Time deposits	1,112.1	1,450.0
Securities sold under repurchase agreements	820.3	697.6
Accrued interest payable	8.1	12.1
Long-term debt	112.4	13.9
Subordinated debentures held by subsidiary trusts	87.0	86.9
Derivative liabilities	63.1	22.2
Deferred compensation plan liabilities	18.9	18.2
Total financial liabilities	14,992.2	12,514.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,860.6	1,076.8
Investment debt securities available-for-sale	3,453.5	2,960.0
Investment debt securities held-to-maturity	58.4	94.5
Accrued interest receivable	56.7	46.7
Mortgage servicing rights, net	24.1	34.8
Loans held for sale	102.0	100.9
Net loans held for investment	9,927.4	8,930.7
Derivative assets	68.3	23.2
Deferred compensation plan assets	18.9	18.2
Total financial assets	15,569.9	13,285.8
Financial liabilities:
Total deposits, excluding time deposits	12,770.3	10,213.5
Time deposits	1,115.4	1,446.6
Securities sold under repurchase agreements	820.3	697.6
Accrued interest payable	8.1	12.1
Long-term debt	111.6	10.4
Subordinated debentures held by subsidiary trusts	82.0	81.3
Derivative liabilities	63.1	22.2
Deferred compensation plan liabilities	18.9	18.2
Total financial liabilities	14,989.7	12,501.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,860.6	1,076.8
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	1,860.6	1,076.8
Financial liabilities:
Total deposits, excluding time deposits	12,770.3	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	12,770.3	10,213.5
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	3,453.5	2,960.0
Investment debt securities held-to-maturity	58.4	94.5
Accrued interest receivable	56.7	46.7
Mortgage servicing rights, net	24.1	34.8
Loans held for sale	102.0	100.9
Net loans held for investment	9,916.0	8,906.7
Derivative assets	68.3	23.2
Deferred compensation plan assets	18.9	18.2
Total financial assets	13,697.9	12,185.0
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,115.4	1,446.6
Securities sold under repurchase agreements	820.3	697.6
Accrued interest payable	8.1	12.1
Long-term debt	111.6	10.4
Subordinated debentures held by subsidiary trusts	82.0	81.3
Derivative liabilities	63.1	22.2
Deferred compensation plan liabilities	18.9	18.2
Total financial liabilities	2,219.4	2,288.4
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	11.4	24.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	11.4	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