Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Financial assets:
Investment debt securities available-for-sale	$ 2,732.2	$ 2,270.7
Investment debt securities held-to-maturity	97.2	400.7
Derivative assets	33.0	10.1
Financial liabilities:
Derivative liabilities	30.8	9.4
Carrying Amount
Financial assets:
Cash and cash equivalents	1,269.6	822.0
Investment debt securities available-for-sale	2,732.2	2,270.7
Investment debt securities held-to-maturity	94.9	406.8
Accrued interest receivable	51.1	44.9
Mortgage servicing rights, net	29.8	27.7
Loans held for sale	108.9	33.3
Net loans held for investment	8,917.6	8,397.4
Derivative assets	33.0	10.1
Deferred compensation plan assets	17.5	12.1
Total financial assets	13,254.6	12,025.0
Financial liabilities:
Total deposits, excluding time deposits	10,264.6	9,363.7
Time deposits	1,535.0	1,317.0
Securities sold under repurchase agreements	636.9	712.4
Accrued interest payable	12.3	7.8
Long-term debt	13.9	15.8
Subordinated debentures held by subsidiary trusts	86.9	86.9
Derivative liabilities	30.8	9.4
Deferred compensation plan liabilities	17.5	12.1
Total financial liabilities	12,597.9	11,525.1
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,269.6	822.0
Investment debt securities available-for-sale	2,732.2	2,270.7
Investment debt securities held-to-maturity	97.2	400.7
Accrued interest receivable	51.1	44.9
Mortgage servicing rights, net	32.1	42.4
Loans held for sale	108.9	33.3
Net loans held for investment	8,953.4	8,439.7
Derivative assets	33.0	10.1
Deferred compensation plan assets	17.5	12.1
Total financial assets	13,295.0	12,075.9
Financial liabilities:
Total deposits, excluding time deposits	10,264.6	9,363.7
Time deposits	1,530.9	1,299.0
Securities sold under repurchase agreements	636.9	712.4
Accrued interest payable	12.3	7.8
Long-term debt	11.3	13.0
Subordinated debentures held by subsidiary trusts	77.9	84.9
Derivative liabilities	30.8	9.4
Deferred compensation plan liabilities	17.5	12.1
Total financial liabilities	12,582.2	11,502.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,269.6	822.0
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	0.0	0.0
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	1,269.6	822.0
Financial liabilities:
Total deposits, excluding time deposits	10,264.6	9,363.7
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	10,264.6	9,363.7
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	2,732.2	2,270.7
Investment debt securities held-to-maturity	97.2	400.7
Accrued interest receivable	51.1	44.9
Mortgage servicing rights, net	32.1	42.4
Loans held for sale	108.9	33.3
Net loans held for investment	8,926.2	8,415.6
Derivative assets	33.0	10.1
Deferred compensation plan assets	17.5	12.1
Total financial assets	11,998.2	11,229.8
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,530.9	1,299.0
Securities sold under repurchase agreements	636.9	712.4
Accrued interest payable	12.3	7.8
Long-term debt	11.3	13.0
Subordinated debentures held by subsidiary trusts	77.9	84.9
Derivative liabilities	30.8	9.4
Deferred compensation plan liabilities	17.5	12.1
Total financial liabilities	2,317.6	2,138.6
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	27.2	24.1
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	27.2	24.1
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