Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial assets:
Investment debt securities available-for-sale	$ 2,609.7	$ 2,270.7
Investment debt securities held-to-maturity	100.8	400.7
Derivative assets	24.5	10.1
Financial liabilities:
Derivative liabilities	22.7	9.4
Carrying Amount
Financial assets:
Cash and cash equivalents	1,128.9	822.0
Investment debt securities available-for-sale	2,609.7	2,270.7
Investment debt securities held-to-maturity	98.8	406.8
Accrued interest receivable	50.2	44.9
Mortgage servicing rights, net	28.6	27.7
Loans held for sale	64.1	33.3
Net loans held for investment	8,921.0	8,397.4
Derivative assets	24.5	10.1
Deferred compensation plan assets	16.6	12.1
Total financial assets	12,942.4	12,025.0
Financial liabilities:
Total deposits, excluding time deposits	9,931.4	9,363.7
Time deposits	1,558.5	1,317.0
Securities sold under repurchase agreements	686.7	712.4
Accrued interest payable	12.4	7.8
Long-term debt	15.8	15.8
Subordinated debentures held by subsidiary trusts	86.9	86.9
Derivative liabilities	22.7	9.4
Deferred compensation plan liabilities	16.6	12.1
Total financial liabilities	12,331.0	11,525.1
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,128.9	822.0
Investment debt securities available-for-sale	2,609.7	2,270.7
Investment debt securities held-to-maturity	100.8	400.7
Accrued interest receivable	50.2	44.9
Mortgage servicing rights, net	35.4	42.4
Loans held for sale	64.1	33.3
Net loans held for investment	8,947.7	8,439.7
Derivative assets	24.5	10.1
Deferred compensation plan assets	16.6	12.1
Total financial assets	12,977.9	12,075.9
Financial liabilities:
Total deposits, excluding time deposits	9,931.4	9,363.7
Time deposits	1,551.9	1,299.0
Securities sold under repurchase agreements	686.7	712.4
Accrued interest payable	12.4	7.8
Long-term debt	14.6	13.0
Subordinated debentures held by subsidiary trusts	83.8	84.9
Derivative liabilities	22.7	9.4
Deferred compensation plan liabilities	16.6	12.1
Total financial liabilities	12,320.1	11,502.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,128.9	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	1,128.9	822.0
Financial liabilities:
Total deposits, excluding time deposits	9,931.4	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	9,931.4	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	2,609.7	2,270.7
Investment debt securities held-to-maturity	100.8	400.7
Accrued interest receivable	50.2	44.9
Mortgage servicing rights, net	35.4	42.4
Loans held for sale	64.1	33.3
Net loans held for investment	8,921.5	8,415.6
Derivative assets	24.5	10.1
Deferred compensation plan assets	16.6	12.1
Total financial assets	11,822.8	11,229.8
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,551.9	1,299.0
Securities sold under repurchase agreements	686.7	712.4
Accrued interest payable	12.4	7.8
Long-term debt	14.6	13.0
Subordinated debentures held by subsidiary trusts	83.8	84.9
Derivative liabilities	22.7	9.4
Deferred compensation plan liabilities	16.6	12.1
Total financial liabilities	2,388.7	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	26.2	24.1
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	26.2	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