Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Financial assets:
Held-to-maturity investment securities, estimated fair value	$ 400.7	$ 483.3
Derivative assets	10.1	8.8
Deferred compensation plan assets	12.1	12.2
Financial liabilities:
Derivative liabilities	9.4	7.9
Deferred compensation plan liabilities	12.1	12.2
Carrying Amount
Financial assets:
Cash and cash equivalents	822.0	759.0
Investment debt securities available-for-sale	2,270.7	2,208.7
Held-to-maturity investment securities, estimated fair value	406.8	484.5
Accrued interest receivable	44.9	38.0
Mortgage servicing rights, net	27.7	24.8
Loans Held-for-sale, Fair Value Disclosure	33.3	46.6
Net loans held for investment	8,397.4	7,495.6
Derivative assets	10.1	8.8
Deferred compensation plan assets	12.1	12.2
Total financial assets	12,025.0	11,078.2
Financial liabilities:
Total deposits, excluding time deposits	9,363.7	8,783.0
Time deposits	1,317.0	1,151.9
Securities sold under repurchase agreements	712.4	643.0
Other borrowed funds		20.0
Accrued interest payable	7.8	5.6
Long-term debt	15.8	13.1
Subordinated debentures held by subsidiary trusts	86.9	82.5
Derivative liabilities	9.4	7.9
Deferred compensation plan liabilities	12.1	12.2
Total financial liabilities	11,525.1	10,719.2
Estimated Fair Value
Financial assets:
Cash and cash equivalents	822.0	759.0
Investment debt securities available-for-sale	2,270.7	2,208.7
Held-to-maturity investment securities, estimated fair value	400.7	483.3
Accrued interest receivable	44.9	38.0
Mortgage servicing rights, net	42.4	40.1
Loans Held-for-sale, Fair Value Disclosure	33.3	46.6
Net loans held for investment	8,439.7	7,252.2
Derivative assets	10.1	8.8
Deferred compensation plan assets	12.1	12.2
Total financial assets	12,075.9	10,848.9
Financial liabilities:
Total deposits, excluding time deposits	9,363.7	8,783.0
Time deposits	1,299.0	1,137.9
Securities sold under repurchase agreements	712.4	643.0
Other borrowed funds		20.0
Accrued interest payable	7.8	5.6
Long-term debt	13.0	11.3
Subordinated debentures held by subsidiary trusts	84.9	76.7
Derivative liabilities	9.4	7.9
Deferred compensation plan liabilities	12.1	12.2
Total financial liabilities	11,502.3	10,697.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	822.0	759.0
Investment debt securities available-for-sale	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	822.0	759.0
Financial liabilities:
Total deposits, excluding time deposits	9,363.7	8,783.0
Time deposits	0.0	0.0
Securities sold under repurchase agreements	0.0	0.0
Other borrowed funds		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	9,363.7	8,783.0
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	2,270.7	2,208.7
Held-to-maturity investment securities, estimated fair value	400.7	483.3
Accrued interest receivable	44.9	38.0
Mortgage servicing rights, net	42.4	40.1
Loans Held-for-sale, Fair Value Disclosure	33.3	46.6
Net loans held for investment	8,415.6	7,219.6
Derivative assets	10.1	8.8
Deferred compensation plan assets	12.1	12.2
Total financial assets	11,229.8	10,057.3
Financial liabilities:
Total deposits, excluding time deposits	0.0	0.0
Time deposits	1,299.0	1,137.9
Securities sold under repurchase agreements	712.4	643.0
Other borrowed funds		20.0
Accrued interest payable	7.8	5.6
Long-term debt	13.0	11.3
Subordinated debentures held by subsidiary trusts	84.9	76.7
Derivative liabilities	9.4	7.9
Deferred compensation plan liabilities	12.1	12.2
Total financial liabilities	2,138.6	1,914.6
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
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	24.1	32.6
Derivative assets	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total financial assets	24.1	32.6
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
Other borrowed funds		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