Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Financial assets:
Estimated Fair Value	$ 2,960.0		$ 2,270.7
Held-to-maturity investment securities, estimated fair value	94.5	$ 275.1	400.7
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Financial liabilities:
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Carrying Amount
Financial assets:
Cash and cash equivalents	1,076.8		822.0
Estimated Fair Value	2,960.0		2,270.7
Held-to-maturity investment securities, estimated fair value	92.3		406.8
Accrued interest receivable	46.7		44.9
Mortgage servicing rights, net	30.2		27.7
Loans Held-for-sale, Fair Value Disclosure	100.9		33.3
Net loans held for investment	8,857.7		8,397.4
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Total financial assets	13,206.0		12,025.0
Financial liabilities:
Total deposits, excluding time deposits	10,213.5		9,363.7
Time deposits	1,450.0		1,317.0
Securities sold under repurchase agreements	697.6		712.4
Other borrowed funds			0.0
Accrued interest payable	12.1		7.8
Long-term debt	13.9		15.8
Subordinated debentures held by subsidiary trusts	86.9		86.9
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Total financial liabilities	12,514.4		11,525.1
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,076.8		822.0
Estimated Fair Value	2,960.0		2,270.7
Held-to-maturity investment securities, estimated fair value	94.5		400.7
Accrued interest receivable	46.7		44.9
Mortgage servicing rights, net	34.8		42.4
Loans Held-for-sale, Fair Value Disclosure	100.9		33.3
Net loans held for investment	8,930.7		8,439.7
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Total financial assets	13,285.8		12,075.9
Financial liabilities:
Total deposits, excluding time deposits	10,213.5		9,363.7
Time deposits	1,446.6		1,299.0
Securities sold under repurchase agreements	697.6		712.4
Other borrowed funds			0.0
Accrued interest payable	12.1		7.8
Long-term debt	10.4		13.0
Subordinated debentures held by subsidiary trusts	81.3		84.9
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Total financial liabilities	12,501.9		11,502.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,076.8		822.0
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	0.0		0.0
Derivative assets	0.0		0.0
Deferred compensation plan assets	0.0		0.0
Total financial assets	1,076.8		822.0
Financial liabilities:
Total deposits, excluding time deposits	10,213.5		9,363.7
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	10,213.5		9,363.7
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0.0		0.0
Estimated Fair Value	2,960.0		2,270.7
Held-to-maturity investment securities, estimated fair value	94.5		400.7
Accrued interest receivable	46.7		44.9
Mortgage servicing rights, net	34.8		42.4
Loans Held-for-sale, Fair Value Disclosure	100.9		33.3
Net loans held for investment	8,906.7		8,415.6
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Total financial assets	12,185.0		11,229.8
Financial liabilities:
Total deposits, excluding time deposits	0.0		0.0
Time deposits	1,446.6		1,299.0
Securities sold under repurchase agreements	697.6		712.4
Other borrowed funds			0.0
Accrued interest payable	12.1		7.8
Long-term debt	10.4		13.0
Subordinated debentures held by subsidiary trusts	81.3		84.9
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Total financial liabilities	2,288.4		2,138.6
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	24.0		24.1
Derivative assets	0.0		0.0
Deferred compensation plan assets	0.0		0.0
Total financial assets	24.0		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
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