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Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Financial assets: |  |
Estimated Fair Value | $ 4,008.7 | $ 2,960.0
Held-to-maturity investment securities, estimated fair value | 55.0 | 94.5
Derivative assets | 55.3 | 23.2
Deferred compensation plan assets | 19.1 | 18.2
Financial liabilities: |  |
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan liabilities | 19.1 | 18.2
Carrying Amount |  |
Financial assets: |  |
Cash and cash equivalents | 2,276.8 | 1,076.8
Estimated Fair Value | 4,008.7 | 2,960.0
Held-to-maturity investment securities, estimated fair value | 51.6 | 92.3
Accrued interest receivable | 51.1 | 46.7
Mortgage servicing rights, net | 24.0 | 30.2
Loans Held-for-sale, Fair Value Disclosure | 74.0 | 100.9
Net loans held for investment | 9,663.2 | 8,857.7
Derivative assets | 55.3 | 23.2
Deferred compensation plan assets | 19.1 | 18.2
Total financial assets | 16,223.8 | 13,206.0
Financial liabilities: |  |
Total deposits, excluding time deposits | 13,158.3 | 10,213.5
Time deposits | 1,058.7 | 1,450.0
Securities sold under repurchase agreements | 1,091.4 | 697.6
Accrued interest payable | 5.8 | 12.1
Long-term debt | 112.4 | 13.9
Subordinated debentures held by subsidiary trusts | 87.0 | 86.9
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan liabilities | 19.1 | 18.2
Total financial liabilities | 15,586.0 | 12,514.4
Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 2,276.8 | 1,076.8
Estimated Fair Value | 4,008.7 | 2,960.0
Held-to-maturity investment securities, estimated fair value | 55.0 | 94.5
Accrued interest receivable | 51.1 | 46.7
Mortgage servicing rights, net | 24.0 | 34.8
Loans Held-for-sale, Fair Value Disclosure | 74.0 | 100.9
Net loans held for investment | 9,785.6 | 8,930.7
Derivative assets | 55.3 | 23.2
Deferred compensation plan assets | 19.1 | 18.2
Total financial assets | 16,349.6 | 13,285.8
Financial liabilities: |  |
Total deposits, excluding time deposits | 13,158.3 | 10,213.5
Time deposits | 1,061.1 | 1,446.6
Securities sold under repurchase agreements | 1,091.4 | 697.6
Accrued interest payable | 5.8 | 12.1
Long-term debt | 116.5 | 10.4
Subordinated debentures held by subsidiary trusts | 81.3 | 81.3
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan liabilities | 19.1 | 18.2
Total financial liabilities | 15,586.8 | 12,501.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 2,276.8 | 1,076.8
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 | 2,276.8 | 1,076.8
Financial liabilities: |  |
Total deposits, excluding time deposits | 13,158.3 | 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 | 13,158.3 | 10,213.5
Significant Other Observable Inputs (Level 2) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0.0 | 0.0
Estimated Fair Value | 4,008.7 | 2,960.0
Held-to-maturity investment securities, estimated fair value | 55.0 | 94.5
Accrued interest receivable | 51.1 | 46.7
Mortgage servicing rights, net | 24.0 | 34.8
Loans Held-for-sale, Fair Value Disclosure | 74.0 | 100.9
Net loans held for investment | 9,770.9 | 8,906.7
Derivative assets | 55.3 | 23.2
Deferred compensation plan assets | 19.1 | 18.2
Total financial assets | 14,058.1 | 12,185.0
Financial liabilities: |  |
Total deposits, excluding time deposits | 0.0 | 0.0
Time deposits | 1,061.1 | 1,446.6
Securities sold under repurchase agreements | 1,091.4 | 697.6
Accrued interest payable | 5.8 | 12.1
Long-term debt | 116.5 | 10.4
Subordinated debentures held by subsidiary trusts | 81.3 | 81.3
Derivative liabilities | 53.3 | 22.2
Deferred compensation plan liabilities | 19.1 | 18.2
Total financial liabilities | 2,428.5 | 2,288.4
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 | 14.7 | 24.0
Derivative assets | 0.0 | 0.0
Deferred compensation plan assets | 0.0 | 0.0
Total financial assets | 14.7 | 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

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