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Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Financial assets: |  |
Held-to-maturity investment securities, estimated fair value | $ 483,349 | $ 513,273
Derivative assets | 8,800 | 2,700
Deferred compensation plan assets | 12,200 | 10,600
Financial liabilities: |  |
Derivative liabilities | 7,900 | 1,300
Deferred compensation plan liabilities | 12,200 | 10,600
Carrying Amount |  |
Financial assets: |  |
Cash and cash equivalents | 759,000 | 782,000
Investment securities available-for-sale | 2,208,700 | 1,611,700
Held-to-maturity investment securities, estimated fair value | 484,500 | 512,800
Accrued interest receivable | 38,000 | 29,900
Mortgage servicing rights, net | 24,800 | 18,500
Net loans | 7,542,200 | 5,402,300
Derivative assets | 8,800 | 2,700
Deferred compensation plan assets | 12,200 | 10,600
Total financial assets | 11,078,200 | 8,370,500
Financial liabilities: |  |
Total deposits, excluding time deposits | 8,783,000 | 6,324,500
Time deposits | 1,151,900 | 1,051,600
Securities sold under repurchase agreements | 643,000 | 537,600
Other borrowed funds | 20,000 | 0
Accrued interest payable | 5,600 | 5,400
Long-term debt | 13,100 | 28,000
Subordinated debentures held by subsidiary trusts | 82,500 | 82,500
Derivative liabilities | 7,900 | 1,300
Deferred compensation plan liabilities | 12,200 | 10,600
Total financial liabilities | 10,719,200 | 8,041,500
Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 759,000 | 782,000
Investment securities available-for-sale | 2,208,700 | 1,611,700
Held-to-maturity investment securities, estimated fair value | 483,300 | 513,300
Accrued interest receivable | 38,000 | 29,900
Mortgage servicing rights, net | 40,100 | 35,700
Net loans | 7,298,800 | 5,309,900
Derivative assets | 8,800 | 2,700
Deferred compensation plan assets | 12,200 | 10,600
Total financial assets | 10,848,900 | 8,295,800
Financial liabilities: |  |
Total deposits, excluding time deposits | 8,783,000 | 6,324,500
Time deposits | 1,137,900 | 1,044,700
Securities sold under repurchase agreements | 643,000 | 537,600
Other borrowed funds | 20,000 | 0
Accrued interest payable | 5,600 | 5,400
Long-term debt | 11,300 | 27,500
Subordinated debentures held by subsidiary trusts | 76,700 | 73,600
Derivative liabilities | 7,900 | 1,300
Deferred compensation plan liabilities | 12,200 | 10,600
Total financial liabilities | 10,697,600 | 8,025,200
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 759,000 | 782,000
Investment securities available-for-sale | 0 | 0
Held-to-maturity investment securities, estimated fair value | 0 | 0
Accrued interest receivable | 0 | 0
Mortgage servicing rights, net | 0 | 0
Net loans | 0 | 0
Derivative assets | 0 | 0
Deferred compensation plan assets | 0 | 0
Total financial assets | 759,000 | 782,000
Financial liabilities: |  |
Total deposits, excluding time deposits | 8,783,000 | 6,324,500
Time deposits | 0 | 0
Securities sold under repurchase agreements | 0 | 0
Other borrowed funds | 0 | 0
Accrued interest payable | 0 | 0
Long-term debt | 0 | 0
Subordinated debentures held by subsidiary trusts | 0 | 0
Derivative liabilities | 0 | 0
Deferred compensation plan liabilities | 0 | 0
Total financial liabilities | 8,783,000 | 6,324,500
Significant Other Observable Inputs (Level 2) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0 | 0
Investment securities available-for-sale | 2,208,700 | 1,611,700
Held-to-maturity investment securities, estimated fair value | 483,300 | 513,300
Accrued interest receivable | 38,000 | 29,900
Mortgage servicing rights, net | 40,100 | 35,700
Net loans | 7,266,200 | 5,270,600
Derivative assets | 8,800 | 2,700
Deferred compensation plan assets | 12,200 | 10,600
Total financial assets | 10,057,300 | 7,474,500
Financial liabilities: |  |
Total deposits, excluding time deposits | 0 | 0
Time deposits | 1,137,900 | 1,044,700
Securities sold under repurchase agreements | 643,000 | 537,600
Other borrowed funds | 20,000 | 0
Accrued interest payable | 5,600 | 5,400
Long-term debt | 11,300 | 27,500
Subordinated debentures held by subsidiary trusts | 76,700 | 73,600
Derivative liabilities | 7,900 | 1,300
Deferred compensation plan liabilities | 12,200 | 10,600
Total financial liabilities | 1,914,600 | 1,700,700
Significant Unobservable Inputs (Level 3) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0 | 0
Investment securities available-for-sale | 0 | 0
Held-to-maturity investment securities, estimated fair value | 0 | 0
Accrued interest receivable | 0 | 0
Mortgage servicing rights, net | 0 | 0
Net loans | 32,600 | 39,300
Derivative assets | 0 | 0
Deferred compensation plan assets | 0 | 0
Total financial assets | 32,600 | 39,300
Financial liabilities: |  |
Total deposits, excluding time deposits | 0 | 0
Time deposits | 0 | 0
Securities sold under repurchase agreements | 0 | 0
Other borrowed funds | 0 | 0
Accrued interest payable | 0 | 0
Long-term debt | 0 | 0
Subordinated debentures held by subsidiary trusts | 0 | 0
Derivative liabilities | 0 | 0
Deferred compensation plan liabilities | 0 | 0
Total financial liabilities | $ 0 | $ 0

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