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Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Financial assets: |  |
Investment debt securities held-to-maturity | $ 407,400 | $ 483,300
Carrying Amount |  |
Financial assets: |  |
Cash and cash equivalents | 965,000 | 759,000
Investment debt securities available-for-sale: | 2,158,800 | 2,208,700
Investment debt securities held-to-maturity | 417,800 | 484,500
Accrued interest receivable | 48,000 | 38,000
Mortgage servicing rights, net | 27,000 | 24,800
Loans held for sale | 37,700 |
Net loans held for investment | 8,480,300 | 7,542,200
Derivative assets | 12,400 | 8,800
Deferred compensation plan assets | 13,000 | 12,200
Total financial assets | 12,160,000 | 11,078,200
Financial liabilities: |  |
Total deposits, excluding time deposits | 9,560,300 | 8,783,000
Time deposits | 1,285,300 | 1,151,900
Securities sold under repurchase agreements | 635,900 | 643,000
Other borrowed funds | 1,500 | 20,000
Accrued interest payable | 6,700 | 5,600
Long-term debt | 22,400 | 13,100
Subordinated debentures held by subsidiary trusts | 86,900 | 82,500
Derivative liabilities | 11,300 | 7,900
Deferred compensation plan liabilities | 13,000 | 12,200
Total financial liabilities | 11,623,300 | 10,719,200
Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 965,000 | 759,000
Investment debt securities available-for-sale: | 2,158,800 | 2,208,700
Investment debt securities held-to-maturity | 407,400 | 483,300
Accrued interest receivable | 48,000 | 38,000
Mortgage servicing rights, net | 41,800 | 40,100
Loans held for sale | 37,700 |
Net loans held for investment | 8,406,800 | 7,298,800
Derivative assets | 12,400 | 8,800
Deferred compensation plan assets | 13,000 | 12,200
Total financial assets | 12,090,900 | 10,848,900
Financial liabilities: |  |
Total deposits, excluding time deposits | 9,560,300 | 8,783,000
Time deposits | 1,269,200 | 1,137,900
Securities sold under repurchase agreements | 635,900 | 643,000
Other borrowed funds | 1,500 | 20,000
Accrued interest payable | 6,700 | 5,600
Long-term debt | 19,600 | 11,300
Subordinated debentures held by subsidiary trusts | 82,400 | 76,700
Derivative liabilities | 11,300 | 7,900
Deferred compensation plan liabilities | 13,000 | 12,200
Total financial liabilities | 11,599,900 | 10,697,600
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 965,000 | 759,000
Investment debt securities available-for-sale: | 0 | 0
Investment debt securities held-to-maturity | 0 | 0
Accrued interest receivable | 0 | 0
Mortgage servicing rights, net | 0 | 0
Loans held for sale | 0 |
Net loans held for investment | 0 | 0
Derivative assets | 0 | 0
Deferred compensation plan assets | 0 | 0
Total financial assets | 965,000 | 759,000
Financial liabilities: |  |
Total deposits, excluding time deposits | 9,560,300 | 8,783,000
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 | 9,560,300 | 8,783,000
Significant Other Observable Inputs (Level 2) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0 | 0
Investment debt securities available-for-sale: | 2,158,800 | 2,208,700
Investment debt securities held-to-maturity | 407,400 | 483,300
Accrued interest receivable | 48,000 | 38,000
Mortgage servicing rights, net | 41,800 | 40,100
Loans held for sale | 37,700 |
Net loans held for investment | 8,378,100 | 7,266,200
Derivative assets | 12,400 | 8,800
Deferred compensation plan assets | 13,000 | 12,200
Total financial assets | 11,097,200 | 10,057,300
Financial liabilities: |  |
Total deposits, excluding time deposits | 0 | 0
Time deposits | 1,269,200 | 1,137,900
Securities sold under repurchase agreements | 635,900 | 643,000
Other borrowed funds | 1,500 | 20,000
Accrued interest payable | 6,700 | 5,600
Long-term debt | 19,600 | 11,300
Subordinated debentures held by subsidiary trusts | 82,400 | 76,700
Derivative liabilities | 11,300 | 7,900
Deferred compensation plan liabilities | 13,000 | 12,200
Total financial liabilities | 2,039,600 | 1,914,600
Significant Unobservable Inputs (Level 3) | Estimated Fair Value |  |
Financial assets: |  |
Cash and cash equivalents | 0 | 0
Investment debt securities available-for-sale: | 0 | 0
Investment debt securities held-to-maturity | 0 | 0
Accrued interest receivable | 0 | 0
Mortgage servicing rights, net | 0 | 0
Loans held for sale | 0 |
Net loans held for investment |  | 32,600
Impaired loans | 28,700 |
Derivative assets | 0 | 0
Deferred compensation plan assets | 0 | 0
Total financial assets | 28,700 | 32,600
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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