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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2018 USD ($) subsidiary segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Property, Plant and Equipment |  |  |
Number of significant subsidiaries | subsidiary | 1 |  |
Cash on deposit with Federal Reserve Bank | $ 574,400,000 | $ 552,000,000 |
Compensating balances with Federal Reserve Bank | 28,100,000 | 16,100,000 |
Mortgage loans held for sale | 33,300,000 | 46,600,000 |
Deferred compensation plan assets | 12,100,000 | 12,200,000 |
Deferred compensation plan liabilities | 12,100,000 | 12,200,000 |
Impairment losses | 0 | 0 | $ 200,000
Write-downs of OREO | (100,000) | (400,000) | (600,000)
Carrying value of foreclosed OREO | 2,000,000 | 2,700,000 |
Federal Reserve Bank stock | 37,500,000 | 10,700,000 |
Federal Home Loan Bank stock | 31,800,000 | 10,200,000 |
Income tax examination, penalties | $ 0 | 0 | 0
Number of operating segments | segment | 1 |  |
Advertising expense | $ 3,200,000 | 3,500,000 | 2,800,000
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition | 5,600,000 | 3,800,000 | 4,400,000
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation |  |  | 2,200,000
Mortgage Loans in Process of Foreclosure, Amount | $ 400,000 |  |
Buildings and improvements | Minimum |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 5 years |  |
Buildings and improvements | Maximum |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 45 years |  |
Furniture and equipment | Minimum |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 4 years |  |
Furniture and equipment | Maximum |  |  |
Property, Plant and Equipment |  |  |
Estimated useful lives | 15 years |  |
Restricted Stock |  |  |
Property, Plant and Equipment |  |  |
Stock compensation expense | $ 5,600,000 | 3,300,000 | 3,900,000
Tax benefits from stock-based compensation | 200,000 | 600,000 | 1,500,000
Common Stock Including Additional Paid in Capital [Member] |  |  |
Property, Plant and Equipment |  |  |
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition |  | 3,800,000 | 4,400,000
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation | $ 1,100,000 | $ 2,600,000 | $ 2,200,000

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