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Commitments and Contingencies - Additional Information (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Other Commitments [Line Items] |  |  |
Data processing and communication expense | $ 10,400,000 | $ 10,400,000 | $ 11,700,000
Merger-related expenses included in data processing and communication charges | 0 | 0 | $ 1,300,000
Unfunded Loan Commitment |  |  |
Other Commitments [Line Items] |  |  |
Allowance for unfunded commitments | $ 2,200,000 | $ 2,600,000 |

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