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Summary of Significant Accounting Policies - Additional Information (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) entity unit subsidiary | Dec. 31, 2016 USD ($) segment
Summary of Significant Accounting Policies [Line Items] |  |  |
Number of wholly owned subsidiaries | subsidiary |  | 3 |
Percentage of outstanding common stock of investees accounted for using equity method of accounting |  | 100.00% |
Number of entities non-banking activities | entity |  | 5 |
Number of operating segments | segment |  |  | 3
Prior period reclassification adjustment |  | $ 0 |
Period for satisfactory payment of TDRs |  | 6 months |
Evaluation for impairment of substandard and nonaccrual commercial loans |  | $ 500,000 |
Look-back period used for assumptions of allowance for loans and lease losses |  | 8 years 6 months |
Number of reporting units | unit |  | 3 |
Goodwill |  | $ 291,670,000 | $ 291,670,000
Percentage of facilities owned |  | 55.00% |
Contractual operating lease commitments |  | $ 71,622,000 |
Securities available-for-sale, at fair value |  | 698,291,000 | 693,487,000
Equity Securities |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Securities available-for-sale, at fair value |  | $ 5,144,000 | $ 11,249,000
Scenario, Forecast |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Provisional estimate for reclassification of AOCI to retain earnings for other income tax effects of Tax Act | $ 3,200,000 |  |
Interest rate lock commitments |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Period for interest rate lock commitment |  | 60 days |
Commercial real estate |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  | 3 years |
Commercial construction |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  | 4 years |
Commercial and industrial |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  | 1 year 3 months |
Consumer real estate |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  | 2 years 9 months |
Other consumer |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Loss emergence period for used for assumptions of allowance for loans and lease losses |  | 1 year 3 months |
Minimum | Core Deposits And Customers Lists |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Weighted average estimated useful of acquired intangibles |  | 10 years |
Maximum |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Amortization period of investments in joint ventures |  | 10 years |
Maximum | Core Deposits And Customers Lists |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Weighted average estimated useful of acquired intangibles |  | 20 years |
Corporate Joint Venture | Common Wealth Trust Life Insurance Company |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |
Percentage of outstanding common stock of investees accounted for using equity method of accounting |  | 50.00% |

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