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Loans and Allowance for Loan Losses - Outstanding and Aging Analysis (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) loan | Sep. 30, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | $ 2,331,000 | $ 301,000
Current | 1,676,682,000 | 1,728,570,000
Total loans | 1,679,013,000 | 1,728,871,000
Non-accrual | $ 406,000 | 386,000
Number of purchase credit impaired (PCI) loans with unpaid balances | loan | 3 |
Unpaid balances on purchase credit impaired (PCI) loans | $ 131,000 | 0
Purchased Credit Impaired (PCI) loans no longer accreting interest | 0 |
Purchased Credit-impaired (PCI) loans accreting interest | 2,100,000 | 2,100,000
Loans past due more than 90 days still accruing | 0 | 0
Deferred loan fees | 818,000 | 704,000
Unaccreted purchase discounts on non-PCI loans | 1,200,000 | 922,000
30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 684,000 | 234,000
60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 1,340,000 | 67,000
90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 307,000 | 0
Commercial loans | Commercial and industrial |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 1,340,000 | 20,000
Current | 234,495,000 | 238,751,000
Total loans | 235,835,000 | 238,771,000
Non-accrual | 0 | 0
Commercial loans | Commercial and industrial | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial loans | Commercial and industrial | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 1,340,000 | 20,000
Commercial loans | Commercial and industrial | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Commercial real estate, owner-occupied |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Current | 300,963,000 | 316,467,000
Total loans | 300,963,000 | 316,467,000
Non-accrual | 0 | 0
Commercial real estate loans | Commercial real estate, owner-occupied | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Commercial real estate, owner-occupied | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Commercial real estate, owner-occupied | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Commercial real estate, investor |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Current | 822,984,000 | 841,493,000
Total loans | 822,984,000 | 841,493,000
Non-accrual | 0 | 0
Commercial real estate loans | Commercial real estate, investor | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Commercial real estate, investor | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Commercial real estate, investor | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Construction |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Current | 63,828,000 | 68,739,000
Total loans | 63,828,000 | 68,739,000
Non-accrual | 0 | 0
Commercial real estate loans | Construction | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Construction | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Commercial real estate loans | Construction | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Residential loans | Home equity |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 406,000 | 121,000
Current | 132,061,000 | 121,122,000
Total loans | 132,467,000 | 121,243,000
Non-accrual | 406,000 | 318,000
Residential loans | Home equity | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 99,000 | 121,000
Residential loans | Home equity | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Residential loans | Home equity | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 307,000 | 0
Residential loans | Other residential |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 255,000 | 0
Current | 95,271,000 | 113,383,000
Total loans | 95,526,000 | 113,383,000
Non-accrual | 0 | 0
Residential loans | Other residential | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 255,000 | 0
Residential loans | Other residential | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Residential loans | Other residential | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 0
Consumer loans | Installment and other consumer |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 330,000 | 160,000
Current | 27,080,000 | 28,615,000
Total loans | 27,410,000 | 28,775,000
Non-accrual | 0 | 68,000
Consumer loans | Installment and other consumer | 30-59 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 330,000 | 113,000
Consumer loans | Installment and other consumer | 60-89 days past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | 0 | 47,000
Consumer loans | Installment and other consumer | 90 days or more past due |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans past due | $ 0 | $ 0

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