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Accounting Policies (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) Payment Quarter Segment | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Certificates of Deposits in Other Banks [Abstract] |  |  |  |  |
Maturity period of certificates of deposit in other banks |  |  | 18 months |  |
Loans [Abstract] |  |  |  |  |
Past due period after which loans must be well secured and in the process of collection to continue accruing interest |  |  | 90 days |  |
Period of current principal and interest payments for reclassifying nonaccrual loans as accruing loans |  |  | 6 months |  |
Allowance for Loan and Lease Losses [Abstract] |  |  |  |  |
Number of delinquent monthly payments before loan charge off | Payment |  |  | 5 |  |
Current value assessment period for past due loans secured against real estate |  |  | 90 days |  |
Threshold period past due for initiation of foreclosure proceedings |  |  | 120 days |  |
Historical loan loss review period | Quarter |  |  | 12 |  |
Goodwill and Core Deposit Intangible [Abstract] |  |  |  |  |
Goodwill |  |  | $ 65,490 | $ 65,490 |
Finite-lived Intangible Assets [Roll Forward] |  |  |  |  |
Beginning balance, January 1 | $ 0 | $ 0 | 133 |  |
Ending balance, December 31 |  |  | $ 0 | $ 133 |
Segment Reporting [Abstract] |  |  |  |  |
Number of operating segments | Segment |  |  | 1 |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Equity securities |  |  | $ 25,000 |  |
Unrealized holding loss on securities |  |  | $ (600) |  |
Federal corporate tax rate |  |  | 35.00% | 35.00% | 35.00%
Minimum [Member] |  |  |  |  |
Other Real Estate Owned [Abstract] |  |  |  |  |
Typical frequency of periodic reviews |  |  | 12 months |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Net present value of all potential lease payments |  |  | $ 10,100 |  |
Maximum [Member] |  |  |  |  |
Other Real Estate Owned [Abstract] |  |  |  |  |
Typical frequency of periodic reviews |  |  | 18 months |  |
Frequency of periodic reviews in general |  |  | 24 months |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Net present value of all potential lease payments |  |  | $ 20,300 |  |
Plan [Member] |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Federal corporate tax rate |  | 21.00% |  |  |
Buildings [Member] | Maximum [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful life |  |  | 40 years |  |
Furniture, Fixtures, and Equipment [Member] | Minimum [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful life |  |  | 2 years |  |
Furniture, Fixtures, and Equipment [Member] | Maximum [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Estimated useful life |  |  | 10 years |  |
Core Deposit Intangible [Member] |  |  |  |  |
Finite-lived Intangible Assets [Roll Forward] |  |  |  |  |
Beginning balance, January 1 | 0 | $ 0 | $ 133 | $ 291 | $ 477
Amortization |  |  | (133) | (158) | (186)
Ending balance, December 31 |  |  | $ 0 | $ 133 | $ 291
Commercial [Member] | Unsecured Commercial Loan [Member] |  |  |  |  |
Allowance for Loan and Lease Losses [Abstract] |  |  |  |  |
Charge off threshold for loans considered uncollectible |  |  | 90 days |  |
Consumer [Member] | Closed-End Consumer Loan [Member] |  |  |  |  |
Allowance for Loan and Lease Losses [Abstract] |  |  |  |  |
Charge off threshold for loans considered uncollectible |  |  | 120 days |  |
ASU 2017-08 [Member] | Forecast [Member] |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Reduction in retained earnings | (150) |  |  |  |
ASU 2017-08 [Member] | Forecast [Member] | Minimum [Member] |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Quarterly increase in amortization expense | 24 |  |  |  |
ASU 2017-08 [Member] | Forecast [Member] | Maximum [Member] |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Quarterly increase in amortization expense | $ 30 |  |  |  |

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