Regulatory Capital Matters - Summary of Actual and Required Capital Amounts and Ratios, Exclusive of Capital Conservation Buffer (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Capital conservation buffer		1.25%	0.625%	0.00%
Consolidated
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual Amount	$ 131,634	$ 100,713
Tier 1 capital (to risk-weighted assets), Actual Amount	121,847	91,510
Common equity Tier 1 capital (to risk-weighted assets), Actual Amount	121,847	91,510
Tier 1 capital (to average assets), Actual Amount	$ 121,847	$ 91,510
Total capital (to risk-weighted assets), Actual Ratio	16.09%	13.49%
Tier 1 capital (to risk-weighted assets), Actual Ratio	14.90%	12.26%
Common equity Tier 1 capital (to risk-weighted assets), Actual Ratio	14.90%	12.26%
Tier 1 capital (to average assets), Actual Ratio	12.91%	10.46%
Total capital (to risk-weighted assets), Amount, Required for Capital Adequacy Purposes	$ 65,433	$ 59,729
Tier 1 capital (to risk-weighted assets), Amount, Required for Capital Adequacy Purposes	49,075	44,797
Common equity Tier 1 capital (to risk-weighted assets), Amount, Required for Capital Adequacy Purposes	36,806	33,597
Tier 1 capital (to average assets), Amount, Required for Capital Adequacy Purposes	$ 37,757	$ 35,009
Capital conservation buffer	8.00%	8.00%
Tier 1 capital (to risk-weighted assets), Ratio, Required for Capital Adequacy Purposes	6.00%	6.00%
Common equity Tier 1 capital (to risk-weighted assets), Ratio, Required for Capital Adequacy Purposes	4.50%	4.50%
Tier 1 capital (to average assets), Ratio, Required for Capital Adequacy Purposes	4.00%	4.00%
Bank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual Amount	$ 131,540	$ 100,648
Tier 1 capital (to risk-weighted assets), Actual Amount	121,753	91,445
Common equity Tier 1 capital (to risk-weighted assets), Actual Amount	121,753	91,445
Tier 1 capital (to average assets), Actual Amount	$ 121,753	$ 91,445
Total capital (to risk-weighted assets), Actual Ratio	16.08%	13.48%
Tier 1 capital (to risk-weighted assets), Actual Ratio	14.89%	12.25%
Common equity Tier 1 capital (to risk-weighted assets), Actual Ratio	14.89%	12.25%
Tier 1 capital (to average assets), Actual Ratio	12.90%	10.45%
Total capital (to risk-weighted assets), Amount, Required for Capital Adequacy Purposes	$ 65,426	$ 59,726
Tier 1 capital (to risk-weighted assets), Amount, Required for Capital Adequacy Purposes	49,069	44,795
Common equity Tier 1 capital (to risk-weighted assets), Amount, Required for Capital Adequacy Purposes	36,802	33,596
Tier 1 capital (to average assets), Amount, Required for Capital Adequacy Purposes	$ 37,753	$ 35,007
Capital conservation buffer	8.00%	8.00%
Tier 1 capital (to risk-weighted assets), Ratio, Required for Capital Adequacy Purposes	6.00%	6.00%
Common equity Tier 1 capital (to risk-weighted assets), Ratio, Required for Capital Adequacy Purposes	4.50%	4.50%
Tier 1 capital (to average assets), Ratio, Required for Capital Adequacy Purposes	4.00%	4.00%
Total capital (to risk-weighted assets), Amount, Minimum To be Considered "Well Capitalized"	$ 81,782	$ 74,658
Tier 1 capital (to risk-weighted assets), Amount, Minimum To be Considered "Well Capitalized"	65,426	59,726
Common equity Tier 1 capital (to risk-weighted assets), Amount, Minimum To be Considered "Well Capitalized"	53,159	48,528
Tier 1 capital (to average assets), Amount, Minimum To be Considered "Well Capitalized"	$ 47,192	$ 43,759
Total capital (to risk-weighted assets), Ratio, Minimum To be Considered "Well Capitalized"	10.00%	10.00%
Tier 1 capital (to risk-weighted assets), Ratio, Minimum To be Considered "Well Capitalized"	8.00%	8.00%
Common equity Tier 1 capital (to risk-weighted assets), Ratio, Minimum To be Considered "Well Capitalized"	6.50%	6.50%
Tier 1 capital (to average assets), Ratio, Minimum To be Considered "Well Capitalized"	5.00%	5.00%

Common equity tier one capital required for capital adequacy to risk weighted assets.

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Common equity tier one capital required to be well capitalized to risk weighted assets.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Amount of Common Equity Tier One Risk-based Capital, as defined by regulatory framework.

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Percentage of Common Equity Tier One Risk-based Capital to total risk-weighted assets, as defined by regulatory framework.

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Amount of Common Equity Tier 1 Risk-based Capital required to be categorized as adequately capitalized under regulatory framework for prompt corrective action.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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