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Note 17 - Regulatory Capital - Capital Ratios (Details)	Dec. 31, 2017	Dec. 31, 2016
Leverage capital, adequately capitalized, ratio	4.00%	4.00%
Tier 1 risk based capital, adequately capitalized, ratio	6.00%	6.00%
Common equity tier 1 capital, adequately capitalized, ratio	4.50%	4.50%
Total risk based capital, adequately capitalized, ratio	8.00%	8.00%
Leverage capital, adequately capitalized plus capital conservation buffer, ratio	4.00%	4.00%
Tier 1 risk based capital, adequately capitalized plus capital conservation buffer, ratio	8.50%	8.50%
Common equity tier 1 capital, adequately capitalized plus capital conservation buffer, ratio	7.00%	7.00%
Total risk based capital, adequately capitalized plus capital conservation buffer, ratio	10.50%	10.50%
Middlefield Banking Company [Member]
Leverage capital, actual, ratio	9.47%	9.29%
Tier 1 risk based capital, actual, ratio	10.88%	13.03%
Common equity tier 1 capital, actual, ratio	10.88%	13.03%
Total risk based capital, actual, ratio	11.64%	14.25%
Leverage capital, well-capitalized, ratio (bank only)	5.00%	5.00%
Tier 1 risk based capital, well-capitalized, ratio (bank only)	8.00%	8.00%
Common equity tier 1 capital, well-capitalized, ratio (bank only)	6.50%	6.50%
Total risk based capital, well-capitalized, ratio (bank only)	10.00%	10.00%
Middlefield Banc Corp [Member]
Leverage capital, actual, ratio	10.20%	9.27%
Tier 1 risk based capital, actual, ratio	11.64%	13.07%
Common equity tier 1 capital, actual, ratio	10.79%	13.07%
Total risk based capital, actual, ratio	12.41%	15.75%

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- Definition

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

+ References

No definition available.

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The minimum common equity tier 1 risk based capital ratio (common equity tier 1 capital divided by risk weighted assets) required for capital adequacy purposes plus capital conservation buffer under the regulatory framework for prompt corrective action.

+ References

No definition available.

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The minimum common equity tier 1 risk based capital ratio (common equity tier 1 capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

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The common equity tier 1 risk based capital ratio (common equity tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

+ References

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The minimum amount of Tier 1 Leverage Capital for capital adequacy plus conservation buffer purposes divided by average assets as defined in the regulations.

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No definition available.

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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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