Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Total capital (to risk weighted assets)
Actual - Amount	$ 277,937us-gaap_Capital	$ 268,770us-gaap_Capital
Actual - Ratio	13.90%us-gaap_CapitalToRiskWeightedAssets	14.00%us-gaap_CapitalToRiskWeightedAssets
For Capital Adequacy Purposes - Amount	160,359	154,052
For Capital Adequacy Purposes - Ratio	8.00%	8.00%
Tier 1 capital (to risk weighted assets)
Actual - Amount	260,148us-gaap_TierOneRiskBasedCapital	252,963us-gaap_TierOneRiskBasedCapital
Actual - Ratio	13.00%	13.10%
For Capital Adequacy Purposes - Amount	120,269	77,026
For Capital Adequacy Purposes - Ratio	6.00%	4.00%
Common equity tier 1 capital
Actual - Amount	260,148gnbc_CommonEquityTierOneCapital
Actual - Ratio	13.00%gnbc_CommonEquityTierOneCapitalRatio
Common Equity Tier One Capital Required For Capital Adequacy	90,202
For Capital Adequacy Purposes - Ratio	4.5
Tier 1 capital (to average assets)
Actual - Amount	260,148us-gaap_TierOneLeverageCapital	252,963us-gaap_TierOneLeverageCapital
Actual - Ratio	12.00%	12.10%
For Capital Adequacy Purposes - Amount	87,032	84,003
For Capital Adequacy Purposes - Ratio	4.00%	4.00%
The Bank [Member]
Total capital (to risk weighted assets)
Actual - Amount	268,912	259,313
Actual - Ratio	13.40%	13.50%
For Capital Adequacy Purposes - Amount	160,178	153,867
For Capital Adequacy Purposes - Ratio	8.00%	8.00%
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Amount	200,223	192,334
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Ratio	10.00%	10.00%
Tier 1 capital (to risk weighted assets)
Actual - Amount	251,123	243,506
Actual - Ratio	12.50%	12.70%
For Capital Adequacy Purposes - Amount	120,134	76,934
For Capital Adequacy Purposes - Ratio	6.00%	4.00%
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Amount	160,178	115,400
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Ratio	8.00%	6.00%
Common equity tier 1 capital
Actual - Amount	251,123
Actual - Ratio	12.50%
Common Equity Tier One Capital Required For Capital Adequacy	90,100
For Capital Adequacy Purposes - Ratio	4.5
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Amount	130,145
Common Equity Tier One Capital Required To Be Well Capitalized Ratio	6.50%
Tier 1 capital (to average assets)
Actual - Amount	251,123	243,506
Actual - Ratio	11.60%	11.60%
For Capital Adequacy Purposes - Amount	86,705	83,738
For Capital Adequacy Purposes - Ratio	4.00%	4.00%
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Amount	108,381	104,673
To be Categorized as Well Capitalized under Prompt Corrective Action Provisions - Ratio	5.00%	5.00%
Dividends paid	$ 0	$ 0

Common equity tier 1 capital as defined in the regulations.

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Common equity tier 1 capital ratio as defined in the regulations.

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The minimum Common equity tier 1 capital ratio required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Common equity tier 1 capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of total capital as defined in the regulations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

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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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- Definition

Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests.

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

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

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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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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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The minimum Tier One Capital Ratio (Tier one 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 amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

+ References

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

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