REGULATORY MATTERS - Summary of Actual Capital Amounts and Ratios (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier 1 leverage capital, Capital Adequacy Ratio	4.00%
Tier 1 risk-based capital, Capital Adequacy Ratio	6.00%
Total risk-based capital, Capital Adequacy Ratio	8.00%
Investar Holding Corporation
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier 1 leverage capital, Actual Amount	$ 161,438	$ 115,312
Tier 1 common equity risk-based capital, Actual Amount	154,938	111,812
Tier 1 risk-based capital, Actual Amount	161,438	115,312
Total risk-based capital, Actual Amount	$ 187,530	$ 122,363
Tier 1 leverage capital, Actual Ratio	10.66%	10.10%
Tier 1 common equity risk-based capital, Actual Ratio	11.75%	11.40%
Tier 1 risk-based capital, Actual Ratio	12.24%	11.75%
Total risk-based capital, Actual Ratio	14.22%	12.47%
Tier 1 leverage capital, Capital Adequacy Amount	$ 60,579	$ 45,689
Tier 1 common equity risk-based capital, Capital Adequacy Amount	59,352	44,144
Tier 1 risk-based capital, Capital Adequacy Amount	79,136	58,858
Total risk-based capital, Capital Adequacy Amount	$ 105,514	$ 78,478
Tier 1 leverage capital, Capital Adequacy Ratio	4.00%	4.00%
Tier 1 common equity risk-based capital, Capital Adequacy Ratio	4.50%	4.50%
Tier 1 risk-based capital, Capital Adequacy Ratio	6.00%	6.00%
Total risk-based capital, Capital Adequacy Ratio	8.00%	8.00%
Investar Bank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier 1 leverage capital, Actual Amount	$ 175,943	$ 114,417
Tier 1 common equity risk-based capital, Actual Amount	175,943	114,417
Tier 1 risk-based capital, Actual Amount	175,943	114,417
Total risk-based capital, Actual Amount	$ 183,867	$ 121,468
Tier 1 leverage capital, Actual Ratio	11.63%	10.03%
Tier 1 common equity risk-based capital, Actual Ratio	13.35%	11.67%
Tier 1 risk-based capital, Actual Ratio	13.35%	11.67%
Total risk-based capital, Actual Ratio	13.95%	12.39%
Tier 1 leverage capital, Capital Adequacy Amount	$ 60,534	$ 45,651
Tier 1 common equity risk-based capital, Capital Adequacy Amount	59,294	44,104
Tier 1 risk-based capital, Capital Adequacy Amount	79,059	58,806
Total risk-based capital, Capital Adequacy Amount	$ 105,411	$ 78,408
Tier 1 leverage capital, Capital Adequacy Ratio	4.00%	4.00%
Tier 1 common equity risk-based capital, Capital Adequacy Ratio	4.50%	4.50%
Tier 1 risk-based capital, Capital Adequacy Ratio	6.00%	6.00%
Total risk-based capital, Capital Adequacy Ratio	8.00%	8.00%
Tier 1 leverage capital, Well Capitalized Amount	$ 75,668	$ 57,063
Tier 1 common equity risk-based capital, Well Capitalized Amount	85,647	63,706
Tier 1 risk-based capital, Well Capitalized Amount	105,411	78,408
Total risk-based capital, Well Capitalized Amount	$ 131,764	$ 98,010
Tier 1 leverage capital, Well Capitalized Ratio	5.00%	5.00%
Tier 1 common equity risk-based capital, Well Capitalized Ratio	6.50%	6.50%
Tier 1 risk-based capital, Well Capitalized Ratio	8.00%	8.00%
Total risk-based capital, Well Capitalized Ratio	10.00%	10.00%

Tier 1 common equity risk based capital.

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Tier 1 common equity risk based capital required for capital adequacy to risk weighted assets.

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Tier 1 common equity risk based capital required to be well capitalized.

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Tier 1 common equity risk based capital required to be well capitalized to risk weighted assets.

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Tier 1 common equity risk based capital 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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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 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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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 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.

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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.

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

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