REGULATORY MATTERS, COMMITMENTS AND CONTINGENCIES- Regulatory Capital Positions (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Holding Company [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets)	$ 2,154,733	$ 2,118,940
Common Equity Tier One Capital (to Risk-Weighted Assets)	1,827,442	1,792,880
Tier One Capital (to Risk-Weighted Assets)	1,863,546	1,828,984
Leverage ratio	1,863,546	1,828,984
Total Capital (to Risk-Weighted Assets) Required For Capital Adequacy	715,368	706,418
Common Equity Tier One Capital (to Risk-Weighted Assets) Required For Capital Adequacy	402,395	397,360
Tier One Capital (to Risk-Weighted Assets) Required For Capital Adequacy	536,526	529,814
Leverage Ratio Required For Capital Adequacy	$ 482,267	$ 475,924
Total Capital (to Risk-Weighted Assets) Ratio	24.10%	24.00%
Tier One Capital (to Risk-Weighted Assets) Ratio	20.84%	20.71%
Common Equity Tier One Capital (to Risk-Weighted Assets) Ratio	20.44%	20.30%
Leverage Ratio	15.46%	15.37%
Total Capital (to Risk-Weighted Assets) Required For Capital Adequacy Ratio	8.00%	8.00%
Common Equity Tier One Capital (to Risk-Weighted Assets) Required for Capiral Adequacy Ratio	4.50%	4.50%
Tier One Capital (to Risk-Weighted Assets) Required For Capital Adequacy Ratio	6.00%	6.00%
Leverage Ratio Required Fo Capital Adequacy Ratio	4.00%	4.00%
FirstBank [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets)	$ 2,111,831	$ 2,075,894
Common Equity Tier One Capital (to Risk-Weighted Assets)	1,691,245	1,656,563
Tier One Capital (to Risk-Weighted Assets)	1,999,245	1,964,563
Leverage ratio	1,999,245	1,964,563
Total Capital (to Risk-Weighted Assets) Required For Capital Adequacy	715,536	706,426
Common Equity Tier One Capital (to Risk-Weighted Assets) Required For Capital Adequacy	402,489	397,365
Tier One Capital (to Risk-Weighted Assets) Required For Capital Adequacy	536,652	529,819
Leverage Ratio Required For Capital Adequacy	481,959	475,490
Total Capital (to Risk-Weighted Assets) Required To Be Well Capitalized	894,420	883,032
Common Equity Tier One Capital (to Risk-Weighted Assets) Required To Be Well Capitalized	581,373	573,971
Tier One Capital (to Risk-Weighted Assets) Required To Be Well Capitalized	715,536	706,426
Leverage Ratio Required To Be Well Capitalized	$ 602,449	$ 594,362
Total Capital (to Risk-Weighted Assets) Ratio	23.61%	23.51%
Tier One Capital (to Risk-Weighted Assets) Ratio	22.35%	22.25%
Common Equity Tier One Capital (to Risk-Weighted Assets) Ratio	18.91%	18.76%
Leverage Ratio	16.59%	16.53%
Total Capital (to Risk-Weighted Assets) Required For Capital Adequacy Ratio	8.00%	8.00%
Common Equity Tier One Capital (to Risk-Weighted Assets) Required for Capiral Adequacy Ratio	4.50%	4.50%
Tier One Capital (to Risk-Weighted Assets) Required For Capital Adequacy Ratio	6.00%	6.00%
Leverage Ratio Required Fo Capital Adequacy Ratio	4.00%	4.00%
Total Capital (to Risk-Weighted Assets) Required To Be Well Capitalized Ratio	10.00%	10.00%
Common Equity Tier One Capital (to Risk-Weighted Assets) Required To Be Well Capitalized Ratio	6.50%	6.50%
Tier One Capital (to Risk-Weighted Assets) Required To Be Well Capitalized Ratio	8.00%	8.00%
Leverage Ratio Required To Be Well Capitalized Ratio	5.00%	5.00%

Common Equity Tier 1 capital as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets for capital adequacy as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets for capital adequacy as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets to be well capitalized as defined by new Basel III rule.

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Common Equity Tier 1 capital divided by risk weighted assets to be well capitalized as defined by new Basel III rule.

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

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