REGULATORY MATTERS, COMMITMENTS AND CONTINGENCIES- Regulatory Capital Positions (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Holding Company [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Capital	$ 1,842,251	$ 1,828,559
Tier One Risk Based Capital	1,516,041	1,546,678
Tier One Leverage Capital	1,516,041	1,546,678
Capital Required For Capital Adequacy	730,576	731,164
Tier One Risk Based Capital Required For Capital Adequacy	547,932	548,373
Tier One Leverage Capital Required For Capital Adequacy	$ 497,157	$ 506,322
Capital To Risk Weighted Assets	20.17%	20.01%
Tier One Risk Based Capital To Risk Weighted Assets	16.60%	16.92%
Tier One Leverage Capital To Average Assets	12.20%	12.22%
Capital Required For Capital Adequacy To Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets	6.00%	6.00%
Tier One Leverage Capital Required For Capital Adequacy To Average Assets	4.00%	4.00%
Common Equity Tier1 Capital To Risk Weight Assets Ratio	16.60%	16.92%
Common Equity Tier 1 Capital To Risk Weight Assets	$ 1,516,041	$ 1,546,678
Common Equity Tier 1 Capital To Risk Weight Assets Ratio Capital Adequacy	4.50%	4.50%
Common Equity Tier 1 Capital To Risk Weight Assets Capital Adequacy	$ 410,949	$ 411,280
FirstBank [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Capital	1,823,238	1,802,711
Tier One Risk Based Capital	1,706,771	1,685,656
Tier One Leverage Capital	1,706,771	1,685,656
Capital Required For Capital Adequacy	730,277	730,824
Tier One Risk Based Capital Required For Capital Adequacy	547,708	548,118
Tier One Leverage Capital Required For Capital Adequacy	496,530	505,648
Capital Required To Be Well Capitalized	912,846	913,530
Tier One Risk Based Capital Required To Be Well Capitalized	$ 730,277	$ 730,824
Capital To Risk Weighted Assets	19.97%	19.73%
Tier One Risk Based Capital To Risk Weighted Assets	18.70%	18.45%
Tier One Leverage Capital To Average Assets	13.75%	13.33%
Capital Required For Capital Adequacy To Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets	6.00%	6.00%
Tier One Leverage Capital Required For Capital Adequacy To Average Assets	4.00%	4.00%
Capital Required To Be Well Capitalized To Risk Weighted Assets	10.00%	10.00%
Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets	8.00%	8.00%
Tier One Leverage Capital Required To Be Well Capitalized To Average Assets	5.00%	5.00%
Tier One Leverage Capital Required To Be Well Capitalized	$ 620,662	$ 632,060
Common Equity Tier1 Capital To Risk Weight Assets Ratio	16.15%	16.35%
Common Equity Tier 1 Capital To Risk Weight Assets	$ 1,474,364	$ 1,493,478
Common Equity Tier 1 Capital To Risk Weight Assets Ratio Capital Adequacy	4.50%	4.50%
Common Equity Tier 1 Capital To Risk Weight Assets Capital Adequacy	$ 410,781	$ 411,088
Common Equity Tier 1 Capital To Risk Weight Assets Well Capitalized	$ 593,350	$ 593,794
Common Equity Tier 1 Capital To Risk Weight Assets Ratio Well Capitalized	6.50%	6.50%

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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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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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 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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