REGULATORY MATTERS, COMMITMENTS AND CONTINGENCIES- Regulatory Capital Positions (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Holding Company [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Capital	$ 1,957,282	$ 1,921,329
Tier One Risk Based Capital	1,642,777	1,597,117
Tier One Leverage Capital	1,642,777	1,597,117
Capital Required For Capital Adequacy	705,895	720,329
Tier One Risk Based Capital Required For Capital Adequacy	529,421	540,247
Tier One Leverage Capital Required For Capital Adequacy	$ 470,583	$ 466,376
Capital To Risk Weighted Assets	22.18%	21.34%
Tier One Risk Based Capital To Risk Weighted Assets	18.62%	17.74%
Tier One Leverage Capital To Average Assets	13.96%	13.70%
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	18.62%	17.74%
Common Equity Tier 1 Capital To Risk Weight Assets	$ 1,642,777	$ 1,597,117
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	$ 397,066	$ 405,185
FirstBank [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Capital	1,914,708	1,872,120
Tier One Risk Based Capital	1,802,621	1,757,642
Tier One Leverage Capital	1,802,621	1,757,642
Capital Required For Capital Adequacy	705,618	720,091
Tier One Risk Based Capital Required For Capital Adequacy	529,213	540,068
Tier One Leverage Capital Required For Capital Adequacy	469,985	465,740
Capital Required To Be Well Capitalized	882,022	900,114
Tier One Risk Based Capital Required To Be Well Capitalized	$ 705,618	$ 720,091
Capital To Risk Weighted Assets	21.71%	20.80%
Tier One Risk Based Capital To Risk Weighted Assets	20.44%	19.53%
Tier One Leverage Capital To Average Assets	15.34%	15.10%
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	$ 587,481	$ 582,174
Common Equity Tier1 Capital To Risk Weight Assets Ratio	17.36%	16.92%
Common Equity Tier 1 Capital To Risk Weight Assets	$ 1,530,926	$ 1,523,332
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	$ 396,910	$ 405,051
Common Equity Tier 1 Capital To Risk Weight Assets Well Capitalized	$ 573,314	$ 585,074
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 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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