REGULATORY MATTERS, COMMITMENTS AND CONTINGENCIES - Regulatory capital positions (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Securitization of FHA/VA mortgage loan production into GNMA mortgage-backed securities	$ 198,700,000
First Ban Corp [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital	1,748,120,000	1,604,548,000
Tier 1 Capital	1,636,004,000	1,484,490,000
Tier 1 Leverage Capital	1,636,004,000	1,484,490,000
Total Capital Required For Capital Adequacy	709,723,000	752,464,000
Tier 1 Risk Based Capital Required for Capital Adequacy	354,861,000	376,232,000
Tier 1 Leverage Capital Required for Capital Adequacy	493,159,000	506,878,000
Total Risk Based Capital Ratio	19.70%	17.06%
Tier 1 Risk Based Capital Ratio	18.44%	15.78%
Tier 1 Leverage Ratio	13.27%	11.71%
Total Risk Based Capital Ratio Adequately Capitalized	8.00%	8.00%
Tier 1 Risk Based Capital Ratio Adequately Capitalized	4.00%	4.00%
Tier 1 Leverage Ratio Adequately Capitalized	4.00%	4.00%
FirstBank [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total Capital	1,717,432,000	1,567,232,000
Tier 1 Capital	1,605,367,000	1,447,262,000
Tier 1 Leverage Capital	1,605,367,000	1,447,262,000
Total Capital Required For Capital Adequacy	709,395,000	751,978,000
Tier 1 Risk Based Capital Required for Capital Adequacy	354,698,000	375,989,000
Tier 1 Leverage Capital Required for Capital Adequacy	492,468,000	506,210,000
Total Capital Required to be Well Capitalized	886,744,000	939,972,000
Tier 1 Risk Based Capital Required to be Well Capitalized	532,046,000	563,983,000
Tier 1 Leverage Capital Required to be Well Capitalized	615,585,000	632,763,000
Total Capital Consent Order Requirement	1,064,093,000	1,127,966,000
Tier 1 Capital Consent Order	886,744,000	939,972,000
Tier 1 Leverage Capital Consent Order	984,937,000	1,012,420,000
Total Risk Based Capital Ratio	19.37%	16.67%
Tier 1 Risk Based Capital Ratio	18.10%	15.40%
Tier 1 Leverage Ratio	13.04%	11.44%
Total Risk Based Capital Ratio Adequately Capitalized	8.00%	8.00%
Tier 1 Risk Based Capital Ratio Adequately Capitalized	4.00%	4.00%
Tier 1 Leverage Ratio Adequately Capitalized	4.00%	4.00%
Total Risk Based Capital Ratio Well Capitalized	10.00%	10.00%
Tier 1 Risk Based Capital to Risk Weighted Well Capitalized	6.00%	6.00%
Tier 1 Leverage Ratio Well Capitalized	5.00%	5.00%
Total Risk Based Capital Ratio Consent Order	12.00%	12.00%
Tier 1 Risk Based Capital Ratio Consent Order	10.00%	10.00%
Leverage Ratio Consent Order	8.00%	8.00%
To originate loans [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	76,235,000	65,184,000
Unused personal lines of credit [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	682,994,000	735,331,000
Commercial lines of credit [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	383,015,000	386,941,000
Commercial Letters Of Credit [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	38,555,000	41,081,000
Standby letters of credit [Member] [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	3,791,000	10,527,000
Commitments To Sell Loans [Member]
Loss Contingencies [Line Items]
Financial instruments whose contract amounts represent credit risk	$ 129,369,000	$ 123,925,000

TThe amount of Tier 1 Risk Based Capital required under the Consent Order.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required under the Consent Order.

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The amount of Tier 1 Leverage Capital required under the Consent Order.

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The amount of Tier 1 Leverage Capital required under the Consent Order.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required under the Consent Agreement.

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The amount of Total Capital required under the Consent Agreement.

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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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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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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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Represents the initial fair value recorded for liabilities incurred on transfer of financial assets in a securitization, asset-backed financing arrangement, or a similar transfer which transaction is recognized as a sale of the transferred financial assets.

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