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EMPLOYEES' BENEFIT PLAN- Narratives (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Employer Matching Contribution, Percent | 25.00% |  |
Employee contribution percent maximum to get employer matching contribution | 4.00% |  |
Total plan expense | $ 900,000 | $ 1,100,000 | $ 1,400,000
P R |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Maximum Annual Contribution Per Employee, Amount | 15,000 | 15,000 | 15,000
United States And Virign Islands Operations [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Contribution Plan, Maximum Annual Contribution Per Employee, Amount | $ 18,000 | $ 18,000 | $ 18,000

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- Definition Administrative expenses and contibutions by the employer to a benefit plan + References No definition available. + Details
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Namespace Prefix: | fbp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition 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. + References No definition available. + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum amount the employee may contribute to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
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- Details
Name: | dei_EntityByLocationAxis=country_PR
Namespace Prefix: |
Data Type: | na
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Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |