DEPOSITS - Narratives (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deposits [Line Items]
Weighted-average interest rate on total interest-bearing	0.55%	1.18%
Unplanned overdrafts of demand deposits that were reclassified as loans	$ 800	$ 4,100
Pre-arranged overdrafts lines of credit	26,000	28,600
CDs in denominations of $100,000 or higher	2,300,000	2,400,000
Brokered certificates of deposit	216,172	435,089
Unamortized broker placement fees	400	900
Deposits	15,317,383	9,348,429
Market value of uninsured portions collateralized by securities and loans	2,100,000	769,600
Amortized cost of uninsured portions collateralized by securities and loans	2,000,000	780,900
Accretion Of Premium From Acquisition	1,000
Letters of credit issued by the FHLB as pledges for public deposits in the Virgin Islands	2,500,000	1,800,000
Amortization Of Broker Placement Fees	537	732	$ 1,163
Time deposits in denomnations of $250,000 or more	$ 1,000,000	$ 1,300,000
CDs including brokered CDs [Member]
Deposits [Line Items]
Weighted Average Interest Rate of Time Deposits, $100,000 or More	2.00%	2.15%
Brokered Cds in denominations of $100,000 or higher [Member]
Deposits [Line Items]
Brokered certificates of deposit	$ 216,100	$ 434,800
Federal Home Loan Bank [Member]
Deposits [Line Items]
Letters of credit issued by the FHLB as pledges for public deposits in the Virgin Islands	2,200,000	1,600,000
Government [Member]
Deposits [Line Items]
Deposits	2,100,000	1,100,000
Puerto Rico [Member]
Deposits [Line Items]
Brokered certificates of deposit	109,000	243,400	441,100
Deposits	12,338,934	6,422,864	6,208,531
Puerto Rico [Member] \| Government [Member]
Deposits [Line Items]
Deposits	1,800,000	826,900
United States [Member]
Deposits [Line Items]
Brokered certificates of deposit	107,100	191,700	114,500
Deposits	1,622,481	1,661,657	1,519,362
United States [Member] \| Government [Member]
Deposits [Line Items]
Deposits	9,700	7,600
Virgin Islands [Member]
Deposits [Line Items]
Deposits	1,355,968	1,263,908	$ 1,266,821
Virgin Islands [Member] \| Federal Home Loan Bank [Member]
Deposits [Line Items]
Letters of credit issued by the FHLB as pledges for public deposits in the Virgin Islands		212,000
Virgin Islands [Member] \| Government [Member]
Deposits [Line Items]
Deposits	$ 280,200	$ 227,700

Represents the amount of accretion of premiums related to time deposits assumed in the Doral bank transaction in 2015.

No definition available.

Name:	fbp_AccretionOfPremiumFromAcquisition
Namespace Prefix:	fbp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amortization of broker placement fees

+ References

No definition available.

+ Details

Name:	fbp_AmortizationOfBrokerPlacementFees
Namespace Prefix:	fbp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Market value amount of uninsured portions collateralized by securities and loans.

+ References

No definition available.

+ Details

Name:	fbp_DepositLiabilitiesCollateralIssuedFinancialInstrumentsMarketValue
Namespace Prefix:	fbp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Deposits [Line Items]

+ References

No definition available.

+ Details

Name:	fbp_DepositsLineItems
Namespace Prefix:	fbp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amounts of deposits held by the entity, for which overdrafts are pre-arranged as lines of credit.

+ References

No definition available.

+ Details

Name:	fbp_OverdraftsPrearrangedLinesOfCredit
Namespace Prefix:	fbp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

unamortized amount of costs for brokerage fees paid for issuance of brokered CDs.

+ References

No definition available.

+ Details

Name:	fbp_UnamortizedBrokerPlacementFees
Namespace Prefix:	fbp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of securities, mortgage loans, or other financial instruments pledged as collateral for deposits.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

+ Details

Name:	us-gaap_DepositLiabilitiesCollateralIssuedFinancialInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of deposits held by the entity that have been reclassified as loan balances receivable, for example, but not limited to, overdrafts receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

+ Details

Name:	us-gaap_DepositLiabilitiesReclassifiedAsLoansReceivable1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_Deposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of interest-bearing domestic third-party deposits placed by or through the assistance of a deposit broker, also known as brokered deposit liabilities.

+ References

+ Details

Name:	us-gaap_InterestBearingDomesticDepositBrokered
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5086-111524

+ Details

Name:	us-gaap_LoansPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations of $100,000 or more.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher SEC
-Name Industry Guide
-Number 3
-Section V
-Paragraph D-E

+ Details

Name:	us-gaap_TimeDeposits100000OrMore
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

+ Details

Name:	us-gaap_TimeDepositsAtOrAboveFDICInsuranceLimit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted average interest rate calculated at point in time for domestic deposit liability.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateDomesticDeposit
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_BankTimeDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=fbp_InterestBearingDomesticDepositBrokeredGreaterThan100000Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_FederalHomeLoanBankAdvancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis=us-gaap_GovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_PR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_VI
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: