BUSINESS COMBINATION - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended			16 Months Ended
	Sep. 01, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Business Acquisition [Line Items]
Business Combination Consideration Transferred Description			The preliminary consideration for the acquisition amounted to approximately (i) $387.8 million for 117.5% of BSPR’s core tangible common equity, comprised of a $57.8 million premium on $330 million of core tangible common equity, plus (ii) $892.2 million for BSPR’s excess capital (paid at par), which represents the estimated closing payment pursuant to the terms of the Stock Purchase Agreement.
Business Combination, Acquisition Related Costs			$ 26,509	$ 11,442	$ 0	$ 37,900
Goodwill		$ 38,632	38,632	$ 28,098		38,632
Intangible assets recognized as part of a business combination		40,900	40,900			40,900
Fair value of PCD held to maturity securities	$ 55,532
Contractual amount PCD held to maturity securities	67,082
Discount on PCD held to maturity securities	10,281
Initial allowance on PCD debt securities	1,269
Fair value of PCD loans	752,826
Contractual amount of PCD loans	$ 786,049
Business Combination Acquisition Related Costs Incurred By The Acquiree			2,000
Adjustments to goodwill			4,231
Core Deposits [Member]
Business Acquisition [Line Items]
Intangible assets recognized as part of a business combination		35,800	35,800			35,800
Customer Relationships [Member] | Credit Card [Member]
Business Acquisition [Line Items]
Intangible assets recognized as part of a business combination		4,700	$ 4,700			$ 4,700
BSPR
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Sep. 01, 2020
Business Acquisition, Name of Acquired Entity	BSPR
Business Combination, Reason for Business Combination	The acquisition of BSPR expands the Corporation’s presence in Puerto Rico, increases its operational scale and strengthens its competitiveness in consumer, commercial, business banking, and residential lending. The acquisition also allowed the Corporation to increase its deposit base at a lower cost, which enhances FirstBank’s funding and risk profile
Goodwill	$ 10,500
Intangible assets recognized as part of a business combination	39,200
Fair value of PCD held to maturity securities	55,500
Contractual amount PCD held to maturity securities	67,100
Initial allowance on PCD debt securities	1,300
Fair value of non-PCD loans	1,800,000
Fair value of PCD loans	752,800
Contractual amount of non-PCD loans	1,800,000
Contractual amount of PCD loans	786,000
Provision for credit losses	38,900
ACL for PCD loans	$ 28,700
Acquired Deposits Weighted Average Useful Life	1 year 2 months 12 days
Adjustments to goodwill	$ 4,200	$ 4,200
BSPR | Core Deposits [Member]
Business Acquisition [Line Items]
Intangible assets recognized as part of a business combination	$ 35,400
Weighted average useful life	5 years 8 months 12 days
BSPR | Customer Relationships [Member] | Credit Card [Member]
Business Acquisition [Line Items]
Intangible assets recognized as part of a business combination	$ 3,800
Weighted average useful life	3 years