INVESTMENT SECURITIES - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Investments [Line Items]
Proceeds from Sale of Available-for-sale Securities		$ 1,080,766,000	$ 0
Percentage Of Debt Securities Government And Government Sponsored Agencies	99.00%	99.00%
Fair value	$ 3,294,649,000	$ 3,294,649,000			$ 2,123,525,000
Gross Unrealized loss	6,941,000	6,941,000			10,056,000
Allowance for Credit Losses	1,386,000	1,386,000		$ 1,631,000	0
Allowance for credit losses, Held-to-Maturity securities	10,176,000	10,176,000			0
Debt Securities, Held-to-maturity, Past Due	$ 0	$ 0
Debt Securities, Held-to-maturity, Threshold Period Past Due	30 days	30 days
Municipal Bonds [Member] | Puerto Rico [Member]
Schedule Of Investments [Line Items]
Allowance for credit losses, Held-to-Maturity securities	$ 10,176,000	$ 10,176,000		9,268,000	0
Held to Maturity Amortized cost	189,156,000	189,156,000			138,675,000
Cash and Cash Equivalents [Member]
Schedule Of Investments [Line Items]
Held to Maturity Amortized cost	0	0
MBS [Member]
Schedule Of Investments [Line Items]
Proceeds from Sale of Available-for-sale Securities		1,100,000
Gain on Sale of Investments		13,400,000
Fair value	2,560,565,000	2,560,565,000			1,783,504,000
Gross Unrealized loss	4,830,000	4,830,000			8,440,000
Allowance for Credit Losses	1,078,000	1,078,000
MBS [Member] | Private label [Member]
Schedule Of Investments [Line Items]
Fair value	8,800,000	8,800,000
Gross Unrealized loss	4,700,000	4,700,000
Allowance for Credit Losses	1,078,000	1,078,000		1,323,000	0
Puerto Rico Government obligations [Member]
Schedule Of Investments [Line Items]
Fair value	6,900,000	6,900,000
Allowance for Credit Losses	308,000	308,000		$ 308,000	$ 0
Net Unrealized loss	1,100,000	1,100,000
Puerto Rico Government obligations [Member] | Residential pass-through mortgage-backed securities [Member]
Schedule Of Investments [Line Items]
Fair value	2,900,000	2,900,000
Gross Unrealized loss	1,100,000	1,100,000
Available for sale Securities [Member]
Schedule Of Investments [Line Items]
Provision For Other Credit Losses Previous Expected Credit Losses	0	1,323,000
Provision For Other Credit Losses No Previous Expected Credit Losses	0	308,000
Available for sale Securities [Member] | MBS [Member] | Private label [Member]
Schedule Of Investments [Line Items]
Provision For Other Credit Losses Previous Expected Credit Losses	0	1,323,000
Provision For Other Credit Losses No Previous Expected Credit Losses	0	$ 0
Available for sale Securities [Member] | MBS [Member] | Private label [Member] | FICO Score, Greater than 700 [Member] | LTV Less than 80 Percent [Member]
Schedule Of Investments [Line Items]
Financing Receivable, Credit Quality, Additional Information		The underlying collateral are fixed-rate, single-family residential mortgage loans in the United States with original FICO scores over 700 and moderate loan-to-value ratios (under 80%), as well as moderate delinquency levels.
Available for sale Securities [Member] | MBS [Member] | Private label [Member] | LIBOR [Member]
Schedule Of Investments [Line Items]
Loans Receivable, Description of Variable Rate Basis		The interest rate on these private-label MBS is variable, tied to 3-month LIBOR and limited to the weighted-average coupon on the underlying collateral.
Available for sale Securities [Member] | Puerto Rico Government obligations [Member]
Schedule Of Investments [Line Items]
Provision For Other Credit Losses Previous Expected Credit Losses	0	$ 0
Provision For Other Credit Losses No Previous Expected Credit Losses	0	308,000
Held-to-maturity Securities [Member] | Municipal Bonds [Member] | Puerto Rico [Member]
Schedule Of Investments [Line Items]
Provision for Other Credit Losses	$ (361,000)	$ 773,000