LOAN PORTFOLIO - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					$ 8,886,873	$ 8,858,123	$ 8,850,476	$ 8,886,873
Loans held for investment transferred to held for sale						90,319	0	10,332
Financing Receivable Recorded Investment Past Due						574,654	848,763
Charge-off						116,792	134,770	136,928
Unpaid Principal Balance						890,301	970,566
Related Allowance					64,421	53,976	51,410	64,421
Proceeds From Sale Of Loans Held For Investment						82,526	53,245	31,852
Provision For Loan And Lease Losses						$ 59,253	144,254	86,733
Threshold Period Past Due Of Financing Receivables To Be Considered Defaulted						2 months 29 days
Troubled Debt Restructurings Sustained Performance Period						6 months
Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans					647,048	$ 582,647	587,219	647,048	$ 661,591
Outstanding unfunded commitments on TDR loans						700
Financing Receivable Loans Restructured Recorded Investment Accruals						$ 9,900		3,000
Troubled Debt Restructurings Sustained Performance Period						6 months
Ab Note Restructure Workout Strategy [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision For Loan And Lease Losses						$ (10,789)	(1,295)	4,279
Total TDR loans					36,971	33,840	35,577	36,971	$ 39,329
PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					165,818	146,640	158,174	165,818
Contractually outstanding principal and interest at acquisition						181,100	196,600
Financing Receivable Recorded Investment Past Due						$ 36,423	46,260
GNMA | Repurchase Option Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Threshold Delinquent Days To Repruchase Gnma Loans						3 months
Commercial And Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						$ 2,148,111	2,083,253
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					3,296,031	3,163,208	3,290,957	3,296,031
Charge-off						24,775	28,186	33,621
Related Allowance					8,633	19,965	22,086	8,633
Provision For Loan And Lease Losses						13,202	50,744	25,090
Residential Portfolio Segment [Member] | Loans In Trial [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						3,400
Residential Portfolio Segment [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					162,676	143,176	153,991	162,676
Financing Receivable Recorded Investment Past Due						33,911	43,071
Residential Portfolio Segment [Member] | Loans Insured or Guaranteed by US Government Authorities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						147,205	138,939
Financing Receivable Recorded Investment Past Due						108,934	109,607
Unpaid Principal Balance						0	0
Related Allowance						0	0
Residential Portfolio Segment [Member] | Loans Insured or Guaranteed by US Government Authorities [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						60,500	62,100
Residential Portfolio Segment [Member] | Loans Insured or Guaranteed by US Government Authorities [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						0	0
Residential Portfolio Segment [Member] | FNMA and FHLMC Loans | Repurchase Option Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Purchases						100	36	700
Residential Portfolio Segment [Member] | GNMA | Repurchase Option Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Purchases						49,100	25,100	29,100
Financing Receivable Recorded Investment Past Due						$ 43,600	62,100
Residential Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Threshold Delinquent Days To Commence Foreclosure Process						3 months 28 days
Financing Receivable Significant Purchases						$ 46,100
Residential Portfolio Segment [Member] | Mortgage Receivable [Member] | Loans Insured or Guaranteed by US Government Authorities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales						233,200
Residential Portfolio Segment [Member] | Mortgage Receivable [Member] | FMMA
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales		$ 9,800
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						3,750,202	3,809,622
Financing Receivable Significant Purchases						21,400	52,600
Commercial Portfolio Segment [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						257,800	354,700
Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					1,568,808	1,522,662	1,614,972	1,568,808
Financing Receivable Significant Sales								20,200
Charge-off						23,911	39,092	20,454
Unpaid Principal Balance						250,369	224,696
Related Allowance					26,172	17,684	9,783	26,172
Provision For Loan And Lease Losses						23,074	30,054	8,688
Commercial Portfolio Segment [Member] | Mortgage Receivable [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						137,949	50,812
Commercial Portfolio Segment [Member] | Mortgage Receivable [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					3,142	3,464	4,183	3,142
Financing Receivable Recorded Investment Past Due						2,512	3,189
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					2,180,455	2,148,111	2,083,253	2,180,455
Financing Receivable Recorded Investment Past Due						38,001	97,703
Charge-off						9,704	19,855	26,579
Unpaid Principal Balance						124,112	143,259
Related Allowance					22,638	9,693	12,359	22,638
Provision For Loan And Lease Losses						(8,440)	1,018	17,075
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						74,921	94,112
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					0	0	0	0
Residential Mortgage Backed Securities [Member] | Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage loans in process of foreclosure						177,200
Residential Mortgage Backed Securities [Member] | Residential Portfolio Segment [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage loans in process of foreclosure						21,300
Residential Mortgage Backed Securities [Member] | Residential Portfolio Segment [Member] | Loans Insured or Guaranteed by US Government Authorities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage loans in process of foreclosure						48,500
Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment transferred to held for sale						74,400
Charge Offs Loans Held For Sale						22,200
Financing Receivable Significant Sales					16,300
Proceeds From Sale Of Loans Held For Investment					11,300
Provision For Loan And Lease Losses					1,800
Classified And Non Performing Loans Sold Outstanding Balance					20,100
Reserves Allocated To Bulk Sale					2,800			2,800
Net charge-offs					4,600
Nonperforming Financial Instruments [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales								16,300
Charge-off								1,300
Nonperforming Financial Instruments [Member] | Loans Tranferred To Held For Sale [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment transferred to held for sale						34,500
Charge Offs Loans Held For Sale						8,500
Nonperforming Financial Instruments [Member] | Commercial And Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment transferred to held for sale						1,800
Charge Offs Loans Held For Sale						1,700
Nonperforming Financial Instruments [Member] | Residential Portfolio Segment [Member] | Loans Insured or Guaranteed by US Government Authorities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						0	0
Nonperforming Financial Instruments [Member] | Commercial Portfolio Segment [Member] | Loans Tranferred To Held For Sale [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 29,700		$ 66,900
Loans held for investment transferred to held for sale	17,200		57,200
Charge Offs Loans Held For Sale	12,500		9,700
Charge Offs Loans Held For Sale Previously Established Reserve	2,400		4,100
Provision Loans Transferred To Held For Sale	10,100		$ 5,600
Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					7,833,150	7,951,214	7,901,994	7,833,150
Performing Financing Receivable [Member] | Ab Note Restructure Workout Strategy [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						31,300
Performing Financing Receivable [Member] | Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					2,691,088	2,616,300	2,703,532	2,691,088
Performing Financing Receivable [Member] | Residential Portfolio Segment [Member] | Loans Insured or Guaranteed by US Government Authorities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						147,205	138,939
Performing Financing Receivable [Member] | Residential Portfolio Segment [Member] | Mortgage Receivable [Member] | FNMA and FHLMC Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales						104,900
Performing Financing Receivable [Member] | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					1,371,275	1,291,772	1,457,875	1,371,275
Performing Financing Receivable [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					2,026,912	2,056,919	1,964,953	2,026,912
Construction loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					124,951	79,429	111,397	124,951
Charge-off						8,296	3,607	1,770
Related Allowance					1,405	760	2,017	1,405
Provision For Loan And Lease Losses						7,032	4,835	497
Construction loans [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					0	0	0	0
Construction loans [Member] | Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						11,151	7,910
Financing Receivable Recorded Investment Past Due						2,102	2,082
Unpaid Principal Balance						2,370	355
Related Allowance						208	55
Construction loans [Member] | Residential Portfolio Segment [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						762	217
Construction loans [Member] | Residential Portfolio Segment [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						0	0
Construction loans [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						47,965	76,556
Financing Receivable Recorded Investment Past Due						0	35,100
Unpaid Principal Balance						0	38,595
Related Allowance						0	560
Construction loans [Member] | Commercial Portfolio Segment [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total TDR loans						0	35,100
Construction loans [Member] | Commercial Portfolio Segment [Member] | PCI loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						0	0
Construction loans [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment transferred to held for sale						33,000
Charge Offs Loans Held For Sale						6,700
Construction loans [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					71,660	72,836	64,131	71,660
Construction loans [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment transferred to held for sale						30,000
Construction loans [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Nonperforming Financial Instruments [Member] | Commercial Portfolio Segment [Member] | Loans Tranferred To Held For Sale [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment transferred to held for sale	7,700
Loss on Sales of Loans, Net	$ 2,700
Puerto Rico Electric Power Authority [Member] | Entity Loan Modification Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-off							10,700
Puerto Rico Electric Power Authority [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales				$ 64,000
Unpaid Principal Balance				75,000
Related Allowance				10,200
Proceeds From Sale Of Loans Held For Investment				53,200
Provision For Loan And Lease Losses				$ 600
Puerto Rico Housing Finance Authority [Member] | Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross						112,100
Puerto Rico [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					6,926,719	$ 6,586,033	6,633,432	6,926,719
Credit risk concentration						74.00%
Outstanding of credit facilities granted						$ 61,600	55,900
Puerto Rico [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales						5,600
Charge-off						1,300
Puerto Rico [Member] | Nonperforming Financial Instruments [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales						34,900
Puerto Rico [Member] | Puerto Rico Government And Political Subdivisions [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding of credit facilities granted						47,200
Puerto Rico [Member] | Puerto Rico Electric Power Authority [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding of credit facilities granted						14,500
Virgin Islands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					627,720	$ 481,188	584,576	627,720
Credit risk concentration						21.00%
Outstanding of credit facilities granted						$ 55,800	70,400
Virgin Islands [Member] | Construction loans [Member] | Nonperforming Financial Instruments [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales						27,000
Virgin Islands [Member] | Public Corporations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding of credit facilities granted						32,600
Virgin Islands [Member] | Government [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding of credit facilities granted						23,200
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross					$ 1,382,440	$ 1,834,088	$ 1,665,448	$ 1,382,440
Credit risk concentration						5.00%
United States [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Significant Sales						$ 9,200