NATURE OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Nature of business (Detail) $ in Millions	Dec. 31, 2020 USD ($) subsidiaries offices numberofbranches
Santander [Member]
Nature Of Business [Line Items]
Assets Held In Custody | $	$ 69.3
FirstBank [Member] | PUERTO RICO
Nature Of Business [Line Items]
Wholly owned subsidiaries | subsidiaries	5
Banking branches	73
FirstBank [Member] | U S [Member]
Nature Of Business [Line Items]
Banking branches	10
FirstBank [Member] | USVI and BVI [Member]
Nature Of Business [Line Items]
Banking branches	11
First Federal Finance Corp [Member] | PUERTO RICO
Nature Of Business [Line Items]
Offices | offices	28