Commitments and contingencies - Commitments - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Commitments and Contingencies Disclosure
Exposure Municipal Loans And Securities	74.00%
Loans And Leases Receivable Gross Carrying Amount	$ 29,344,620	$ 29,588,430
Commitments To Extend Credit
Commitments and Contingencies Disclosure
Reserve of potential losses associated with unfunded loan commitments	10,000	16,000
Noncredit commitment
Commitments and Contingencies Disclosure
Investments	1,400	1,400
Residential mortgage loans
Commitments and Contingencies Disclosure
Loans And Leases Receivable Gross Carrying Amount	1,700,000	1,800,000
SBA loans under PPP program
Commitments and Contingencies Disclosure
Loans And Leases Receivable Gross Carrying Amount	1,200,000	1,300,000
BVI | Retail and commerical loans
Commitments and Contingencies Disclosure
Loans And Leases Receivable Gross Carrying Amount	250,000	251,000
BVI | Loan with BVI government
Commitments and Contingencies Disclosure
Loans And Leases Receivable Gross Carrying Amount	18,000	19,000
Insured or guaranteed by the U.S. Government or its agencies | Commercial Loans
Commitments and Contingencies Disclosure
Loans And Leases Receivable Gross Carrying Amount	60,000	60,000
PR Government direct exposure
Commitments and Contingencies Disclosure
Outstanding concentration of risk	374,570
Total concentration of risk	374,570
PR Government direct exposure | Loans
Commitments and Contingencies Disclosure
Outstanding concentration of risk	341,925	342,000
PR Government direct exposure | Securities Investment
Commitments and Contingencies Disclosure
Outstanding concentration of risk	32,645	35,000
PR Government direct exposure | PR government and its instrumentalities and municipalities
Commitments and Contingencies Disclosure
Outstanding concentration of risk	375,000	377,000
PR Government Indirect Exposure
Commitments and Contingencies Disclosure
Outstanding concentration of risk	311,000	317,000
Loans and Leases Receivable, Consumer, Mortgage with PRHA	255,000	260,000
PR Government Indirect Exposure | PR Housing Bonds backed by second mortgage loans
Commitments and Contingencies Disclosure
Outstanding concentration of risk	45,000	46,000
PR Government Indirect Exposure | Commercial real estate
Commitments and Contingencies Disclosure
Outstanding concentration of risk	11,000	11,000
USVI Government Direct Exposure | From USVI Government And Public Corporations
Commitments and Contingencies Disclosure
Total concentration of risk	$ 103,000	$ 105,000