Commitments and contingencies - Commitments - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2023
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount	$ 35,486,161	$ 35,420,879		$ 35,420,879
FDIC deposit insurance expense	23,887		$ 8,865
FDIC Special Assessment
Commitments and Contingencies Disclosure [Line Items]
FDIC Indemnification Asset	20,400,000	16,300,000		16,300,000
FDIC deposit insurance expense	14,300	71,400
FDIC deposit insurance expense net of tax	9,100	45,300
Commitments to extend credit
Commitments and Contingencies Disclosure [Line Items]
Reserve of potential losses associated with unfunded loan commitments	16,800	17,000		17,000
Noncredit commitment
Commitments and Contingencies Disclosure [Line Items]
Investments	3,300	3,300		3,300
Residential mortgage loans
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount	1,900,000	1,900,000		1,900,000
BVI | Retail and commerical loans
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount	208,000	205,000		205,000
Insured or guaranteed by the U.S. Government or its agencies | Commercial Loans
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount	92,400	89,200		89,200
PR Government direct exposure
Commitments and Contingencies Disclosure [Line Items]
Total concentration of risk	388,091			362,000
Outstanding concentration of risk	$ 363,091	333,000		333,000
Exposure Municipal Loans And Securities	78.00%
PR Government direct exposure | Loans
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk	$ 347,525	314,000		314,000
PR Government direct exposure | Securities Investment
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk	15,566	19,000		19,000
PR Government Indirect Exposure
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk	233,000	238,000		238,000
Loans and Leases Receivable, Consumer, Mortgage with PRHA	187,000	191,000		191,000
PR Government Indirect Exposure | PR Housing Bonds backed by second mortgage loans
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk	39,000	$ 40,000		40,000
USVI Government Direct Exposure | From USVI Government And Public Corporations
Commitments and Contingencies Disclosure [Line Items]
Total concentration of risk	$ 28,000			$ 28,000