Commitments and contingencies - Commitments - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended				9 Months Ended		12 Months Ended
	Jul. 24, 2024	Sep. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount		$ 36,599,612		$ 35,420,879		$ 36,599,612		$ 35,420,879
FDIC deposit insurance expense		10,433			$ 8,932	44,901	$ 24,600
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		18,000		17,000		18,000		17,000
Noncredit commitment
Commitments and Contingencies Disclosure [Line Items]
Investments		2,000		3,300		2,000		3,300
Residential mortgage loans
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount		2,000,000		1,900,000		2,000,000		1,900,000
BVI | Retail and commerical loans
Commitments and Contingencies Disclosure [Line Items]
Loans And Leases Receivable Gross Carrying Amount		200,000		205,000		200,000		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		88,400		89,200		88,400		89,200
PR Government direct exposure
Commitments and Contingencies Disclosure [Line Items]
Total concentration of risk						336,113		362,000
Outstanding concentration of risk		$ 336,113		333,000		$ 336,113		333,000
Exposure Municipal Loans And Securities		80.00%				80.00%
Principal payment from various obligations	$ 40,000
PR Government direct exposure | Loans
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk		$ 323,452				$ 323,452
PR Government direct exposure | Loans | Adjustment
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk		323,000		314,000		323,000		314,000
PR Government direct exposure | Securities Investment
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk		12,661		19,000		12,661		19,000
PR Government Indirect Exposure
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk		225,000		238,000		225,000		238,000
Loans and Leases Receivable, Consumer, Mortgage with PRHA		179,000		191,000		179,000		191,000
PR Government Indirect Exposure | PR Housing Bonds backed by second mortgage loans
Commitments and Contingencies Disclosure [Line Items]
Outstanding concentration of risk		$ 38,000		$ 40,000		38,000		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