Fair value of financial instruments - Carrying or notional amounts and estimated fair values for financial instruments (Details) - USD ($)	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Financial Assets:
Cash and due from banks	$ 427,594,000	$ 420,462,000
Money market investments	6,530,788,000	6,998,871,000
Debt securities available-for-sale	17,186,123,000	16,729,044,000
Debt securities held-to-maturity	7,859,864,000	8,188,555,000
Equity securities:
Equity securities, at lower of cost or realizable value	198,191,000	193,726,000
Loans held in portfolio	35,450,647,000	34,335,630,000
Mortgage servicing rights	108,827,000	118,109,000	$ 119,030,000	$ 128,350,000
Deposits
Time deposits	9,080,990,000	8,501,892,000
Deposits	63,668,501,000	63,618,243,000
Assets sold under agreements to repurchase	55,360,000	91,384,000
Other short-term borrowings	0	0
Notes payable:
Notes payable	918,376,000	986,948,000
US Treasury Securities
Financial Assets:
Debt securities available-for-sale	11,524,430,000	10,747,061,000
Debt securities held-to-maturity	7,799,966,000	8,121,411,000
Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	52,415,000	59,628,000
Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities available-for-sale	128,553,000	134,686,000
Debt securities held-to-maturity	1,523,000	1,556,000
Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960,000	5,960,000
Other
Financial Assets:
Debt securities available-for-sale	2,000,000	2,511,000
Carrying amount
Financial Assets:
Cash and due from banks	427,594,000	420,462,000
Money market investments	6,530,788,000	6,998,871,000
Trading account debt securities, excluding derivatives	30,836,000	31,568,000
Debt securities available-for-sale	17,186,123,000	16,729,044,000
Debt securities held-to-maturity	7,859,864,000	8,188,555,000
Equity securities:
FHLB stock	46,668,000	49,549,000
FRB stock	98,957,000	98,948,000
Other investments	52,566,000	45,229,000
Equity securities, at lower of cost or realizable value	198,191,000	193,726,000
Loans held-for-sale	5,509,000	4,301,000
Loans held in portfolio	35,450,647,000	34,335,630,000
Mortgage servicing rights	108,827,000	118,109,000
Derivatives	25,941,000	24,419,000
Deposits
Demand deposits	54,587,511,000	55,116,351,000
Time deposits	9,080,990,000	8,501,892,000
Deposits	63,668,501,000	63,618,243,000
Assets sold under agreements to repurchase	55,360,000	91,384,000
Notes payable:
FHLB advances	325,095,000	394,665,000
Unsecured senior debt securities	394,915,000	393,937,000
Junior subordinated deferrable interest debentures (related to trust preferred securities)	198,366,000	198,346,000
Notes payable	918,376,000	986,948,000
Derivatives	23,608,000	21,103,000
Carrying amount | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	7,799,966,000	8,121,411,000
Carrying amount | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	52,415,000	59,628,000
Carrying amount | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,523,000	1,556,000
Carrying amount | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960,000	5,960,000
Fair Value
Financial Assets:
Cash and due from banks	427,594,000	420,462,000
Money market investments	6,530,788,000	6,998,871,000
Trading account debt securities, excluding derivatives	30,836,000	31,568,000
Debt securities available-for-sale	17,186,123,000	16,729,044,000
Debt securities held-to-maturity	7,915,928,000	8,159,385,000
Equity securities:
FHLB stock	46,668,000	49,549,000
FRB stock	98,957,000	98,948,000
Other investments	53,295,000	46,144,000
Equity securities, at lower of cost or realizable value	198,920,000	194,641,000
Loans held-for-sale	5,509,000	4,328,000
Loans held in portfolio	34,681,826,000	33,376,255,000
Mortgage servicing rights	108,827,000	118,109,000
Derivatives	25,941,000	24,419,000
Deposits
Demand deposits	54,587,511,000	55,116,351,000
Time deposits	8,872,969,000	8,154,823,000
Deposits	63,460,480,000	63,271,174,000
Assets sold under agreements to repurchase	55,369,000	91,386,000
Notes payable:
FHLB advances	318,450,000	377,851,000
Unsecured senior debt securities	417,316,000	400,848,000
Junior subordinated deferrable interest debentures (related to trust preferred securities)	189,592,000	180,076,000
Notes payable	925,358,000	958,775,000
Derivatives	23,608,000	21,103,000
Fair Value | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	7,854,735,000	8,092,339,000
Fair Value | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	53,835,000	59,678,000
Fair Value | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,398,000	1,408,000
Fair Value | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960,000	5,960,000
Fair Value | Level 1
Financial Assets:
Cash and due from banks	427,594,000	420,462,000
Money market investments	6,522,716,000	6,991,758,000
Trading account debt securities, excluding derivatives	7,675,000	16,859,000
Debt securities available-for-sale	5,879,911,000	3,936,036,000
Debt securities held-to-maturity	0	0
Equity securities:
FHLB stock	0	0
FRB stock	0	0
Other investments	0	0
Equity securities, at lower of cost or realizable value	0	0
Loans held-for-sale	0	0
Loans held in portfolio	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Deposits
Demand deposits	0	0
Time deposits	0	0
Deposits	0	0
Assets sold under agreements to repurchase	0	0
Notes payable:
FHLB advances	0	0
Unsecured senior debt securities	0	0
Junior subordinated deferrable interest debentures (related to trust preferred securities)	0	0
Notes payable	0	0
Derivatives	0	0
Fair Value | Level 1 | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | Level 1 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | Level 1 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | Level 1 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | Level 2
Financial Assets:
Cash and due from banks	0	0
Money market investments	8,072,000	7,113,000
Trading account debt securities, excluding derivatives	22,923,000	14,425,000
Debt securities available-for-sale	11,303,654,000	12,789,902,000
Debt securities held-to-maturity	7,869,023,000	8,106,701,000
Equity securities:
FHLB stock	46,668,000	49,549,000
FRB stock	98,957,000	98,948,000
Other investments	46,248,000	37,965,000
Equity securities, at lower of cost or realizable value	191,873,000	186,462,000
Loans held-for-sale	5,509,000	4,328,000
Loans held in portfolio	0	0
Mortgage servicing rights	0	0
Derivatives	25,941,000	24,419,000
Deposits
Demand deposits	54,587,511,000	55,116,351,000
Time deposits	8,872,969,000	8,154,823,000
Deposits	63,460,480,000	63,271,174,000
Assets sold under agreements to repurchase	55,369,000	91,386,000
Notes payable:
FHLB advances	318,450,000	377,851,000
Unsecured senior debt securities	417,316,000	400,848,000
Junior subordinated deferrable interest debentures (related to trust preferred securities)	189,592,000	180,076,000
Notes payable	925,358,000	958,775,000
Derivatives	23,608,000	21,103,000
Fair Value | Level 2 | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	7,854,735,000	8,092,339,000
Fair Value | Level 2 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	6,930,000	7,007,000
Fair Value | Level 2 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,398,000	1,395,000
Fair Value | Level 2 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960,000	5,960,000
Fair Value | Level 3
Financial Assets:
Cash and due from banks	0	0
Money market investments	0	0
Trading account debt securities, excluding derivatives	238,000	284,000
Debt securities available-for-sale	2,558,000	3,106,000
Debt securities held-to-maturity	46,905,000	52,684,000
Equity securities:
FHLB stock	0	0
FRB stock	0	0
Other investments	6,662,000	7,869,000
Equity securities, at lower of cost or realizable value	6,662,000	7,869,000
Loans held-for-sale	0	0
Loans held in portfolio	34,681,826,000	33,376,255,000
Mortgage servicing rights	108,827,000	118,109,000
Derivatives	0	0
Deposits
Demand deposits	0	0
Time deposits	0	0
Deposits	0	0
Assets sold under agreements to repurchase	0	0
Notes payable:
FHLB advances	0	0
Unsecured senior debt securities	0	0
Junior subordinated deferrable interest debentures (related to trust preferred securities)	0	0
Notes payable	0	0
Derivatives	0	0
Fair Value | Level 3 | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | Level 3 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	46,905,000	52,671,000
Fair Value | Level 3 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	0	13,000
Fair Value | Level 3 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | NAV
Financial Assets:
Cash and due from banks	0	0
Money market investments	0	0
Trading account debt securities, excluding derivatives	0	0
Debt securities available-for-sale	0	0
Debt securities held-to-maturity	0	0
Equity securities:
FHLB stock	0	0
FRB stock	0	0
Other investments	385,000	310,000
Equity securities, at lower of cost or realizable value	385,000	310,000
Loans held-for-sale	0	0
Loans held in portfolio	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Deposits
Demand deposits	0	0
Time deposits	0	0
Deposits	0	0
Assets sold under agreements to repurchase	0	0
Notes payable:
FHLB advances	0	0
Unsecured senior debt securities	0	0
Junior subordinated deferrable interest debentures (related to trust preferred securities)	0	0
Notes payable	0	0
Derivatives	0	0
Fair Value | NAV | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | NAV | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | NAV | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	0	0
Fair Value | NAV | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	$ 0	$ 0