Fair value of financial instruments - Carrying or notional amounts and estimated fair values for financial instruments (Details) - USD ($)	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Financial Assets:
Cash and due from banks	$ 320,486,000	$ 420,462,000
Money market investments	5,928,578,000	6,998,871,000
Debt securities available-for-sale	18,017,424,000	16,729,044,000
Debt securities held-to-maturity	8,077,429,000	8,188,555,000
Equity securities:
Equity securities, at lower of cost or realizable value	195,747,000	193,726,000
Loans held in portfolio	34,379,194,000	34,335,630,000
Mortgage servicing rights	114,964,000	118,109,000	$ 127,475,000	$ 128,350,000
Deposits
Time deposits	8,755,072,000	8,501,892,000
Deposits	63,808,784,000	63,618,243,000
Assets sold under agreements to repurchase	66,090,000	91,384,000
Other short-term borrowings	0	0
Notes payable:
Notes payable	966,303,000	986,948,000
US Treasury Securities
Financial Assets:
Debt securities available-for-sale	12,289,211,000	10,747,061,000
Debt securities held-to-maturity	8,013,952,000	8,121,411,000
Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	55,981,000	59,628,000
Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities available-for-sale	127,608,000	134,686,000
Debt securities held-to-maturity	1,536,000	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	2,005,000	2,511,000
Carrying amount
Financial Assets:
Cash and due from banks	320,486,000	420,462,000
Money market investments	5,928,578,000	6,998,871,000
Trading account debt securities, excluding derivatives	27,303,000	31,568,000
Debt securities available-for-sale	18,017,424,000	16,729,044,000
Debt securities held-to-maturity	8,077,429,000	8,188,555,000
Equity securities:
FHLB stock	48,604,000	49,549,000
FRB stock	99,920,000	98,948,000
Other investments	47,223,000	45,229,000
Equity securities, at lower of cost or realizable value	195,747,000	193,726,000
Loans held-for-sale	5,352,000	4,301,000
Loans held in portfolio	34,379,194,000	34,335,630,000
Mortgage servicing rights	114,964,000	118,109,000
Derivatives	24,045,000	24,419,000
Deposits
Demand deposits	55,053,712,000	55,116,351,000
Time deposits	8,755,072,000	8,501,892,000
Deposits	63,808,784,000	63,618,243,000
Assets sold under agreements to repurchase	66,090,000	91,384,000
Notes payable:
FHLB advances	373,665,000	394,665,000
Unsecured senior debt securities	394,285,000	393,937,000
Junior subordinated deferrable interest debentures (related to trust preferred securities)	198,353,000	198,346,000
Notes payable	966,303,000	986,948,000
Derivatives	21,784,000	21,103,000
Carrying amount | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	8,013,952,000	8,121,411,000
Carrying amount | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	55,981,000	59,628,000
Carrying amount | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,536,000	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	320,486,000	420,462,000
Money market investments	5,928,578,000	6,998,871,000
Trading account debt securities, excluding derivatives	27,303,000	31,568,000
Debt securities available-for-sale	18,017,424,000	16,729,044,000
Debt securities held-to-maturity	7,958,326,000	8,159,385,000
Equity securities:
FHLB stock	48,604,000	49,549,000
FRB stock	99,920,000	98,948,000
Other investments	47,800,000	46,144,000
Equity securities, at lower of cost or realizable value	196,324,000	194,641,000
Loans held-for-sale	5,532,000	4,328,000
Loans held in portfolio	33,249,195,000	33,376,255,000
Mortgage servicing rights	114,964,000	118,109,000
Derivatives	24,045,000	24,419,000
Deposits
Demand deposits	55,053,712,000	55,116,351,000
Time deposits	8,430,548,000	8,154,823,000
Deposits	63,484,260,000	63,271,174,000
Assets sold under agreements to repurchase	66,088,000	91,386,000
Notes payable:
FHLB advances	358,046,000	377,851,000
Unsecured senior debt securities	410,228,000	400,848,000
Junior subordinated deferrable interest debentures (related to trust preferred securities)	187,383,000	180,076,000
Notes payable	955,657,000	958,775,000
Derivatives	21,784,000	21,103,000
Fair Value | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	7,894,800,000	8,092,339,000
Fair Value | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	56,202,000	59,678,000
Fair Value | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,364,000	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	320,486,000	420,462,000
Money market investments	5,920,785,000	6,991,758,000
Trading account debt securities, excluding derivatives	8,316,000	16,859,000
Debt securities available-for-sale	6,262,480,000	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	7,793,000	7,113,000
Trading account debt securities, excluding derivatives	18,737,000	14,425,000
Debt securities available-for-sale	11,752,337,000	12,789,902,000
Debt securities held-to-maturity	7,909,042,000	8,106,701,000
Equity securities:
FHLB stock	48,604,000	49,549,000
FRB stock	99,920,000	98,948,000
Other investments	40,933,000	37,965,000
Equity securities, at lower of cost or realizable value	189,457,000	186,462,000
Loans held-for-sale	5,532,000	4,328,000
Loans held in portfolio	0	0
Mortgage servicing rights	0	0
Derivatives	24,045,000	24,419,000
Deposits
Demand deposits	55,053,712,000	55,116,351,000
Time deposits	8,430,548,000	8,154,823,000
Deposits	63,484,260,000	63,271,174,000
Assets sold under agreements to repurchase	66,088,000	91,386,000
Notes payable:
FHLB advances	358,046,000	377,851,000
Unsecured senior debt securities	410,228,000	400,848,000
Junior subordinated deferrable interest debentures (related to trust preferred securities)	187,383,000	180,076,000
Notes payable	955,657,000	958,775,000
Derivatives	21,784,000	21,103,000
Fair Value | Level 2 | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	7,894,800,000	8,092,339,000
Fair Value | Level 2 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	6,918,000	7,007,000
Fair Value | Level 2 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,364,000	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	250,000	284,000
Debt securities available-for-sale	2,607,000	3,106,000
Debt securities held-to-maturity	49,284,000	52,684,000
Equity securities:
FHLB stock	0	0
FRB stock	0	0
Other investments	6,531,000	7,869,000
Equity securities, at lower of cost or realizable value	6,531,000	7,869,000
Loans held-for-sale	0	0
Loans held in portfolio	33,249,195,000	33,376,255,000
Mortgage servicing rights	114,964,000	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	49,284,000	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	336,000	310,000
Equity securities, at lower of cost or realizable value	336,000	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