Fair value of financial instruments - Carrying or notional amounts and estimated fair values for financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Financial Assets:
Cash and due from banks	$ 420,462	$ 469,501
Money market investments	6,998,871	5,614,595
Debt securities available-for-sale	16,729,044	17,804,374
Debt securities held-to-maturity	8,188,555	8,518,455
Equity securities:
Equity securities, at lower of cost or realizable value	193,726	195,854
Loans held in portfolio	34,335,630	31,357,467
Mortgage servicing rights	118,109	128,350	$ 128,350	$ 121,570
Deposits
Time deposits	8,501,892	6,781,401
Deposits	63,618,243	61,227,227
Assets sold under agreements to repurchase	91,384	148,609
Other short-term borrowings	0	365,000
Notes payable:
Notes payable	986,948	886,710
Derivatives	21,103	17,000
US Treasury Securities
Financial Assets:
Debt securities available-for-sale	10,747,061	11,180,948
Debt securities held-to-maturity	8,121,411	8,453,467
Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	59,628	59,010
Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities available-for-sale	134,686	165,196
Debt securities held-to-maturity	1,556	19
Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960	5,959
Other
Financial Assets:
Debt securities available-for-sale	2,511	1,060
Carrying amount
Financial Assets:
Cash and due from banks	420,462	469,501
Money market investments	6,998,871	5,614,595
Trading account debt securities, excluding derivatives	31,568	27,723
Debt securities available-for-sale	16,729,044	17,804,374
Debt securities held-to-maturity	8,188,555	8,518,455
Equity securities:
FHLB stock	49,549	65,861
FRB stock	98,948	96,206
Other investments	45,229	33,787
Equity securities, at lower of cost or realizable value	193,726	195,854
Loans held-for-sale	4,301	5,381
Loans held in portfolio	34,335,630	31,357,467
Mortgage servicing rights	118,109	128,350
Derivatives	24,419	19,229
Deposits
Demand deposits	55,116,351	54,445,825
Time deposits	8,501,892	6,781,402
Deposits	63,618,243	61,227,227
Assets sold under agreements to repurchase	91,384	148,609
Other short-term borrowings		365,000
Notes payable:
FHLB advances	394,665	389,282
Unsecured senior debt securities	393,937	299,109
Junior subordinated deferrable interest debentures (related to trust preferred securities)	198,346	198,319
Notes payable	986,948	886,710
Derivatives	21,103	17,000
Carrying amount | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	8,121,411	8,453,467
Carrying amount | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	59,628	59,010
Carrying amount | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,556	19
Carrying amount | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960	5,959
Fair Value
Financial Assets:
Cash and due from banks	420,462	469,501
Money market investments	6,998,871	5,614,595
Trading account debt securities, excluding derivatives	31,568	27,723
Debt securities available-for-sale	16,729,044	17,804,374
Debt securities held-to-maturity	8,159,385	8,440,196
Equity securities:
FHLB stock	49,549	65,861
FRB stock	98,948	96,206
Other investments	46,144	34,598
Equity securities, at lower of cost or realizable value	194,641	196,665
Loans held-for-sale	4,328	5,404
Loans held in portfolio	33,376,255	29,366,365
Mortgage servicing rights	118,109	128,350
Derivatives	24,419	19,229
Deposits
Demand deposits	55,116,351	54,445,825
Time deposits	8,154,823	6,464,943
Deposits	63,271,174	60,910,768
Assets sold under agreements to repurchase	91,386	148,566
Other short-term borrowings		365,000
Notes payable:
FHLB advances	377,851	361,951
Unsecured senior debt securities	400,848	300,027
Junior subordinated deferrable interest debentures (related to trust preferred securities)	180,076	173,938
Notes payable	958,775	835,916
Derivatives	21,103	17,000
Fair Value | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	8,092,339	8,372,601
Fair Value | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	59,678	61,617
Fair Value | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,408	19
Fair Value | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960	5,959
Fair Value | Level 1
Financial Assets:
Cash and due from banks	420,462	469,501
Money market investments	6,991,758	5,607,937
Trading account debt securities, excluding derivatives	16,859	13,069
Debt securities available-for-sale	3,936,036	1,908,589
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
Other short-term borrowings		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	7,113	6,658
Trading account debt securities, excluding derivatives	14,425	14,119
Debt securities available-for-sale	12,789,902	15,894,074
Debt securities held-to-maturity	8,106,701	8,378,560
Equity securities:
FHLB stock	49,549	65,861
FRB stock	98,948	96,206
Other investments	37,965	29,302
Equity securities, at lower of cost or realizable value	186,462	191,369
Loans held-for-sale	4,328	0
Loans held in portfolio	0	0
Mortgage servicing rights	0	0
Derivatives	24,419	19,229
Deposits
Demand deposits	55,116,351	54,445,825
Time deposits	8,154,823	6,464,943
Deposits	63,271,174	60,910,768
Assets sold under agreements to repurchase	91,386	148,566
Other short-term borrowings		365,000
Notes payable:
FHLB advances	377,851	361,951
Unsecured senior debt securities	400,848	300,027
Junior subordinated deferrable interest debentures (related to trust preferred securities)	180,076	173,938
Notes payable	958,775	835,916
Derivatives	21,103	17,000
Fair Value | Level 2 | US Treasury Securities
Financial Assets:
Debt securities held-to-maturity	8,092,339	8,372,601
Fair Value | Level 2 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity	7,007	0
Fair Value | Level 2 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	1,395	0
Fair Value | Level 2 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity	5,960	5,959
Fair Value | Level 3
Financial Assets:
Cash and due from banks	0	0
Money market investments	0	0
Trading account debt securities, excluding derivatives	284	535
Debt securities available-for-sale	3,106	1,711
Debt securities held-to-maturity	52,684	61,636
Equity securities:
FHLB stock	0	0
FRB stock	0	0
Other investments	7,869	4,966
Equity securities, at lower of cost or realizable value	7,869	4,966
Loans held-for-sale	0	5,404
Loans held in portfolio	33,376,255	29,366,365
Mortgage servicing rights	118,109	128,350
Derivatives	0	0
Deposits
Demand deposits	0	0
Time deposits	0	0
Deposits	0	0
Assets sold under agreements to repurchase	0	0
Other short-term borrowings		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	52,671	61,617
Fair Value | Level 3 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity	13	19
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	310	330
Equity securities, at lower of cost or realizable value	310	330
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
Other short-term borrowings		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