Fair value of financial instruments - Carrying or notional amounts and estimated fair values for financial instruments (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Carrying amount
Financial Assets:
Cash and due from banks	$ 445,551,000	$ 388,311,000
Money market investments	5,941,716,000	3,262,286,000
Trading account debt securities, excluding derivatives	42,545,000	40,321,000
Debt Securities Available for Sale, at Fair Value	15,813,301,000	17,648,473,000
Debt securities held-to-maturity, net	81,873,000	97,662,000
Equity securities:
FHLB Stock	46,316,000	43,787,000
FRB stock	93,838,000	93,470,000
Other investments	22,904,000	22,630,000
Equity securities	163,058,000	159,887,000
Loans held-for-sale	87,855,000	59,203,000
Loans held in portfolio, net	26,742,556,000	26,929,165,000
Mortgage servicing rights	147,311,000	150,906,000
Derivatives	9,534,000	17,966,000
Deposits
Demand deposits	36,269,352,000	36,083,809,000
Time deposits	8,527,824,000	7,674,797,000
Total deposits	44,797,176,000	43,758,606,000
Assets sold under agreements to repurchase	178,766,000	193,378,000
Other short-term borrowings	100,000,000
Notes payable:
Advances with the FHLB	377,598,000	421,399,000
Unsecured senior debt securities	295,624,000	295,307,000
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	384,909,000	384,902,000
Notes Payable	1,058,131,000	1,101,608,000
Derivatives	12,037,000	16,619,000
Carrying amount | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity, net	69,771,000	85,556,000
Carrying amount | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity, net	41,000	45,000
Carrying amount | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity, net	11,561,000	11,561,000
Carrying amount | Other
Financial Assets:
Debt securities held-to-maturity, net	500,000	500,000
Cash and due from banks	445,551,000	388,311,000
Money market investments	5,941,716,000	3,262,286,000
Trading account debt securities, excluding derivatives	42,545,000	40,321,000
Debt Securities Available for Sale, at Fair Value	15,813,301,000	17,648,473,000
Debt securities held-to-maturity, net	81,873,000	97,662,000
Equity securities:
Equity securities	163,058,000	159,887,000
Loans held in portfolio, net	26,742,556,000	26,929,165,000
Mortgage servicing rights	147,311,000	150,906,000	$ 167,813,000	$ 169,777,000
Deposits
Time deposits	8,527,824,000	7,674,797,000
Total deposits	44,797,176,000	43,758,606,000
Assets sold under agreements to repurchase	178,766,000	193,378,000
Other short-term borrowings	100,000,000	0
Notes payable:
Notes Payable	1,058,131,000	1,101,608,000
Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt Securities Available for Sale, at Fair Value		6,975,000
Debt securities held-to-maturity, net	69,771,000
Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt Securities Available for Sale, at Fair Value	560,836,000	586,175,000
Debt securities held-to-maturity, net	41,000
Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity, net	11,561,000
Other
Financial Assets:
Debt Securities Available for Sale, at Fair Value	316,000	350,000
Debt securities held-to-maturity, net	500,000
Fair Value
Financial Assets:
Cash and due from banks	445,551,000	388,311,000
Money market investments	5,941,716,000	3,262,286,000
Trading account debt securities, excluding derivatives	42,545,000	40,321,000
Debt Securities Available for Sale, at Fair Value	15,813,301,000	17,648,473,000
Debt securities held-to-maturity, net	80,400,000	105,110,000
Equity securities:
FHLB Stock	46,316,000	43,787,000
FRB stock	93,838,000	93,470,000
Other investments	28,305,000	28,695,000
Equity securities	168,459,000	165,952,000
Loans held-for-sale	91,182,000	60,030,000
Loans held in portfolio, net	24,977,460,000	25,051,400,000
Mortgage servicing rights	147,311,000	150,906,000
Derivatives	9,534,000	17,966,000
Deposits
Demand deposits	36,269,352,000	36,083,809,000
Time deposits	8,398,572,000	7,598,732,000
Total deposits	44,667,924,000	43,682,541,000
Assets sold under agreements to repurchase	178,104,000	193,271,000
Other short-term borrowings	100,000,000
Notes payable:
Advances with the FHLB	393,674,000	429,718,000
Unsecured senior debt securities	309,228,000	323,415,000
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	371,086,000	395,216,000
Notes Payable	1,073,988,000	1,148,349,000
Derivatives	12,037,000	16,619,000
Fair Value | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity, net	68,296,000	93,002,000
Fair Value | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity, net	43,000	47,000
Fair Value | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity, net	11,561,000	11,561,000
Fair Value | Other
Financial Assets:
Debt securities held-to-maturity, net	500,000	500,000
Fair Value | Level 1
Financial Assets:
Cash and due from banks	445,551,000	388,311,000
Money market investments	5,935,945,000	3,256,274,000
Trading account debt securities, excluding derivatives	1,000	7,081,000
Debt Securities Available for Sale, at Fair Value	2,099,856,000	3,841,715,000
Debt securities held-to-maturity, net	0	0
Equity securities:
FHLB Stock	0	0
FRB stock	0	0
Other investments	0	0
Equity securities	0	0
Loans held-for-sale	0	0
Loans held in portfolio, net	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Deposits
Demand deposits	0	0
Time deposits	0	0
Total deposits	0	0
Assets sold under agreements to repurchase	0	0
Other short-term borrowings	0
Notes payable:
Advances with the FHLB	0	0
Unsecured senior debt securities	0	0
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	0	0
Notes Payable	0	0
Derivatives	0	0
Fair Value | Level 1 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 1 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 1 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 1 | Other
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 2
Financial Assets:
Cash and due from banks	0	0
Money market investments	5,771,000	6,012,000
Trading account debt securities, excluding derivatives	41,649,000	32,270,000
Debt Securities Available for Sale, at Fair Value	13,712,268,000	13,805,576,000
Debt securities held-to-maturity, net	12,061,000	12,061,000
Equity securities:
FHLB Stock	46,316,000	43,787,000
FRB stock	93,838,000	93,470,000
Other investments	21,601,000	21,328,000
Equity securities	161,755,000	158,585,000
Loans held-for-sale	0	0
Loans held in portfolio, net	0	0
Mortgage servicing rights	0	0
Derivatives	9,534,000	17,966,000
Deposits
Demand deposits	36,269,352,000	36,083,809,000
Time deposits	8,398,572,000	7,598,732,000
Total deposits	44,667,924,000	43,682,541,000
Assets sold under agreements to repurchase	178,104,000	193,271,000
Other short-term borrowings	100,000,000
Notes payable:
Advances with the FHLB	393,674,000	429,718,000
Unsecured senior debt securities	309,228,000	323,415,000
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	371,086,000	395,216,000
Notes Payable	1,073,988,000	1,148,349,000
Derivatives	12,037,000	16,619,000
Fair Value | Level 2 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 2 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 2 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity, net	11,561,000	11,561,000
Fair Value | Level 2 | Other
Financial Assets:
Debt securities held-to-maturity, net	500,000	500,000
Fair Value | Level 3
Financial Assets:
Cash and due from banks	0	0
Money market investments	0	0
Trading account debt securities, excluding derivatives	895,000	970,000
Debt Securities Available for Sale, at Fair Value	1,177,000	1,182,000
Debt securities held-to-maturity, net	68,339,000	93,049,000
Equity securities:
FHLB Stock	0	0
FRB stock	0	0
Other investments	6,704,000	7,367,000
Equity securities	6,704,000	7,367,000
Loans held-for-sale	91,182,000	60,030,000
Loans held in portfolio, net	24,977,460,000	25,051,400,000
Mortgage servicing rights	147,311,000	150,906,000
Derivatives	0	0
Deposits
Demand deposits	0	0
Time deposits	0	0
Total deposits	0	0
Assets sold under agreements to repurchase	0	0
Other short-term borrowings	0
Notes payable:
Advances with the FHLB	0	0
Unsecured senior debt securities	0	0
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	0	0
Notes Payable	0	0
Derivatives	0	0
Fair Value | Level 3 | Obligations of Puerto Rico, States and political subdivisions
Financial Assets:
Debt securities held-to-maturity, net	68,296,000	93,002,000
Fair Value | Level 3 | Collateralized Mortgage Obligations - Federal agencies
Financial Assets:
Debt securities held-to-maturity, net	43,000	47,000
Fair Value | Level 3 | Securities in wholly owned statutory business trusts
Financial Assets:
Debt securities held-to-maturity, net	0	0
Fair Value | Level 3 | Other
Financial Assets:
Debt securities held-to-maturity, net	$ 0	$ 0