Debt securities held-to-maturity - Schedule of Debt Securities, Held-to-Maturity (Details) - USD ($)	Jun. 30, 2026	Dec. 31, 2025
Amortized cost
Amortized cost, Total	$ 6,405,803,000	$ 7,620,074,000
Book Value
Debt securities held-to-maturity (fair value 2026 - $6,176,976; 2025 - $7,363,587)	6,204,020,000	7,327,529,000
Allowance for Credit Losses
Allowance for credit losses, Total	6,230,000	5,812,000
Carrying Value Net of Allowance
Debt securities held-to-maturity, net	6,197,790,000	7,321,717,000
Gross unrealized gains
Gross unrealized gains, Total	2,490,000	44,085,000
Gross unrealized losses
Gross unrealized losses, Total	23,304,000	2,215,000
Fair value
Fair value, Total	$ 6,176,976,000	$ 7,363,587,000
Weighted average yield
Weighted average yield, Total	1.23%	1.29%
Debt securities net unrealized loss remaining in AOCI included in book value for AFS securities transferred to HTM securities	$ 201,800,000	$ 293,000,000.0
Debt securities pledged to secure government and trust deposits	6,405,803,000	7,620,074,000
Asset Pledged as Collateral
Amortized cost
Amortized cost, Total	6,100,000,000	7,300,000,000
Weighted average yield
Debt securities pledged to secure government and trust deposits	6,100,000,000	7,300,000,000
Asset Not Pledged as Collateral
Amortized cost
Amortized cost, Total	99,700,000	98,800,000
Weighted average yield
Debt securities pledged to secure government and trust deposits	99,700,000	98,800,000
U.S. Treasury securities
Amortized cost
Amortized cost - Within 1 year	2,550,122,000	2,558,293,000
Amortized cost - After 1 to 5 years	3,806,389,000	5,003,219,000
Amortized cost, Total	6,356,511,000	7,561,512,000
Book Value
Book Value - within 1 year	2,514,507,000	2,519,071,000
Book Value - After 1 to 5 years	3,640,221,000	4,749,896,000
Debt securities held-to-maturity (fair value 2026 - $6,176,976; 2025 - $7,363,587)	6,154,728,000	7,268,967,000
Allowance for Credit Losses
Allowance for credit losses - Within 1 Year	0	0
Allowance for credit losses - After 1 to 5 years	0	0
Allowance for credit losses, Total	0	0
Carrying Value Net of Allowance
Carrying Value Net of Allowance - Within 1 Year	2,514,507,000	2,519,071,000
Carrying Value Net of Allowance - After 1 to 5 Years	3,640,221,000	4,749,896,000
Debt securities held-to-maturity, net	6,154,728,000	7,268,967,000
Gross unrealized gains
Gross unrealized gains - Within 1 year	0	5,224,000
Gross unrealized gains - After 1 to 5 years	0	35,910,000
Gross unrealized gains, Total	0	41,134,000
Gross unrealized losses
Gross unrealized losses - Within 1 year	2,273,000	110,000
Gross unrealized losses - After 1 to 5 years	19,074,000	0
Gross unrealized losses, Total	21,347,000	110,000
Fair value
Fair value - within 1 year	2,512,234,000	2,524,185,000
Fair value - After 1 to 5 years	3,621,147,000	4,785,806,000
Fair value, Total	$ 6,133,381,000	$ 7,309,991,000
Weighted average yield
Weighted average yield - Within 1 Year	1.06%	1.31%
Weighted average yield - After 1 to 5 years	1.33%	1.27%
Weighted average yield, Total	1.22%	1.28%
Debt securities pledged to secure government and trust deposits	$ 6,356,511,000	$ 7,561,512,000
Obligations of Puerto Rico, States and political subdivisions
Amortized cost
Amortized cost - Within 1 year	2,720,000	2,605,000
Amortized cost - After 1 to 5 years	3,910,000	12,508,000
Amortized cost - After 5 to 10 years	450,000	450,000
Amortized cost - After 10 years	34,769,000	35,544,000
Amortized cost, Total	41,849,000	51,107,000
Book Value
Book Value - within 1 year	2,720,000	2,605,000
Book Value - After 1 to 5 years	3,910,000	12,508,000
Book Value - After 5 to 10 years	450,000	450,000
Book Value - After 10 years	34,769,000	35,544,000
Debt securities held-to-maturity (fair value 2026 - $6,176,976; 2025 - $7,363,587)	41,849,000	51,107,000
Allowance for Credit Losses
Allowance for credit losses - Within 1 Year	8,000	5,000
Allowance for credit losses - After 1 to 5 years	24,000	39,000
Allowance for credit losses - After 5 to 10 years	14,000	15,000
Allowance for credit losses - After 10 years	6,184,000	5,753,000
Allowance for credit losses, Total	6,230,000	5,812,000
Carrying Value Net of Allowance
Carrying Value Net of Allowance - Within 1 Year	2,712,000	2,600,000
Carrying Value Net of Allowance - After 1 to 5 Years	3,886,000	12,469,000
Carrying Value Net of Allowance - After 5 to 10 Years	436,000	435,000
Carrying Value Net of Allowance - After 10 Years	28,585,000	29,791,000
Debt securities held-to-maturity, net	35,619,000	45,295,000
Gross unrealized gains
Gross unrealized gains - Within 1 year	8,000	4,000
Gross unrealized gains - After 1 to 5 years	11,000	24,000
Gross unrealized gains - After 5 to 10 years	6,000	15,000
Gross unrealized gains - After 10 years	2,465,000	2,908,000
Gross unrealized gains, Total	2,490,000	2,951,000
Gross unrealized losses
Gross unrealized losses - Within 1 year	0	0
Gross unrealized losses - After 1 to 5 years	0	87,000
Gross unrealized losses - After 5 to 10 years	0	0
Gross unrealized losses - After 10 years	1,770,000	1,829,000
Gross unrealized losses, Total	1,770,000	1,916,000
Fair value
Fair value - within 1 year	2,720,000	2,604,000
Fair value - After 1 to 5 years	3,897,000	12,406,000
Fair value - After 5 to 10 years	442,000	450,000
Fair value - After 10 years	29,280,000	30,870,000
Fair value, Total	$ 36,339,000	$ 46,330,000
Weighted average yield
Weighted average yield - Within 1 Year	6.45%	6.43%
Weighted average yield - After 1 to 5 years	1.84%	3.49%
Weighted average yield - After 5 to 10 years	5.81%	5.81%
Weighted average yield - After 10 years	1.43%	1.43%
Weighted average yield, Total	1.84%	2.22%
Debt securities pledged to secure government and trust deposits	$ 41,849,000	$ 51,107,000
Collateralized mortgage obligations - federal agencies
Amortized cost
Amortized cost - After 10 years	1,483,000	1,495,000
Amortized cost, Total	1,483,000	1,495,000
Book Value
Book Value - After 10 years	1,483,000	1,495,000
Debt securities held-to-maturity (fair value 2026 - $6,176,976; 2025 - $7,363,587)	1,483,000	1,495,000
Allowance for Credit Losses
Allowance for credit losses - After 10 years	0	0
Allowance for credit losses, Total	0	0
Carrying Value Net of Allowance
Carrying Value Net of Allowance - After 10 Years	1,483,000	1,495,000
Debt securities held-to-maturity, net	1,483,000	1,495,000
Gross unrealized gains
Gross unrealized gains - After 10 years	0	0
Gross unrealized gains, Total	0	0
Gross unrealized losses
Gross unrealized losses - After 10 years	187,000	189,000
Gross unrealized losses, Total	187,000	189,000
Fair value
Fair value - After 10 years	1,296,000	1,306,000
Fair value, Total	$ 1,296,000	$ 1,306,000
Weighted average yield
Weighted average yield - After 10 years	2.87%	2.87%
Weighted average yield, Total	2.87%	2.87%
Debt securities pledged to secure government and trust deposits	$ 1,483,000	$ 1,495,000
Securities in wholly owned statutory business trusts
Amortized cost
Amortized cost - After 5 to 10 years	5,960,000	5,960,000
Amortized cost, Total	5,960,000	5,960,000
Book Value
Book Value - After 5 to 10 years	5,960,000	5,960,000
Debt securities held-to-maturity (fair value 2026 - $6,176,976; 2025 - $7,363,587)	5,960,000	5,960,000
Allowance for Credit Losses
Allowance for credit losses - After 5 to 10 years	0	0
Allowance for credit losses, Total	0	0
Carrying Value Net of Allowance
Carrying Value Net of Allowance - After 5 to 10 Years	5,960,000	5,960,000
Debt securities held-to-maturity, net	5,960,000	5,960,000
Gross unrealized gains
Gross unrealized gains - After 5 to 10 years	0	0
Gross unrealized gains, Total	0	0
Gross unrealized losses
Gross unrealized losses - After 5 to 10 years	0	0
Gross unrealized losses, Total	0	0
Fair value
Fair value - After 5 to 10 years	5,960,000	5,960,000
Fair value, Total	$ 5,960,000	$ 5,960,000
Weighted average yield
Weighted average yield - After 5 to 10 years	6.33%	6.33%
Weighted average yield, Total	6.33%	6.33%
Debt securities pledged to secure government and trust deposits	$ 5,960,000	$ 5,960,000