Debt Securities Held-to-maturity - Weighted average yield and contractual maturities of debt securities HTM (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Amortized cost
Amortized cost	$ 8,390,024		$ 8,852,825
Book Value
Book Value	7,865,294		8,194,335
Allowance for credit losses
Allowance for credit losses	5,430		5,780
Net of Allowance
Net of Allowance	7,859,864		8,188,555
Gross unrecognized gains
Gross Unrealized Gains	65,914		3,578
Gross unrealized losses
Gross Unrealized Losses	9,850		32,748
Fair Value
Fair value	$ 7,915,928		$ 8,159,385
Weighted Average Yield
Weighted Average Yield	1.42%		1.48%
US Treasury Securities
Amortized cost
Amortized cost - Within 1 year	$ 599,518		$ 597,768
Amortized cost - After 1 to 5 year	7,725,178		7,971,072
Amortized cost - After 5 to 10 years			211,061
Amortized cost	8,324,696		8,779,901
Book Value
Book Value - within 1 year	599,518		597,768
Book Value - Within 1 to 5 years	7,200,448		7,335,159
Book Value - After 5 to 10 years			188,484
Book Value	7,799,966		8,121,411
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 - After 5 to 10 years			0
Allowance for credit losses	0		0
Net of Allowance
Net of Allowance - Within 1 Year	599,518		597,768
Net of Allowance - After 1 to 5 Years	7,200,448		7,335,159
Net of Allowance - After 5 to 10 Years			188,484
Net of Allowance	7,799,966		8,121,411
Gross unrecognized gains
Gross unrealized gains - Within 1 year	0		0
Gross unrealized gains - After 1 to 5 years	62,265		637
Gross unrealized gains - After 5 to 10 years			0
Gross Unrealized Gains	62,265		637
Gross unrealized losses
Gross unrealized losses - Within 1 year	4,538		7,526
Gross unrealized losses - After 1 to 5 years	2,958		21,996
Gross unrealized losses - After 5 to 10 years			187
Gross Unrealized Losses	7,496		29,709
Fair Value
Fair Value - within 1 year	594,980		590,242
Fair Value - After 1 to 5 years	7,259,755		7,313,800
Fair Value - After 5 to 10 years			188,297
Fair value	$ 7,854,735		$ 8,092,339
Weighted Average Yield
Weighted average yield - Within 1 Year	2.70%		2.58%
Weighted average yield - After 1 to 5 years	1.31%		1.39%
Weighted average yield - After 5 to 10 years			1.50%
Weighted Average Yield	1.41%		1.47%
Obligations of Puerto Rico, States and political subdivisions
Amortized cost
Amortized cost - Within 1 year	$ 2,440		$ 4,820
Amortized cost - After 1 to 5 year	16,474		20,171
Amortized cost - After 5 to 10 years	655		845
Amortized cost - After 10 years	38,276		39,572
Amortized cost	57,845		65,408
Book Value
Book Value - within 1 year	2,440		4,820
Book Value - Within 1 to 5 years	16,474		20,171
Book Value - After 5 to 10 years	655		845
Book Value - After 10 years	38,276		39,572
Book Value	57,845		65,408
Allowance for credit losses
Allowance for credit losses - Within 1 Year	7		9
Allowance for credit losses - After 1 to 5 years	84		147
Allowance for credit losses - After 5 to 10 years	22		28
Allowance for credit losses - After 10 years	5,317		5,596
Allowance for credit losses	5,430	$ 6,251	5,780	$ 6,057	$ 6,145	$ 6,911
Net of Allowance
Net of Allowance - Within 1 Year	2,433		4,811
Net of Allowance - After 1 to 5 Years	16,390		20,024
Net of Allowance - After 5 to 10 Years	633		817
Net of Allowance - After 10 Years	32,959		33,976
Net of Allowance	52,415		59,628
Gross unrecognized gains
Gross unrealized gains - Within 1 year	4		3
Gross unrealized gains - After 1 to 5 years	62		96
Gross unrealized gains - After 5 to 10 years	22		28
Gross unrealized gains - After 10 years	3,561		2,814
Gross Unrealized Gains	3,649		2,941
Gross unrealized losses
Gross unrealized losses - Within 1 year	2		0
Gross unrealized losses - After 1 to 5 years	83		125
Gross unrealized losses - After 5 to 10 years	0		0
Gross unrealized losses - After 10 years	2,144		2,766
Gross Unrealized Losses	2,229		2,891
Fair Value
Fair Value - within 1 year	2,435		4,814
Fair Value - After 1 to 5 years	16,369		19,995
Fair Value - After 5 to 10 years	655		845
Fair Value - After 10 years	34,376		34,024
Fair value	$ 53,835		$ 59,678
Weighted Average Yield
Weighted average yield - Within 1 Year	6.39%		6.17%
Weighted average yield - After 1 to 5 years	3.69%		3.80%
Weighted average yield - After 5 to 10 years	5.81%		5.80%
Weighted average yield - After 10 years	1.42%		1.41%
Weighted Average Yield	2.33%		2.55%
Collateralized Mortgage Obligations - Federal agencies
Amortized cost
Amortized cost - Within 1 year			$ 13
Amortized cost - After 10 years	$ 1,523		1,543
Amortized cost	1,523		1,556
Book Value
Book Value - within 1 year			13
Book Value - After 10 years	1,523		1,543
Book Value	1,523		1,556
Allowance for credit losses
Allowance for credit losses - Within 1 Year			0
Allowance for credit losses - After 10 years	0		0
Allowance for credit losses	0		0
Net of Allowance
Net of Allowance - Within 1 Year			13
Net of Allowance - After 10 Years	1,523		1,543
Net of Allowance	1,523		1,556
Gross unrecognized gains
Gross unrealized gains - Within 1 year			0
Gross unrealized gains - After 10 years	0		0
Gross Unrealized Gains	0		0
Gross unrealized losses
Gross unrealized losses - Within 1 year			0
Gross unrealized losses - After 10 years	125		148
Gross Unrealized Losses	125		148
Fair Value
Fair Value - within 1 year			13
Fair Value - After 10 years	1,398		1,395
Fair value	$ 1,398		$ 1,408
Weighted Average Yield
Weighted average yield - Within 1 Year			6.44%
Weighted average yield - After 10 years	2.87%		2.87%
Weighted Average Yield	2.87%		2.90%
Securities in wholly owned statutory business trusts
Amortized cost
Amortized cost - After 10 years	$ 5,960		$ 5,960
Amortized cost	5,960		5,960
Book Value
Book Value - After 10 years	5,960		5,960
Book Value	5,960		5,960
Allowance for credit losses
Allowance for credit losses - After 10 years	0		0
Allowance for credit losses	0		0
Net of Allowance
Net of Allowance - After 10 Years	5,960		5,960
Net of Allowance	5,960		5,960
Gross unrecognized gains
Gross unrealized gains - After 10 years	0		0
Gross Unrealized Gains	0		0
Gross unrealized losses
Gross unrealized losses - After 10 years	0		0
Gross Unrealized Losses	0		0
Fair Value
Fair Value - After 10 years	5,960		5,960
Fair value	$ 5,960		$ 5,960
Weighted Average Yield
Weighted average yield - After 10 years	6.33%		6.33%
Weighted Average Yield	6.33%		6.33%