Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Deferred tax assets:
Tax credits available for carryforward	$ 11,012	$ 10,544
Net operating loss and other carryforward available	673,309	743,616
Postretirement and pension benefits	37,314	38,121
Allowance for credit losses	276,432	273,178
Depreciation	13,425	13,352
FDIC-assisted transaction	152,665	152,665
Lease liability	47,552	49,562
Unrealized net loss on investment securities	329,597	331,620
Difference in outside basis from pass-through entities	44,008	46,056
Mortgage servicing rights	14,378	14,085
Other temporary differences	59,641	57,304
Total gross deferred tax assets	1,659,333	1,730,103
Deferred tax liabilities:
Intangibles	138,279	136,579
Right of use assets	42,779	44,678
Deferred loan origination fees/cost	1,262	430
Loans acquired	19,703	20,430
Other temporary differences	7,276	6,824
Total gross deferred tax liabilities	209,299	208,941
Valuation allowance	450,290	513,382
Net deferred tax asset	999,744	1,007,780
PR
Deferred tax assets:
Tax credits available for carryforward	263	263
Net operating loss and other carryforward available	53,852	122,634
Postretirement and pension benefits	37,314	38,121
Allowance for credit losses	246,566	244,956
Depreciation	6,774	6,774
FDIC-assisted transaction	152,665	152,665
Lease liability	28,280	29,070
Unrealized net loss on investment securities	309,191	312,583
Difference in outside basis from pass-through entities	44,008	46,056
Mortgage servicing rights	14,378	14,085
Other temporary differences	49,600	47,679
Total gross deferred tax assets	942,891	1,014,886
Deferred tax liabilities:
Intangibles	85,564	84,635
Right of use assets	25,839	26,648
Deferred loan origination fees/cost	(823)	(1,056)
Loans acquired	19,703	20,430
Other temporary differences	6,854	6,402
Total gross deferred tax liabilities	137,137	137,059
Valuation allowance	71,380	139,347
Net deferred tax asset	734,374	738,480
US
Deferred tax assets:
Tax credits available for carryforward	10,749	10,281
Net operating loss and other carryforward available	619,457	620,982
Postretirement and pension benefits	0	0
Allowance for credit losses	29,866	28,222
Depreciation	6,651	6,578
FDIC-assisted transaction	0	0
Lease liability	19,272	20,492
Unrealized net loss on investment securities	20,406	19,037
Difference in outside basis from pass-through entities	0	0
Mortgage servicing rights	0	0
Other temporary differences	10,041	9,625
Total gross deferred tax assets	716,442	715,217
Deferred tax liabilities:
Intangibles	52,715	51,944
Right of use assets	16,940	18,030
Deferred loan origination fees/cost	2,085	1,486
Loans acquired	0	0
Other temporary differences	422	422
Total gross deferred tax liabilities	72,162	71,882
Valuation allowance	378,910	374,035
Net deferred tax asset	$ 265,370	$ 269,300