Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred tax assets:
Tax credits available for carryforward	$ 6,499	$ 8,272
Net operating loss and other carryforward available	788,329	823,197
Postretirement and pension benefits	73,189	80,179
Deferred loan origination fees	7,254	9,427
Allowance for credit losses	286,613	411,616
Accelerated depreciation	11,725	8,829
FDIC-assisted transaction	152,665	152,665
Deferred Tax Assets Intercompany deferred gains	1,458	1,728
Lease liability	52,427	41,640
Difference in outside basis from pass-through entities	56,358	61,222
Other temporary differences	49,132	46,298
Total gross deferred tax assets	1,485,649	1,645,073
Deferred tax liabilities:
Indefinite-lived intangibles	126,365	111,050
Unrealized net gain (loss) on trading and available-for-sale securities	40,977	75,598
Right of use assets	46,629	36,218
Other temporary differences	54,130	51,416
Total gross deferred tax liabilities	268,101	274,282
Valuation allowance	541,364	520,096
Net deferred tax asset	676,184	850,695
PR
Deferred tax assets:
Tax credits available for carryforward	3,718	3,003
Net operating loss and other carryforward available	106,024	124,355
Postretirement and pension benefits	73,189	80,179
Deferred loan origination fees	11,251	12,079
Allowance for credit losses	254,120	373,010
Accelerated depreciation	4,400	3,439
FDIC-assisted transaction	152,665	152,665
Deferred Tax Assets Intercompany deferred gains	1,458	1,728
Lease liability	27,670	22,790
Difference in outside basis from pass-through entities	56,358	61,222
Other temporary differences	40,689	38,954
Total gross deferred tax assets	731,542	873,424
Deferred tax liabilities:
Indefinite-lived intangibles	75,803	73,305
Unrealized net gain (loss) on trading and available-for-sale securities	35,095	67,003
Right of use assets	25,564	20,708
Other temporary differences	52,623	50,247
Total gross deferred tax liabilities	189,085	211,263
Valuation allowance	124,351	112,871
Net deferred tax asset	418,106	549,290
US
Deferred tax assets:
Tax credits available for carryforward	2,781	5,269
Net operating loss and other carryforward available	682,305	698,842
Postretirement and pension benefits	0	0
Deferred loan origination fees	(3,997)	(2,652)
Allowance for credit losses	32,493	38,606
Accelerated depreciation	7,325	5,390
FDIC-assisted transaction	0	0
Deferred Tax Assets Intercompany deferred gains	0	0
Lease liability	24,757	18,850
Difference in outside basis from pass-through entities	0	0
Other temporary differences	8,443	7,344
Total gross deferred tax assets	754,107	771,649
Deferred tax liabilities:
Indefinite-lived intangibles	50,562	37,745
Unrealized net gain (loss) on trading and available-for-sale securities	5,882	8,595
Right of use assets	21,065	15,510
Other temporary differences	1,507	1,169
Total gross deferred tax liabilities	79,016	63,019
Valuation allowance	417,013	407,225
Net deferred tax asset	$ 258,078	$ 301,405