Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Tax credits available for carryforward	$ 5,784	$ 8,272
Net operating loss and other carryforward available	827,419	823,197
Postretirement and pension benefits	77,832	80,179
Deferred loan origination fees	13,949	9,427
Allowance for credit losses	355,526	411,616
Accelerated depreciation	10,896	8,829
FDIC-assisted transaction	152,665	152,665
Deferred Tax Assets Intercompany deferred gains	1,412	1,728
Lease liability	40,811	41,640
Difference in outside basis from pass-through entities	56,932	61,222
Other temporary differences	45,912	46,298
Total gross deferred tax assets	1,589,138	1,645,073
Deferred tax liabilities:
Indefinite-lived intangibles	117,690	111,050
Unrealized net gain (loss) on trading and available-for-sale securities	44,563	75,598
Right of use assets	37,064	36,218
Other temporary differences	52,885	51,416
Total gross deferred tax liabilities	252,202	274,282
Valuation allowance	533,654	520,096
Net deferred tax asset	803,282	850,695
PR
Deferred tax assets:
Tax credits available for carryforward	3,003	3,003
Net operating loss and other carryforward available	130,535	124,355
Postretirement and pension benefits	77,832	80,179
Deferred loan origination fees	17,041	12,079
Allowance for credit losses	321,981	373,010
Accelerated depreciation	4,400	3,439
FDIC-assisted transaction	152,665	152,665
Deferred Tax Assets Intercompany deferred gains	1,412	1,728
Lease liability	21,967	22,790
Difference in outside basis from pass-through entities	56,932	61,222
Other temporary differences	37,968	38,954
Total gross deferred tax assets	825,736	873,424
Deferred tax liabilities:
Indefinite-lived intangibles	74,138	73,305
Unrealized net gain (loss) on trading and available-for-sale securities	39,536	67,003
Right of use assets	19,883	20,708
Other temporary differences	51,553	50,247
Total gross deferred tax liabilities	185,110	211,263
Valuation allowance	118,575	112,871
Net deferred tax asset	522,051	549,290
US
Deferred tax assets:
Tax credits available for carryforward	2,781	5,269
Net operating loss and other carryforward available	696,884	698,842
Postretirement and pension benefits	0	0
Deferred loan origination fees	(3,092)	(2,652)
Allowance for credit losses	33,545	38,606
Accelerated depreciation	6,496	5,390
FDIC-assisted transaction	0	0
Deferred Tax Assets Intercompany deferred gains	0	0
Lease liability	18,844	18,850
Difference in outside basis from pass-through entities	0	0
Other temporary differences	7,944	7,344
Total gross deferred tax assets	763,402	771,649
Deferred tax liabilities:
Indefinite-lived intangibles	43,552	37,745
Unrealized net gain (loss) on trading and available-for-sale securities	5,027	8,595
Right of use assets	17,181	15,510
Other temporary differences	1,332	1,169
Total gross deferred tax liabilities	67,092	63,019
Valuation allowance	415,079	407,225
Net deferred tax asset	$ 281,231	$ 301,405