Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Deferred tax assets:
Tax credits available for carryforward	$ 24,199	$ 23,657
Net operating loss and other carryforward available	840,406	837,087
Postretirement and pension benefits	74,833	83,390
Allowance for loan losses	444,535	535,255
Accelerated depreciation	7,589	7,749
FDIC-assisted transaction	85,609	95,851
Difference in outside basis from pass-through entities	18,570	20,209
Other temporary differences	36,911	32,479
Total gross deferred tax assets	1,536,472	1,641,682
Deferred tax liabilities:
Indefinite-lived intangibles	79,742	73,678
Unrealized net gain (loss) on trading and available-for-sale securities	41,869	11,040
Deferred loan origination costs	286
Other temporary differences	13,541	11,688
Total gross deferred tax liabilities	135,438	96,406
Valuation allowance	492,409	496,307
Net deferred tax asset	908,625	1,048,969
PUERTO RICO
Deferred tax assets:
Tax credits available for carryforward	16,442	15,900
Net operating loss and other carryforward available	128,899	116,154
Postretirement and pension benefits	74,833	83,390
Allowance for loan losses	425,855	516,643
Accelerated depreciation	1,963	1,963
FDIC-assisted transaction	85,609	95,851
Difference in outside basis from pass-through entities	18,570	20,209
Other temporary differences	28,722	24,957
Total gross deferred tax assets	782,270	879,801
Deferred tax liabilities:
Indefinite-lived intangibles	36,578	34,081
Unrealized net gain (loss) on trading and available-for-sale securities	40,840	23,823
Deferred loan origination costs	(2,741)
Other temporary differences	12,432	10,579
Total gross deferred tax liabilities	87,109	68,483
Valuation allowance	99,766	89,852
Net deferred tax asset	595,395	721,466
UNITED STATES
Deferred tax assets:
Tax credits available for carryforward	7,757	7,757
Net operating loss and other carryforward available	711,507	720,933
Postretirement and pension benefits	0	0
Allowance for loan losses	18,680	18,612
Accelerated depreciation	5,626	5,786
FDIC-assisted transaction	0	0
Difference in outside basis from pass-through entities	0	0
Other temporary differences	8,189	7,522
Total gross deferred tax assets	754,202	761,881
Deferred tax liabilities:
Indefinite-lived intangibles	43,164	39,597
Unrealized net gain (loss) on trading and available-for-sale securities	1,029	(12,783)
Deferred loan origination costs	3,027
Other temporary differences	1,109	1,109
Total gross deferred tax liabilities	48,329	27,923
Valuation allowance	392,643	406,455
Net deferred tax asset	313,230	327,503
Deferred loans origination fees
Deferred tax assets:
Deferred income		1,936
Deferred loans origination fees | PUERTO RICO
Deferred tax assets:
Deferred income		3,216
Deferred loans origination fees | UNITED STATES
Deferred tax assets:
Deferred income		(1,280)
Deferred gain
Deferred tax assets:
Deferred income	2,443	2,551
Deferred gain | PUERTO RICO
Deferred tax assets:
Deferred income	0	0
Deferred gain | UNITED STATES
Deferred tax assets:
Deferred income	2,443	2,551
Intercompany deferred gains
Deferred tax assets:
Deferred income	1,377	1,518
Intercompany deferred gains | PUERTO RICO
Deferred tax assets:
Deferred income	1,377	1,518
Intercompany deferred gains | UNITED STATES
Deferred tax assets:
Deferred income	$ 0	$ 0