Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Tax credits available for carryforward	$ 7,637	$ 7,637
Net operating loss and other carryforward available	835,492	829,599
Postretirement and pension benefits	81,912	82,623
Allowance for loan losses	504,011	416,456
Accelerated depreciation	8,347	8,353
FDIC-assisted transaction	82,936	82,684
Operating lease liabilities	45,483	46,081
Difference in outside basis from pass-through entities	53,892	21,670
Other temporary differences	39,595	34,014
Total gross deferred tax assets	1,659,519	1,530,481
Deferred tax liabilities:
Indefinite-lived intangibles	75,395	73,469
Unrealized net gain (loss) on trading and available-for-sale securities	88,383	16,067
Operating ROU assets	41,441	42,028
Other temporary differences	13,106	13,957
Total gross deferred tax liabilities	218,325	145,521
Valuation allowance	513,244	499,975
Net deferred tax asset	927,950	884,985
PUERTO RICO
Deferred tax assets:
Tax credits available for carryforward	2,368	2,368
Net operating loss and other carryforward available	117,927	112,803
Postretirement and pension benefits	81,912	82,623
Allowance for loan losses	465,054	405,475
Accelerated depreciation	3,439	3,439
FDIC-assisted transaction	82,936	82,684
Operating lease liabilities	22,847	22,694
Difference in outside basis from pass-through entities	53,892	21,670
Other temporary differences	31,893	26,554
Total gross deferred tax assets	865,646	764,433
Deferred tax liabilities:
Indefinite-lived intangibles	38,243	37,411
Unrealized net gain (loss) on trading and available-for-sale securities	81,077	15,635
Operating ROU assets	20,739	20,598
Other temporary differences	11,924	12,778
Total gross deferred tax liabilities	151,983	86,422
Valuation allowance	105,165	100,175
Net deferred tax asset	608,498	577,836
UNITED STATES
Deferred tax assets:
Tax credits available for carryforward	5,269	5,269
Net operating loss and other carryforward available	717,565	716,796
Postretirement and pension benefits	0	0
Allowance for loan losses	38,957	10,981
Accelerated depreciation	4,908	4,914
FDIC-assisted transaction	0	0
Operating lease liabilities	22,636	23,387
Difference in outside basis from pass-through entities	0	0
Other temporary differences	7,702	7,460
Total gross deferred tax assets	793,873	766,048
Deferred tax liabilities:
Indefinite-lived intangibles	37,152	36,058
Unrealized net gain (loss) on trading and available-for-sale securities	7,306	432
Operating ROU assets	20,702	21,430
Other temporary differences	1,182	1,179
Total gross deferred tax liabilities	66,342	59,099
Valuation allowance	408,079	399,800
Net deferred tax asset	319,452	307,149
Deferred loans origination fees
Deferred tax assets:
Deferred income	(1,121)	(240)
Deferred loans origination fees | PUERTO RICO
Deferred tax assets:
Deferred income	2,043	2,519
Deferred loans origination fees | UNITED STATES
Deferred tax assets:
Deferred income	(3,164)	(2,759)
Intercompany deferred gains
Deferred tax assets:
Deferred income	1,335	1,604
Intercompany deferred gains | PUERTO RICO
Deferred tax assets:
Deferred income	1,335	1,604
Intercompany deferred gains | UNITED STATES
Deferred tax assets:
Deferred income	$ 0	$ 0