Income taxes - Differences between income tax expense (benefit) in Puerto Rico (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Amount
Income tax expense	$ 3,097	$ 50,223
PUERTO RICO
Amount
Computed income tax at statutory rates	14,025	81,806
Net benefit of net tax exempt interest income	(32,896)	(26,944)
Deferred tax asset valuation allowance	5,538	5,482
Difference in tax rates due to multiple jurisdictions	8,875	(2,862)
Effect of income subject to preferential tax rate	(1,900)	(2,928)
Adjustments in net deferred tax due to change in tax law	9,004	(3,130)
State and local taxes	(555)	1,624
Other	1,006	(2,825)
Income tax expense	$ 3,097	$ 50,223
% of pre-tax income
Computed income tax at statutory rates	38.00%	38.00%
Net benefit of net tax exempt interest income	(88.00%)	(12.00%)
Deferred tax asset valuation allowance	15.00%	2.00%
Difference in tax rates due to multiple jurisdictions	24.00%	(1.00%)
Effect of income subject to preferential tax rate	(5.00%)	(1.00%)
Adjustments in net deferred tax due to change in tax law	24.00%	(2.00%)
State and local taxes	(2.00%)	0.00%
Others	2.00%	(1.00%)
Income tax expense	8.00%	23.00%