Income taxes - Differences between income tax expense (benefit) in Puerto Rico (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Amount
Income tax expense	$ 41,370	$ 42,018	$ 131,923	$ 35,613
% of pre-tax income
Computed income tax at statutory rates			37.50%		39.00%
PUERTO RICO
Amount
Computed income tax at statutory rates	77,508	71,240	$ 238,602	213,474
Net benefit of net tax exempt interest income	(38,103)	(24,941)	(95,986)	(70,341)
Deferred tax asset valuation allowance	3,170	5,606	9,915	17,018
Difference in tax rates due to multiple jurisdictions	(4,233)	(5,203)	(9,851)	(10,400)
Effect of income subject to preferential tax rate	(2,243)	(2,031)	(7,458)	(108,087)
Unrecognized tax benefits	0	(1,621)	0	(1,621)
State and local taxes	2,180	3,115	5,535	6,196
Other	3,091	(4,147)	(8,834)	(10,626)
Income tax expense	$ 41,370	$ 42,018	$ 131,923	$ 35,613
% of pre-tax income
Computed income tax at statutory rates	38.00%	39.00%	38.00%	39.00%
Net benefit of net tax exempt interest income	(18.00%)	(14.00%)	(15.00%)	(13.00%)
Deferred tax asset valuation allowance	1.00%	3.00%	2.00%	3.00%
Difference in tax rates due to multiple jurisdictions	(2.00%)	(3.00%)	(2.00%)	(2.00%)
Effect of income subject to preferential tax rate	(1.00%)	(1.00%)	(1.00%)	(20.00%)
Unrecognized tax benefits	0.00%	(100.00%)	0.00%	0.00%
State and local taxes	1.00%	2.00%	1.00%	1.00%
Others	1.00%	(2.00%)	(2.00%)	(2.00%)
Income tax (benefit) expense	20.00%	23.00%	21.00%	6.00%