Other comprehensive loss - Other Comprehensive Loss and Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other comprehensive loss
Unrealized (loss) gain on investment securities available-for-sale	$ 2,611	$ (839)	$ 5,050	$ (3,215)
Net gain on the sale of investment securities available-for-sale	(23)		(23)
Other comprehensive loss	3,031	(839)	5,533	(3,215)
Income tax	637	(176)	1,162	(677)
Other comprehensive loss	$ 2,394	$ (663)	$ 4,371	$ (2,538)

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