Other comprehensive loss - Other Comprehensive Loss and Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Other comprehensive loss
Unrealized (loss) gain on investment securities available-for-sale	$ 161			$ (1,179)			$ 5,211	$ (4,394)
Net gain on the sale of investment securities available-for-sale							(23)
Other comprehensive loss	314			(1,179)			5,847	(4,394)
Income tax	66			(248)			1,228	(925)
Other comprehensive loss	$ 248	$ 2,395	$ 1,976	$ (931)	$ (663)	$ (1,875)	$ 4,619	$ (3,469)

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