Income taxes - Schedule of Net Deferred Tax liability (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Allowance for loan losses	$ 7,539	$ 6,264
Amortization of core deposit intangible	1,012	759
Deferred compensation	5,878	6,158
Unrealized loss on available-for-sale debt securities	3,299	988
Other	1,998	3,599
Subtotal	19,726	17,768
Deferred tax liabilities:
FHLB stock dividends	(550)	(550)
Depreciation	(4,812)	(4,115)
Cash flow hedges	(736)	0
Mortgage servicing rights	(23,146)	(19,830)
Other	(7,145)	(5,131)
Subtotal	(36,389)	(29,626)
Net deferred tax liability	$ (16,663)	$ (11,858)