Goodwill and intangible assets - Schedule of Core Deposit and Other Intangibles (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Finite-lived Intangible Assets [Roll Forward]
Beginning Balance			$ 11,628
Amortization of intangibles	$ (1,197)	$ (777)	(3,180)	$ (2,432)
Ending Balance	18,748		18,748
Core Deposit and Other Intangibles
Finite-lived Intangible Assets [Roll Forward]
Beginning Balance	19,945	13,203	11,628	14,902
Finite-lived intangible assets acquired	0	0	10,760	0
Reclassification of leasehold intangible	0	0	(460)	0
Amortization of intangibles	(1,197)	(800)	(3,180)	(2,499)
Ending Balance	18,748	$ 12,403	18,748	$ 12,403
Lease Agreements | Occupancy and equipment expense
Finite-lived Intangible Assets [Roll Forward]
Amortization of intangibles	$ (23)		$ (67)