Goodwill and intangible assets - Schedule of Core Deposit and Other Intangibles (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 42,190	$ 42,190
Accumulated Amortization	(30,562)	(27,288)
Net Carrying Amount	11,628	14,902
Core deposit intangible
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	38,915	38,915
Accumulated Amortization	(29,901)	(27,121)
Net Carrying Amount	9,014	11,794
Leasehold intangible
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	587	587
Accumulated Amortization	(127)	(38)
Net Carrying Amount	460	549
Customer base trust intangible
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,600	1,600
Accumulated Amortization	(227)	(67)
Net Carrying Amount	1,373	1,533
Manufactured housing servicing intangible
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,088	1,088
Accumulated Amortization	(307)	(62)
Net Carrying Amount	$ 781	$ 1,026