Goodwill and other intangible assets - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Finite-lived Intangible Assets [Roll Forward]
CDI current period amortization	$ (374)	$ (205)
Core Deposits
Finite-Lived Intangible Assets [Line Items]
Gross CDI balance, beginning of period			$ 14,966	$ 8,196
Accumulated amortization, beginning of period			$ (2,649)	$ (1,333)
Finite-lived Intangible Assets [Roll Forward]
CDI, net, beginning of period	12,317	6,863
Established through acquisitions	0	6,427
CDI current period amortization	(374)	(205)
Total CDI, end of period	$ 11,943	$ 13,085