Goodwill and other intangible assets - Intangible Assets (Details) - Core Deposits - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross CDI balance, beginning of period					$ 14,623	$ 8,196	$ 8,196	$ 8,196
Accumulated amortization, beginning of period					$ (1,903)	$ (1,333)	$ (923)	$ (513)
Finite-lived Intangible Assets [Roll Forward]
CDI, net, beginning of period	$ 12,720	$ 7,273	$ 6,863	$ 7,683
Established through acquisitions	342	0	6,770	0
CDI current period amortization	(371)	(205)	(942)	(615)
Total CDI, end of period	$ 12,691	$ 7,068	$ 12,691	$ 7,068