Goodwill and other intangible assets - Intangible Assets (Details) - Core Deposits - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross core deposit intangibles balance, beginning of period			$ 8,196	$ 8,196
Accumulated amortization, beginning of period			$ (1,333)	$ (513)
Finite-lived Intangible Assets [Roll Forward]
Core deposit intangible, net, beginning of period	$ 6,863	$ 7,683
Established through acquisitions	6,427	0
CDI current period amortization	(205)	(205)
Total core deposit intangible, end of period	$ 13,085	$ 7,478