Goodwill and other intangible assets (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Finite-lived Intangible Assets [Roll Forward]      
Total core deposit intangible, end of period $ 7,683 $ 529  
Other Goodwill Disclosures      
Goodwill 80,082 0  
Core Deposits      
Finite-Lived Intangible Assets [Line Items]      
Gross core deposit intangibles balance, beginning of period   529 0
Accumulated amortization, beginning of period   0 0
Finite-lived Intangible Assets [Roll Forward]      
Core deposit intangible, net, beginning of period 529 0  
Established through acquisitions 7,667 529  
CDI current period amortization (513) 0 0
Total core deposit intangible, end of period $ 7,683 $ 529 $ 0
Other Goodwill Disclosures      
Core deposit intangible, useful life 10 years