Commitments, guarantees and contingencies (Summary of the Bank's Off-balance Sheet Financial Instruments) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet financial instruments $ 404,894 $ 272,388
Commitments to extend credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet financial instruments 371,871 245,906
Commitments under credit card lines of credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet financial instruments 28,822 24,321
Standby letters of credit    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Off-balance sheet financial instruments $ 4,201 $ 2,161