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Investment securities - Summary of Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Amortized Cost of Held-to-maturity Securities    
Amortized Cost, After ten years, Held to maturity $ 6,860  
Amortized Cost, Held to maturity 6,860  
Amortized Cost, Mortgage-backed securities, Held to maturity 3,008  
Held-to-maturity, Amortized Cost 9,868 $ 10,517
Fair Value of Held-to-maturity Securities    
Fair Value, After ten years, Held to maturity 7,020  
Fair Value, Held to maturity 7,020  
Fair Value, Mortgage-backed securities, Held to maturity 3,201  
Held to maturity, Fair Value $ 10,221 $ 10,714