Investment securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale
Available-for-sale, Amortized cost, Due in one year or less	$ 1,667
Available-for-sale, Amortized cost, Due after one year through five years	22,899
Available-for-sale, Amortized cost, Due after five years through ten years	48,731
Available-for-sale, Amortized cost, Due after ten years	233,826
Available-for-sale, Amortized cost, Mutual fund	525
Available-for-sale, Amortized cost, Tax credit investments	0
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	307,648
Available-for-sale, Estimated fair value, Due in one year or less	1,665
Available-for-sale, Estimated fair value, Due after one year through five years	22,904
Available-for-sale, Estimated fair value, Due after five years through ten years	48,536
Available-for-sale, Estimated fair value, Due after ten years	236,620
Available-for-sale, Estimated fair value, Mutual fund	537
Available-for-sale, Estimated fair value, Tax credit investments	0
Available-for-sale Securities, Debt Securities	310,262
Held-to-maturity
Held-to-maturity Securities, Amortized cost, Due in one year or less	0
Held-to-maturity Securities, Amortized cost, Due after one year through five years	29,868
Held-to-maturity Securities, Amortized cost, Due after five years through ten years	88,925
Held-to-maturity Securities, Amortized cost, Due after ten years	20,210
Held-to-maturity Securities, Amortized cost, Mutual fund	0
Held-to-maturity Securities, Amortized cost, Tax credit investments	421
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount	139,424	$ 152,579
Held-to-maturity Securities, Estimated fair value, Due in one year or less	0
Held-to-maturity Securities, Estimated fair value, Due after one year through five years	30,188
Held-to-maturity Securities, Estimated fair value, Due after five years through ten years	91,070
Held-to-maturity Securities, Estimated fair value, Due after ten years	20,581
Held-to-maturity Securities, Estimated fair value, Mutual fund	0
Held-to-maturity Securities Estimated fair value, Tax credit investments	421
Held-to-maturity Securities, Debt Maturities, Fair Value	$ 142,260	$ 155,555