Investment securities (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale
Available-for-sale, Amortized cost, Due in one year or less	$ 3,397
Available-for-sale, Amortized cost, Due after one year through five years	75,489
Available-for-sale, Amortized cost, Due after five years through ten years	37,600
Available-for-sale, Amortized cost, Due after ten years	379,513
Available-for-sale, Amortized cost, Mutual fund	537
Available-for-sale, Amortized cost, Tax credit investments	0
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	496,536
Available-for-sale, Estimated fair value, Due in one year or less	3,411
Available-for-sale, Estimated fair value, Due after one year through five years	75,632
Available-for-sale, Estimated fair value, Due after five years through ten years	37,341
Available-for-sale, Estimated fair value, Due after ten years	377,911
Available-for-sale, Estimated fair value, Mutual fund	524
Available-for-sale, Estimated fair value, Tax credit investments	0
Available-for-sale Securities, Debt Securities	494,819
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	34,955
Held-to-maturity Securities, Amortized cost, Due after five years through ten years	87,817
Held-to-maturity Securities, Amortized cost, Due after ten years	17,498
Held-to-maturity Securities, Amortized cost, Mutual fund	0
Held-to-maturity Securities, Amortized cost, Tax credit investments	287
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount	140,557	$ 139,424
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	35,205
Held-to-maturity Securities, Estimated fair value, Due after five years through ten years	89,323
Held-to-maturity Securities, Estimated fair value, Due after ten years	17,457
Held-to-maturity Securities, Estimated fair value, Mutual fund	0
Held-to-maturity Securities Estimated fair value, Tax credit investments	287
Held-to-maturity Securities, Debt Maturities, Fair Value	$ 142,272	$ 142,260