Loans and reserve for credit losses (Past Due Financing Receivables) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 8,015	$ 6,914
Current	2,107,557	2,097,498
Total loans	2,115,572	2,104,412
30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4,639	5,138
90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,376	1,776
Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,334	1,012
Current	379,884	390,199
Total loans	381,218	391,211
Commercial real estate | Owner occupied | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	244	522
Commercial real estate | Owner occupied | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,090	490
Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	786	100
Current	623,102	612,297
Total loans	623,888	612,397
Commercial real estate | Non-owner occupied | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	786	100
Commercial real estate | Non-owner occupied | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,120	1,112
Current	1,002,986	1,002,496
Total loans	1,005,106	1,003,608
Commercial real estate | Commercial real estate | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,030	622
Commercial real estate | Commercial real estate | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,090	490
Construction | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	417	490
Current	215,842	210,548
Total loans	216,259	211,038
Construction | Construction | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	41	97
Construction | Construction | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	376	393
Residential real estate | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,074	4,084
Current	433,833	427,758
Total loans	436,907	431,842
Residential real estate | Residential real estate | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,114	3,541
Residential real estate | Residential real estate | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	960	543
Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,172	885
Current	412,563	413,551
Total loans	414,735	414,436
Commercial and industrial | Commercial and industrial | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,251	547
Commercial and industrial | Commercial and industrial | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	921	338
Consumer | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	232	343
Current	42,333	43,145
Total loans	42,565	43,488
Consumer | Consumer | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	203	331
Consumer | Consumer | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	29	12
Originated Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,951	3,718
Current	1,852,339	1,815,616
Total loans	1,855,290	1,819,334
Originated Loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,574	2,502
Originated Loans | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,377	1,216
Originated Loans | Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	734	695
Current	296,371	298,026
Total loans	297,105	298,721
Originated Loans | Commercial real estate | Owner occupied | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	244	205
Originated Loans | Commercial real estate | Owner occupied | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	490	490
Originated Loans | Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	100
Current	529,025	514,263
Total loans	529,025	514,363
Originated Loans | Commercial real estate | Non-owner occupied | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	100
Originated Loans | Commercial real estate | Non-owner occupied | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Originated Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	734	795
Current	825,396	812,289
Total loans	826,130	813,084
Originated Loans | Commercial real estate | Commercial real estate | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	244	305
Originated Loans | Commercial real estate | Commercial real estate | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	490	490
Originated Loans | Construction | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	417	473
Current	212,395	200,181
Total loans	212,812	200,654
Originated Loans | Construction | Construction | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	41	97
Originated Loans | Construction | Construction | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	376	376
Originated Loans | Residential real estate | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	491	1,488
Current	387,016	375,886
Total loans	387,507	377,374
Originated Loans | Residential real estate | Residential real estate | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	351	1,488
Originated Loans | Residential real estate | Residential real estate | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	140	0
Originated Loans | Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,082	628
Current	386,466	385,522
Total loans	387,548	386,150
Originated Loans | Commercial and industrial | Commercial and industrial | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	740	290
Originated Loans | Commercial and industrial | Commercial and industrial | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	342	338
Originated Loans | Consumer | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	227	334
Current	41,066	41,738
Total loans	41,293	42,072
Originated Loans | Consumer | Consumer | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	198	322
Originated Loans | Consumer | Consumer | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	29	12
Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5,064	3,196
Current	255,218	281,882
Total loans	260,282	285,078
Acquired Loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	3,065	2,636
Acquired Loans | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,999	560
Acquired Loans | Commercial real estate | Owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	600	317
Current	83,513	92,173
Total loans	84,113	92,490
Acquired Loans | Commercial real estate | Owner occupied | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	317
Acquired Loans | Commercial real estate | Owner occupied | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	600	0
Acquired Loans | Commercial real estate | Non-owner occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	786	0
Current	94,077	98,034
Total loans	94,863	98,034
Acquired Loans | Commercial real estate | Non-owner occupied | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	786	0
Acquired Loans | Commercial real estate | Non-owner occupied | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Acquired Loans | Commercial real estate | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,386	317
Current	177,590	190,207
Total loans	178,976	190,524
Acquired Loans | Commercial real estate | Commercial real estate | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	786	317
Acquired Loans | Commercial real estate | Commercial real estate | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	600	0
Acquired Loans | Construction | Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	17
Current	3,447	10,367
Total loans	3,447	10,384
Acquired Loans | Construction | Construction | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	0
Acquired Loans | Construction | Construction | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0	17
Acquired Loans | Residential real estate | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	2,583	2,596
Current	46,817	51,872
Total loans	49,400	54,468
Acquired Loans | Residential real estate | Residential real estate | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,763	2,053
Acquired Loans | Residential real estate | Residential real estate | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	820	543
Acquired Loans | Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	1,090	257
Current	26,097	28,029
Total loans	27,187	28,286
Acquired Loans | Commercial and industrial | Commercial and industrial | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	511	257
Acquired Loans | Commercial and industrial | Commercial and industrial | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	579	0
Acquired Loans | Consumer | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5	9
Current	1,267	1,407
Total loans	1,272	1,416
Acquired Loans | Consumer | Consumer | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5	9
Acquired Loans | Consumer | Consumer | 90 days or more past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	$ 0