Loans - Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Average recorded investment
Impaired loans with a valuation allowance	$ 51,526	$ 64,609	$ 55,997	$ 66,383
Impaired loans with no related valuation allowance	59,219	42,184	55,036	39,969
Total impaired loans	110,745	106,793	111,033	106,352
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	359	491	769	1,176
Impaired loans with no related valuation allowance	295	162	542	304
Total impaired loans	654	653	1,311	1,480
Commercial
Average recorded investment
Impaired loans with a valuation allowance	18,139	24,493	19,271	25,103
Impaired loans with no related valuation allowance	20,078	11,010	19,102	10,153
Total impaired loans	38,217	35,503	38,373	35,256
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	111	201	276	425
Impaired loans with no related valuation allowance	147	48	242	78
Total impaired loans	258	249	518	503
Total commercial real estate
Average recorded investment
Impaired loans with a valuation allowance	16,503	19,026	19,070	19,530
Impaired loans with no related valuation allowance	27,544	25,183	25,920	24,329
Total impaired loans	44,047	44,209	44,990	43,859
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	108	202	216	405
Impaired loans with no related valuation allowance	119	99	240	203
Total impaired loans	227	301	456	608
Real estate construction and land development
Average recorded investment
Impaired loans with a valuation allowance	243	141	234	151
Impaired loans with no related valuation allowance	1,994	106	1,051	93
Total impaired loans	2,237	247	1,285	244
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	2	3	4	5
Impaired loans with no related valuation allowance	2	0	3	0
Total impaired loans	4	3	7	5
Residential mortgage
Average recorded investment
Impaired loans with a valuation allowance	12,762	16,243	13,183	16,821
Impaired loans with no related valuation allowance	7,075	4,581	6,606	4,194
Total impaired loans	19,837	20,824	19,789	21,015
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	115	68	232	302
Impaired loans with no related valuation allowance	25	10	48	17
Total impaired loans	140	78	280	319
Consumer installment
Average recorded investment
Impaired loans with a valuation allowance	1,067	682	986	731
Impaired loans with no related valuation allowance	41	142	91	179
Total impaired loans	1,108	824	1,077	910
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	2	1	3	2
Impaired loans with no related valuation allowance	0	0	0	0
Total impaired loans	2	1	3	2
Home equity
Average recorded investment
Impaired loans with a valuation allowance	2,812	4,024	3,253	4,047
Impaired loans with no related valuation allowance	2,487	1,162	2,266	1,021
Total impaired loans	5,299	5,186	5,519	5,068
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	21	16	38	37
Impaired loans with no related valuation allowance	2	5	9	6
Total impaired loans	23	21	47	43
Owner-occupied | Total commercial real estate
Average recorded investment
Impaired loans with a valuation allowance	12,949	14,188	13,510	14,423
Impaired loans with no related valuation allowance	14,565	9,712	14,967	9,789
Total impaired loans	27,514	23,900	28,477	24,212
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	81	150	163	304
Impaired loans with no related valuation allowance	81	12	137	12
Total impaired loans	162	162	300	316
Non-owner occupied | Total commercial real estate
Average recorded investment
Impaired loans with a valuation allowance	2,246	2,973	3,058	3,180
Impaired loans with no related valuation allowance	9,539	10,387	8,349	9,760
Total impaired loans	11,785	13,360	11,407	12,940
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	10	27	21	51
Impaired loans with no related valuation allowance	38	81	103	167
Total impaired loans	48	108	124	218
Vacant land | Total commercial real estate
Average recorded investment
Impaired loans with a valuation allowance	1,308	1,865	2,502	1,927
Impaired loans with no related valuation allowance	3,440	5,084	2,604	4,780
Total impaired loans	4,748	6,949	5,106	6,707
Interest income recognized while on impaired status
Impaired loans with a valuation allowance	17	25	32	50
Impaired loans with no related valuation allowance	0	6	0	24
Total impaired loans	$ 17	$ 31	$ 32	$ 74