Loans (Summary of Impaired Loans by Classes of Loans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impaired loans with a valuation allowance:
Recorded investment	$ 78,621	$ 74,740
Unpaid principal balance	87,956	85,264
Related valuation allowance	8,002	9,392
Average annual recorded investment	66,592	39,582	$ 33,090
Interest income recognized while on impaired status	2,250	1,285	1,312
Impaired loans with no related valuation allowance:
Recorded investment	33,256	32,129
Unpaid principal balance	35,824	38,794
Average annual recorded investment	41,151	73,864	87,093
Interest income recognized while on impaired status	325	2,601	2,577
Total impaired loans:
Recorded investment	111,877	106,869
Unpaid principal balance	123,780	124,058
Related valuation allowance	8,002	9,392
Average annual recorded investment	107,743	113,446	120,183
Interest income recognized while on impaired status	2,575	3,886	3,889
Commercial
Impaired loans with a valuation allowance:
Recorded investment	28,897	28,925
Unpaid principal balance	31,655	33,209
Related valuation allowance	2,296	3,128
Average annual recorded investment	25,099	7,829	9,511
Interest income recognized while on impaired status	939	0	0
Impaired loans with no related valuation allowance:
Recorded investment	8,504	7,435
Unpaid principal balance	9,291	11,153
Average annual recorded investment	10,196	29,559	27,778
Interest income recognized while on impaired status	28	1,343	1,005
Total impaired loans:
Recorded investment	37,401	36,360
Unpaid principal balance	40,946	44,362
Related valuation allowance	2,296	3,128
Average annual recorded investment	35,295	37,388	37,289
Interest income recognized while on impaired status	967	1,343	1,005
Commercial real estate
Impaired loans with a valuation allowance:
Recorded investment	28,003	21,318
Unpaid principal balance	34,580	27,558
Related valuation allowance	3,227	2,102
Average annual recorded investment	19,983	5,658	2,918
Interest income recognized while on impaired status	677	0	0
Impaired loans with no related valuation allowance:
Recorded investment	18,080	20,588
Unpaid principal balance	19,861	23,535
Average annual recorded investment	24,658	41,646	50,079
Interest income recognized while on impaired status	245	1,236	1,547
Total impaired loans:
Recorded investment	46,083	41,906
Unpaid principal balance	54,441	51,093
Related valuation allowance	3,227	2,102
Average annual recorded investment	44,641	47,304	52,997
Interest income recognized while on impaired status	922	1,236	1,547
Real estate construction and land development
Impaired loans with a valuation allowance:
Recorded investment	313	177
Unpaid principal balance	313	177
Related valuation allowance	14	4
Average annual recorded investment	175	19	0
Interest income recognized while on impaired status	10	0	0
Impaired loans with no related valuation allowance:
Recorded investment	0	80
Unpaid principal balance	0	80
Average annual recorded investment	78	585	889
Interest income recognized while on impaired status	0	22	25
Total impaired loans:
Recorded investment	313	257
Unpaid principal balance	313	257
Related valuation allowance	14	4
Average annual recorded investment	253	604	889
Interest income recognized while on impaired status	10	22	25
Residential mortgage
Impaired loans with a valuation allowance:
Recorded investment	15,872	20,864
Unpaid principal balance	15,872	20,864
Related valuation allowance	1,487	3,528
Average annual recorded investment	16,390	23,958	20,661
Interest income recognized while on impaired status	538	1,285	1,312
Impaired loans with no related valuation allowance:
Recorded investment	4,902	3,252
Unpaid principal balance	4,902	3,252
Average annual recorded investment	4,622	1,519	6,027
Interest income recognized while on impaired status	38	0	0
Total impaired loans:
Recorded investment	20,774	24,116
Unpaid principal balance	20,774	24,116
Related valuation allowance	1,487	3,528
Average annual recorded investment	21,012	25,477	26,688
Interest income recognized while on impaired status	576	1,285	1,312
Consumer installment
Impaired loans with a valuation allowance:
Recorded investment	966	879
Unpaid principal balance	966	879
Related valuation allowance	120	240
Average annual recorded investment	744	359	0
Interest income recognized while on impaired status	4	0	0
Impaired loans with no related valuation allowance:
Recorded investment	0	0
Unpaid principal balance	0	0
Average annual recorded investment	205	0	448
Interest income recognized while on impaired status	0	0	0
Total impaired loans:
Recorded investment	966	879
Unpaid principal balance	966	879
Related valuation allowance	120	240
Average annual recorded investment	949	359	448
Interest income recognized while on impaired status	4	0	0
Home equity
Impaired loans with a valuation allowance:
Recorded investment	4,570	2,577
Unpaid principal balance	4,570	2,577
Related valuation allowance	858	390
Average annual recorded investment	4,201	1,759	0
Interest income recognized while on impaired status	82	0	0
Impaired loans with no related valuation allowance:
Recorded investment	1,770	774
Unpaid principal balance	1,770	774
Average annual recorded investment	1,392	555	1,872
Interest income recognized while on impaired status	14	0	0
Total impaired loans:
Recorded investment	6,340	3,351
Unpaid principal balance	6,340	3,351
Related valuation allowance	858	390
Average annual recorded investment	5,593	2,314	1,872
Interest income recognized while on impaired status	$ 96	$ 0	$ 0