Loans and Allowance for Loan Losses (Summary of All Troubled Debt Restructuring Loans (Accruing and Non Accruing) by Portfolio Segment) (Details)	3 Months Ended	12 Months Ended
Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($) loan
Financing Receivable, Modifications [Line Items]
Financing Receivable Modifications Number Of Contracts Outstanding \| loan		36	35
Financing Receivable, Modifications, Recorded Investment		$ 6,232,000	$ 5,747,000
Specific Reserve		$ 468,000	$ 483,000
Number of Contracts		5	6	1
Pre-Modification Outstanding Recorded Investment		$ 895,000	$ 2,973,000	$ 74,000
Post-Modification Outstanding Recorded Investment		914,000	2,973,000	78,000
Specific Reserve		45,000	1,400,000	27,000
Recorded investment of troubled-debt restructurings that subsequently defaulted		$ 0	$ 0	$ 0
Commercial loans
Financing Receivable, Modifications [Line Items]
Financing Receivable Modifications Number Of Contracts Outstanding \| loan		7	10
Financing Receivable, Modifications, Recorded Investment		$ 1,345,000	$ 1,502,000
Specific Reserve		$ 0	$ 0
Home equity loans
Financing Receivable, Modifications [Line Items]
Financing Receivable Modifications Number Of Contracts Outstanding \| loan		2	1
Financing Receivable, Modifications, Recorded Investment		$ 307,000	$ 16,000
Specific Reserve		$ 0	$ 0
Residential real estate
Financing Receivable, Modifications [Line Items]
Financing Receivable Modifications Number Of Contracts Outstanding \| loan		24	21
Financing Receivable, Modifications, Recorded Investment		$ 3,604,000	$ 3,221,000
Specific Reserve		$ 452,000	$ 483,000
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable Modifications Number Of Contracts Outstanding \| loan		3	3
Financing Receivable, Modifications, Recorded Investment		$ 976,000	$ 1,008,000
Specific Reserve		$ 16,000	$ 0
Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| loan		1	0	0
Pre-Modification Outstanding Recorded Investment		$ 148,000	$ 0	$ 0
Post-Modification Outstanding Recorded Investment		$ 156,000	0	0
Specific Reserve	$ 30,000		$ 0	$ 0
Home Equity Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| loan		1	0	0
Pre-Modification Outstanding Recorded Investment		$ 315,000	$ 0	$ 0
Post-Modification Outstanding Recorded Investment		315,000	0	0
Specific Reserve		$ 0	$ 0	$ 0
Extended Maturity [Member] \| Residential real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts		2	0	0
Pre-Modification Outstanding Recorded Investment		$ 298,000	$ 0	$ 0
Post-Modification Outstanding Recorded Investment		298,000	0	0
Specific Reserve		$ 15,000	$ 0	$ 0
Extended Maturity [Member] \| Commercial loans
Financing Receivable, Modifications [Line Items]
Number of Contracts		0	6	0
Pre-Modification Outstanding Recorded Investment		$ 0	$ 2,973,000	$ 0
Post-Modification Outstanding Recorded Investment		0	2,973,000	0
Specific Reserve		$ 0	$ 1,400,000	$ 0
Court Order [Member] \| Residential real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts		0	0	1
Pre-Modification Outstanding Recorded Investment		$ 0	$ 0	$ 74,000
Post-Modification Outstanding Recorded Investment		0	0	78,000
Specific Reserve		$ 0	$ 0	$ 27,000
Contractual Interest Rate Reduction [Member] \| Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| loan		1	0	0
Pre-Modification Outstanding Recorded Investment		$ 134,000	$ 0	$ 0
Post-Modification Outstanding Recorded Investment		145,000	0	0
Specific Reserve		$ 0	$ 0	$ 0

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