Loans and Allowance for Loan Losses - ALL by Loan Classification (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 6,477,000	$ 5,860,000	$ 5,105,000
All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	6,477,000	5,844,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	0	16,000
Performing [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	6,108,000	5,399,000
Impaired Loans [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	369,000	445,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	2,925,000	2,465,000	2,369,000
Commercial Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses		16,000
Commercial Real Estate [Member] | Performing [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	2,925,000	2,444,000
Commercial Real Estate [Member] | Impaired Loans [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	0	5,000
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	1,519,000	1,596,000	1,382,000
Consumer Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses		0
Consumer Real Estate [Member] | Performing [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	1,327,000	1,340,000
Consumer Real Estate [Member] | Impaired Loans [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	192,000	256,000
Construction and Land Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	627,000	521,000	717,000
Construction and Land Development [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses		0
Construction and Land Development [Member] | Performing [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	627,000	521,000
Construction and Land Development [Member] | Impaired Loans [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	0	0
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	1,210,000	1,062,000	520,000
Commercial and Industrial [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses		0
Commercial and Industrial [Member] | Performing [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	1,111,000	890,000
Commercial and Industrial [Member] | Impaired Loans [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	99,000	172,000
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	196,000	216,000	$ 117,000
Consumer and Other [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses		0
Consumer and Other [Member] | Performing [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	118,000	204,000
Consumer and Other [Member] | Impaired Loans [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 78,000	$ 12,000