Loans and Allowance for Loan Losses - Loan Summary (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 1,575,435,000	$ 1,323,258,000
Less: Allowance for loan losses	(7,073,937)	(5,860,291)	$ (5,105,000)
Loans, net	1,568,360,556	1,317,397,909
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	32,658,000	32,794,000
Less: Allowance for loan losses	(19,000)	(16,000)
Loans, net	32,639,000	32,778,000
All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,542,777,000	1,290,464,000
Less: Allowance for loan losses	(7,055,000)	(5,844,000)
Loans, net	1,535,722,000	1,284,620,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	745,864,000	642,988,000
Less: Allowance for loan losses	(3,135,000)	(2,465,000)	(2,369,000)
Commercial Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	18,474,000	17,903,000
Less: Allowance for loan losses	(19,000)	(16,000)
Commercial Real Estate [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	727,390,000	625,085,000
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	355,876,000	293,457,000
Less: Allowance for loan losses	(1,528,000)	(1,596,000)	(1,382,000)
Consumer Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,987,000	7,450,000
Less: Allowance for loan losses	0	0
Consumer Real Estate [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	348,889,000	286,007,000
Construction and Land Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	179,431,000	135,409,000
Less: Allowance for loan losses	(744,000)	(521,000)	(717,000)
Construction and Land Development [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	5,690,000	5,120,000
Less: Allowance for loan losses	0	0
Construction and Land Development [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	173,741,000	130,289,000
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	279,771,000	238,087,000
Less: Allowance for loan losses	(1,367,000)	(1,062,000)	(520,000)
Commercial and Industrial [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	821,000	858,000
Less: Allowance for loan losses	0	0
Commercial and Industrial [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	278,950,000	237,229,000
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	14,493,000	13,317,000
Less: Allowance for loan losses	(300,000)	(216,000)	$ (117,000)
Consumer and Other [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	686,000	1,463,000
Less: Allowance for loan losses	0	0
Consumer and Other [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 13,807,000	$ 11,854,000