Loans and Allowance for Loan Losses - Loan Summary (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 871,679,000	$ 813,376,000
Less: Allowance for loan losses	(5,393,000)	(5,105,000)	$ (4,354,000)
Loans, net	866,286,380	808,271,003
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	22,006,000	27,193,000
Less: Allowance for loan losses	0	0
Loans, net	22,006,000	27,193,000
All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	849,673,000	786,183,000
Less: Allowance for loan losses	(5,393,000)	(5,105,000)
Loans, net	844,280,000	781,078,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	447,620,000	415,208,000
Less: Allowance for loan losses	(2,543,000)	(2,369,000)	(1,906,000)
Commercial Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	13,202,000	14,943,000
Commercial Real Estate [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	434,418,000	400,265,000
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	199,704,000	187,802,000
Less: Allowance for loan losses	(1,415,000)	(1,382,000)	(1,015,000)
Consumer Real Estate [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,143,000	9,004,000
Consumer Real Estate [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	193,561,000	178,798,000
Construction and Land Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	98,212,000	117,869,000
Less: Allowance for loan losses	(565,000)	(717,000)	(627,000)
Construction and Land Development [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,576,000	1,678,000
Construction and Land Development [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	96,636,000	116,191,000
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	119,782,000	85,022,000
Less: Allowance for loan losses	(745,000)	(520,000)	(777,000)
Commercial and Industrial [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,085,000	1,568,000
Commercial and Industrial [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	118,697,000	83,454,000
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	6,361,000	7,475,000
Less: Allowance for loan losses	(125,000)	(117,000)	$ (29,000)
Consumer and Other [Member] | Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	0	0
Consumer and Other [Member] | All Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 6,361,000	$ 7,475,000