Acquisitions - Consideration Paid, Assets Acquired, and Liabilities Assumed (Details) - USD ($) $ in Thousands	Oct. 01, 2016	Feb. 29, 2016	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair Value Adjustments, Identifiable liabilities assumed:
Preliminary goodwill resulting from acquisition			$ 10,989		$ 6,198
HCB and Subsidiary
Business Acquisition [Line Items]
Cash consideration paid		$ 21,800
Identifiable assets acquired:
Total identifiable assets acquired				$ 123,000
Identifiable liabilities assumed:
Total identifiable liabilities assumed				$ 107,000
Fair Value Adjustments, Identifiable liabilities assumed:
Preliminary goodwill resulting from acquisition	$ 4,790
Highland County Bankshares, Inc [Member]
Identifiable assets acquired:
Cash and cash equivalents	53,235
Securities available for sale, at fair value	5,932
Loans, net of unearned income, Purchased performing	58,931
Loans, net of unearned income, Purchased credit impaired	2,910
Allowance for loan losses	(1,040)
Premises and equipment	1,925
Property held for sale	41
Core deposit intangibles	0
Other assets	906
Total identifiable assets acquired	122,840
Identifiable liabilities assumed:
Deposits	106,907
Other liabilities	332
Total identifiable liabilities assumed	107,239
Net identifiable assets acquired	15,601
As Recorded By Summit [Member]
Business Acquisition [Line Items]
Cash consideration paid	21,826
Identifiable assets acquired:
Cash and cash equivalents	53,235
Securities available for sale, at fair value	5,932
Loans, net of unearned income, Purchased performing	58,464
Loans, net of unearned income, Purchased credit impaired	2,382
Allowance for loan losses	0
Premises and equipment	1,618
Property held for sale	23
Core deposit intangibles	1,610
Other assets	865
Total identifiable assets acquired	124,129
Identifiable liabilities assumed:
Deposits	106,762
Other liabilities	332
Total identifiable liabilities assumed	107,094
Net identifiable assets acquired	17,035
Fair Value Adjustments, Identifiable assets acquired
Fair Value Adjustments, Loans, net of unearned income, Purchased performing	(467)
Fair Value Adjustments, Loans, net of unearned income, Purchased credit impaired	(528)
Fair Value Adjustments, Allowance for loan losses	1,040
Fair Value Adjustments, Premises and equipment	(307)
Fair Value Adjustments, Property held for sale	(18)
Fair Value Adjustments, Core deposit intangibles	1,610
Fair Value Adjustments, Other assets	(41)
Fair Value Adjustments, Total identifiable assets acquired	1,289
Fair Value Adjustments, Identifiable liabilities assumed:
Fair Value Adjustments, Deposits	(145)
Fair Value Adjustments, Other liabilities	0
Total identifiable liabilities assumed	(145)
Fair Value Adjustments, Net identifiable assets (liabilities) assumed	1,434
Preliminary goodwill resulting from acquisition	$ 4,791