Acquisitions - Consideration Paid, Assets Acquired, and Liabilities Assumed (Details) - USD ($) $ in Thousands	Apr. 01, 2017	Oct. 01, 2016	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value Adjustments, Identifiable liabilities assumed:
Goodwill resulting from acquisition			$ 15,272		$ 10,989
HCB and Subsidiary
Identifiable assets acquired:
Total identifiable assets acquired						$ 123,000
Identifiable liabilities assumed:
Total identifiable liabilities assumed						$ 107,000
Fair Value Adjustments, Identifiable liabilities assumed:
Goodwill resulting from acquisition		$ 4,820
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	$ 48,116
Identifiable assets acquired:
Cash and cash equivalents	54,042	53,235
Securities available for sale, at fair value	101,317	5,932
Loans, net of unearned income, Purchased performing	222,116	58,464
Loans, net of unearned income, Purchased credit impaired	3,627	2,382
Allowance for loan losses	0	0
Premises and equipment	6,174	1,618
Property held for sale	2,377	23
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Goodwill	0
Core deposit intangibles	10,916	1,612
Other assets	5,102	865
Total identifiable assets acquired	405,671	124,131
Identifiable liabilities assumed:
Deposits	350,533	106,795
Other liabilities	11,274	332
Total identifiable liabilities assumed	361,807	107,127
Net identifiable assets acquired	43,864	17,004
Fair Value Adjustments, Identifiable assets acquired
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Fair Value Adjustment, Securities Available for Sale	295
Fair Value Adjustments, Loans, net of unearned income, Purchased performing	2,693	467
Fair Value Adjustments, Loans, net of unearned income, Purchased credit impaired	(540)	(528)
Fair Value Adjustments, Allowance for loan losses	2,511	1,040
Fair Value Adjustments, Premises and equipment	(4,222)	(307)
Fair Value Adjustments, Property held for sale	(2,219)	(18)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Fair Value Adjustment, Goodwill	(5,183)
Fair Value Adjustments, Core deposit intangibles	10,916	1,612
Fair Value Adjustments, Other assets	652	(41)
Fair Value Adjustments, Total identifiable assets acquired	(483)	1,291
Fair Value Adjustments, Identifiable liabilities assumed:
Fair Value Adjustments, Deposits	807	(112)
Fair Value Adjustments, Other liabilities	58	0
Total identifiable liabilities assumed	865	(112)
Fair Value Adjustments, Net identifiable assets (liabilities) assumed	(1,348)	1,403
Goodwill resulting from acquisition	4,252	4,822
First Century Bankshares, Inc [Member]
Identifiable assets acquired:
Cash and cash equivalents	54,042
Securities available for sale, at fair value	101,022
Loans, net of unearned income, Purchased performing	224,809
Loans, net of unearned income, Purchased credit impaired	4,167
Allowance for loan losses	(2,511)
Premises and equipment	10,396
Property held for sale	4,596
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Goodwill	5,183
Core deposit intangibles	0
Other assets	4,450
Total identifiable assets acquired	406,154			$ 406,000
Identifiable liabilities assumed:
Deposits	349,726
Other liabilities	11,216
Total identifiable liabilities assumed	360,942			$ 361,000
Net identifiable assets acquired	45,212
Fair Value Adjustments, Identifiable liabilities assumed:
Goodwill resulting from acquisition	4,250
Cash Consideration [Member] | HCB and Subsidiary
Business Acquisition [Line Items]
Cash consideration paid		21,800
Cash Consideration [Member] | As Recorded By Summit [Member]
Business Acquisition [Line Items]
Cash consideration paid	14,989	$ 21,826
Cash Consideration [Member] | First Century Bankshares, Inc [Member]
Business Acquisition [Line Items]
Cash consideration paid	15,000
Stock Consideration [Member] | As Recorded By Summit [Member]
Business Acquisition [Line Items]
Cash consideration paid	33,127
Stock Consideration [Member] | First Century Bankshares, Inc [Member]
Business Acquisition [Line Items]
Cash consideration paid	$ 33,100