Acquisition of MainStreet Bankshares, Inc. - (Details)		6 Months Ended
	Jan. 01, 2015 USD ($) bank $ / shares shares	Jun. 30, 2016 USD ($) shares	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	[1]
Business Acquisition [Line Items]
Number of shares granted in conjunction with the acquisition (in shares) | shares		0
Net loans		$ 1,045,285,000		$ 992,924,000
Assets		1,601,651,000	$ 1,524,356,000	1,547,599,000
Deposits		1,298,385,000		$ 1,262,660,000
MainStreet BankShares, Inc. [Member]
Business Acquisition [Line Items]
Net loans	$ 122,000,000
Assets	164,000,000
Deposits	$ 137,000,000
Franklin Community Bank, N.A. [Member] | Virginia [Member]
Business Acquisition [Line Items]
Number of banking offices served (banks) | bank	3
MainStreet BankShares, Inc. [Member]
Business Acquisition [Line Items]
Shares received by acquiree entity in exchange of acquirer entity's common stock (in dollars per share) | $ / shares	$ 3.46
Shares received by acquiree entity in exchange of acquirer entity's common stock (in shares) | shares	0.482
Share exchange ratio on acquisitions	0.643
Cash dividends received for purchase of acquisition	$ 6,000,000
Amount of loan taken to fund merger	$ 0
Number of shares granted in conjunction with the acquisition (in shares) | shares	43,086
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Net impact to income before taxes		361,000	872,000
MainStreet BankShares, Inc. [Member] | Amortization of intangible asset [Member]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Intangibles		(124,000)	(147,000)
MainStreet BankShares, Inc. [Member] | CD [Member] | Interest on deposits [Member]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Financial assets		0	0
MainStreet BankShares, Inc. [Member] | Brokered CD [Member] | Interest on deposits [Member]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Financial assets		0	0
MainStreet BankShares, Inc. [Member] | Purchase Acquired Impaired Loans [Member] | Interest and fees on loans [Member]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Financial assets		370,000	803,000
MainStreet BankShares, Inc. [Member] | Performing [Member] | Interest and fees on loans [Member]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract]
Financial assets		$ 115,000	$ 216,000

[1] Derived from audited consolidated financial statements.