Acquisition of MainStreet BankShares, Inc. - Narrative (Details)		3 Months Ended
	Jan. 01, 2015 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 43,872,000	$ 43,872,000	$ 39,043,000
MainStreet BankShares, Inc.
Business Acquisition [Line Items]
Shares received by acquiree in exchange for common stock (in dollars per share) | $ / shares	$ 3.46
Shares received by acquiree in exchange for common stock (in shares) | shares	0.482
Share exchange ratio on acquisition	0.643
Cash dividends received as part of acquisition	$ 6,000,000
Amount of loan used to fund merger	$ 0
Option awards acquired as part of merger (in shares) | shares	43,086
Goodwill	$ 5,300,000	4,829,000
Increase (decrease) in goodwill		(461,000)
Loans | MainStreet BankShares, Inc.
Business Acquisition [Line Items]
Increase (decrease) in goodwill		(723,000)
Deferred Income Taxes | MainStreet BankShares, Inc.
Business Acquisition [Line Items]
Increase (decrease) in goodwill		$ 262,000