Merger with Middleburg Financial Corporation - Narrative (Details)		12 Months Ended
	Apr. 01, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Business Acquisition [Line Items]
Exercise of stock options (in shares) | shares		155,769	43,801	29,975
Impaired loans		$ 4,626,000	$ 6,922,000
Contractual principal and interest at acquisition	$ 7,835,000
Cash flows expected to be collected	$ 3,427,000
Time deposits		368,622,000	251,706,000
Merger related expenses		6,794,000	$ 984,000	$ 0
Middleburg Financial Corporation
Business Acquisition [Line Items]
Share exchange ratio	1.3314
Corporation's common shares issued (in shares) | shares	9,516,097
Common shares issued (9,516,097)	$ 285,679,000
Option exercise price (in dollars per share) | $ / shares	$ 40.04
Exercise of stock options (in shares) | shares	23,362
Cash payments to settle options	$ 608,000
Acquired performing loans	810,900,000
Impaired loans	4,200,000
Contractual principal and interest at acquisition	7,800,000
Cash flows expected to be collected	3,400,000
Fair value adjustment related to bank premises	2,500,000
Unfavorable lease liability	5,300,000
Time deposits	293,600
Federal home loan bank advances discount	$ 107,600
Federal home loan bank advances discount amortization period	1 year 8 months 12 days
Trust preferred debentures discount	$ 1,300,000
Trust preferred debentures discount amortization period	16 years 9 months 18 days
Merger related expenses		$ 6,800,000
Middleburg Financial Corporation | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible amortization period	9 years