Merger with Middleburg Financial Corporation - Schedule of Fair Value of Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($)	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of liabilities assumed:
Goodwill resulting from merger with Middleburg		$ 166,549,000	$ 1,501,000	$ 1,501,000
Middleburg Financial Corporation
Consideration paid:
Common shares issued (9,516,097)	$ 285,679,000
Cash paid to shareholders	608,000
Value of consideration	286,287,000
Fair value of assets acquired:
Cash and cash equivalents	90,940,000
Investment securities	244,123,000
Restricted stock	4,119,000
Loans	815,785,000
Bank premises and equipment	21,920,000
OREO	3,919,000
Intangibles	21,436,000
Bank owned life insurance	24,080,000
Other assets	26,343,000
Total assets	1,252,665,000
Fair value of liabilities assumed:
Deposits	1,056,619,000
Short-term borrowings	26,033,000
Long-term borrowings	29,892,000
Trust preferred debentures	3,824,000
Other liabilities	16,721,000
Total liabilities	1,133,089,000
Net assets acquired	119,576,000
Goodwill resulting from merger with Middleburg	$ 166,103,000
Corporation's common shares issued (in shares)	9,516,097,000