Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Thousands	Oct. 15, 2018	Sep. 30, 2018
Summary Of Significant Accounting Policies [Line Items]
Accumulated Other Comprehensive Income Loss Reclassification of Stranded Tax Effects		$ 48,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		56,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		7,900,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Assets		$ 51,400,000
Powhatan Community Bancshares Inc [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock Issued During Period, Shares, Acquisitions	6.9233
Business Combination Consideration Transferred Cash Paid Per Share	$ 28,520
Business Acquisition Equity Interest Issued or Issuable Number of Shares Exchanged	52,955
Business Combination, Consideration Transferred	$ 6,800,000
Business CombinationConsideration Transferred Percentage Of Stock	70.00%
Business CombinationConsideration Transferred Percentage Of Cash	30.00%
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable	$ 366,623