Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) ServiceBankingBranch LoanProductionCenter ManagementOffice Segment	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Number of full service banking branches | ServiceBankingBranch	35
Number of loan production centers | LoanProductionCenter	13
Number of wealth management offices | ManagementOffice	3
Percentage value of estimated real estate	90.00%
Recognized tax benefit	50.00%
Postretirement medical benefits number of years of service for eligibility	20 years
Postretirement medical benefits number of age of service for eligibility	60 years
Number of reportable operating segment | Segment	1
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Recognized tax benefit in income tax expense related to vesting of restricted stock awards and exercised stock options	$ 246,000
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Rreclassification of other comprehensive income to retained earnings	$ 352,000
Customer List [Member] | James & Sons Insurance Company [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets estimated useful life		10 years
Customer List [Member] | Premier Bank and Trust [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets estimated useful life	10 years
Minimum [Member] | Buildings [Member]
Summary Of Significant Accounting Policies [Line Items]
Term of lease	20 years
Minimum [Member] | Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Term of lease	3 years
Maximum [Member] | Buildings [Member]
Summary Of Significant Accounting Policies [Line Items]
Term of lease	40 years
Maximum [Member] | Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Term of lease	5 years
HSB Insurance, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Operations commenced date	Jun. 01, 2017