Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) ServiceBankingBranch LoanProductionCenter ManagementOffice	Dec. 31, 2017 USD ($)	Dec. 31, 2016	Jan. 01, 2019 USD ($)
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
Increase in assets due to adopting new standard	$ 2,811,357	$ 2,649,905
Increase in liabilities due to adopting new standard	$ 2,502,023	$ 2,355,640
ASU 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase in assets due to adopting new standard				$ 5,100
Increase in liabilities due to adopting new standard				$ 5,400
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