Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Minimum corporation reserves for commercial loan	$ 500	$ 500
Core deposit intangibles amortization period, years	10 years
Qualified affordable housing project investments	$ 20,900	14,000
Qualified affordable housing project investments, unfunded commitments	$ 67,200	$ 9,800
Largest amount recognized in the financial statement of tax benefit threshold, minimum	Greater than 50%
Total commercial loans
Summary Of Significant Accounting Policies [Line Items]
Number of days placed on non-accrual	90 days	90 days
Installment Loans
Summary Of Significant Accounting Policies [Line Items]
Number of days placed on non-accrual	120 days	120 days
Residential Mortgages
Summary Of Significant Accounting Policies [Line Items]
Number of days placed on non-accrual	180 days	180 days
Minimum
Summary Of Significant Accounting Policies [Line Items]
Useful life of the asset, years	3 years
Estimated useful life of intangibles, years	8 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Useful life of the asset, years	40 years
Estimated useful life of intangibles, years	13 years