Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 21, 2020 $ / shares	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Other-than-temporary impairment recognized	$ 0	$ 0			$ 0
Loan past due	90 days
Nonaccrual loans and loans past due	90 days
Non-accrual status	90 days
Shared national credit portfolio	$ 136,658,000
Change in loans held-in-portfolio (as a percent)	(45.10%)
Principal amount outstanding of loans held-in-portfolio		249,033,000
Payments for loans held-in-portfolio	$ 112,375,000
Tax Cuts and Jobs Act additional income tax expense		$ 5,323,000
Operating lease, right-of-use asset	39,594,000
Operating leases	$ 41,308,000
Cumulative effect of accounting change				$ (2,244,000)
Subsequent Event | Merger Agreement
Summary Of Significant Accounting Policies [Line Items]
Business combination, equity issuable exchange ratio			0.9650
Common stock, par value (in dollars per share) | $ / shares			$ 1.00
Business combination, per share cash consideration (in dollars per share) | $ / shares			$ 2.00
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of premises and equipment	3 years
Estimated useful lives of intangible assets	7 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of premises and equipment	15 years
Estimated useful lives of intangible assets	10 years
ASU 2018-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset				43,723,000
Operating leases				43,723,000
ASU 2017-08
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect of accounting change				$ 2,244,000