Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Deferred income tax (benefit) expense	$ 0	$ 0
Unrealized losses on investments in debt and equity	4,350,000	2,084,000
Postretirement benefit obligations	300,000	300,000
Unrealized losses on derivatives	800,000	800,000
Change in tax rate, deferred tax asset, provisional income tax expense		14,500,000
Tax benefit, adjustment	900,000
Deferred tax asset	15,560,000	16,645,000
Accrued income taxes related to BOLI surrender	7,500,000
Excess income tax benefits from stock compensation	3,500,000	2,300,000	$ 1,500,000
Unrecognized tax benefits	0
Amortization of low-income housing credit investments reflected as income tax expense	1,900,000	1,700,000	$ 1,600,000
Affordable housing tax benefits	300,000
Carrying value of investment in affordable housing credits	16,900,000	$ 13,800,000
Federal
Income Tax Examination [Line Items]
Net operating loss carryforwards	800,000
Federal net operating loss	100,000
General Business Tax Credit Carryforward [Member]
Income Tax Examination [Line Items]
Tax credit	$ 1,700,000