Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 01, 2019
Income Tax Examination [Line Items]
Deferred income tax (benefit) expense	$ 0	$ 0
Unrealized gains	8,700,000
Postretirement benefit obligations	1,200,000	300,000
Unrealized losses on investments in debt and equity	0	4,350,000
Derivatives	182,000	775,000
Unrealized losses on derivatives		(800,000)
Deferred tax asset	18,244,000	15,560,000
Unrecognized tax benefits	0
Change in tax rate, deferred tax asset, provisional income tax expense			$ 14,500,000
Tax benefit, adjustment		900,000
Accrued income taxes related to BOLI surrender		7,500,000
Excess income tax benefits from stock compensation	2,000,000.0	3,500,000	2,300,000
Amortization of low-income housing credit investments reflected as income tax expense	3,000,000.0	1,900,000	$ 1,700,000
Affordable housing tax benefits	800,000
Carrying value of investment in affordable housing credits	25,800,000	$ 16,900,000
Federal
Income Tax Examination [Line Items]
Net operating loss carryforwards	700,000
Federal net operating loss	100,000
General Business Tax Credit Carryforward
Income Tax Examination [Line Items]
Tax credit	2,600,000
Beneficial
Income Tax Examination [Line Items]
Deferred tax asset				$ 18,900,000
Federal net operating loss				12,700,000
State operating loss				$ 1,000,000.0
Unrecognized tax benefits	$ 1,700,000