Comprehensive Income - Components of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), net of tax	$ 1,499	$ (17,879)
Net Unrealized Loss on Available for Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	6,537	(20,974)
Accumulated other comprehensive loss, tax effect	(1,932)	6,201
Accumulated other comprehensive income (loss), net of tax	4,605	(14,773)
Unfunded Status of Supplemental Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(4,802)	(4,802)
Accumulated other comprehensive loss, tax effect	1,420	1,420
Accumulated other comprehensive income (loss), net of tax	(3,382)	(3,382)
Joint Beneficiary Agreement Liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	276	276
Accumulated other comprehensive income (loss), net of tax	$ 276	$ 276