Comprehensive Income - Components of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (5,228)	$ (17,879)
Net Unrealized Loss on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(3,409)	(20,974)
Stranded OCI from tax rate change	425
Accumulated other comprehensive loss, tax effect	1,008	6,201
Accumulated other comprehensive loss	(1,976)	(14,773)
Unfunded Status of Supplemental Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(5,352)	(4,802)
Stranded OCI from tax rate change	668
Accumulated other comprehensive loss, tax effect	1,582	1,420
Accumulated other comprehensive loss	(3,102)	(3,382)
Joint Beneficiary Agreement Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, before tax	(150)	276
Accumulated other comprehensive loss	$ (150)	$ 276