Pension Plans and Other Postretirement Benefits - Fair Values of Company's Pension Plan Assets by Asset Category (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest and dividend receivable on plan assets	$ 1,774,000	$ 1,831,000
Investment Grade Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment in mutual funds	77.00%	75.00%
High-Yielding Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment in mutual funds	23.00%	25.00%