CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Unrealized gain (loss) on available-for-sale securities, tax	$ 74,000	$ 7,000	$ 72,000
Actuarial loss on retirement plan, tax	1,841,000	1,047,000	945,000
Plan amendment cost, tax	0	154,000	0
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, tax	$ 457,000	$ 754,000	$ 591,000