CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Unrealized loss on available-for-sale securities, tax	$ 7,000	$ 72,000	$ 46,000
Actuarial loss on retirement plan, tax	1,047,000	945,000	218,000
Plan amendment cost, tax	154,000	0	3,305,000
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, tax	$ 754,000	$ 591,000	$ 553,000