CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Unrealized loss on available-for-sale securities, tax	$ 8,000	$ 82,000
Actuarial gain (loss), tax	0	469,000
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, tax	$ 163,000	$ 159,000