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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]    
Unrealized gain (loss) on available-for-sale securities, tax $ 15 $ 8
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, tax $ 117 $ 163