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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]        
Unrealized loss on available-for-sale securities, tax $ 14 $ 92 $ 64 $ 139
Actuarial gain (loss), tax 154 (5) 1,060 195
Amortization of prior service cost and actuarial loss for retirement plan included in net periodic pension costs, tax $ 199 $ 141 $ 549 $ 443