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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE EARNINGS (PARENTHETICAL) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Statement of Comprehensive Income [Abstract]        
Postretirement benefit plan reclassification, tax benefit $ 825 $ 878 $ 1,650 $ 1,757