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CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE EARNINGS (PARENTHETICAL) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Statement of Comprehensive Income [Abstract]        
Postretirement benefit plan re-measurement, net of tax expense $29,172 (see note 8) $ 0 $ 29,172 $ 0 $ 29,172
Postretirement benefit plan reclassification, tax benefit $ 825 $ 962 $ 2,475 $ 2,720