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Statements of Consolidated Comprehensive Income (Unaudited) (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Comprehensive Income Net Of Tax [Abstract]        
Net of tax provision on recognized net actuarial gain $ 30 $ 55 $ 119 $ 147