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Statements of Consolidated Comprehensive Income (Unaudited) (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]        
Natural gas, tax benefit $ (150) $ (955) $ (413) $ (2,640)
Interest rate, tax expense 10 26 54 78
Other post-retirement benefits liability adjustment, tax expense $ 29 $ 49 $ 89 $ 148