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Statements of Consolidated Comprehensive Income (Unaudited) (Parenthetical) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Statement of Comprehensive Income [Abstract]    
Natural gas, net of tax benefit of $(584) and $(8,339) $ (584) $ (8,339)
Interest rate, net of tax expense of $25 and $25 25 25
Pension and other post-retirement benefits liability adjustment, net of tax expense of $49 and $135 $ 49 $ 135