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Pension and Other Postretirement Benefits (Schedule Of Changes In Plan Assets And Projected Benefit Obligations Recognized In Other Comprehensive Income) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Net prior service cost/(credit) $ 0.0 $ 0.0  
Net actuarial (gain)/loss (78.2) (53.8)  
Settlements (10.5) (9.5) $ (18.5)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (0.7) (0.2) 0.4
Less: amortization of net actuarial (gain) loss (33.8) (45.2) (40.6)
Gain Attributable to spinoff to Eversource (33.1) 0.0  
Prior Service Cost attributable to spinoff to Eversource (2.0) 0.0  
Total Recognized in Other Comprehensive Income or Regulatory Asset or Liability (158.3) (108.7)  
Amount Recognized in Net Periodic Benefits Cost and Other Comprehensive Income or Regulatory Asset or Liability (141.3) (61.1)  
Other Postretirement Benefits      
Net prior service cost/(credit) 0.0 5.1  
Net actuarial (gain)/loss 2.9 45.1  
Settlements (1.5) 0.0 0.0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 2.1 3.2 4.0
Less: amortization of net actuarial (gain) loss (4.9) (2.0) $ (3.8)
Gain Attributable to spinoff to Eversource 0.0 0.0  
Prior Service Cost attributable to spinoff to Eversource 0.0 0.0  
Total Recognized in Other Comprehensive Income or Regulatory Asset or Liability (1.4) 51.4  
Amount Recognized in Net Periodic Benefits Cost and Other Comprehensive Income or Regulatory Asset or Liability $ 10.5 $ 61.4