Pension And Other Postretirement Benefits (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 508.4 $ 516.4  
Decrease In Net Periodic Benefit Cost   (1.2)    
Accumulated benefit obligation   327.6    
Regulatory assets   139.1    
Regulatory liabilities   310.9    
Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   294.3 304.8  
Defined Benefit Plan, Actuarial Gain (Loss) [1]   7.3    
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)   (0.9)    
Defined Benefit Plan, Net Periodic Benefit Cost   1.5    
Employer contributions   16.5    
Expected contribution   0.1    
Funded status of plan   $ (33.3) (40.5)  
Expected return on plan assets   8.20%    
Settlement loss   $ 0.0    
Change in Net Periodic Benefit Cost due to Interim Measurement       $ 1.2
Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   213.9 211.6  
Defined Benefit Plan, Actuarial Gain (Loss) [1]   11.6    
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)   0.1    
Defined Benefit Plan, Net Periodic Benefit Cost   (12.5)    
Employer contributions   11.3    
Expected contribution   0.9    
Increase in funded status other postretirement benefit plans   17.8    
Funded status of plan   $ 120.5    
Expected return on plan assets   8.06%    
Settlement loss   $ 0.0    
Significant Unobservable Inputs (Level 3) [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0.0 $ 15.3 16.4
Percentage of investments   0.00% 3.00%  
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets   $ 0.0 $ 0.0  
Unrecognized Pension Benefit And Other Postretirement Benefit Costs        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory liabilities   0.6    
Scenario, Forecast | Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Actuarial Gain (Loss) $ 10.1      
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (0.9)      
Amortization of transition obligation 0.0      
Scenario, Forecast | Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Actuarial Gain (Loss) 0.2      
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 0.1      
Amortization of transition obligation $ 0.0      
Predecessor        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated benefit obligation     345.2  
Regulatory assets     158.0  
Regulatory liabilities     295.6  
Predecessor | Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets     304.7 299.1
Defined Benefit Plan, Actuarial Gain (Loss) [1]     (20.0)  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)     (1.0) (0.9)
Defined Benefit Plan, Net Periodic Benefit Cost     0.3 $ 17.5
Employer contributions     6.7  
Funded status of plan     $ (40.5)  
Expected return on plan assets     8.30% 8.30%
Settlement loss     $ 0.0 $ 12.4
Predecessor | Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Fair Value of Plan Assets     211.6 198.8
Defined Benefit Plan, Actuarial Gain (Loss) [1]     (4.6)  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)     0.1 0.1
Defined Benefit Plan, Net Periodic Benefit Cost     (10.8) $ (6.0)
Employer contributions     10.8  
Funded status of plan     $ 102.7  
Expected return on plan assets     8.14% 8.15%
Settlement loss     $ 0.0 $ 0.0
Predecessor | Unrecognized Pension Benefit And Other Postretirement Benefit Costs        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory liabilities     8.3  
Unrecognized Pension Benefit And Other Postretirement Benefit Costs        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory assets   $ 127.1    
Unrecognized Pension Benefit And Other Postretirement Benefit Costs | Predecessor        
Defined Benefit Plan Disclosure [Line Items]        
Regulatory assets     $ 120.9  
[1] The change in benefit obligation for Pension Benefits represents the change in Projected Benefit Obligation while the change in benefit obligation for Other Postretirement Benefits represents the change in Accumulated Postretirement Benefit Obligation.