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Pension and Other Postretirement Benefit Plans - Assumptions Used to Determine Benefit Obligation and Net Periodic Benefit Costs (Details)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Minimum [Member]        
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Discount Rate   4.72% 4.30% 4.93%
Maximum [Member]        
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Discount Rate   4.73% 4.33% 4.96%
Pension [Member]        
Assumptions Used to Determine Benefit Obligation [Abstract]        
Discount Rate   4.53% 4.72%  
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Expected Long-Term Return on Plan Assets [1]   8.00% 8.00% 8.00%
Pension [Member] | Minimum [Member]        
Assumptions Used to Determine Benefit Obligation [Abstract]        
Rate of Compensation Increase   3.70% 3.70%  
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Rate of Compensation Increase   3.70% 3.70% 3.70%
Pension [Member] | Maximum [Member]        
Assumptions Used to Determine Benefit Obligation [Abstract]        
Rate of Compensation Increase   4.30% 4.30%  
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Rate of Compensation Increase   4.30% 4.30% 4.30%
Postretirement Health and Life [Member]        
Assumptions Used to Determine Benefit Obligation [Abstract]        
Discount Rate   4.57% 4.73%  
Health Care Trend Rates [Abstract]        
Trend Rate     6.50%  
Ultimate Trend Rate   4.50% 5.00%  
Year Ultimate Trend Rate Effective   2038 2022  
Postretirement Health and Life [Member] | Minimum [Member]        
Health Care Trend Rates [Abstract]        
Trend Rate   5.00%    
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Expected Long-Term Return on Plan Assets [1]   6.40% 6.40% 6.40%
Postretirement Health and Life [Member] | Maximum [Member]        
Health Care Trend Rates [Abstract]        
Trend Rate   7.00%    
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Expected Long-Term Return on Plan Assets [1]   8.00% 8.00% 8.00%
Subsequent Event [Member] | Pension [Member]        
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Expected Long-Term Return on Plan Assets 7.50%      
Subsequent Event [Member] | Postretirement Health and Life [Member] | Minimum [Member]        
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Expected Long-Term Return on Plan Assets 6.00%      
Subsequent Event [Member] | Postretirement Health and Life [Member] | Maximum [Member]        
Assumptions Used to Determine Net Periodic Benefit Costs [Abstract]        
Expected Long-Term Return on Plan Assets 7.50%      
[1] The expected long-term rates of return used to determine net periodic benefit expense for 2017 have been reduced to 7.50 percent for pension expense and 6.00 percent to 7.50 percent for postretirement health and life expense.