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Benefit Plans and Other Postretirement Benefits, Pension Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Benefits [Abstract]        
Expected average long-term rate of return on assets for next fiscal year (as a percent)   7.10%    
Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan        
Pension Benefits [Abstract]        
Total benefit obligation   $ 1.0 $ 1.0  
Pension Plan [Member]        
Pension Benefits [Abstract]        
Total benefit obligation   139.8 156.8 $ 157.5
Net benefit cost recognized for financial reporting   $ 10.7 $ 10.5 $ 7.6
Minimum number of years historical achieved weighted average annual returns are used to determine investment return assumptions (in years)   20 years    
Expected average long-term rate of return on assets (as a percent)   7.10% 7.10% 7.10%
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   100.00% 100.00%  
Pension Plan [Member] | Domestic and international equity securities        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   37.00% 38.00%  
Pension Plan [Member] | Long-duration fixed income and interest rate swap securities        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   28.00% 23.00%  
Pension Plan [Member] | Short-to-intermediate fixed income securities        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   18.00% 21.00%  
Pension Plan [Member] | Alternative investments        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   15.00% 16.00%  
Pension Plan [Member] | Cash        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   2.00% 2.00%  
Scenario, Forecast [Member] | Pension Plan [Member]        
Pension Benefits [Abstract]        
Expected average long-term rate of return on assets for next fiscal year (as a percent) 7.10%      
Parent Company [Member] | Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan        
Pension Benefits [Abstract]        
Total benefit obligation   $ 33.0 $ 37.0  
Net benefit cost recognized for financial reporting   $ 4.0 $ 5.0