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Pension and Postretirement Health and Life Benefits (Schedule of Fair Value of Plan Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
US Government Securities $ 16.4 $ 32.7  
Corporate Debt 104.2 121.1  
Municipal Bonds 2.1 2.1  
Mortgage and asset backed securities 6.4 11.0  
Fair Value of Investments by Hierarchy Level 129.1 166.9  
Plan investments measured at NAV 504.4 453.5  
Accrued Interest Receivable 1.3 1.8  
Unsettled Transactions Receivable (1.8) (1.0)  
Defined Benefit Plan, Plan Assets, Amount 633.0 621.2  
Pension Plan, Defined Benefit [Member]      
Defined Benefit Plan, Plan Assets, Amount 633.1 621.2 $ 563.5
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0 $ 0.0
Fair Value, Inputs, Level 2 [Member]      
US Government Securities 16.4 32.7  
Corporate Debt 104.2 121.1  
Municipal Bonds 2.1 2.1  
Mortgage and asset backed securities 6.4 11.0  
Fair Value of Investments by Hierarchy Level $ 129.1 $ 166.9