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Pension and Postretirement Health and Life Benefits (Schedule of Fair Value of Plan Assets) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mortgage and asset backed securities $ 0.3 $ 5.4  
Pension Plan, Defined Benefit [Member]      
US Government Securities 34.6 84.7  
Corporate Debt 119.7 217.0  
Municipal Bonds 3.5 6.2  
Fair Value of Investments by Hierarchy Level 158.1 313.3  
Investments measured at NAV 494.7 318.3  
Accrued Interest Receivable 1.9 3.5  
Unsettled Transactions Receivable (0.1) (1.0)  
Defined Benefit Plan, Fair Value of Plan Assets 654.6 634.1 $ 656.3
Other Postretirement Benefit Plan, Defined Benefit [Member]      
Money Market Funds 0.0 0.2  
Bond Mutual Funds 1.8 7.5  
Equity Mutual Funds 0.0 0.0  
Fair Value of Investments by Hierarchy Level 1.8 7.7  
Defined Benefit Plan, Fair Value of Plan Assets 1.8 7.7 $ 13.8
Fair Value, Inputs, Level 1 [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]      
Money Market Funds 0.0 0.2  
Bond Mutual Funds 1.8 7.5  
Equity Mutual Funds 0.0 0.0  
Fair Value of Investments by Hierarchy Level 1.8 7.7  
Fair Value, Inputs, Level 2 [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]      
US Government Securities 34.6 84.7  
Corporate Debt 119.7 217.0  
Mortgage and asset backed securities 0.3 5.4  
Municipal Bonds 3.5 6.2  
Fair Value of Investments by Hierarchy Level $ 158.1 $ 313.3