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Employee Benefits (Fair Value) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets $ 878.7 $ 867.1 $ 998.3 $ 878.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 878.7 867.1    
Fair value of plan assets at year end 998.3 878.7    
Fair Value, Measurements, Recurring [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     562.6 496.3
Fair value of plan assets 878.7 878.7 998.3 878.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 878.7      
Fair value of plan assets at year end 998.3 878.7    
Fair Value, Measurements, Recurring [Member] | Level 1        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 164.3 164.3 170.3 164.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 164.3      
Fair value of plan assets at year end 170.3 164.3    
Fair Value, Measurements, Recurring [Member] | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 176.3 176.3 212.8 176.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 176.3      
Fair value of plan assets at year end 212.8 176.3    
Fair Value, Measurements, Recurring [Member] | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 41.8 44.2 52.6 41.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 41.8 44.2    
Purchases 3.9 6.7    
Sales (1.1) (0.6)    
Unrealized gain (loss) 8.0 (8.5)    
Fair value of plan assets at year end 52.6 41.8    
Fair Value, Measurements, Recurring [Member] | Equities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 87.9 87.9 104.1 87.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 87.9      
Fair value of plan assets at year end 104.1 87.9    
Fair Value, Measurements, Recurring [Member] | Equities | Level 1        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 87.9 87.9 101.5 87.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 87.9      
Fair value of plan assets at year end 101.5 87.9    
Fair Value, Measurements, Recurring [Member] | Equities | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.0 0.0 2.6 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.0      
Fair value of plan assets at year end 2.6 0.0    
Fair Value, Measurements, Recurring [Member] | Equities | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.0 0.0 0.0 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.0      
Fair value of plan assets at year end 0.0 0.0    
Fair Value, Measurements, Recurring [Member] | Private equities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     143.9 94.6
Unfunded Commitments     8.2 11.0
Fair value of plan assets 38.3 38.3 46.6 38.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 38.3      
Fair value of plan assets at year end 46.6 38.3    
Fair Value, Measurements, Recurring [Member] | Private equities | Level 1        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.0 0.0 0.0 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.0      
Fair value of plan assets at year end 0.0 0.0    
Fair Value, Measurements, Recurring [Member] | Private equities | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.0 0.0 0.0 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.0      
Fair value of plan assets at year end 0.0 0.0    
Fair Value, Measurements, Recurring [Member] | Private equities | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 38.3 40.0 46.6 38.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 38.3 40.0    
Purchases 1.9 4.0    
Sales (1.1) (0.6)    
Unrealized gain (loss) 7.5 (5.1)    
Fair value of plan assets at year end 46.6 38.3    
Fair Value, Measurements, Recurring [Member] | Corporate        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 24.8 24.8 34.2 24.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 24.8      
Fair value of plan assets at year end 34.2 24.8    
Fair Value, Measurements, Recurring [Member] | Corporate | Level 1        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 5.4 5.4 9.6 5.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 5.4      
Fair value of plan assets at year end 9.6 5.4    
Fair Value, Measurements, Recurring [Member] | Corporate | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 15.9 15.9 18.6 15.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 15.9      
Fair value of plan assets at year end 18.6 15.9    
Fair Value, Measurements, Recurring [Member] | Corporate | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 3.5 3.5 6.0 3.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 3.5      
Fair value of plan assets at year end 6.0 3.5    
Fair Value, Measurements, Recurring [Member] | Government        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 184.1 184.1 210.3 184.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 184.1      
Fair value of plan assets at year end 210.3 184.1    
Fair Value, Measurements, Recurring [Member] | Government | Level 1        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 52.4 52.4 54.2 52.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 52.4      
Fair value of plan assets at year end 54.2 52.4    
Fair Value, Measurements, Recurring [Member] | Government | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 131.7 131.7 156.1 131.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 131.7      
Fair value of plan assets at year end 156.1 131.7    
Fair Value, Measurements, Recurring [Member] | Government | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.0 0.0 0.0 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.0      
Fair value of plan assets at year end 0.0 0.0    
Fair Value, Measurements, Recurring [Member] | Interest bearing        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 47.3 47.3 40.5 47.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 47.3      
Fair value of plan assets at year end 40.5 47.3    
Fair Value, Measurements, Recurring [Member] | Interest bearing | Level 1        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 18.6 18.6 5.0 18.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 18.6      
Fair value of plan assets at year end 5.0 18.6    
Fair Value, Measurements, Recurring [Member] | Interest bearing | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 28.7 28.7 35.5 28.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 28.7      
Fair value of plan assets at year end 35.5 28.7    
Fair Value, Measurements, Recurring [Member] | Interest bearing | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.0 0.0 0.0 0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.0      
Fair value of plan assets at year end 0.0 0.0    
Fair Value, Measurements, Recurring [Member] | Fixed income        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     181.0 186.2
Unfunded Commitments     0.5 6.2
Fair Value, Measurements, Recurring [Member] | Fixed income | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 3.5 4.2 6.0 3.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 3.5 4.2    
Purchases 2.0 2.7    
Sales 0.0 0.0    
Unrealized gain (loss) 0.5 (3.4)    
Fair value of plan assets at year end $ 6.0 $ 3.5    
Fair Value, Measurements, Recurring [Member] | Equity Funds [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     112.7 91.7
Fair Value, Measurements, Recurring [Member] | Absolute Return        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     $ 125.0 $ 123.8
Minimum | Fair Value, Measurements, Recurring [Member] | Equity Funds [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investment redemption, notice period 3 days 3 days    
Minimum | Fair Value, Measurements, Recurring [Member] | Absolute Return        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investment redemption, notice period 2 days 2 days    
Maximum | Fair Value, Measurements, Recurring [Member] | Equity Funds [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investment redemption, notice period 30 days 30 days    
Maximum | Fair Value, Measurements, Recurring [Member] | Absolute Return        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investment redemption, notice period 45 days 45 days