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Employee Benefits (Fair Value) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets $ 867.1 $ 899.3 $ 878.7 $ 867.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 867.1 899.3    
Fair value of plan assets at year end 878.7 867.1    
Fair Value, Measurements, Recurring [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     496.3 497.4
Fair value of plan assets 867.1 867.1 878.7 867.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 867.1      
Fair value of plan assets at year end 878.7 867.1    
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 187.6 187.6 184.8 187.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 187.6      
Fair value of plan assets at year end 184.8 187.6    
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 137.9 137.9 155.8 137.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 137.9      
Fair value of plan assets at year end 155.8 137.9    
Fair Value, Measurements, Recurring [Member] | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     496.3 497.4
Fair value of plan assets 44.2 41.4 41.8 44.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 44.2 41.4    
Purchases 6.7 4.5    
Sales (0.6) (0.7)    
Unrealized gain (loss) (8.5) (1.0)    
Fair value of plan assets at year end 41.8 44.2    
Fair Value, Measurements, Recurring [Member] | Equities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 83.2 83.2 87.9 83.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 83.2      
Fair value of plan assets at year end 87.9 83.2    
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 83.2 83.2 87.9 83.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 83.2      
Fair value of plan assets at year end 87.9 83.2    
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 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] | 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]        
Fair value of plan assets 40.0 40.0 38.3 40.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 40.0      
Fair value of plan assets at year end 38.3 40.0    
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]        
Unfunded Commitments     11.0 11.3
Investments measured at net asset value     94.6 91.1
Fair value of plan assets 40.0 37.0 38.3 40.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 40.0 37.0    
Purchases 4.0 4.5    
Sales (0.6) (0.7)    
Unrealized gain (loss) (5.1) (0.8)    
Fair value of plan assets at year end 38.3 40.0    
Fair Value, Measurements, Recurring [Member] | Corporate        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 35.9 35.9 24.8 35.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 35.9      
Fair value of plan assets at year end 24.8 35.9    
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 16.0 16.0 5.4 16.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 16.0      
Fair value of plan assets at year end 5.4 16.0    
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 15.7 15.7 15.9 15.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 15.7      
Fair value of plan assets at year end 15.9 15.7    
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 4.2 4.2 3.5 4.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 4.2      
Fair value of plan assets at year end 3.5 4.2    
Fair Value, Measurements, Recurring [Member] | Government        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 194.5 194.5 184.1 194.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 194.5      
Fair value of plan assets at year end 184.1 194.5    
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 73.0 73.0 72.9 73.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 73.0      
Fair value of plan assets at year end 72.9 73.0    
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 121.5 121.5 111.2 121.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 121.5      
Fair value of plan assets at year end 111.2 121.5    
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] | Absolute Return        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.7 0.7 0.0 0.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.7      
Fair value of plan assets at year end 0.0 0.7    
Fair Value, Measurements, Recurring [Member] | Absolute Return | 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] | Absolute Return | Level 2        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Fair value of plan assets 0.7 0.7 0.0 0.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 0.7      
Fair value of plan assets at year end 0.0 0.7    
Fair Value, Measurements, Recurring [Member] | Absolute Return | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     123.8 109.8
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 15.4 15.4 47.3 15.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 15.4      
Fair value of plan assets at year end 47.3 15.4    
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 15.4 15.4 18.6 15.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 15.4      
Fair value of plan assets at year end 18.6 15.4    
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 0.0 0.0 28.7 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 28.7 0.0    
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 | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Unfunded Commitments     6.2 4.0
Investments measured at net asset value     186.2 193.1
Fair value of plan assets 4.2 4.4 3.5 4.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]        
Fair value of plan assets beginning of year 4.2 4.4    
Purchases 2.7 0.0    
Sales 0.0 0.0    
Unrealized gain (loss) (3.4) (0.2)    
Fair value of plan assets at year end $ 3.5 $ 4.2    
Fair Value, Measurements, Recurring [Member] | Equity Funds [Member] | Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Investments measured at net asset value     $ 91.7 $ 103.4