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Pension and Other Postretirement Benefit Plans (Tables)
 3 Months Ended
Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Expense (Income) [Table Text Block]
 
Pension
 
Other
Postretirement
Components of Net Periodic Benefit Expense (Income)
2017
 
2016
 
2017
 
2016
Millions
 
 
 
 
 
 
 
Three Months Ended March 31,
 
 
 
 
 
 
 
Service Cost

$2.5

 

$2.0

 

$1.1

 

$1.0

Interest Cost
8.1

 
8.1

 
1.9

 
1.9

Expected Return on Plan Assets
(10.6
)
 
(10.6
)
 
(2.6
)
 
(2.8
)
Amortization of Prior Service Credits
—

 
—

 
(0.5
)
 
(0.7
)
Amortization of Net Loss
2.5

 
2.4

 
0.1

 
—

Net Periodic Benefit Expense (Income)

$2.5

 

$1.9

 
—

 
$(0.6)