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Pension and Other Postretirement Benefit Plans (Tables)
 9 Months Ended
Sep. 30, 2017
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Expense (Income) [Table Text Block]
 
Pension
 
Other
Postretirement
Components of Net Periodic Benefit Cost (Income)
2017
 
2016
 
2017
 
2016
Millions
 
 
 
 
 
 
 
Quarter Ended September 30,
 
 
 
 
 
 
 
Service Cost

$2.6

 

$2.0

 

$1.1

 

$1.0

Interest Cost
8.1

 
8.2

 
2.0

 
1.9

Expected Return on Plan Assets
(10.6
)
 
(10.7
)
 
(2.6
)
 
(2.8
)
Amortization of Prior Service Credits
—

 
—

 
(0.5
)
 
(0.7
)
Amortization of Net Loss
2.5

 
2.4

 
—

 
—

Net Periodic Benefit Cost (Income)

$2.6

 

$1.9

 
—

 
$(0.6)
 
 
 
 
 
 
 
 
Nine Months Ended September 30,
 
 
 
 
 
 
 
Service Cost

$7.7

 

$6.1

 

$3.3

 

$3.0

Interest Cost
24.4

 
24.4

 
5.8

 
5.6

Expected Return on Plan Assets
(31.8
)
 
(32.0
)
 
(7.9
)
 
(8.4
)
Amortization of Prior Service Credits
—

 
—

 
(1.5
)
 
(2.2
)
Amortization of Net Loss
7.4

 
7.3

 
0.2

 
0.1

Net Periodic Benefit Cost (Income)

$7.7

 

$5.8

 
$(0.1)
 
$(1.9)