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Pension and Other Postretirement Benefit Plans (Tables)
 6 Months Ended
Jun. 30, 2017
Compensation and Retirement Disclosure [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 June 30,
 
 
 
 
 
 
 
Service Cost

$2.6

 

$2.1

 

$1.1

 

$1.0

Interest Cost
8.2

 
8.1

 
1.9

 
1.8

Expected Return on Plan Assets
(10.6
)
 
(10.7
)
 
(2.7
)
 
(2.8
)
Amortization of Prior Service Credits
—

 
—

 
(0.5
)
 
(0.8
)
Amortization of Net Loss
2.4

 
2.5

 
0.1

 
0.1

Net Periodic Benefit Cost (Income)

$2.6

 

$2.0

 
$(0.1)
 
$(0.7)
 
 
 
 
 
 
 
 
Six Months Ended June 30,
 
 
 
 
 
 
 
Service Cost

$5.1

 

$4.1

 

$2.2

 

$2.0

Interest Cost
16.3

 
16.2

 
3.8

 
3.7

Expected Return on Plan Assets
(21.2
)
 
(21.3
)
 
(5.3
)
 
(5.6
)
Amortization of Prior Service Credits
—

 
—

 
(1.0
)
 
(1.5
)
Amortization of Net Loss
4.9

 
4.9

 
0.2

 
0.1

Net Periodic Benefit Cost (Income)

$5.1

 

$3.9

 
$(0.1)
 
$(1.3)