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Pension and Postretirement Health and Life Benefits (Tables)
 6 Months Ended
Jun. 30, 2018
Pension Plan, Defined Benefit [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Benefit Costs

The components of net periodic benefit expense (income) for the defined benefit pension plans were as follows (in millions):
 
Three Months Ended 
 June 30,
 
Six Months Ended 
 June 30,
 
2018
 
2017
 
2018
 
2017
Service costs
$
0.9

 
$
1.0

 
$
1.7

 
$
1.9

Interest costs
7.2

 
8.0

 
14.4

 
16.1

Expected gains on plan assets
(10.3
)
 
(10.2
)
 
(20.5
)
 
(20.4
)
Net periodic benefit income
$
(2.2
)
 
$
(1.2
)
 
$
(4.4
)
 
$
(2.4
)
Other Postretirement Benefits Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Schedule of Net Benefit Costs
The components of net periodic benefit expense for the postretirement health and life benefit plans were as follows (in millions):
 
Three Months Ended 
 June 30,
 
Six Months Ended 
 June 30,
 
2018
 
2017
 
2018
 
2017
Interest costs
$
1.5

 
$
1.8

 
$
2.9

 
$
3.6

Amortization of prior service credits
(0.1
)
 
—

 
(0.1
)
 
(0.1
)
Net periodic benefit expense
$
1.4

 
$
1.8

 
$
2.8

 
$
3.5