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Pension and Postretirement Benefit Costs (Tables)
 6 Months Ended
Jun. 30, 2019
Defined benefit pension plans  
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]  
Schedule of Net Benefit Costs
Net periodic pension cost (benefit) included the following components:
 
Three Months Ended June 30,
 
Six Months Ended June 30,
 
2019
 
2018
 
2019
 
2018
 
(Dollars in millions)
Service cost for benefits earned
$
0.5

 
$
0.5

 
$
1.0

 
$
1.1

Interest cost on projected benefit obligation
8.4

 
7.9

 
16.7

 
15.7

Expected return on plan assets
(7.8
)
 
(10.7
)
 
(15.6
)
 
(21.4
)
Net periodic pension cost (benefit)
$
1.1

 
$
(2.3
)
 
$
2.1

 
$
(4.6
)
Postretirement benefit plans  
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]  
Schedule of Net Benefit Costs
Net periodic postretirement benefit cost included the following components:
 
Three Months Ended June 30,
 
Six Months Ended June 30,
 
2019
 
2018
 
2019
 
2018
 
(Dollars in millions)
Service cost for benefits earned
$
1.2

 
$
2.1

 
$
2.4

 
$
4.1

Interest cost on accumulated postretirement benefit obligation
6.3

 
7.1

 
12.6

 
14.2

Expected return on plan assets
(0.1
)
 
—

 
(0.2
)
 
—

Amortization of prior service credit
(2.2
)
 
—

 
(4.4
)
 
—

Net periodic postretirement benefit cost
$
5.2

 
$
9.2

 
$
10.4

 
$
18.3