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Pension and Postretirement Benefit Plans (Tables)
 9 Months Ended
Sep. 30, 2018
Retirement Benefits [Abstract]  
Company's Share of Net Periodic Cost Components The following table sets forth the components of net periodic cost for the Company’s pension and postretirement benefit plans for the three and nine months ended September 30, 2018 and 2017:
($ millions)
Pension
 
Postretirement
 
Pension
 
Postretirement
 
Three months ended September 30
 
Nine months ended September 30
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
Service cost
$
1.2

 
$
1.4

 
$
—

 
$
—

 
$
3.8

 
$
4.4

 
$
—

 
$
—

Interest cost
2.7

 
2.9

 
0.2

 
0.1

 
8.0

 
8.6

 
0.5

 
0.5

Expected return on plan assets
(4.5
)
 
(4.2
)
 
—

 
—

 
(13.5
)
 
(12.6
)
 
—

 
—

Amortization of:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Negative prior service cost
—

 
—

 
(1.4
)
 
(1.3
)
 
—

 
—

 
(4.1
)
 
(4.1
)
Net actuarial loss
2.1

 
2.0

 
0.1

 
0.1

 
6.2

 
5.9

 
0.2

 
0.2

Net periodic cost (benefit)
$
1.5

 
$
2.1

 
$
(1.1
)
 
$
(1.1
)
 
$
4.5

 
$
6.3

 
$
(3.4
)
 
$
(3.4
)