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Pension and Postretirement Benefit Plans (Tables)
 3 Months Ended
Mar. 31, 2020
Retirement Benefits [Abstract]  
Company's Share of Net Periodic Cost Components for the three months ended March 31, 2020 and 2019:
($ millions)
Pension
 
Postretirement
 
Three months ended March 31
 
2020
 
2019
 
2020
 
2019
Service cost
$
1.2

 
$
1.0

 
$
—

 
$
—

Interest cost
2.4

 
2.9

 
0.1

 
0.2

Expected return on plan assets
(4.4
)
 
(4.2
)
 
—

 
—

Amortization of:
 
 
 
 
 
 
 
Negative prior service cost
—

 
—

 
(1.4
)
 
(1.4
)
Net actuarial loss
2.3

 
1.5

 
0.1

 
0.1

Net periodic cost (benefit)
$
1.5

 
$
1.2

 
$
(1.2
)
 
$
(1.1
)