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Components of Net Periodic Benefit Cost (Tables)
 9 Months Ended
Sep. 30, 2018
Retirement Benefits, Description [Abstract]  
Components of net periodic benefit cost
 
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
 
 
Pension Benefits
 
Other Benefits
 
Pension Benefits
 
Other Benefits
(in millions)
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
 
2018
 
2017
Service cost - benefits earned during the period
 
$
1.9

 
$
1.8

 
$
—

 
$
—

 
$
5.7

 
$
5.5

 
$
—

 
$
—

Interest cost on projected benefit obligation
 
6.7

 
7.7

 
0.1

 
—

 
20.1

 
22.9

 
0.1

 
0.1

Expected return on plan assets
 
(10.8
)
 
(12.1
)
 
—

 
—

 
(32.4
)
 
(36.3
)
 
—

 
—

Settlement loss
 
—

 
—

 
—

 
—

 
—

 
0.3

 
—

 
—

Amortization:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Prior service cost
 
0.2

 
0.2

 
(0.1
)
 
—

 
0.7

 
0.6

 
(0.2
)
 
(0.2
)
Actuarial net loss (gain)
 
4.2

 
3.4

 
(0.3
)
 
(0.2
)
 
12.5

 
10.4

 
(0.9
)
 
(0.6
)
Net periodic benefit cost
 
$
2.2

 
$
1.0

 
$
(0.3
)
 
$
(0.2
)
 
$
6.6

 
$
3.4

 
$
(1.0
)
 
$
(0.7
)