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Components of net periodic benefit cost
 6 Months Ended
Jun. 30, 2018
Components of net periodic benefit cost [Abstract]  
Components of net periodic benefit cost
Components of net periodic benefit cost
 
Pension Benefits
 
Three months ended June 30,

Six months ended June 30,
 
2018
 
2017

2018
 
2017
Service cost
$
1.0

 
$
1.0

 
$
2.1

 
$
2.2

Interest cost
3.0

 
3.3

 
6.1

 
6.7

Expected return on plan assets
(5.3
)
 
(5.1
)
 
(10.6
)
 
(9.9
)
Amortization of prior service costs
0.1

 
0.0

 
0.1

 
0.1

Amortization of net loss
1.1

 
1.5

 
2.6

 
2.7

Net periodic benefit cost
$
(0.1
)
 
$
0.7

 
$
0.3

 
$
1.8


 
Other Postretirement Benefits ("OPEB")
 
Three months ended June 30,
 
Six months ended June 30,
 
2018
 
2017
 
2018
 
2017
Service cost
$
0.0

 
$
0.2

 
$
0.1

 
$
0.4

Interest cost
(1.8
)
 
3.0

 
(0.7
)
 
4.3

Amortization of prior service cost
(1.5
)
 
(0.7
)
 
(3.3
)
 
(1.0
)
Amortization of net loss
3.3

 
(1.0
)
 
4.4

 
0.0

Net periodic benefit cost
$
0.0

 
$
1.5

 
$
0.5

 
$
3.7