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Components of net periodic benefit cost (Tables)
 3 Months Ended
Mar. 31, 2018
Components of net periodic benefit cost [Abstract]  
Schedule of net periodic benefit cost
 
Pension Benefits
 
Three months ended March 31,
 
2018
 
2017
Service cost
$
1.1

 
$
1.2

Interest cost
3.1

 
3.3

Expected return on plan assets
(5.3
)
 
(4.7
)
Amortization of prior service costs
0.0
 
0.0
Amortization of net loss
1.5

 
1.2

Net periodic benefit cost
$
0.4

 
$
1.0


 
Other Postretirement Benefits ("OPEB")
 
Three months ended March 31,
 
2018
 
2017
Service cost
$
0.1

 
$
0.2

Interest cost
1.1

 
1.4

Amortization of prior service cost
(1.8
)
 
(0.3
)
Amortization of net loss
1.1

 
0.9

Net periodic benefit cost
$
0.5

 
$
2.2