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Employee Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2016
Compensation and Retirement Disclosure [Abstract]  
Components of Net Periodic Benefit Costs
The components of net periodic benefit cost for our pension plans were as follows (dollars in millions):
 
Pension Plans
 
Three Months Ended March 31
 
2016
 
2015
Service cost
$
6.1

 
$
5.9

Interest cost
10.2

 
11.5

Expected return on plan assets
(12.4
)
 
(13.3
)
Net amortization of unrecognized amounts
 
 
 
Prior service cost
1.4

 
1.4

Actuarial loss
1.4

 
2.1

Net periodic benefit cost
$
6.7

 
$
7.6



The components of net periodic benefit cost for our postretirement plans were as follows (dollars in millions):
 
Postretirement Plans
 
Three Months Ended March 31
 
2016
 
2015
Service cost
$
0.2

 
$
0.4

Interest cost
0.2

 
0.3

Net amortization of unrecognized amounts
 
 
 
Prior service cost
—

 
—

Actuarial loss
(0.2
)
 
0.1

Net periodic benefit cost
$
0.2

 
$
0.8