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Pension and Other Postretirement Benefits (Notes)
 3 Months Ended
Mar. 31, 2018
Retirement Benefits, Description [Abstract]  
Pension and Other Postretirement Benefits Pension and Other Postretirement Benefits
 

Net periodic benefit (income) expense for pension and other postretirement benefits was as follows:
 
Three Months Ended March 31,
 
2018
 
2017
Pension Benefits
 
 
 
Service cost
$
0.8

 
$
0.7

Interest cost
23.7

 
27.0

Expected return on assets
(38.8
)
 
(37.6
)
Amortization of prior service cost
1.0

 
1.2

Amortization of loss
3.9

 
2.7

Net periodic benefit (income) expense
$
(9.4
)
 
$
(6.0
)
 
 
 
 
Other Postretirement Benefits
 
 
 
Service cost
$
1.1

 
$
1.2

Interest cost
3.9

 
4.3

Amortization of prior service cost (credit)
(3.4
)
 
(14.6
)
Amortization of (gain) loss
(0.3
)
 
(1.1
)
Net periodic benefit (income) expense
$
1.3

 
$
(10.2
)


We are required to contribute $49.9 to the master pension trust during 2018, of which $6.0 was contributed in the first quarter of 2018 and another $9.2 was contributed in April 2018. Based on current actuarial assumptions, we estimate that our required pension contributions will be approximately $35.0 and $10.0 in 2019 and 2020. Factors that affect our estimates of future funding projections include return on plan assets, actuarial data and assumptions relating to plan participants, the discount rate used to measure the pension obligations and changes to regulatory funding requirements.