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Postretirement Benefits
 3 Months Ended
Mar. 31, 2020
Retirement Benefits [Abstract]  
Postretirement Benefits Postretirement Benefits
 
Pension plans
Our pension plans consist of a noncontributory defined benefit pension plan covering substantially all employees and an unfunded supplemental employee retirement plan for certain members of executive and senior management. Although we are the sponsor of these postretirement plans and record the funded status of these plans, the Exchange and its subsidiaries reimburse us for approximately 57% of the annual benefit expense of these plans, which represents pension benefits for employees performing administrative services and their allocated share of costs for employees in departments that support the administrative functions.
 
The cost of our pension plans are as follows:
 
 
Three months ended March 31,
(in thousands)
 
2020
 
2019
Service cost for benefits earned
 
$
10,873

 
$
8,463

Interest cost on benefits obligation
 
9,395

 
9,827

Expected return on plan assets
 
(12,353
)
 
(11,871
)
Prior service cost amortization
 
336

 
349

Net actuarial loss amortization
 
3,031

 
1,278

Pension plan cost (1)
 
$
11,282

 
$
8,046

 
(1)
The components of pension plan costs other than the service cost component are included in the line item "Other (expense) income" in the Statements of Operations after reimbursements from the Exchange and its subsidiaries.