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Pension and other post-retirement benefits
 6 Months Ended
Jun. 30, 2020
Retirement Benefits [Abstract]  
Pension and other post-retirement benefits
Pension and other post-employment benefits
The following table lists the components of net benefit costs for the pension plans and other post-employment benefits ("OPEB") in the unaudited interim consolidated statements of operations for the three and six-month periods ended June 30:
 
Pension benefits
 
Three months ended June 30
 
Six months ended June 30
 
2020
 
2019
 
2020
 
2019
Service cost
$
4,136

 
$
2,795

 
$
7,703

 
$
6,060

Non-service costs
 
 
 
 
 
 
 
Interest cost
4,112

 
3,670

 
8,903

 
8,429

Expected return on plan assets
(6,261
)
 
(3,088
)
 
(12,510
)
 
(10,211
)
Amortization of net actuarial loss
1,145

 
2,060

 
2,290

 
1,744

Amortization of prior service credits
(402
)
 
(412
)
 
(804
)
 
(390
)
Amortization of regulatory assets/liabilities
4,769

 
1,194

 
8,307

 
4,271

 
$
3,363

 
$
3,424

 
$
6,186

 
$
3,843

Net benefit cost
$
7,499

 
$
6,219

 
$
13,889

 
$
9,903













8.
Pension and other post-employment benefits (continued)
 
OPEB
 
Three months ended June 30
 
Six months ended June 30
 
2020
 
2019
 
2020
 
2019
Service cost
$
1,466

 
$
1,203

 
$
2,933

 
$
2,402

Non-service costs
 
 
 
 
 
 
 
Interest cost
1,755

 
1,834

 
3,574

 
3,618

Expected return on plan assets
(2,193
)
 
(1,371
)
 
(4,385
)
 
(3,301
)
Amortization of net actuarial gain
(14
)
 
(424
)
 
(27
)
 
(476
)
Amortization of prior service credits
—

 
(10
)
 
—

 
(105
)
Amortization of regulatory assets/liabilities
706

 
294

 
1,625

 
1,461

 
$
254

 
$
323

 
$
787

 
$
1,197

Net benefit cost
$
1,720

 
$
1,526

 
$
3,720

 
$
3,599


The service cost components of pension plans and OPEB are shown as part of operating expenses within operating income in the unaudited interim consolidated statements of operations. The remaining components of net benefit cost are considered non-service costs and have been included outside of operating income in the unaudited interim consolidated statements of operations.