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Pension and other post-retirement benefits (Tables)
 6 Months Ended
Jun. 30, 2019
Retirement Benefits [Abstract]  
Components of Net Benefit Costs For Pension Plans and OPEB Recorded as Part of Administrative Expenses
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.
 
Pension benefits
 
Three Months Ended June 30
 
Six Months Ended June 30
 
2019
 
2018
 
2019
 
2018
Service cost
$
2,795

 
$
3,614

 
$
6,060

 
$
7,228

Non-service costs
 
 
 
 
 
 
 
Interest cost
3,670

 
4,459

 
8,429

 
8,390

Expected return on plan assets
(3,088
)
 
(7,005
)
 
(10,211
)
 
(14,011
)
Amortization of net actuarial loss
2,060

 
111

 
1,744

 
223

Amortization of prior service credits
(412
)
 
(156
)
 
(390
)
 
(311
)
Amortization of regulatory assets/liability
1,194

 
3,091

 
4,271

 
5,157

 
3,424

 
500

 
3,843

 
(552
)
Net benefit cost
$
6,219

 
$
4,114

 
$
9,903

 
$
6,676



8.
Pension and other post-employment benefits (continued)
 
OPEB
 
Three Months Ended June 30
 
Six Months Ended June 30
 
2019
 
2018
 
2019
 
2018
Service cost
$
1,203

 
$
1,487

 
$
2,402

 
$
2,974

Non-service costs
 
 
 
 
 
 
 
Interest cost
1,834

 
1,625

 
3,618

 
3,251

Expected return on plan assets
(1,371
)
 
(1,849
)
 
(3,301
)
 
(3,697
)
Amortization of net actuarial gain
(424
)
 
(38
)
 
(476
)
 
(77
)
Amortization of prior service credits
(10
)
 
(65
)
 
(105
)
 
(131
)
Amortization of regulatory assets/liability
294

 
347

 
1,461

 
1,534

 
323

 
20

 
1,197

 
880

Net benefit cost
$
1,526

 
$
1,507

 
$
3,599

 
$
3,854