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Pension and other post-retirement benefits
 6 Months Ended
Jun. 30, 2021
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 months ended June 30:
 Pension benefits
Three months ended June 30Six months ended June 30
 2021202020212020
Service cost$4,508 $4,136 $8,336 $7,703 
Non-service costs
Interest cost3,511 4,112 10,217 8,903 
Expected return on plan assets(6,616)(6,261)(17,780)(12,510)
Amortization of net actuarial loss2,540 1,145 4,812 2,290 
Amortization of prior service credits(406)(402)(813)(804)
Impact of regulatory accounts4,825 4,769 11,009 8,307 
$3,854 $3,363 $7,445 $6,186 
Net benefit cost$8,362 $7,499 $15,781 $13,889 

 OPEB
Three months ended June 30Six months ended June 30
 2021202020212020
Service cost$1,772 $1,466 $3,544 $2,933 
Non-service costs
Interest cost3,031 1,755 4,052 3,574 
Expected return on plan assets(2,510)(2,193)(5,021)(4,385)
Amortization of net actuarial loss (gain)436 (14)873 (27)
Impact of regulatory accounts(950)706 196 1,625 
$7 $254 $100 $787 
Net benefit cost$1,779 $1,720 $3,644 $3,720 
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.