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Derivative instruments (Tables)
 12 Months Ended
Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Schedule of notional volume of natural gas derivatives that is expected to settle or mature each year
The following table reflects the volume activity of our natural gas derivatives as of December 31, 2024 that is expected to settle or mature each year:
YearNatural Gas
Swaps
(MMBTUs)
(in millions)
202526.1 
202622.8 
202711.8 
20281.2 
20291.5 
Total
63.4 
Schedule of fair value of derivative instruments and effect on consolidated balance sheets
The table below reflects the fair value of derivative instruments subject to the right of setoff and their effect on our consolidated balance sheets at December 31, 2024 and 2023. We have elected to offset the recognized fair value amounts for multiple derivative instruments executed with the same counterparty in our financial statements when a legal right of setoff exists.
Consolidated
Balance Sheet
Location
Fair Value
20242023
AssetsLiabilitiesNet Carrying Value Presented on the Balance SheetAssetsLiabilitiesNet Carrying Value Presented on the Balance Sheet
Assets(dollars in thousands)
Natural gas swapsOther current assets$19,527 $(1,563)$17,964 $— $— $— 
Natural gas swapsOther deferred charges17,566 (5,279)12,287 32,177 (6,718)25,459 
Liabilities
Natural gas swapsOther current liabilities$290 $(1,917)$(1,627)$3,798 $(14,168)$(10,370)
Natural gas swapsOther deferred credits— — — — (1,644)(1,644)
Total
$37,383 $(8,759)$28,624 $35,975 $(22,530)$13,445 
Schedule of the realized gains and (losses) on derivative instruments recognized in margin
The following table presents the realized gains and (losses) on derivative instruments recognized in margin for the years ended December 31, 2024, 2023 and 2022.
Consolidated
Statement of
Revenues and
Expenses Location		202420232022
(dollars in thousands)
Natural gas swaps gainsFuel$1,213 $2,001 $121,626 
Natural gas swaps lossesFuel(25,510)(22,924)(6,587)
Total
$(24,297)$(20,923)$115,039 
Schedule of unrealized gains and (losses) on derivative instruments deferred on the balance sheet
The following table presents the unrealized (gains) and losses on derivative instruments deferred on the consolidated balance sheets at December 31, 2024 and 2023.
Consolidated Balance
Sheet Location
20242023
(dollars in thousands)
Natural gas swapsRegulatory liability$28,624 $13,445 
Total
$28,624 $13,445