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Derivative instruments (Tables)
 12 Months Ended
Dec. 31, 2022
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, 2022 that is expected to settle or mature each year:
YearNatural Gas
Swaps
(MMBTUs)
(in millions)
202332.1 
202427.7 
202523.3 
202618.2 
20276.0 
Total107.3 
Schedule of fair value of derivative instruments and effect on consolidated balance sheets The table below reflects the fair value of derivative instruments and their effect on our consolidated balance sheets at December 31, 2022 and 2021.
Consolidated Balance Sheet
Location		Fair Value
20222021
(dollars in thousands)
Assets
Natural gas swapsOther current assets$35,285 $23,596 
Natural gas swapsOther deferred charges$99,725 $40,398 
Liabilities
Natural gas swapsOther current liabilities$3,206 $— 
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, 2022, 2021 and 2020.
Consolidated
Statement of
Revenues and
Expenses Location		202220212020
(dollars in thousands)
Natural gas swaps gainsFuel$121,626 $31,440 $830 
Natural gas swaps lossesFuel(6,587)(1,431)(21,179)
Total$115,039 $30,009 $(20,349)
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, 2022 and 2021.
Consolidated Balance
Sheet Location
20222021
(dollars in thousands)
Natural gas swapsRegulatory liability$131,804 $63,994 
Total$131,804 $63,994