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Derivative Instruments (Tables)
 3 Months Ended
Mar. 31, 2021
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 notional volume of our natural gas derivatives as of March 31, 2021 that is expected to settle or mature each year:
Year
 Natural Gas Swaps
(MMBTUs)
 (in millions)
202124.0 
202224.0 
202323.3 
202421.4 
202517.7 
20266.9 
Total117.3 
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 March 31, 2021 and December 31, 2020.
 Balance Sheet
Location
Fair Value
 20212020
 (dollars in thousands)
Assets:   
Natural gas swapsOther current assets$1,290 $222 
Natural gas swapsOther deferred charges$125 $— 
Liabilities:   
Natural gas swapsOther current liabilities$577 $2,305 
Natural gas swapsOther deferred credits$2,700 $8,165 
Schedule of the realized gains and (losses) on derivative instruments recognized in margin The following table presents the gross realized gains and (losses) on derivative instruments recognized in net margins for the three months ended March 31, 2021 and 2020.
Statement of
Revenues and
Expenses
Location		Three Months Ended
March 31,
 20212020
 (dollars in thousands)
Natural gas swaps gainsFuel$101 $— 
Natural gas swaps lossesFuel(1,244)(4,452)
Total $(1,143)$(4,452)
Schedule of unrealized losses on derivative instruments deferred on the balance sheet The following table presents the unrealized losses on derivative instruments deferred on the balance sheet at March 31, 2021 and December 31, 2020.
Balance Sheet Location20212020
 (dollars in thousands)
Natural gas swapsRegulatory asset$1,862 $10,248 
Total $1,862 $10,248