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Derivative instruments (Tables)
 12 Months Ended
Dec. 31, 2019
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, 2019 that is expected to settle or mature each year:
YearNatural Gas
Swaps
(MMBTUs)
(in millions)
202025.1  
202122.6  
202215.5  
202310.5  
20248.2  
Total81.9  
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, 2019 and 2018.
Balance Sheet
Location		Fair Value
20192018
(dollars in thousands)
Assets
Natural gas swapsOther current assets$—  $226  
Liabilities
Natural gas swapsOther current liabilities$12,898  $2,066  
Natural gas swapsOther deferred credits$19,358  $11,314  
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, 2019, 2018 and 2017.
Consolidated
Statement of
Revenues and
Expenses Location		201920182017
(dollars in thousands)
Natural Gas SwapsFuel$224  $6,088  $3,818  
Natural Gas SwapsFuel(9,308) (956) (1,677) 
Total$(9,084) $5,132  $2,141  
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 balance sheet at December 31, 2019 and 2018.
Consolidated Balance
Sheet Location
20192018
(dollars in thousands)
Natural Gas SwapsRegulatory asset$32,256  $13,154  
Total$32,256  $13,154