The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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Amount of unrealized gain (loss) arising from the change in fair value of price risk derivatives designated as cash flow hedging instruments recorded in accumulated other comprehensive income in the period, before taxes, to the extent that the cash flow hedge is determined to be effective.

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Risk Management and Derivatives: Derivatives Designated as Hedge Instruments (Details) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ (381)	$ (890)	$ (1,782)	$ (2,872)
Designated as Hedging Instrument
Derivative [Line Items]
Unrealized Gain (Loss) on Price Risk Cash Flow Derivatives, before Tax	6,248	1,403	9,034	3,097
Interest rate swaps \| Interest Expense
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(712)	(712)	(2,138)	(2,138)
Interest rate swaps \| Designated as Hedging Instrument
Derivative [Line Items]
Unrealized Gain (Loss) on Price Risk Cash Flow Derivatives, before Tax	712	712	2,138	2,138
Commodity Contract \| Fuel, purchased power and cost of natural gas sold
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	331	(178)	356	(734)
Commodity Contract \| Designated as Hedging Instrument
Derivative [Line Items]
Unrealized Gain (Loss) on Price Risk Cash Flow Derivatives, before Tax	$ 5,536	$ 691	6,896	$ 959
Interest Rate Swap and Commodity Derivative
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			$ 4,000