Accounting for Derivative Instruments and Hedging Activities Derivative Assets and Liabilities (Details) $ in Thousands, MMBTU in Millions		Sep. 30, 2021 USD ($) MMBTU	Dec. 31, 2020 USD ($) MMBTU
Derivative [Line Items]
Current, Assets		$ 306,685	$ 33,015
Long-term, Assets		55,842	8,805
Current, Liabilities		50,447	31,441
Long-term, Liabilities		12,007	29,833
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Current, Assets		306,685	33,015
Long-term, Assets		55,842	8,805
Assets	[1]	362,527	41,820
Current, Liabilities		50,447	31,441
Long-term, Liabilities		12,007	29,833
Derivative Liability	[2]	$ 62,454	$ 61,274
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU		315.0	320.0
Not Designated as Hedging Instrument [Member] | Electric Portfolio [Member]
Derivative [Line Items]
Assets		$ 193,765	$ 22,544
Derivative Liability		45,349	46,922
Not Designated as Hedging Instrument [Member] | Natural Gas Portfolio [Member]
Derivative [Line Items]
Assets		168,762	19,276
Derivative Liability		$ 17,105	$ 14,352
Not Designated as Hedging Instrument [Member] | Electric Generation Fuel [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU		208.4	212.2
Not Designated as Hedging Instrument [Member] | Purchased Electricity [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU		8.1	6.6

[1] _______________ 1 Balance sheet classification: Current and Long-term Unrealized gain on derivative instruments.

[2] 2 Balance sheet classification: Current and Long-term Unrealized loss on derivative instruments.