Accounting for Derivative Instruments and Hedging Activities (Schedule of Derivative Assets and Liabilities) (Details) $ in Thousands, MWh in Millions, MMBTU in Millions	Dec. 31, 2020 USD ($) MMBTU MWh	Dec. 31, 2019 USD ($) MMBTU MWh
Derivative [Line Items]
Assets, Current	$ 33,015	$ 23,626
Assets, Long-term	8,805	7,682
Liabilities, Current	31,441	13,428
Liabilities, Long-term	29,833	12,693
Not Designated as Hedging Instrument
Derivative [Line Items]
Total derivative assets	41,820	31,308
Total derivative liabilities	61,274	26,121
Not Designated as Hedging Instrument | Parent Company [Member]
Derivative [Line Items]
Assets, Current	33,015	23,626
Assets, Long-term	8,805	7,682
Liabilities, Current	31,441	13,428
Liabilities, Long-term	$ 29,833	$ 12,693
Not Designated as Hedging Instrument | Electric Generation Fuel
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	212.2	229.3
Not Designated as Hedging Instrument | Purchased Electricity
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MWh	6.6	10.4
Not Designated as Hedging Instrument | Natural Gas Portfolio
Derivative [Line Items]
Total derivative assets	$ 19,276	$ 11,375
Total derivative liabilities	14,352	8,617
Not Designated as Hedging Instrument | Electric Portfolio
Derivative [Line Items]
Total derivative assets	22,544	19,933
Total derivative liabilities	$ 46,922	$ 17,504
Not Designated as Hedging Instrument | Gas Derivatives
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	320.0	316.0