Accounting for Derivative Instruments and Hedging Activities (Schedule of Derivative Assets and Liabilities) (Details) $ in Thousands, MWh in Millions, MMBTU in Millions	Dec. 31, 2019 USD ($) MWh MMBTU	Dec. 31, 2018 USD ($) MMBTU MWh
Derivative [Line Items]
Assets, Current	$ 23,626	$ 46,507
Assets, Long-term	7,682	2,512
Liabilities, Current	13,428	46,661
Liabilities, Long-term	12,693	11,095
Not Designated as Hedging Instrument
Derivative [Line Items]
Total derivative assets	31,308	49,019
Total derivative liabilities	26,121	57,756
Not Designated as Hedging Instrument | Parent Company [Member]
Derivative [Line Items]
Assets, Current	23,626	46,507
Assets, Long-term	7,682	2,512
Liabilities, Current	13,428	46,661
Liabilities, Long-term	$ 12,693	$ 11,095
Not Designated as Hedging Instrument | Electric Generation Fuel
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	229.3	194.8
Not Designated as Hedging Instrument | Purchased Electricity
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MWh	10.4	6.6
Not Designated as Hedging Instrument | Natural Gas Portfolio
Derivative [Line Items]
Total derivative assets	$ 11,375	$ 15,732
Total derivative liabilities	8,617	30,472
Not Designated as Hedging Instrument | Electric Portfolio
Derivative [Line Items]
Total derivative assets	19,933	33,287
Total derivative liabilities	$ 17,504	$ 27,284
Not Designated as Hedging Instrument | Gas Derivatives
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	316.0	337.0