Derivative Instruments and Hedging Activities (Details) MWh in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) MWh	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Other Comprehensive Income (Loss), Net of Tax	$ 29,594,000	$ (36,313,000)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	30,050,000	(36,313,000)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(456,000)	0
Accumulated Other Comprehensive Income (Loss), Net of Tax	(13,826,000)	(56,063,000)	$ (43,420,000)	$ (19,750,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	$ 30,050,000	(36,313,000)
FTR [Member] | Not Designated as Hedging Instrument [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	1,046
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0
Derivative, Nonmonetary Notional Amount, Purchase (Sales), Net | MWh	1,046
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Purchase of Derivative Instruments Interest Rate Swap	$ 400,000,000
Sale of Derivative Instruments Interest Rate Swap	0
Derivative, Notional Amount, Purchase (Sales), Net	400,000,000
Interest Rate Contract [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(13,826,000)	$ (56,063,000)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ (5,375,000)
Maximum Length of Time Hedged in Cash Flow Hedge	42 months
Other Current Assets [Member] | FTR [Member] | Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Gross Asset	$ 139,000,000		543,000,000
Derivative Liability, Noncurrent | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative Liability, Fair Value, Gross Liability	$ 23,511,000,000		$ 63,215,000,000