Derivative Instruments and Hedging Activities (Details) MWh in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) MWh	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	$ (6,810,000)	$ 0	$ 0
Number of Interest Rate Swaps	3
Other Comprehensive Income (Loss), Net of Tax	$ (19,750,000)	0	0
Interest Rate Contract [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(19,750,000)
Other Comprehensive Income (Loss), Net of Tax	(19,750,000)
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 0
Maximum Length of Time Hedged in Cash Flow Hedge	7 months
Not Designated as Hedging Instrument [Member] | FTR [Member]
Derivative [Line Items]
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	5,707
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0
Derivative, Nonmonetary Notional Amount, Purchase (Sales), Net | MWh	5,707
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
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
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | FTR [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	864,000,000	3,099,000,000
Other Current Assets [Member] | Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 26,560,000,000	0
Parent Company [Member]
Number of Interest Rate Swaps	3
Other Comprehensive Income (Loss), Net of Tax	$ (19,750,000)	0	$ 0
Parent Company [Member] | Interest Rate Contract [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(19,750,000)	0
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 0
Maximum Length of Time Hedged in Cash Flow Hedge	7 months
Parent Company [Member] | Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
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
Subsidiaries [Member] | Not Designated as Hedging Instrument [Member] | FTR [Member]
Derivative [Line Items]
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	5,707
Derivative, Nonmonetary Notional Amount, Purchase (Sales), Net | MWh	5,707
Subsidiaries [Member] | Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | FTR [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 864,000,000	$ 3,099,000,000