Summary of Significant Accounting Policies and Other Matters - Long-Lived and Intangible Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	$ 228,000,000	$ 126,000,000
Amortization of intangible assets	102,000,000	99,000,000	$ 14,000,000
Amortization expense, 2020	102,000,000
Amortization expense, 2021	102,000,000
Amortization expense, 2022	102,000,000
Amortization expense, 2023	102,000,000
Amortization expense, 2024	102,000,000
Impairment charges	0	0	$ 2,000,000
Net non-utility plant	46,555,000,000	43,889,000,000
Intangible assets	1,460,000,000	1,557,000,000
Long-term debt	22,349,000,000	19,973,000,000
Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net	3,000,000
Accumulated amortization	8,000,000	7,000,000
Power Purchase Agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net	1,457,000,000	1,554,000,000
Accumulated amortization	$ 220,000,000	$ 119,000,000