Organization and Basis of Presentation - Narrative (Details) mi in Thousands, customer in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) customer company transmissionCenter mi MW	Mar. 31, 2020 USD ($)
Property, Plant and Equipment [Line Items]
Length of transmission lines | mi	24
Number of regional transmission centers | transmissionCenter	2
Capitalized cost of equity | $	$ 7	$ 11
Capitalized interest | $	$ 6	$ 7
PN | Waverly, New York
Property, Plant and Equipment [Line Items]
Number of customers served by utility operating companies | customer	4
Regulated Distribution
Property, Plant and Equipment [Line Items]
Number of existing utility operating companies | company	10
Number of customers served by utility operating companies | customer	6,000
Plant capacity (in MW's) | MW	3,580