Summary of Significant Accounting Policies and Other Matters - Reconciliation of Cash, Temporary Investments and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash and temporary cash investments	$ 76	$ 1,272	$ 1,395
Restricted cash	81		179
Total cash, temporary cash investments and restricted cash	157	1,436	1,574	$ 1,217
CEB | Cash Collateral Held for Project Finance Agreements
Cash and Cash Equivalents [Line Items]
Restricted cash	81		179
CECONY
Cash and Cash Equivalents [Line Items]
Cash and temporary cash investments	30	1,067	1,303
Restricted cash	0		0
Total cash, temporary cash investments and restricted cash	$ 30	$ 1,067	$ 1,303	$ 933