Summary of Significant Accounting Policies and Other Matters - Reconciliation of Cash, Temporary Investments and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash and temporary cash investments	$ 1,272	$ 981
Restricted cash	164	236
Total cash, temporary cash investments and restricted cash	1,436	1,217	$ 1,006	$ 844
Cash Collateral Held For Project Finance Agreements | Clean Energy Businesses
Cash and Cash Equivalents [Line Items]
Restricted cash	164	236
CECONY
Cash and Cash Equivalents [Line Items]
Cash and temporary cash investments	1,067	933
Restricted cash	0	0
Total cash, temporary cash investments and restricted cash	$ 1,067	$ 933	$ 818	$ 730