Summary of Significant Accounting Policies - Reconciliation of Cash, Temporary Investments and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and temporary cash investments	$ 651	$ 797	$ 67
Restricted cash	32		35
Total cash, temporary cash investments and restricted cash	683	844	102	$ 830
CEB | Cash Collateral Held for Project Finance Agreements
Cash and Cash Equivalents [Line Items]
Restricted cash	31		31
CECONY
Cash and Cash Equivalents [Line Items]
Cash and temporary cash investments	574	730	17
Restricted cash	0		2
Total cash, temporary cash investments and restricted cash	574	$ 730	19	$ 704
CECONY | Reserve for Construction Expenditures
Cash and Cash Equivalents [Line Items]
Restricted cash			2
RECO | Transition Bond
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 1		$ 2