Summary of Significant Accounting Policies - Investments (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 488	$ 574
Supplemental retirement income plan assets	273	243
Deferred income plan assets	60	55
Other	9	9
Total investments	1,921	884
Stagecoach Gas Services, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	992
Mountain Valley Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	48
CET | Stagecoach Gas Services, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	992	0
CET | New York Transco, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	51	3
CET | Mountain Valley Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	48	0
CECONY
Schedule of Equity Method Investments [Line Items]
Equity method investments	0	0
Supplemental retirement income plan assets	246	221
Deferred income plan assets	60	55
Other	9	10
Total investments	315	286
CECONY | CET | Stagecoach Gas Services, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	0	0
CECONY | CET | New York Transco, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	0	0
CECONY | CET | Mountain Valley Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 0	$ 0