Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
OCI before reclassifications, Tax			$ 1
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax	1	1	1
CECONY [Member]
OCI before reclassifications, Tax
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax

Amount of tax (expense) benefit of reclassification adjustment from accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

No definition available.

Name:	us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration