Other Postretirement Benefits - Total Periodic Postretirement Benefit Costs (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5us-gaap_DefinedBenefitPlanServiceCost	$ 5us-gaap_DefinedBenefitPlanServiceCost
Interest cost on accumulated other postretirement benefit obligation	13us-gaap_DefinedBenefitPlanInterestCost	15us-gaap_DefinedBenefitPlanInterestCost
Expected return on plan assets	(20)	(19)
Recognition of net actuarial loss	8	14
Recognition of prior service cost	(5)	(5)
TOTAL PERIODIC OTHER POSTRETIREMENT BENEFIT COST	1	10
Cost capitalized	(1)ed_DefinedBenefitPlanCostCapitalized	(4)ed_DefinedBenefitPlanCostCapitalized
Reconciliation to rate level	4ed_EffectOfReconciliationToRateLevel	3ed_EffectOfReconciliationToRateLevel
Cost charged to operating expenses	4	9
CECONY [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4
Interest cost on accumulated other postretirement benefit obligation	11	13
Expected return on plan assets	(17)	(17)
Recognition of net actuarial loss	7	13
Recognition of prior service cost	(4)	(4)
TOTAL PERIODIC OTHER POSTRETIREMENT BENEFIT COST	1	9
Cost capitalized	(1)	(3)
Reconciliation to rate level	2
Cost charged to operating expenses	$ 2	$ 6

Defined Benefit Plan Cost Capitalized

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- Definition

Defined Benefit Plan Cost Charged To Operating Expenses

+ References

No definition available.

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- Definition

Effect of Reconciliation to Rate Level

+ References

No definition available.

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- Definition

The amount of gains or losses recognized in net periodic benefit cost.

+ References

+ Details

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- Definition

The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.

+ References

+ Details

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512136

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 60

-Glossary Expected Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512171

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (h)(3)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References

+ Details

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

+ References

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (h)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Service Cost (Component of Net Periodic Pension Cost)

-URI http://asc.fasb.org/extlink&oid=6525008

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

+ Details

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