Mergers, Acquisitions, and Dispositions - Narrative (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended			12 Months Ended	0 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014	Jan. 21, 2015	Nov. 14, 2014	Jun. 30, 2014	Jun. 11, 2014	May 01, 2014	Jul. 18, 2014
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross	$ 15,000,000		$ 0
Business Combination, Integration Related Costs	69,000,000
Temporary Equity, Share Subscriptions							57.5	57.5
Line of Credit Facility, Maximum Borrowing Capacity	8,500,000,000	[1]
Bridge Loan	3,200,000,000								7,200,000,000
Other noncurrent assets	1,234,000,000			1,160,000,000
Business Combination, Separately Recognized Transactions, Expenses and Losses Recognized	289,000,000
MARYLAND
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Acquisition, Preacquisition Contingency, Amount of Settlement	94,400,000
Customer Loyalty Program Liability, Noncurrent	4,000,000
BusinessAcquisitionPreacquisitionContingencyCapacityOfSettlementMwH	15
MARYLAND | Stability Fund [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Acquisition, Preacquisition Contingency, Amount of Settlement	19,800,000
Prince George County [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
BusinessAcquisitionPreacquisitionContingencyCapacityOfSettlementMwH	5
Pepco Holdings [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Acquisition, Transaction Costs								220,000,000
Cash Funding From Non Core Asset Sale				1,000,000,000
Expected Debt Issuance				3,500,000,000
Temporary Equity, Share Subscriptions								23.0
Line of Credit Facility, Maximum Borrowing Capacity								7,200,000,000
Cash offered in exchange for each share of PHI stock				$ 35						$ 27.25
Other Long-term Investments										18,000,000
Other noncurrent assets	144,000,000
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds	7,200,000,000
Business Acquisition, Preacquisition Contingency, Amount of Settlement	300,000,000
Pepco Holdings [Member] | Energy Efficiency Program [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Customer Loyalty Program Liability, Noncurrent	57,600,000
Pepco Holdings [Member] | Rate Bill Credits [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Customer Loyalty Program Liability, Noncurrent	36,800,000
Pepco Holdings [Member] | Minimum [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Exit Costs				259,000,000
Pepco Holdings [Member] | Maximum [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Exit Costs				293,000,000
Other Long-term Investments										180,000,000
Delmarva Power & Light [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Acquisition, Preacquisition Contingency, Amount of Settlement	40,000,000
Delmarva Power & Light [Member] | Workforce Development [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Customer Loyalty Program Liability, Noncurrent	2,000,000
Delmarva Power & Light [Member] | Energy Efficiency Program [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Customer Loyalty Program Liability, Noncurrent	2,000,000
Atlantic City Electric [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Acquisition, Preacquisition Contingency, Amount of Settlement	50,000,000
Exelon Generation Co L L C [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross	15,000,000		0
Equity Method Investment, Ownership Percentage			50.01%
Other noncurrent assets	776,000,000			731,000,000
Proceeds from Sale of Productive Assets					1,800,000,000
Exelon Generation Co L L C [Member] | Integrys Energy Services [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross						319,000,000
Business Combination, Consideration Transferred, Liabilities Incurred						13,000,000
Business Combination, Consideration Transferred						332,000,000
Junior Subordinated Debt [Member] | Pepco Holdings [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Subordinated Debt, Amount				$ 1,200,000,000

[1] Excludes additional credit facility agreements for Generation, ComEd, PECO and BGE with aggregate commitments of $50 million, $34 million, $34 million and $5 million, respectively, arranged with minority and community banks located primarily within ComEd’s, PECO’s and BGE’s service territories. These facilities expire on October 16, 2015. These facilities are solely utilized to issue letters of credit. As of March 31, 2015, letters of credit issued under these agreements for Generation, ComEd, PECO and BGE totaled $7 million, $16 million, $21 million and $1 million, respectively. Also, excludes the unsecured bridge credit facility of $3.2 billion to support the PHI transaction discussed below.