Stock-Based Compensation Plans - Narrative (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) warrant $ / shares shares	Dec. 31, 2014 USD ($) warrant $ / shares shares	Dec. 31, 2013 USD ($) $ / shares		Dec. 31, 2012	Jun. 30, 2015 USD ($) $ / shares shares
Common Stock Narrative Information [Line Items]
Allocated Share-based Compensation Expense	$ 119,000,000	$ 127,000,000	$ 118,000,000	[1]
Common stock shares reserved for warrant conversion | shares	24,478
Equity offering | shares						57,500,000
Price per share (in usd per share) | $ / shares						$ 35
Junior Subordinated Notes, Noncurrent						$ 1,150,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares | shares						23,000,000
Treasury Stock, Number of Shares Held | shares	35,000,000
Treasury stock, at cost	$ 2,327,000,000	2,327,000,000
Expiration period	10 years
Obligations of outstanding restricted stock not settled	$ 97,000,000	85,000,000
Settlement of restricted stock	75,000,000	43,000,000	28,000,000
Unrecognized compensation costs of nonvested restricted stock	56,000,000
Weighted average grant date fair value of performance shares (in usd per share)	35.88	28.75	31.55
Settlement of performance shares	46,000,000	27,000,000	26,000,000
Settlement of performance shares in cash	29,000,000	13,000,000	12,000,000
Unrecognized compensation costs of performance based shares	$ 27,000,000
Weighted-average period of recognition of unrecognized compensation costs of performance based shares	1 year 4 months 24 days
Commonwealth Edison Co [Member]
Common Stock Narrative Information [Line Items]
Allocated Share-based Compensation Expense	$ 6,000,000	$ 7,000,000	9,000,000	[1]
Warrants outstanding | warrant	73,434	73,533
Common stock shares reserved for warrant conversion | shares		24,511
Performance Shares [Member]
Common Stock Narrative Information [Line Items]
Allocated Share-based Compensation Expense	$ 41,000,000	$ 59,000,000	$ 48,000,000
Mix of awards			67.00%
Percentage to be settled as common stock	50.00%
Percentage to be settled as cash	50.00%
Percentage to be settled as cash if ownership requirements are met	100.00%
Weighted average grant date fair value (in usd per share) | $ / shares	$ 35.88
Nonvested Stock Option [Member]
Common Stock Narrative Information [Line Items]
Unrecognized compensation costs	$ 0
Employee Stock Option [Member]
Common Stock Narrative Information [Line Items]
Allocated Share-based Compensation Expense	$ 1,000,000	$ 2,000,000	$ 3,000,000
Requisite service period	4 years
Restricted Stock [Member]
Common Stock Narrative Information [Line Items]
Mix of awards			33.00%
Weighted-average period of recognition of unrecognized compensation costs	2 years
Weighted average grant date fair value (in usd per share) | $ / shares	$ 36.55	$ 28.71	$ 31.06
Restricted Stock Units (RSUs) [Member]
Common Stock Narrative Information [Line Items]
Allocated Share-based Compensation Expense	$ 71,000,000	$ 61,000,000	$ 61,000,000
Stock Compensation Plan [Member]
Common Stock Narrative Information [Line Items]
Allocated Share-based Compensation Expense	$ 6,000,000	$ 5,000,000	$ 6,000,000
Minimum [Member]
Common Stock Narrative Information [Line Items]
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value						$ 1,870,000,000
Minimum [Member] | Restricted Stock [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	3 years
Maximum [Member] | Restricted Stock [Member]
Common Stock Narrative Information [Line Items]
Requisite service period	5 years
LTIP [Member]
Common Stock Narrative Information [Line Items]
Shares authorized | shares	16,000,000
2013 Plan [Member] | Performance Shares [Member]
Common Stock Narrative Information [Line Items]
Vesting period			3 years
2012 Plan [Member] | Performance Shares [Member]
Common Stock Narrative Information [Line Items]
Vesting period					3 years
Performance period					1 year
Performance Share Transition Award [Member]
Common Stock Narrative Information [Line Items]
Percentage to be settled as common stock			50.00%
Percentage to be settled as cash			50.00%
Percentage to be settled as cash if ownership requirements are met			100.00%
Performance Share Transition Award [Member] | Performance Shares [Member]
Common Stock Narrative Information [Line Items]
Vesting period			1 year
Performance period			2 years

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