Variable Interest Entities Variable Interest Entities - Assets and Liabilities of VIEs which Creditors or Beneficiaries have No Recourse (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Cash and cash equivalents		$ 1,349	$ 898	$ 635	$ 6,502
Restricted cash		247	207	253	205
Customer		4,607	4,445
Other		1,256	1,132
Mark-to-market derivative assets, current		804	976
Other current assets		1,238	1,260
Total current assets		13,360	11,896
Property, plant and equipment, net		76,707	74,202
Nuclear decommissioning trust funds		11,661	13,272
Goodwill		6,677	6,677	6,677
Mark-to-market derivative assets, noncurrent		452	337
Other noncurrent assets		1,575	1,330
Total assets	[1]	119,666	116,770
Long-term debt due within one year		1,349	2,088
Accounts payable		3,800	3,532
Accrued expenses		2,112	1,837
Mark-to-market derivative liabilities, current		475	232
Unamortized energy contract liabilities		149	231
Other current liabilities		1,035	1,069
Total current liabilities		11,404	10,798
Long-term debt		34,075	32,176
Asset retirement obligations		9,679	10,029
Pension obligations		3,988	3,736
Unamortized energy contract liabilities		463	609
Other noncurrent liabilities		2,130	2,097
Total liabilities	[1]	86,596	84,583
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	[2]	414	126
Restricted cash	[2]	66	64
Customer	[2]	146	170
Other	[2]	23	25
Materials and supplies	[2]	212	205
Other current assets	[2]	52	45
Total current assets	[2]	913	635
Property, plant and equipment, net	[2]	6,145	6,186
Nuclear decommissioning trust funds	[2]	2,351	2,502
Other noncurrent assets	[2]	258	274
Total noncurrent assets	[2]	8,754	8,962
Total assets	[2]	9,667	9,597
Long-term debt due within one year	[2]	87	102
Accounts payable	[2]	96	114
Accrued expenses	[2]	72	67
Unamortized energy contract liabilities	[2]	15	18
Other current liabilities	[2]	3	7
Total current liabilities	[2]	273	308
Long-term debt	[2]	1,072	1,154
Asset retirement obligations	[2]	2,160	2,035
Unamortized energy contract liabilities		1
Other noncurrent liabilities	[2]	42	121
Total noncurrent liabilities	[2]	3,275	3,310
Total liabilities	[2]	3,548	3,618
Exelon Generation Co L L C [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents		750	416	290	431
Restricted cash		153	138	158	123
Customer		2,941	2,697
Other		562	321
Mark-to-market derivative assets, current		804	976
Other current assets		883	933
Total current assets		8,433	6,882
Property, plant and equipment, net		23,981	24,906
Nuclear decommissioning trust funds		11,661	13,272
Goodwill		47	47
Mark-to-market derivative assets, noncurrent		452	334
Other noncurrent assets		755	670
Total assets	[3]	47,556	48,457
Long-term debt due within one year		906	346
Accounts payable		1,847	1,773
Accrued expenses		898	1,022
Mark-to-market derivative liabilities, current		449	211
Unamortized energy contract liabilities		31	43
Other current liabilities		279	265
Total current liabilities		5,769	4,191
Long-term debt		6,989	7,734
Asset retirement obligations		9,450	9,844
Unamortized energy contract liabilities		20	48
Other noncurrent liabilities		610	658
Total liabilities	[3]	32,048	32,498
Exelon Generation Co L L C [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents		414	126
Restricted cash		62	58
Customer		146	170
Other		23	25
Materials and supplies		212	205
Other current assets		49	41
Total current assets		906	625
Property, plant and equipment, net		6,145	6,186
Nuclear decommissioning trust funds		2,351	2,502
Other noncurrent assets		232	243
Total noncurrent assets		8,728	8,931
Total assets		9,634	9,556
Long-term debt due within one year		66	67
Accounts payable		96	114
Accrued expenses		72	66
Unamortized energy contract liabilities		15	18
Other current liabilities		3	7
Total current liabilities		252	272
Long-term debt		1,025	1,088
Asset retirement obligations		2,160	2,035
Unamortized energy contract liabilities		1
Other noncurrent liabilities		42	121
Total noncurrent liabilities		3,228	3,244
Total liabilities		3,480	3,516
Pepco Holdings LLC [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents		124	30
Restricted cash		43	42
Customer		453	486
Other		177	206
Other current assets		75	75
Total current assets		1,533	1,551
Property, plant and equipment, net		13,446	12,498
Goodwill		4,005	4,005	4,005
Other noncurrent assets		60	70
Total assets	[4]	21,984	21,247
Long-term debt due within one year		125	396
Accounts payable		496	348
Accrued expenses		256	261
Unamortized energy contract liabilities		119	188
Other current liabilities		123	123
Total current liabilities		1,592	1,931
Long-term debt		6,134	5,478
Asset retirement obligations		52	16
Unamortized energy contract liabilities		442	561
Other noncurrent liabilities		369	389
Total liabilities	[4]	12,702	12,422
Pepco Holdings LLC [Member] | Successor [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents				170
Restricted cash				43
Pepco Holdings LLC [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	[2]	0	0
Restricted cash	[2]	4	6
Customer	[2]	0	0
Other	[2]	0	0
Materials and supplies	[2]	0	0
Other current assets	[2]	3	4
Total current assets	[2]	7	10
Property, plant and equipment, net	[2]	0	0
Nuclear decommissioning trust funds	[2]	0	0
Other noncurrent assets	[2]	26	31
Total noncurrent assets	[2]	26	31
Total assets	[2]	33	41
Long-term debt due within one year	[2]	21	35
Accounts payable	[2]	0	0
Accrued expenses	[2]	1	1
Unamortized energy contract liabilities	[2]	0	0
Other current liabilities	[2]	0	0
Total current liabilities	[2]	22	36
Long-term debt	[2]	47	66
Asset retirement obligations	[2]	0	0
Unamortized energy contract liabilities		0
Other noncurrent liabilities	[2]	0	0
Total noncurrent liabilities	[2]	47	66
Total liabilities	[2]	69	102
Atlantic City Electric Company [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents		7	2	101	3
Restricted cash		4	6	$ 9	$ 12
Customer		95	92
Other		55	56
Other current assets		5	2
Total current assets		240	258
Property, plant and equipment, net		2,966	2,706
Other noncurrent assets		40	45
Total assets	[5]	3,699	3,445
Long-term debt due within one year		18	281
Accounts payable		154	118
Accrued expenses		35	33
Other current liabilities		4	8
Total current liabilities		422	619
Long-term debt		1,170	840
Other noncurrent liabilities		27	25
Total liabilities	[5]	2,573	2,402
Atlantic City Electric Company [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents		0	0
Restricted cash		4	6
Customer		0	0
Other		0	0
Materials and supplies		0	0
Other current assets		0	0
Total current assets		4	6
Property, plant and equipment, net		0	0
Nuclear decommissioning trust funds		0	0
Other noncurrent assets		19	23
Total noncurrent assets		19	23
Total assets		23	29
Long-term debt due within one year		18	31
Accounts payable		0	0
Accrued expenses		1	1
Unamortized energy contract liabilities		0	0
Other current liabilities		0	0
Total current liabilities		19	32
Long-term debt		40	58
Asset retirement obligations		0	0
Unamortized energy contract liabilities		0
Other noncurrent liabilities		0	0
Total noncurrent liabilities		40	58
Total liabilities		$ 59	$ 90

[1] Exelon’s consolidated assets include $9,667 million and $9,597 million at December 31, 2018 and 2017, respectively, of certain VIEs that can only be used to settle the liabilities of the VIE. Exelon’s consolidated liabilities include $3,548 million and $3,618 million at December 31, 2018 and 2017, respectively, of certain VIEs for which the VIE creditors do not have recourse to Exelon. See Note 2–Variable Interest Entities for additional information.

[2] Includes certain purchase accounting adjustments not pushed down to the ACE standalone entity.

[3] Generation’s consolidated assets include $9,634 million and $9,556 million at December 31, 2018 and 2017, respectively, of certain VIEs that can only be used to settle the liabilities of the VIE. Generation’s consolidated liabilities include $3,480 million and $3,516 million at December 31, 2018 and 2017, respectively, of certain VIEs for which the VIE creditors do not have recourse to Generation. See Note 2–Variable Interest Entities for additional information.

[4] PHI’s consolidated total assets include $33 million and $41 million at December 31, 2018 and 2017, respectively, of PHI's consolidated VIE that can only be used to settle the liabilities of the VIE. PHI’s consolidated total liabilities include $69 million and $102 million at December 31, 2018 and 2017, respectively, of PHI's consolidated VIE for which the VIE creditors do not have recourse to PHI. See Note 2 - Variable Interest Entities for additional information.

[5] ACE’s consolidated assets include $23 million and $29 million at December 31, 2018 and 2017, respectively, of ACE’s consolidated VIE that can only be used to settle the liabilities of the VIE. ACE’s consolidated liabilities include $59 million and $90 million at December 31, 2018 and 2017, respectively, of ACE’s consolidated VIE for which the VIE creditors do not have recourse to ACE. See Note 2 - Variable Interest Entities for additional information.