Changes in Accumulated Other Comprehensive Income Changes in Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		$ 32,187		$ 27,640		$ 27,330
Amounts reclassified from AOCI	[1]	182		144
Other comprehensive income (loss)		42		171		(36)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings				539
Ending Balance		33,070		32,187		27,640
Gains and (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(14)		(17)
OCI before reclassifications	[2]	11		(1)
Amounts reclassified from AOCI	[2],[3]	1		4
Other comprehensive income (loss)	[2]	12		3
Ending Balance	[2]	(2)		(14)		(17)
Gains and (Losses) on Cash Flow Hedges | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Unrealized Gains and (losses) on Marketable Securities
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	10		4
OCI before reclassifications	[2]	0		6
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		6
Ending Balance	[2]	0		10		4
Unrealized Gains and (losses) on Marketable Securities | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	(10)
Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(2,998)		(2,610)
OCI before reclassifications	[2]	(143)		11
Amounts reclassified from AOCI	[1],[2],[3]	181		140
Other comprehensive income (loss)	[2]	38		151
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings	[2],[5]			(539)
Ending Balance	[2]	(2,960)		(2,998)		(2,610)
Pension and Non-Pension Postretirement Benefit Plan Items | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(23)		(30)
OCI before reclassifications	[2]	(10)		7
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	(10)		7
Ending Balance	[2]	(33)		(23)		(30)
Foreign Currency Items | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
AOCI of Investments Unconsolidated Affiliates
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(1)		(7)
OCI before reclassifications	[2]	1		6
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	1		6
Ending Balance	[2]	0		(1)		(7)
AOCI of Investments Unconsolidated Affiliates | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance		(3,026)	[2]	(2,660)	[2]	(2,624)
OCI before reclassifications	[2]	(141)		29
Amounts reclassified from AOCI	[2],[3]	182		144
Other comprehensive income (loss)		41	[2]	173	[2]	(36)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings	[2],[5]			(539)
Ending Balance	[2]	(2,995)		(3,026)		(2,660)
Total | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	(10)
Exelon Generation Co L L C [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from AOCI	[1]	1		4
Other comprehensive income (loss)		3		15		9
Exelon Generation Co L L C [Member] | Gains and (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(16)		(19)
OCI before reclassifications	[2]	11		(1)
Amounts reclassified from AOCI	[2],[3]	1		4
Other comprehensive income (loss)	[2]	12		3
Ending Balance	[2]	(4)		(16)		(19)
Exelon Generation Co L L C [Member] | Gains and (Losses) on Cash Flow Hedges | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Exelon Generation Co L L C [Member] | Unrealized Gains and (losses) on Marketable Securities
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	3		2
OCI before reclassifications	[2]	0		1
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		1
Ending Balance	[2]	0		3		2
Exelon Generation Co L L C [Member] | Unrealized Gains and (losses) on Marketable Securities | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	(3)
Exelon Generation Co L L C [Member] | Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	0		0
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[1],[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		0		0
Exelon Generation Co L L C [Member] | Pension and Non-Pension Postretirement Benefit Plan Items | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Exelon Generation Co L L C [Member] | Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(23)		(30)
OCI before reclassifications	[2]	(10)		7
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	(10)		7
Ending Balance	[2]	(33)		(23)		(30)
Exelon Generation Co L L C [Member] | Foreign Currency Items | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Exelon Generation Co L L C [Member] | AOCI of Investments Unconsolidated Affiliates
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(1)		(7)
OCI before reclassifications	[2]	0		6
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		6
Ending Balance	[2]	(1)		(1)		(7)
Exelon Generation Co L L C [Member] | AOCI of Investments Unconsolidated Affiliates | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
Exelon Generation Co L L C [Member] | Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	(37)		(54)
OCI before reclassifications	[2]	1		13
Amounts reclassified from AOCI	[2],[3]	1		4
Other comprehensive income (loss)		2	[2]	17	[2]	9
Ending Balance	[2]	(38)		(37)		(54)
Exelon Generation Co L L C [Member] | Total | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	(3)
PECO Energy Co [Member] | Gains and (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	0		0
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		0		0
PECO Energy Co [Member] | Gains and (Losses) on Cash Flow Hedges | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
PECO Energy Co [Member] | Unrealized Gains and (losses) on Marketable Securities
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	1		1
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		1		1
PECO Energy Co [Member] | Unrealized Gains and (losses) on Marketable Securities | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	(1)
PECO Energy Co [Member] | Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	0		0
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		0		0
PECO Energy Co [Member] | Pension and Non-Pension Postretirement Benefit Plan Items | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
PECO Energy Co [Member] | Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	0		0
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		0		0
PECO Energy Co [Member] | Foreign Currency Items | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
PECO Energy Co [Member] | AOCI of Investments Unconsolidated Affiliates
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	0		0
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		0		0
PECO Energy Co [Member] | AOCI of Investments Unconsolidated Affiliates | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	0
PECO Energy Co [Member] | Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	[2]	1		1
OCI before reclassifications	[2]	0		0
Amounts reclassified from AOCI	[2],[3]	0		0
Other comprehensive income (loss)	[2]	0		0
Ending Balance	[2]	0		$ 1		$ 1
PECO Energy Co [Member] | Total | Accounting Standards Update 2016-01 [Member]
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Impact of adoption of Recognition and Measurement of Financial Assets and Liabilities standard	[2],[4]	$ (1)

[1] Amounts in parenthesis represent a decrease in net income.

[2] All amounts are net of tax and noncontrolling interests. Amounts in parenthesis represent a decrease in AOCI.

[3] See next tables for details about these reclassifications.

[4] Exelon prospectively adopted the new standard Recognition and Measurement of Financial Assets and Financial Liabilities. The standard was adopted as of January 1, 2018, which resulted in an increase to Retained earnings and Accumulated other comprehensive loss of $10 million, $3 million and $1 million for Exelon, Generation and PECO, respectively. The amounts reclassified related to Rabbi Trusts. See Note 1 — Significant Accounting Policies for additional information.

[5] Exelon early adopted the new standard Reclassification of Certain Tax Effects from AOCI. The standard was adopted retrospectively as of December 31, 2017, which resulted in an increase to Exelon’s Retained earnings and Accumulated other comprehensive loss of $539 million, primarily related to deferred income taxes associated with Exelon’s pension and OPEB obligations. See Note 1 — Significant Accounting Policies for additional information.