Supplemental Financial Information - Supplemental Balance Sheet Information (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Mar. 31, 2014	Dec. 31, 2013
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	$ 0			$ 1,925
Total equity method investments	50			2,295
Net investment in leases	361			698
Total investments	544			3,112
Compensation-related accruals	832			683
Taxes accrued	305		285	315
Interest accrued	240			234
Severance accrued	49			66
Other accrued expenses	113			335
Total accrued expenses	1,539			1,633
Antelope Valle [Member]
Supplemental Balance Sheet Information [Line Items]
Other accrued expenses	19			228
Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	22	[1]		22	[1]
Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	13
Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	9
Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	1			2
Sunnyside [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	5
Keystone Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				32
Conemaugh Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				21
CENG [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				1,925
Safe Harbor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				285
Malacha [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				8
Trust for Benefit of Employees [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	85	[2]		90	[2]
Other Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	42	[3]		22	[3]
Exelon Generation Co L L C [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0			1,925
Total equity method investments	28			2,273
Net investment in leases	7			7
Total investments	104			2,325
Compensation-related accruals	447			337
Taxes accrued	248			212
Interest accrued	66			72
Severance accrued	33			31
Other accrued expenses	92			324
Total accrued expenses	886			976
Exelon Generation Co L L C [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0	[1]		0	[1]
Exelon Generation Co L L C [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	13
Exelon Generation Co L L C [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	9
Exelon Generation Co L L C [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	1			2
Exelon Generation Co L L C [Member] | Sunnyside [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	5
Exelon Generation Co L L C [Member] | Keystone Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				32
Exelon Generation Co L L C [Member] | Conemaugh Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				21
Exelon Generation Co L L C [Member] | CENG [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				1,925
Exelon Generation Co L L C [Member] | Safe Harbor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				285
Exelon Generation Co L L C [Member] | Malacha [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				8
Exelon Generation Co L L C [Member] | Trust for Benefit of Employees [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	27	[2]		23	[2]
Exelon Generation Co L L C [Member] | Other Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	42	[3]		22	[3]
Commonwealth Edison Co [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments	6			6
Net investment in leases	0			0
Total investments	6			11
Compensation-related accruals	153			135
Taxes accrued	59			62
Interest accrued	102			95
Severance accrued	2			3
Other accrued expenses	15			12
Total accrued expenses	331			307
Commonwealth Edison Co [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	6	[1]		6	[1]
Commonwealth Edison Co [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
Commonwealth Edison Co [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
Commonwealth Edison Co [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0			0
Commonwealth Edison Co [Member] | Keystone Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Commonwealth Edison Co [Member] | Conemaugh Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Commonwealth Edison Co [Member] | CENG [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Commonwealth Edison Co [Member] | Safe Harbor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Commonwealth Edison Co [Member] | Malacha [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Commonwealth Edison Co [Member] | Trust for Benefit of Employees [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	0	[2]		5	[2]
Commonwealth Edison Co [Member] | Other Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	0	[3]		0	[3]
PECO Energy Co [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments	8			8
Net investment in leases	0			0
Total investments	31			31
Compensation-related accruals	50			47
Taxes accrued	3			24
Interest accrued	33			32
Severance accrued	1			1
Other accrued expenses	4			2
Total accrued expenses	91			106
PECO Energy Co [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	8	[1]		8	[1]
PECO Energy Co [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
PECO Energy Co [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
PECO Energy Co [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0			0
PECO Energy Co [Member] | Sunnyside [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
PECO Energy Co [Member] | Keystone Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
PECO Energy Co [Member] | Conemaugh Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
PECO Energy Co [Member] | CENG [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
PECO Energy Co [Member] | Safe Harbor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
PECO Energy Co [Member] | Malacha [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
PECO Energy Co [Member] | Trust for Benefit of Employees [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	23	[2]		23	[2]
PECO Energy Co [Member] | Other Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	0	[3]		0	[3]
Baltimore Gas and Electric Company [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments	8			8
Net investment in leases	0			0
Total investments	12			13
Compensation-related accruals	58			55
Taxes accrued	42			16
Interest accrued	29			29
Severance accrued	2			4
Other accrued expenses	0			7
Total accrued expenses	131			111
Baltimore Gas and Electric Company [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	8	[1]		8	[1]
Baltimore Gas and Electric Company [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
Baltimore Gas and Electric Company [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
Baltimore Gas and Electric Company [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0			0
Baltimore Gas and Electric Company [Member] | Sunnyside [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	0
Baltimore Gas and Electric Company [Member] | Keystone Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Baltimore Gas and Electric Company [Member] | Conemaugh Fuels [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Baltimore Gas and Electric Company [Member] | CENG [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Baltimore Gas and Electric Company [Member] | Safe Harbor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Baltimore Gas and Electric Company [Member] | Malacha [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments				0
Baltimore Gas and Electric Company [Member] | Trust for Benefit of Employees [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	4	[2]		5	[2]
Baltimore Gas and Electric Company [Member] | Other Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	0	[3]		0	[3]
Exelon Consolidations [Member]
Supplemental Balance Sheet Information [Line Items]
Net investment in leases	$ 367			$ 705

[1] Includes investments in affiliated financing trusts, which were not consolidated within the financial statements of Exelon and are shown as investments on the Consolidated Balance Sheets. See Note 1 — Significant Accounting Policies for additional information.

[2] The Registrants’ investments in these marketable securities are recorded at fair market value.

[3] Includes cost method and available-for-sale investments.