Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2012

Asset Retirement Obligations Tables [Line Items]

Nuclear decommissioning asset retirement obligation rollforward

		Exelon and Generation
Nuclear decommissioning ARO at January 1, 2011	$	3,276
Accretion expense		209
Net increase due to changes in, and timing of, estimated future cash flows		198
Costs incurred to decommission retired plants		(3)
Nuclear decommissioning ARO at December 31, 2011 (a)		3,680
Accretion expense		231
Net increase due to changes in, and timing of, estimated future cash flows		833
Costs incurred to decommission retired plants		(3)
Nuclear decommissioning ARO at December 31, 2012 (a)	$	4,741

(a) Includes $10million and $5 million as the current portion of the ARO at December 31, 2012and 2011, respectively, which isincluded in other current liabilities on Exelon's and Generation's Consolidated Balance Sheets.

Unrealized Gains (Losses) on nuclear decommissioning trust funds

		Exelon and Generation
		For the Years Ended December 31,
		2012		2011		2010
Net unrealized gains (losses) on decommissioning
	trust funds — Regulatory Agreement Units (a) (b) (c)	$	386	$	(74)	$	294
Net unrealized gains (losses) on decommissioning
	trust funds — Non-Regulatory Agreement Units (c)		105		(4)		104

__________

(a)       Net unrealized gains (losses) related to Generation's NDT funds associated with Regulatory Agreement Units are included in regulatory liabilities on Exelon's Consolidated Balance Sheets and noncurrent payables to affiliates on Generation's Consolidated Balance Sheets.

(b)       Excludes$73million and $48 million of net unrealized gains (losses) related to the Zion Station pledged assets in 2012 and 2011. Net unrealized gains (losses) related to Zion Station pledged assets are included inthe payable for Zion Station decommissioning on Exelon's and Generation's Consolidated Balance Sheets.

(c)       Net unrealized gains (losses) related to Generation's NDT funds are included within other, net in Exelon's and Generation's Consolidated Statements of Operations and Comprehensive Income.

Non-nuclear asset retirement obligation rollforward

		Exelon		Generation			ComEd		PECO		BGE
Non-nuclear AROs at January 1, 2011		$	223	$	86	$	105	$	32	$	1
Net decrease due to changes in, and timing of,
	estimated future cash flows (a)		(24)		(3)		(17)		(4)		—
Development projects			7		7		—		—		—
Accretion expense (b)			9		5		3		1		—
Payments			(6)		(3)		(2)		(1)		—
Non-nuclear AROs at December 31, 2011			209		92		89		28		1
Net increase due to changes in, and timing of,
	estimated future cash flows (a)		27		18		8		1		7
Development projects			47		47		—		—		—
Accretion expense(b)			13		8		4		1		—
Merger with Constellation (c)			58		50
Payments			(11)		(8)		(2)		(1)		—
Non-nuclear AROs at December 31, 2012		$	343	$	207	$	99	$	29	$	8

• During the year ended December 31, 2011, PECO recorded a reduction in operating and maintenance expense of $3 million. Generation and ComEd did not record any reductions in operating and maintenance expense for the year ended December 31, 2011. During the year ended December 31, 2012, Generation recorded a reduction in operating and maintenance expense of $8 million. ComEd and PECO did not record any adjustments in operating and maintenance expense for the year ended December 31, 2012.
• For ComEd and PECO, the majority of the accretion is recorded as an increase to a regulatory asset due to the associated regulatory treatment.
• Exelon's ARO includes $8 million of BGE costs incurred prior to the closing of Exelon's merger with Constellation. Refer to Note 4 –Merger and Acquisitions for additional information.

Zion Station pledged assets

	Exelon and Generation
	2012		2011
Carrying value of Zion Station pledged assets	$	614	$	734
Payable to Zion Solutions (a)		564		691
Current portion of payable to Zion Solutions (b)		132		128
Withdrawals by Zion Solutions to pay decommissioning costs		192		143

(a) Excludes a liability recorded within Generation's Consolidated Balance Sheets related to the tax obligation on the unrealized activity associated with the Zion Station NDT Funds. The NDT Funds will be utilized to satisfy the tax obligations as gains and losses are realized.

(b) Included in other current liabilities within Generation's Consolidated Balance Sheets