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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Nuclear Decommissioning Asset Retirement Obligation Rollforward
The following table provides a rollforward of the nuclear decommissioning ARO reflected on Exelon’s and Generation’s Consolidated Balance Sheets, from January 1, 2014 to December 31, 2015:
 
 
Exelon and
Generation
Nuclear decommissioning ARO at January 1, 2014
$
4,855

Consolidation of CENG (a)
1,760

Accretion expense
334

Net increase due to changes in, and timing of, estimated future cash flows
19

Costs incurred to decommission retired plants
(7
)
Nuclear decommissioning ARO at December 31, 2014 (b)
6,961

Accretion expense
387

Net increase due to changes in, and timing of, estimated future cash flows
901

Costs incurred to decommission retired plants
(3
)
Nuclear decommissioning ARO at December 31, 2015 (b)

$
8,246

_________________________
(a)
Represents the fair value of the CENG ARO liability as of April 1, 2014, the date of consolidation. See Note 5 — Investment in Constellation Energy Nuclear Group, LLC for additional information.
(b)
Includes $7 million and $8 million as the current portion of the ARO at December 31, 2015 and 2014, respectively, which is included in Other current liabilities on Exelon’s and Generation’s Consolidated Balance Sheets.
Unrealized Gains Losses On Nuclear Decommissioning Trust Funds
The following table provides unrealized gains on NDT funds for 2015, 2014 and 2013:
 
 
Exelon and Generation
 
For the Years Ended December 31,
 
2015
 
2014
 
2013
Net unrealized gains (losses) on decommissioning trust
funds—Regulatory Agreement Units (a)
$
(282
)
 
$
180

 
$
406

Net unrealized gains (losses) on decommissioning trust
funds—Non-Regulatory Agreement Units (b)(c)
(197
)
 
134

 
146

_______________________
(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 $7 million, $29 million and $7 million of net unrealized gains related to the Zion Station pledged assets in 2015, 2014 and 2013, respectively. Net unrealized gains related to Zion Station pledged assets are included in the 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 with Non-Regulatory Agreement Units are included within Other, net in Exelon’s and Generation’s Consolidated Statements of Operations and Comprehensive Income.
Nuclear Decommissioning Pledged Assets
The following table provides the pledged assets and payables to ZionSolutions, and withdrawals by ZionSolutions at December 31, 2015 and 2014:
 
 
Exelon and Generation
 
2015
 
2014
Carrying value of Zion Station pledged assets
$
206

 
$
319

Payable to Zion Solutions (a)
189

 
292

Current portion of payable to Zion Solutions (b)
99

 
137

Cumulative withdrawals by Zion Solutions to pay decommissioning costs (c)
786

 
666

___________________ 
(a)
Excludes a liability recorded within Exelon’s and 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 Exelon’s and Generation’s Consolidated Balance Sheets.
(c)
Includes project expenses to decommission Zion Station and estimated tax payments on Zion Station NDT fund earnings.
Non Nuclear Decommissioning Asset Retirement Obligation Rollforward
The following table provides a rollforward of the non-nuclear AROs reflected on the Registrants’ Consolidated Balance Sheets from January 1, 2014 to December 31, 2015:
 
 
Exelon
 
Generation
 
ComEd
 
PECO
 
BGE
Non-nuclear AROs at January 1, 2014
$
351

 
$
201


$
101


$
30


$
19

Net increase (decrease) due to changes in, and
   timing of, estimated future cash flows (a)
(1
)
 
(2
)

2


—


(1
)
Development projects (b)
11

 
11


—


—


—

Accretion expense (c)
15

 
11


3


1


—

Liabilities held for sale (d)
(4
)
 
(4
)
 
—

 
—

 
—

Sale of generating assets (e)
(20
)
 
(20
)
 
—

 
—

 
—

Payments
(6
)
 
(3
)

(2
)

(1
)

—

Non-nuclear AROs at December 31, 2014 (f)

346

 
194


104


30


18

Net increase (decrease) due to changes in, and
   timing of, estimated future cash flows (a)
(10
)
 
(12
)

6


(4
)

—

Development projects (b)
10

 
10


—


—


—

Accretion expense (c)
16

 
10


5


1


—

Sale of generating assets (e)
(2
)
 
(2
)
 
—

 
—

 
—

Payments
(5
)
 
(3
)

(2
)

—


—

Non-nuclear AROs at December 31, 2015 (f)
$
355

 
$
197


$
113


$
27


$
18

________________________
(a)
During the year ended December 31, 2015, Generation recorded a decrease of $(2) million in Operating and maintenance expense. ComEd, PECO, and BGE did not record any adjustments in Operating and maintenance expense for the year ended December 31, 2015. During the year ended December 31, 2014, Generation recorded a decrease of $(2) million and ComEd recorded an increase of $1 million in Operating and maintenance expense. PECO and BGE did not record any adjustments in Operating and maintenance expense for the year ended December 31, 2014.
(b)
Relates to new AROs recorded due to the construction of solar, wind and other non-nuclear generating sites.
(c)
For ComEd, PECO, and BGE, the majority of the accretion is recorded as an increase to a regulatory asset due to the associated regulatory treatment.
(d)
Represents AROs related to generating stations classified as held for sale. See Note 4 — Mergers, Acquisitions, and Dispositions for further information.
(e)
Reflects a reduction to the ARO resulting primarily from the sales of Schuylkill generating station in 2015 and Keystone and Conemaugh generating stations in 2014. See Note 4 — Mergers, Acquisitions, and Dispositions for further information.
(f)
Excludes $5 million, $2 million, $0 million and $1 million as the current portion of the ARO at December 31, 2015 for Generation, ComEd, PECO and BGE, respectively. Excludes $1 million, $1 million, $1 million and $1 million as the current portion of the ARO at December 31, 2014 for Generation, ComEd, PECO and BGE, respectively. This is included in Other current liabilities on the Registrants' respective Consolidated Balance Sheets.