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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract] |
Nuclear Decommissioning Asset Retirement Obligation Rollforward | The following table provides a rollforward of the non-nuclear AROs reflected in the Registrants’ Consolidated Balance Sheets from January 1, 2017 to December 31, 2018:
| Exelon |  | Generation |  | ComEd |  | PECO |  | BGE |  | PHI |  | Pepco |  | DPL |  | ACE
Non-nuclear AROs at January 1, 2017 | $ | 393 |  |  | $ | 199 |  |  | $ | 121 |  |  | $ | 28 |  |  | $ | 24 |  |  | $ | 14 |  |  | $ | 2 |  |  | $ | 9 |  |  | $ | 3 |
Net (decrease) increase due to changes in, and timing of, estimated future cash flows | (11 | ) |  | (1 | ) |  | (13 | ) |  | (1 | ) |  | 2 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | — |
Development projects | 1 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Accretion expense(a) | 18 |  |  | 10 |  |  | 7 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Deconsolidation of EGTP | (7 | ) |  | (7 | ) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Payments | (10 | ) |  | (5 | ) |  | (2 | ) |  | (1 | ) |  | (2 | ) |  | — |  |  | — |  |  | — |  |  | — |
Non-nuclear AROs at December 31, 2017 | 384 |  |  | 197 |  |  | 113 |  |  | 27 |  |  | 24 |  |  | 16 |  |  | 3 |  |  | 10 |  |  | 3 |
Net increase due to changes in, and timing of, estimated future cash flows(b) | 80 |  |  | 35 |  |  | 7 |  |  | — |  |  | 2 |  |  | 36 |  |  | 34 |  |  | 1 |  |  | 1 |
Accretion expense(a) | 16 |  |  | 10 |  |  | 4 |  |  | 1 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | — |
Asset divestitures | (3 | ) |  | (3 | ) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Payments | (6 | ) |  | (1 | ) |  | (3 | ) |  | — |  |  | (2 | ) |  | — |  |  | — |  |  | — |  |  | — |
Non-nuclear AROs at December 31, 2018 | $ | 471 |  |  | $ | 238 |  |  | $ | 121 |  |  | $ | 28 |  |  | $ | 25 |  |  | $ | 52 |  |  | $ | 37 |  |  | $ | 11 |  |  | $ | 4 |
__________
(a) | For ComEd and PECO, the majority of the accretion is recorded as an increase to a regulatory asset due to the associated regulatory treatment.
(b) | In 2018, Pepco recorded an increase of $22 million in Operating and maintenance expense primarily related to asbestos identified at its Buzzard Point property as part of an annual ARO study. Buzzard Point is a waterfront property in the District of Columbia occupied by an active substation and former Pepco operated steam plant building, which Pepco retired and closed in 1981.
The following table provides a rollforward of the nuclear decommissioning ARO reflected in Exelon’s and Generation’s Consolidated Balance Sheets, from January 1, 2017 to December 31, 2018:
Nuclear decommissioning ARO at January 1, 2017 | $ | 8,734 |
Accretion expense | 458 |
Acquisition of FitzPatrick | 444 |
Net increase due to changes in, and timing of, estimated future cash flows | 34 |
Costs incurred related to decommissioning plants | (8 | )
Nuclear decommissioning ARO at December 31, 2017 (a) | 9,662 |
Accretion expense | 478 |
Net decrease due to changes in, and timing of, estimated future cash flows | (77 | )
Costs incurred related to decommissioning plants | (58 | )
Nuclear decommissioning ARO at December 31, 2018 (a) (b) | $ | 10,005 |
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(a) | Includes $22 million and $13 million as the current portion of the ARO at December 31, 2018 and 2017, respectively, which is included in Other current liabilities in Exelon’s and Generation’s Consolidated Balance Sheets.
Unrealized Gains Losses On Nuclear Decommissioning Trust Funds | The following table provides unrealized (losses) gains on NDT funds of Exelon and Generation for the years ended 2018, 2017 and 2016:
| 2018 |  | 2017 |  | 2016
Net unrealized (losses) gains on NDT funds—Regulatory Agreement Units (a) | $ | (715 | ) |  | $ | 455 |  |  | $ | 216 |
Net unrealized (losses) gains on NDT funds—Non-Regulatory Agreement Units (b) | (483 | ) |  | 521 |  |  | 194 |
__________
(a) | Net unrealized (losses) gains related to Generation’s NDT funds associated with Regulatory Agreement Units are included in Regulatory liabilities in Exelon’s Consolidated Balance Sheets and Noncurrent payables to affiliates in Generation’s Consolidated Balance Sheets.
(b) | Net unrealized (losses) gains 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 Exelon's and Generation's pledged assets and payables to ZionSolutions, and withdrawals by ZionSolutions at December 31, 2018 and 2017:
| 2018 |  | 2017
Carrying value of Zion Station pledged assets | $ | 9 |  |  | $ | 39 |
Current payable to ZionSolutions (a) | 9 |  |  | 37 |
Cumulative withdrawals by ZionSolutions to pay decommissioning costs (b) | 965 |  | 942
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(a) | Included in Other current liabilities within Exelon's and Generation's Consolidated Balance Sheets. Excludes a liability recorded within Exelon’s and Generation’s Consolidated Balance Sheets related to the tax obligation on the unrealized gains and losses 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) | Includes project expenses to decommission Zion Station and estimated tax payments on Zion Station NDT fund earnings.

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- Definition Tabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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