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Derivative Financial Instruments - Summary of Derivative Fair Value Balances (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Derivative [Line Items] |  |  |  |  |
Derivative, Collateral, Right to Reclaim Cash |  | $ 50 |  | $ 77 |
Mark-to-market derivative assets (current assets) |  | 804 |  | 976 |
Mark-to-market derivative assets (noncurrent assets) |  | 452 |  | 337 |
Derivative Liability, Current |  | (475) |  | (232) |
Derivative Liability, Noncurrent |  | (479) |  | (409) |
Total mark-to-market derivative net assets (liabilities) |  | 1 |  | 6 |
Margin Deposit Assets |  | (94) |  | (117) |
Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) |  | 804 |  | 976 |
Mark-to-market derivative assets (noncurrent assets) |  | 452 |  | 334 |
Derivative Liability, Current |  | (449) |  | (211) |
Derivative Liability, Noncurrent |  | (252) |  | (174) |
Derivative, Collateral, Obligation to Return Cash |  | 2 |  | 4 |
Commonwealth Edison Co [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative Liability, Current |  | (26) |  | (21) |
Derivative Liability, Noncurrent |  | (223) |  | (235) |
Commodity Contract [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) |  | 801 |  | 973 |
Mark-to-market derivative assets (noncurrent assets) |  | 445 |  | 331 |
Total mark-to-market derivative assets |  | 1,246 |  | 1,304 |
Derivative Liability, Current |  | (473) |  | (230) |
Derivative Liability, Noncurrent |  | (474) |  | (407) |
Total mark-to-market derivative liabilities |  | (947) |  | (637) |
Total mark-to-market derivative net assets (liabilities) |  | 299 |  | 667 |
Commodity Contract [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) |  | 801 | [1] | 973 | [2]
Mark-to-market derivative assets (noncurrent assets) |  | 445 | [1] | 331 | [2]
Total mark-to-market derivative assets |  | 1,246 | [1] | 1,304 | [2]
Derivative Liability, Current |  | (447) | [1] | (209) | [2]
Derivative Liability, Noncurrent |  | (251) | [1] | (172) | [2]
Total mark-to-market derivative liabilities |  | (698) | [1] | (381) | [2]
Total mark-to-market derivative net assets (liabilities) |  | 548 | [1] | 923 | [2]
Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset |  | (357) |  | (466) |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Commodity Contract [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) |  | 3,505 |  | 3,061 |
Mark-to-market derivative assets (noncurrent assets) |  | 1,266 |  | 1,164 |
Total mark-to-market derivative assets |  | 4,771 |  | 4,225 |
Derivative Liability, Current |  | (3,429) |  | (2,646) |
Derivative Liability, Noncurrent |  | (1,203) |  | (1,137) |
Total mark-to-market derivative liabilities |  | (4,632) |  | (3,783) |
Total mark-to-market derivative net assets (liabilities) |  | 139 |  | 442 |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Commodity Contract [Member] | Commonwealth Edison Co [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) | [3] | 0 |  | 0 |
Mark-to-market derivative assets (noncurrent assets) | [3] | 0 |  | 0 |
Total mark-to-market derivative assets | [3] | 0 |  | 0 |
Derivative Liability, Current | [3] | (26) |  | (21) |
Derivative Liability, Noncurrent | [3] | (223) |  | (235) |
Total mark-to-market derivative liabilities | [3] | (249) |  | (256) |
Total mark-to-market derivative net assets (liabilities) | [3] | (249) |  | (256) |
Not Designated as Hedging Instrument, Trading [Member] | Commodity Contract [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) |  | 105 |  | 56 |
Mark-to-market derivative assets (noncurrent assets) |  | 41 |  | 12 |
Total mark-to-market derivative assets |  | 146 |  | 68 |
Derivative Liability, Current |  | (74) |  | (43) |
Derivative Liability, Noncurrent |  | (20) |  | (10) |
Total mark-to-market derivative liabilities |  | (94) |  | (53) |
Total mark-to-market derivative net assets (liabilities) |  | 52 |  | 15 |
Other Current Assets [Member] | Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Collateral, Right to Reclaim Cash |  | 121 |  | 169 |
Margin Deposit Assets |  | (94) |  | (117) |
Other Noncurrent Assets [Member] | Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Collateral, Right to Reclaim Cash |  | 51 |  | 53 |
Other Current Liabilities [Member] | Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Collateral, Right to Reclaim Cash |  | 125 |  | 167 |
Other Noncurrent Liabilities [Member] | Designated as Hedging Instrument [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative, Collateral, Right to Reclaim Cash |  | 60 |  | 77 |
Collateral and Netting | Commodity Contract [Member] | Exelon Generation Co L L C [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Mark-to-market derivative assets (current assets) | [4] | (2,809) | [5] | (2,144) | [6]
Mark-to-market derivative assets (noncurrent assets) | [4] | (862) | [5] | (845) | [6]
Total mark-to-market derivative assets | [4] | (3,671) | [5] | (2,989) | [6]
Derivative Liability, Current | [4] | 3,056 | [5] | 2,480 | [6]
Derivative Liability, Noncurrent | [4] | 972 | [5] | 975 | [6]
Total mark-to-market derivative liabilities | [4] | 4,028 | [5] | 3,455 | [6]
Total mark-to-market derivative net assets (liabilities) | [4] | $ 357 | [5] | $ 466 | [6]
[1] | Current and noncurrent assets are shown net of collateral of $121 million and $51 million, respectively, and current and noncurrent liabilities are shown net of collateral of $125 million and $60 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $357 million at December 31, 2018.
[2] | Current and noncurrent assets are shown net of collateral of $169 million and $53 million, respectively, and current and noncurrent liabilities are shown net of collateral of $167 million and $77 million, respectively. The total cash collateral posted, net of cash collateral received and offset against mark-to-market assets and liabilities was $466 million at December 31, 2017.
[3] | Includes current and noncurrent liabilities relating to floating-to-fixed energy swap contracts with unaffiliated suppliers.
[4] | Exelon and Generation net all available amounts allowed under the derivative authoritative guidance on the balance sheet. These amounts include unrealized derivative transactions with the same counterparty under legally enforceable master netting agreements and cash collateral. In some cases, Exelon and Generation may have other offsetting exposures, subject to a master netting or similar agreement, such as trade receivables and payables, transactions that do not qualify as derivatives, letters of credit and other forms of non-cash collateral. These are not reflected in the table above.
[5] | Of the collateral posted/(received), $(94) million represents variation margin on the exchanges.
[6] | Of the collateral posted/(received), $(117) million represents variation margin on the exchanges

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