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Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities - Fair Value of Financial Liabilities Recorded at the Carrying Amount (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) |  | $ 795 | $ 865
Spent Nuclear Fuel Obligation, Noncurrent |  | 1,171 | 1,147
Debt Issuance Costs, Net |  | 216 | 201
Financing Trust Member [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 0 | (1)
Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 714 | 929
Long-term debt (including amounts due within one year)(a) | [1] | 35,869 | 36,705
Due to Related Parties, Noncurrent | [2] | 400 | 431
Spent Nuclear Fuel Obligation, Noncurrent |  | 949 | 936
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Due to Related Parties, Noncurrent | [2] | 0 | 0
Spent Nuclear Fuel Obligation, Noncurrent |  | 0 | 0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 714 | 929
Long-term debt (including amounts due within one year)(a) | [1] | 33,711 | 34,735
Due to Related Parties, Noncurrent | [2] | 0 | 0
Spent Nuclear Fuel Obligation, Noncurrent |  | 949 | 936
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 2,158 | 1,970
Due to Related Parties, Noncurrent | [2] | 400 | 431
Spent Nuclear Fuel Obligation, Noncurrent |  | 0 | 0
Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 714 | 929
Long-term debt (including amounts due within one year)(a) | [1] | 35,424 | 34,264
Due to Related Parties, Noncurrent | [2] | 390 | 389
Spent Nuclear Fuel Obligation, Noncurrent |  | 1,171 | 1,147
Exelon Generation Co L L C [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) |  | 91 | 103
Spent Nuclear Fuel Obligation, Noncurrent |  | 1,171 | 1,147
Debt Issuance Costs, Net |  | 51 | 60
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 2
Long-term debt (including amounts due within one year)(a) | [1] | 8,910 | 9,512
Spent Nuclear Fuel Obligation, Noncurrent |  | 949 | 936
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Spent Nuclear Fuel Obligation, Noncurrent |  | 0 | 0
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 2
Long-term debt (including amounts due within one year)(a) | [1] | 7,467 | 7,839
Spent Nuclear Fuel Obligation, Noncurrent |  | 949 | 936
Exelon Generation Co L L C [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 0
Long-term debt (including amounts due within one year)(a) | [1] | 1,443 | 1,673
Spent Nuclear Fuel Obligation, Noncurrent |  | 0 | 0
Exelon Generation Co L L C [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 2
Long-term debt (including amounts due within one year)(a) | [1] | 8,793 | 8,990
Spent Nuclear Fuel Obligation, Noncurrent |  | 1,171 | 1,147
Commonwealth Edison Co [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 63 | 52
Commonwealth Edison Co [Member] | Financing Trust Member [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | (1) | (1)
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 8,390 | 8,418
Due to Related Parties, Noncurrent | [2] | 209 | 227
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Due to Related Parties, Noncurrent | [2] | 0 | 0
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 8,390 | 8,418
Due to Related Parties, Noncurrent | [2] | 0 | 0
Commonwealth Edison Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Due to Related Parties, Noncurrent | [2] | 209 | 227
Commonwealth Edison Co [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 8,101 | 7,601
Due to Related Parties, Noncurrent | [2] | 205 | 205
PECO Energy Co [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 23 | 17
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 3,207 | 3,194
Due to Related Parties, Noncurrent |  | 191 | 204
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Due to Related Parties, Noncurrent |  | 0 | 0
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 3,157 | 3,194
Due to Related Parties, Noncurrent |  | 0 | 0
PECO Energy Co [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 50 | 0
Due to Related Parties, Noncurrent |  | 191 | 204
PECO Energy Co [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) | [1] | 3,084 | 2,903
Due to Related Parties, Noncurrent |  | 184 | 184
Baltimore Gas and Electric Company [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 18 | 17
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 35 | 77
Long-term debt (including amounts due within one year)(a) | [1] | 2,950 | 2,825
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 35 | 77
Long-term debt (including amounts due within one year)(a) | [1] | 2,950 | 2,825
Baltimore Gas and Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Baltimore Gas and Electric Company [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 35 | 77
Long-term debt (including amounts due within one year)(a) | [1] | 2,876 | 2,577
Pepco Holdings LLC [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term debt (including amounts due within one year)(a) |  | 633 | 686
Pepco Holdings LLC [Member] | Successor [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 14 | 6
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 179 | 350
Long-term debt (including amounts due within one year)(a) | [1] | 6,101 | 6,019
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 179 | 350
Long-term debt (including amounts due within one year)(a) | [1] | 5,436 | 5,722
Pepco Holdings LLC [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 665 | 297
Pepco Holdings LLC [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 179 | 350
Long-term debt (including amounts due within one year)(a) | [1] | 6,259 | 5,874
Potomac Electric Power Company [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 34 | 32
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 40 | 26
Long-term debt (including amounts due within one year)(a) | [1] | 3,097 | 3,123
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 40 | 26
Long-term debt (including amounts due within one year)(a) | [1] | 2,901 | 3,114
Potomac Electric Power Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 196 | 9
Potomac Electric Power Company [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 40 | 26
Long-term debt (including amounts due within one year)(a) | [1] | 2,719 | 2,540
Delmarva Power & Light Company [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 12 | 11
Delmarva Power & Light Company [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 216
Long-term debt (including amounts due within one year)(a) | [1] | 1,496 | 1,393
Delmarva Power & Light Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Delmarva Power & Light Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 216
Long-term debt (including amounts due within one year)(a) | [1] | 1,303 | 1,393
Delmarva Power & Light Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 0
Long-term debt (including amounts due within one year)(a) | [1] | 193 | 0
Delmarva Power & Light Company [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  |  | 216
Long-term debt (including amounts due within one year)(a) | [1] | 1,494 | 1,300
Atlantic City Electric Company [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Issuance Costs, Net |  | 7 | 5
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 139 | 108
Long-term debt (including amounts due within one year)(a) | [1] | 1,262 | 1,237
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 0 | 0
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 139 | 108
Long-term debt (including amounts due within one year)(a) | [1] | 987 | 949
Atlantic City Electric Company [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 0 | 0
Long-term debt (including amounts due within one year)(a) | [1] | 275 | 288
Atlantic City Electric Company [Member] | Reported Value Measurement [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Short-term Debt, Fair Value |  | 139 | 108
Long-term debt (including amounts due within one year)(a) | [1] | $ 1,188 | $ 1,121
[1] | Includes unamortized debt issuance costs which are not fair valued of $216 million, $51 million, $63 million, $23 million, $18 million, $14 million, $34 million, $12 million and $7 million for Exelon, Generation, ComEd, PECO, BGE, PHI, Pepco, DPL and ACE respectively, as of December 31, 2018. Includes unamortized debt issuance costs which are not fair valued of $201 million, $60 million, $52 million, $17 million, $17 million, $6 million, $32 million, $11 million and $5 million for Exelon, Generation, ComEd, PECO, BGE, PHI, Pepco, DPL and ACE respectively, as of December 31, 2017.
[2] | Includes unamortized debt issuance costs which are not fair valued of $0 million and $1 million for Exelon and ComEd, respectively, as of December 31, 2018. Includes unamortized debt issuance costs which are not fair valued of $1 million and $1 million for Exelon and ComEd, respectively, as of December 31, 2017.

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