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Accounting for Derivative Instruments and Hedging Activities - Net Notional Volume Buy/Sell of Open Derivative Transactions (Details) shares in Millions, bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) MWh MMBTU T shares bbl | Dec. 31, 2017 USD ($) MWh MMBTU T shares bbl
Long | Emissions |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) |  | 1
Long | Renewables Energy Certificates |  |
Derivative [Line Items] |  |
Derivative, non-monetary notional amount (in shares) | shares | 1 | 0
Long | Coal |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | 13 | 21
Long | Oil |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, volume (in barrels) | bbl | 1 | 0
Long | Power |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh | 1 | 23
Long | Interest |  |
Derivative [Line Items] |  |
Derivative, notional amount (in usd) | $ | $ 1,000 | $ 1,060
Long | Equity |  |
Derivative [Line Items] |  |
Derivative, non-monetary notional amount (in shares) | shares | 0 | 1
Short | Emissions |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | 2 |
Short | Natural Gas |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MMBTU | 330 | 20
Short | Capacity |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh | 1 | 1

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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