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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, 2017 USD ($) MWh MMBTU T shares bbl | Dec. 31, 2016 USD ($) MWh MMBTU T shares bbl
Long | Emissions |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 1 | 0
Long | Coal |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 21 | 35
Long | Oil |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, volume (in barrels) | bbl | 0 | 1
Long | Power |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh | 14 | 7
Long | Interest |  |
Derivative [Line Items] |  |
Derivative, notional amount (in usd) | $ | $ 3,876 | $ 3,429
Long | Equity |  |
Derivative [Line Items] |  |
Derivative, non-monetary notional amount (in shares) | shares | 1 | 1
Short | Natural Gas |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MMBTU | 17 | 53
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 Derivative, Non-monetary Notional Amount, Other + References No definition available. + 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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- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
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