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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) shares in Millions, mw_per_day in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions | 6 Months Ended | 12 Months Ended
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Jun. 30, 2018 USD ($) MMBTU MWh mw_per_day T shares | Dec. 31, 2017 USD ($) MMBTU MWh mw_per_day T shares
Long | Emissions |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, mass (ton) | 2 | 1
Long | Coal |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, mass (ton) | 12 | 21
Long | Power |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MWh | 16 | 14
Long | Interest |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, notional amount (in usd) | $ | $ 4,016 | $ 3,876
Long | Equity |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, non-monetary notional amount (in shares) | shares | 0 | 1
Short | Natural Gas |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MMBTU | 551 | 17
Short | Capacity |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, flow rate (in MW/Day) | mw_per_day | (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, Nonmonetary Notional Amount, Energy Flow Rate + References No definition available. + Details
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- Definition Volumetric Underlying Derivative Transactions [Abstract] + 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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