v3.8.0.1

Derivative Instruments (Details) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) MMBTU MWh t | Dec. 31, 2014 MMBTU MWh t | Dec. 31, 2016 USD ($)
Natural Gas [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | (398) |  | (613) |
Environmental Credits [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount, Mass | t | 19 |  | 16 |
Interest Rate Hedging Instruments |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount | $ | $ 4,600 | [1] |  | $ 3,721
Short [Member] | Power [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh | (98) |  | (86) |
[1] | We entered into interest rate hedging instruments during the first quarter of 2017 to hedge approximately $1.0 billion of variable rate debt for 2018 through 2020 and approximately $500 million of variable rate debt for 2021 through 2022. We also extended the tenor of certain interest rate hedging instruments, which effectively places a ceiling on LIBOR on $2.5 billion of variable rate corporate debt through 2020 and $1.25 billion of variable rate corporate debt in 2021.

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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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Balance Type: | na
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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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Namespace Prefix: | us-gaap_
Data Type: | num:energyItemType
Balance Type: | na
Period Type: | duration

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- Definition Nominal mass used to calculate payments on a derivative instrument. + References No definition available. + Details
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- Details
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- Details
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- Details
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- Details
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- Details
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