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Fair Value of Financial Instruments - Counterparty Credit Risk (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Derivative Fair Value Meaurements and Concentration of Credit Risk |  |  |
Counterparty credit exposure, excluding credit risk exposure under certain long term agreements | $ 220 |  |
Counterparty credit exposure, collateral held (cash and letters of credit) against positions | 30 |  |
Counterparty credit exposure, net | $ 196 |  |
Company's exposure before collateral expected to roll off by the end of 2015 (as a percent) | 73.00% |  |
Net exposure (as a percent) | 100.00% |  |
Counterparty credit risk exposure to certain counterparties, threshold (as a percent) | 10.00% |  |
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage | $ 37 |  |
Estimated counterparty credit risk exposure under certain long term agreements, including California tolling agreements, South Central load obligations and solar power purchase agreements for the next 5 years | $ 4,100 |  |
Period of estimated counterparty credit risk exposure under certain long term agreements, including California tolling agreements, South Central load obligations and solar power purchase agreements (in years) | 5 years |  |
Provision for bad debts | $ 68 | $ 48 | $ 64
Investment grade |  |  |
Derivative Fair Value Meaurements and Concentration of Credit Risk |  |  |
Net exposure (as a percent) | 69.00% |  |
Non-Investment grade/Non-Rated |  |  |
Derivative Fair Value Meaurements and Concentration of Credit Risk |  |  |
Net exposure (as a percent) | 31.00% |  |
Financial Institutions |  |  |
Derivative Fair Value Meaurements and Concentration of Credit Risk |  |  |
Net exposure (as a percent) | 14.00% |  |
Utilities, energy merchants, marketers and other |  |  |
Derivative Fair Value Meaurements and Concentration of Credit Risk |  |  |
Net exposure (as a percent) | 86.00% |  |
NRG Yield, Inc. |  |  |
Derivative Fair Value Meaurements and Concentration of Credit Risk |  |  |
Estimated counterparty credit risk exposure under certain long term agreements, including California tolling agreements, South Central load obligations and solar power purchase agreements for the next 5 years | $ 2,600 |  |

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- Definition The amount of credit exposure to a significant portion of all counterparties. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The aggregate amount of net counterparty credit risk exposure to certain counterparties representing more than 10% of total net counterparty exposure. + References No definition available. + Details
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Period Type: | instant

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- Definition Carrying amount as of the balance sheet date of cash collateral and letters of credit received by the entity on deposit from third parties in support of energy risk management. + References No definition available. + Details
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Balance Type: | debit
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- Definition The percentage of net counterparty credit exposure that rolls-off by the end of 2012. + References No definition available. + Details
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- Definition Represents the threshold percentage for disclosure of counterparty credit exposure. + References No definition available. + Details
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- Definition Represents the percentage of net counterparty credit exposure of the entity. + References No definition available. + Details
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- Definition This element represents the estimated amount of the counterparty credit risk exposure. + References No definition available. + Details
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- Definition Estimated Counterparty Credit Risk Exposure to Certain Counterparties, Period + References No definition available. + Details
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- Definition Represents the net exposure of the entity determined as total credit exposure to all counterparties less collateral (cash and letters of credit) held against those positions as of the reporting date. + References No definition available. + Details
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