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Accounting for Derivative Instruments and Hedging Activities - Effect on the Company's of Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives, Net (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives |  |  |
Accumulated OCI beginning balance | $ (101) |  |
Reclassified from accumulated OCI to income: |  |  |
Due to realization of previously deferred amounts | 21 |  |
Mark-to-market of cash flow hedge accounting contracts | 14 |  |
Sale of NRG Yield and Renewables | 25 | $ 0 | $ 0
Accumulated OCI ending balance, net of $0, $8 and $16 tax | (66) | (101) |
Accumulated OCI income (loss), tax | 0 | (8) | (16)
2018 |  |  |
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives |  |  |
Accumulated OCI beginning balance | (54) | (66) |
Reclassified from accumulated OCI to income: |  |  |
Due to realization of previously deferred amounts | 8 | 12 |
Mark-to-market of cash flow hedge accounting contracts | 21 | 0 |
Accumulated OCI ending balance, net of $0, $8 and $16 tax | $ 0 | $ (54) | $ (66)

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- Definition Accumulated Other Comprehensive Income (Loss), Cumulative Change in Gain (Loss) from Cash Flow Hedges, Tax Amount + References No definition available. + Details
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- Definition Amount, after tax, of accumulated gain (loss) on derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580 + Details
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- Definition Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e689-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + Details
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- Definition Amount, before tax, of reclassification from accumulated other comprehensive income (AOCI) for gain (loss) from derivative designated and qualifying as net investment hedge. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4CCC -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109998896-113959 + Details
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