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Nuclear Decommissioning Trust Fund | 12 Months Ended
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Dec. 31, 2017
Nuclear Decommissioning Trust Fund Disclosure [Abstract] |
Nuclear Decommissioning Trust Fund | Nuclear Decommissioning Trust Fund NRG's Nuclear Decommissioning Trust Fund assets, which are for the decommissioning of STP, are comprised of securities classified as available-for-sale and recorded at fair value based on actively quoted market prices. Although NRG is responsible for managing the decommissioning of its 44% interest in STP, the predecessor utilities that owned STP are authorized by the PUCT to collect decommissioning funds from their ratepayers to cover decommissioning costs on behalf of NRG. NRC requirements determine the decommissioning cost estimate which is the minimum required level of funding. In the event that funds from the ratepayers that accumulate in the nuclear decommissioning trust are ultimately determined to be inadequate to decommission the STP facilities, the utilities will be required to collect through rates charged to rate payers all additional amounts, with no obligation from NRG, provided that NRG has complied with PUCT rules and regulations regarding decommissioning trusts. Following completion of the decommissioning, if surplus funds remain in the decommissioning trusts, any excess will be refunded to the respective ratepayers of the utilities. NRG accounts for the Nuclear Decommissioning Trust Fund in accordance with ASC 980, Regulated Operations, or ASC 980, because the Company's nuclear decommissioning activities are subject to approval by the PUCT, with regulated rates that are designed to recover all decommissioning costs and that can be charged to and collected from the ratepayers per PUCT mandate. Since the Company is in compliance with PUCT rules and regulations regarding decommissioning trusts and the cost of decommissioning is the responsibility of the Texas ratepayers, not NRG, all realized and unrealized gains or losses (including other-than-temporary impairments) related to the Nuclear Decommissioning Trust Fund are recorded to the Nuclear Decommissioning Trust liability and are not included in net income or accumulated other comprehensive income, consistent with regulatory treatment. The following table summarizes the aggregate fair values and unrealized gains and losses (including other-than-temporary impairments) for the securities held in the trust funds, as well as information about the contractual maturities of those securities.
| As of December 31, 2017 |  | As of December 31, 2016
(In millions, except otherwise noted) | Fair Value |  | Unrealized Gains |  | Unrealized Losses |  | Weighted- average maturities (in years) |  | Fair Value |  | Unrealized Gains |  | Unrealized Losses |  | Weighted- average maturities (in years)
Cash and cash equivalents | $ | 47 |  |  | $ | — |  |  | $ | — |  |  | — |  |  | $ | 25 |  |  | $ | — |  |  | $ | — |  |  | — |
U.S. government and federal agency obligations | 43 |  |  | 1 |  |  | — |  |  | 11 |  |  | 73 |  |  | 1 |  |  | — |  |  | 11 |
Federal agency mortgage-backed securities | 82 |  |  | 1 |  |  | 1 |  |  | 23 |  |  | 62 |  |  | 1 |  |  | 1 |  |  | 25 |
Commercial mortgage-backed securities | 13 |  |  | — |  |  | — |  |  | 20 |  |  | 17 |  |  | — |  |  | 1 |  |  | 26 |
Corporate debt securities | 99 |  |  | 2 |  |  | 1 |  |  | 11 |  |  | 84 |  |  | 1 |  |  | 2 |  |  | 11 |
Equity securities | 403 |  |  | 272 |  |  | — |  |  | — |  |  | 346 |  |  | 214 |  |  | — |  |  | — |
Foreign government fixed income securities | 5 |  |  | — |  |  | — |  |  | 9 |  |  | 3 |  |  | — |  |  | — |  |  | 9 |
Total | $ | 692 |  |  | $ | 276 |  |  | $ | 2 |  |  |  |  |  | $ | 610 |  |  | $ | 217 |  |  | $ | 4 |  |  |  |
The following table summarizes proceeds from sales of available-for-sale securities and the related realized gains and losses from these sales. The cost of securities sold is determined using the specific identification method.
| Year Ended December 31,
| 2017 |  | 2016 |  | 2015
| (In millions)
Realized gains | $ | 22 |  |  | $ | 26 |  |  | $ | 21 |
Realized losses | 8 |  |  | 11 |  |  | 14 |
Proceeds from sale of securities | 501 |  |  | 510 |  |  | 631 |

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- Definition This item represents the disclosure regarding the nuclear decommissioning trust fund assets investments held in trust to pay for the cost of decommissioning a facility at the end of its economic life as a single block text. The disclosure may include the following: the aggregate fair values and unrealized gains and losses (including other-than-temporary impairments) for the securities held as well as information about the contractual maturities of those securities; summarized proceeds from sales of available for sale securities and the related realized gains and losses from these sales. The disclosure also includes that such funds are contributed and owned by the ratepayers but reported by the entity. + References No definition available. + Details
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