Variable Interest Entities	12 Months Ended
Dec. 31, 2017
Variable Interest Entities
Variable Interest Entities	18. Variable Interest Entities Following is a description of our financial interests in unconsolidated and consolidated variable interest entities that we consider significant: Low-Income Housing Properties and Refined Coal Facility Investments We have investments in entities established to manage low-income housing properties and a refined coal facility. We support the operations of these entities in exchange for a pro-rata share of the tax credits they generate. We do not consolidate these entities as we have determined we are not the primary beneficiary of these entities as we do not have the power to individually direct the activities of these entities. Accordingly, we account for these investments under the equity method of accounting. Our aggregate investment balance in these two entities was $59 million and $84 million as of December 31, 2017 and 2016, respectively. The debt balance related to our investment in low-income housing properties was $34 million and $57 million as of December 31, 2017 and 2016, respectively. Additional information related to these investments is discussed in Note 8. Trust Funds for Final Capping, Closure, Post-Closure or Environmental Remediation Obligations We have significant financial interests in trust funds that were created to settle certain of our final capping, closure, post-closure or environmental remediation obligations. These trust funds are established such that we are either the sole beneficiary of these restricted balances or we share benefit with the host community in which we operate. We have determined that these trust funds are variable interest entities; however, we are not the primary beneficiary of certain of these entities, as described further below. As the party with primary responsibility to fund the related final capping, closure, post-closure or environmental remediation activities for these trust funds, we are exposed to risk of loss if there are declines in the fair value of the assets of the trust. We currently expect the trust funds to continue to meet the statutory requirements for which they were established. Unconsolidated Variable Interest Entities — Trust funds that are established for both the benefit of the Company and the host community in which we operate are not consolidated because we are not the primary beneficiary of these entities as we either do not have the (i) power to direct the significant activities of the trusts or (ii) power over the trusts’ significant activities is shared. Our interests in these trusts are accounted for as investments in unconsolidated entities and receivables. These amounts are recorded in other receivables, investments in unconsolidated entities and long-term other assets in our Consolidated Balance Sheets, as appropriate. We also reflect our share of the unrealized gains and losses on available-for-sale securities held by these trusts as a component of our accumulated other comprehensive income (loss). Our investments and receivables related to these trusts had an aggregate carrying value of $99 million and $93 million as of December 31, 2017 and 2016, respectively. Consolidated Variable Interest Entities — Trust funds for which we are the sole beneficiary are consolidated because we are the primary beneficiary. These trust funds are recorded in restricted trust and escrow accounts in our Consolidated Balance Sheets. Unrealized gains and losses on available-for-sale securities held by these trusts are recorded as a component of accumulated other comprehensive income (loss). These trusts had a fair value of $101 million and $95 million as of December 31, 2017 and 2016, respectively.

18. Variable Interest Entities

Following is a description of our financial interests in unconsolidated and consolidated variable interest entities that we consider significant:

Low-Income Housing Properties and Refined Coal Facility Investments

We have investments in entities established to manage low-income housing properties and a refined coal facility. We support the operations of these entities in exchange for a pro-rata share of the tax credits they generate. We do not consolidate these entities as we have determined we are not the primary beneficiary of these entities as we do not have the power to individually direct the activities of these entities. Accordingly, we account for these investments under the equity method of accounting. Our aggregate investment balance in these two entities was $59 million and $84 million as of December 31, 2017 and 2016, respectively. The debt balance related to our investment in low-income housing properties was $34 million and $57 million as of December 31, 2017 and 2016, respectively. Additional information related to these investments is discussed in Note 8.

Trust Funds for Final Capping, Closure, Post-Closure or Environmental Remediation Obligations

We have significant financial interests in trust funds that were created to settle certain of our final capping, closure, post-closure or environmental remediation obligations. These trust funds are established such that we are either the sole beneficiary of these restricted balances or we share benefit with the host community in which we operate. We have determined that these trust funds are variable interest entities; however, we are not the primary beneficiary of certain of these entities, as described further below. As the party with primary responsibility to fund the related final capping, closure, post-closure or environmental remediation activities for these trust funds, we are exposed to risk of loss if there are declines in the fair value of the assets of the trust. We currently expect the trust funds to continue to meet the statutory requirements for which they were established.

Unconsolidated Variable Interest Entities — Trust funds that are established for both the benefit of the Company and the host community in which we operate are not consolidated because we are not the primary beneficiary of these entities as we either do not have the (i) power to direct the significant activities of the trusts or (ii) power over the trusts’ significant activities is shared. Our interests in these trusts are accounted for as investments in unconsolidated entities and receivables. These amounts are recorded in other receivables, investments in unconsolidated entities and long-term other assets in our Consolidated Balance Sheets, as appropriate. We also reflect our share of the unrealized gains and losses on available-for-sale securities held by these trusts as a component of our accumulated other comprehensive income (loss). Our investments and receivables related to these trusts had an aggregate carrying value of $99 million and $93 million as of December 31, 2017 and 2016, respectively.

Consolidated Variable Interest Entities — Trust funds for which we are the sole beneficiary are consolidated because we are the primary beneficiary. These trust funds are recorded in restricted trust and escrow accounts in our Consolidated Balance Sheets. Unrealized gains and losses on available-for-sale securities held by these trusts are recorded as a component of accumulated other comprehensive income (loss). These trusts had a fair value of $101 million and $95 million as of December 31, 2017 and 2016, respectively.