Tennessee Valley Authority PARRS D 2028 (TVC) 10-K 15 Nov 17 2017 FY Annual report Financial data

DEI Document

DEI Document shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)shares
Document Information [Line Items]
Entity Registrant Name	Tennessee Valley Authority
Entity Central Index Key	1,376,986
Current Fiscal Year End Date	--09-30
Entity Filer Category	Non-accelerated Filer
Document Type	10-K
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding | shares	0
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float | $	$ 0

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Operating revenues
Revenue from sales of electricity	$ 10,586	$ 10,461	$ 10,847
Other revenue	153	155	156
Total operating revenues	10,739	10,616	11,003
Operating expenses
Fuel	2,169	2,126	2,444
Purchased power	991	964	950
Operating and maintenance	3,362	2,842	2,838
Depreciation and amortization	1,717	1,836	2,031
Tax equivalents	525	522	525
Total operating expenses	8,764	8,290	8,788
Operating income	1,975	2,326	2,215
Other income (expense), net	56	43	29
Interest expense
Interest expense	1,346	1,371	1,347
Allowance for funds used during construction	0	(235)	(214)
Net interest expense	1,346	1,136	1,133
Net income (loss)	$ 685	$ 1,233	$ 1,111

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Current assets
Cash and cash equivalents	$ 300	$ 300
Accounts receivable, net	1,569	1,747
Inventories, net	1,065	993
Regulatory assets	447	536
Other current assets	65	68
Total current assets	3,446	3,644
Property, plant, and equipment
Completed plant	58,947	51,564
Less accumulated depreciation	(28,404)	(27,592)
Net completed plant	30,543	23,972
Construction in progress	2,842	8,458
Nuclear fuel	1,401	1,450
Capital leases	161	163
Total property, plant, and equipment, net	34,947	34,043
Investment funds	2,603	2,257
Regulatory and other long-term assets
Regulatory assets	8,698	10,164
Other long-term assets	323	386
Total regulatory and other long-term assets	9,021	10,550
Total assets	50,017	50,494
Current liabilities
Accounts payable and accrued liabilities	1,940	2,163
Accrued interest	346	363
Current portion of leaseback obligations	37	58
Current portion of energy prepayment obligations	100	100
Regulatory liabilities	163	154
Short-term debt, net of discounts	1,998	1,407
Total Current maturities of power bonds issued at par	1,728	1,555
Current maturities of long-term debt of variable interest entities issued at par	36	35
Current maturities of notes payable	53	27
Total current liabilities	6,401	5,862
Other liabilities
Post-retirement and post-employment benefit obligations	5,477	6,929
Asset retirement obligations	4,176	3,840
Other long-term liabilities	3,055	2,773
Leaseback obligations	302	409
Energy prepayment obligations	10	110
Non-current regulatory liabilities	25	3
Total other liabilities	13,045	14,064
Long-term debt, net
Long-term power bonds, net	20,205	20,901
Long-term debt of variable interest entities, net	1,164	1,199
Long-term notes payable	69	48
Total long-term debt, net	21,438	22,148
Total liabilities	40,884	42,074
Commitments and contingencies (Note 21)
Proprietary capital
Power program appropriation investment	258	258
Power program retained earnings	8,282	7,594
Total power program proprietary capital	8,540	7,852
Nonpower programs appropriation investment, net	572	580
Accumulated other comprehensive income (loss)	21	(12)
Total proprietary capital	9,133	8,420
Total liabilities and proprietary capital	$ 50,017	$ 50,494

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net income (loss)	$ 685	$ 1,233	$ 1,111
Adjustments to reconcile net income (loss) to net cash provided by operating activities
Depreciation and amortization (including amortization of debt issuance costs and premiums/discounts)	1,763	1,882	2,077
Amortization of nuclear fuel cost	341	287	277
Non-cash retirement benefit expense	837	327	332
Prepayment credits applied to revenue	(100)	(100)	(100)
Fuel cost adjustment deferral	98	(83)	(6)
Fuel cost tax equivalents	5	(16)	(18)
Changes in current assets and liabilities
Accounts receivable, net	230	(83)	93
Inventories and other current assets, net	1	50	(12)
Accounts payable and accrued liabilities	(119)	(4)	(121)
Accrued interest	(17)	(3)	(13)
Regulatory asset costs	(50)	(31)	(23)
Pension contributions	(805)	(281)	(282)
Settlements of asset retirement obligations	(123)	(139)	(58)
Other, net	(10)	3	58
Net cash provided by operating activities	2,736	3,042	3,315
Cash flows from investing activities
Construction expenditures	(2,153)	(2,710)	(2,850)
Combustion turbine asset acquisition	0	0	(342)
Nuclear fuel expenditures	(305)	(300)	(350)
Purchases of investments	(49)	(50)	(52)
Loans and other receivables
Advances	(11)	(10)	(17)
Repayments	8	7	8
Other, net	(26)	(50)	18
Net cash used in investing activities	(2,536)	(3,113)	(3,585)
Long-term debt
Issues of power bonds	999	0	973
Redemptions and repurchases of power bonds	(1,558)	(76)	(1,180)
Redemptions of notes payable	(27)	0	0
Payments on debt of variable interest entities	(35)	(33)	(32)
Short-term debt issues (redemptions), net	583	370	437
Payments on leases and leasebacks	(136)	(159)	(80)
Financing costs, net	(4)	0	(7)
Payments to U.S. Treasury	(5)	(6)	(5)
Other, net	(17)	(25)	(36)
Net cash (used in) provided by financing activities	(200)	71	70
Net change in cash and cash equivalents	0	0	(200)
Cash and cash equivalents at beginning of the year	300	300	500
Cash and cash equivalents at end of the year	$ 300	$ 300	$ 300

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN PROPRIETARY CAPITAL - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Balance at beginning of year		$ 8,420		$ 7,203	$ 8,420	$ 7,203	$ 6,104
Net income (loss)	$ 139	102	$ 661	(37)	685	1,233	1,111
Total other comprehensive income (loss)					33	(10)	(7)
Return on power program appropriation investment					(5)	(6)	(5)
Balance at end of year	9,133		8,420		9,133	8,420	7,203
Power Program Appropriation Investment
Balance at beginning of year		258		258	258	258	258
Net income (loss)					0	0	0
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					0	0	0
Balance at end of year	258		258		258	258	258
Power Program Retained Earnings
Balance at beginning of year		7,594		6,357	7,594	6,357	5,240
Net income (loss)					693	1,243	1,122
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					(5)	(6)	(5)
Balance at end of year	8,282		7,594		8,282	7,594	6,357
Nonpower Programs Appropriation Investment, Net
Balance at beginning of year		580		590	580	590	601
Net income (loss)					(8)	(10)	(11)
Total other comprehensive income (loss)					0	0	0
Return on power program appropriation investment					0	0	0
Balance at end of year	572		580		572	580	590
Accumulated Other Comprehensive Income (Loss) Net Gains (Losses) on Cash Flow Hedges
Balance at beginning of year		$ (12)		$ (2)	(12)	(2)	5
Net income (loss)					0	0	0
Total other comprehensive income (loss)					33	(10)	(7)
Return on power program appropriation investment					0	0	0
Balance at end of year	$ 21		$ (12)		$ 21	$ (12)	$ (2)

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) Statement - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 685	$ 1,233	$ 1,111
Net unrealized gain (loss) on cash flow hedges	59	(139)	(72)
Reclassification to earnings from cash flow hedges	(26)	129	65
Total other comprehensive income (loss)	33	(10)	(7)
Total comprehensive income (loss)	$ 718	$ 1,223	$ 1,104

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	General The Tennessee Valley Authority ("TVA") is a corporate agency and instrumentality of the United States that was created in 1933 by legislation enacted by the United States ("U.S.") Congress in response to a request by President Franklin D. Roosevelt. TVA was created to, among other things, improve navigation on the Tennessee River, reduce the damage from destructive flood waters within the Tennessee River system and downstream on the lower Ohio and Mississippi Rivers, further the economic development of TVA's service area in the southeastern United States, and sell the electricity generated at the facilities TVA operates. Today, TVA operates the nation's largest public power system and supplies power in most of Tennessee, northern Alabama, northeastern Mississippi, and southwestern Kentucky and in portions of northern Georgia, western North Carolina, and southwestern Virginia to a population of over nine million people. TVA also manages the Tennessee River, its tributaries, and certain shorelines to provide, among other things, year-round navigation, flood damage reduction, and affordable and reliable electricity. Consistent with these primary purposes, TVA also manages the river system and public lands to provide recreational opportunities, adequate water supply, improved water quality, cultural and natural resource protection, and economic development. The power program has historically been separate and distinct from the stewardship programs. It is required to be self-supporting from power revenues and proceeds from power financings, such as proceeds from the issuance of bonds, notes, or other evidences of indebtedness ("Bonds") . Although TVA does not currently receive congressional appropriations, it is required to make annual payments to the United States Department of the Treasury ("U.S. Treasury") as a return on the government's appropriation investment in TVA's power facilities (the "Power Program Appropriation Investment") . In the 1998 Energy and Water Development Appropriations Act, Congress directed TVA to fund essential stewardship activities related to its management of the Tennessee River system and nonpower or stewardship properties with power revenues in the event that there were insufficient appropriations or other available funds to pay for such activities in any fiscal year. Congress has not provided any appropriations to TVA to fund such activities since 1999. Consequently, during 2000, TVA began paying for essential stewardship activities primarily with power revenues, with the remainder funded with user fees and other forms of revenues derived in connection with those activities. The activities related to stewardship properties do not meet the criteria of an operating segment under accounting principles generally accepted in the United States of America ("GAAP") . Accordingly, these assets and properties are included as part of the power program, TVA's only operating segment. Power rates are established by the TVA Board of Directors (the "TVA Board") as authorized by the Tennessee Valley Authority Act of 1933, as amended, 16 U.S.C. §§ 831-831ee (the “TVA Act”). The TVA Act requires TVA to charge rates for power that will produce gross revenues sufficient to provide funds for operation, maintenance, and administration of its power system; payments to states and counties in lieu of taxes ("tax equivalents") ; debt service on outstanding indebtedness; payments to the U.S. Treasury in repayment of and as a return on the Power Program Appropriation Investment; and such additional margin as the TVA Board may consider desirable for investment in power system assets, retirement of outstanding Bonds in advance of maturity, additional reduction of the Power Program Appropriation Investment, and other purposes connected with TVA's power business. In setting TVA's rates, the TVA Board is charged by the TVA Act to have due regard for the primary objectives of the TVA Act, including the objective that power shall be sold at rates as low as are feasible. Rates set by the TVA Board are not subject to review or approval by any state or other federal regulatory body. Fiscal Year TVA's fiscal year ends September 30. Years ( 2017 , 2016 , etc.) refer to TVA's fiscal years unless they are preceded by “CY,” in which case the references are to calendar years. Cost-Based Regulation Since the TVA Board is authorized by the TVA Act to set rates for power sold to its customers, TVA is self-regulated. Additionally, TVA's regulated rates are designed to recover its costs. Based on current projections, TVA believes that rates, set at levels that will recover TVA's costs, can be charged and collected. As a result of these factors, TVA records certain assets and liabilities that result from the regulated ratemaking process that would not be recorded under GAAP for non-regulated entities. Regulatory assets generally represent incurred costs that have been deferred because such costs are probable of future recovery in customer rates. Regulatory liabilities generally represent obligations to make refunds to customers for previous collections for costs that are not likely to be incurred or deferral of gains that will be credited to customers in future periods. TVA assesses whether the regulatory assets are probable of future recovery by considering factors such as applicable regulatory changes, potential legislation, and changes in technology. Based on these assessments, TVA believes the existing regulatory assets are probable of recovery. This determination reflects the current regulatory and political environment and is subject to change in the future. If future recovery of regulatory assets ceases to be probable, or any of the other factors described above cease to be applicable, TVA would no longer be considered to be a regulated entity and would be required to write off these costs. All regulatory asset write offs would be required to be recognized in earnings in the period in which future recovery ceases to be probable. Basis of Presentation The accompanying consolidated financial statements, which have been prepared in accordance with GAAP, include the accounts of TVA, wholly-owned direct subsidiaries, and variable interest entities ("VIE") of which TVA is the primary beneficiary. See Note 9 and Note 10 . Intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of financial statements requires TVA to estimate the effects of various matters that are inherently uncertain as of the date of the consolidated financial statements. Although the consolidated financial statements are prepared in conformity with GAAP, TVA is required to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities, and the amounts of revenues and expenses reported during the reporting period. Each of these estimates varies in regard to the level of judgment involved and its potential impact on TVA's financial results. Estimates are considered critical either when a different estimate could have reasonably been used, or where changes in the estimate are reasonably likely to occur from period to period, and such use or change would materially impact TVA's financial condition, results of operations, or cash flows. Reclassifications Amounts previously presented in Cash flows from operating activities as Insurance recoveries of $ 7 million and $ 63 million for the years ended September 30, 2016 and 2015 , respectively, are currently reported in Other, net. Cash and Cash Equivalents Cash includes cash on hand and non-interest bearing cash and deposit accounts. All highly liquid investments with original maturities of three months or less are considered cash equivalents. Allowance for Uncollectible Accounts The allowance for uncollectible accounts reflects TVA's estimate of probable losses inherent in its accounts and loans receivable balances. TVA determines the allowance based on known accounts, historical experience, and other currently available information including events such as customer bankruptcy and/or a customer failing to fulfill payment arrangements after 90 days . It also reflects TVA's corporate credit department's assessment of the financial condition of customers and the credit quality of the receivables. The allowance for uncollectible accounts was less than $ 1 million and $ 1 million at September 30, 2017 , and 2016 , for accounts receivable. Additionally, loans receivable of $118 million and $ 141 million at September 30, 2017 , and 2016 , respectively, are included in Accounts receivable, net and Other long-term assets, for the current and long-term portions, respectively, and are reported net of allowances for uncollectible accounts of less than $ 1 million and $ 8 million at September 30, 2017 , and 2016 , respectively. Revenues Revenues from power sales are recorded as electricity is delivered to customers. In addition to power sales invoiced and recorded during the month, TVA accrues estimated unbilled revenues for power sales provided to six customers whose billing date occurs prior to the end of the month. Exchange power sales are presented in the accompanying consolidated statements of operations as a component of Sales of electricity. Exchange power sales are sales of excess power after meeting TVA native load and directly served requirements. Native load refers to the customers on whose behalf a company, by statute, franchise, regulatory requirement, or contract, has undertaken an obligation to serve. From time to time TVA transfers fiber optic capacity on TVA’s network to telecommunications service carriers and local power company customers of TVA ("LPCs") . These transactions are structured as indefeasible rights of use ("IRUs") , which are the exclusive right to use a specified amount of fiber optic capacity for a specified term. TVA accounts for the consideration received on transfers of fiber optic capacity for cash and on all of the other elements deliverable under an IRU as revenue ratably over the term of the agreement. TVA does not recognize revenue on any contemporaneous exchanges of its fiber optic capacity for an IRU of fiber optic capacity of the counterparty to the exchange. TVA engages in a wide array of arrangements in addition to power sales. TVA records revenue when it is realized or realizable and earned when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the price or fee is fixed or determinable; and collectability is reasonably assured. Revenues from activities related to TVA’s overall mission are recorded as other operating revenue versus those that are not related to the overall mission, which are recorded in Other income (expense), net. Pre-Commercial Plant Operations As part of the process of completing the construction of a generating unit, the electricity produced is used to serve the demands of the electric system. TVA estimates revenue from such pre-commercial generation based on the guidance provided by Federal Energy Regulatory Commission ("FERC") regulations. Watts Bar Nuclear Plant ("Watts Bar") Unit 2 commenced pre-commercial plant operations on June 3, 2016, and commercial operations began on October 19, 2016. In addition, the Paradise Combined Cycle Plant commenced pre-commercial plant operations on October 10, 2016, and commercial operations began on April 7, 2017. The Allen Combined Cycle Plant began pre-commercial operations on September 9, 2017. Estimated revenue of $ 22 million and $ 18 million related to these projects was capitalized to offset project costs for the years ended September 30, 2017 , and 2016, respectively. TVA also capitalized related fuel costs for these three construction projects of approximately $ 14 million and $ 6 million during the years ended September 30, 2017 , and 2016, respectively. Inventories Certain Fuel, Materials, and Supplies . Materials and supplies inventories are valued using an average unit cost method. A new average cost is computed after each inventory purchase transaction, and inventory issuances are priced at the latest moving weighted average unit cost. Coal, fuel oil, and natural gas inventories are valued using an average cost method. A new weighted average cost is computed monthly, and monthly issues are priced accordingly. Allowance for Inventory Obsolescence . TVA reviews material and supplies inventories by category and usage on a periodic basis. Each category is assigned a probability of becoming obsolete based on the type of material and historical usage data. Based on the estimated value of the inventory, TVA adjusts its allowance for inventory obsolescence. Emission Allowances . TVA has emission allowances for sulfur dioxide ("SO 2 ") and nitrogen oxides ("NO x ") which are accounted for as inventory. The average cost of allowances used each month is charged to operating expense based on tons of SO 2 and NO x emitted during the respective compliance periods. Allowances granted to TVA by the Environmental Protection Agency ("EPA") are recorded at zero cost. Renewable Energy Credits. TVA accounts for Renewable Energy Credits ("RECs") using the specific identification cost method. RECs that are acquired through power purchases are recorded as inventory and charged to purchased power expense when the RECs are subsequently used or sold. TVA assigns a value to the RECs at the inception of power purchase arrangement using a relative fair value approach. RECs created through TVA-owned asset generation are recorded at zero cost. Property, Plant, and Equipment, and Depreciation Property, Plant, and Equipment. Additions to plant are recorded at cost, which includes direct and indirect costs and may include allowance for funds used during construction ("AFUDC") , if eligible. The cost of current repairs and minor replacements is charged to operating expense. Nuclear fuel inventories, which are included in Property, plant, and equipment, are valued using the average cost method for raw materials and the specific identification method for nuclear fuel in a reactor. Amortization of nuclear fuel in a reactor is calculated on a units-of-production basis and is included in fuel expense. Depreciation. TVA accounts for depreciation of its properties using the composite depreciation convention of accounting. Accordingly, the original cost of property retired is charged to accumulated depreciation. Depreciation is generally computed on a straight-line basis over the estimated service lives of the various classes of assets. Depreciation rates are determined based on an external depreciation study. TVA concluded and implemented a new depreciation study effective October 1, 2016, resulting in approximately $ 224 million less depreciation expense in 2017 compared to the prior year. This study will be updated at least every five years. Depreciation expense for the years ended September 30, 2017 , 2016 , and 2015 was $ 1.3 billion , $ 1.4 billion , and $ 1.7 billion , respectively. Depreciation expense expressed as a percentage of the average annual depreciable completed plant was 2.49 percent for 2017 , 2.97 percent for 2016 , and 3.71 percent for 2015 . Average depreciation rates by asset class are as follows: Property, Plant, and Equipment Depreciation Rates At September 30 (percent) 2017 2016 2015 Asset Class Nuclear 2.66 2.37 2.81 Coal-fired 2.33 3.50 5.50 Hydroelectric 1.58 1.29 1.30 Gas and oil-fired 3.27 3.09 3.18 Transmission 1.34 2.80 2.78 Other 6.12 8.97 8.65 Nuclear . In September 2015, the Nuclear Regulatory Commission ("NRC") approved renewed licenses for Sequoyah Nuclear Plant ("Sequoyah") Units 1 and 2, which allow both units to operate for an additional 20 years, and TVA adjusted prospectively the Sequoyah depreciation rate. Coal-Fired . In April 2011, TVA entered into two substantively similar agreements, one with the EPA and the other with Alabama, Kentucky, North Carolina, Tennessee, and three environmental advocacy groups (collectively, the "Environmental Agreements”). See Note 21 — Legal Proceedings — Environmental Agreements . Under the Environmental Agreements, TVA committed to retire 18 coal-fired units on a phased schedule, among other things. Since its November 2013 meeting, the TVA Board has approved the retirement of certain coal-fired units. Units subsequently retired include: Widows Creek Fossil Plant ("Widows Creek") Units 7 and 8 on September 30, 2015; Colbert Fossil Plant ("Colbert") Units 1-5 on April 16, 2016; and Paradise Fossil Plant ("Paradise") Units 1 and 2 on April 15, 2017. Other pending TVA Board actions at September 30, 2017 are the retirement of Johnsonville Fossil Plant ("Johnsonville") Units 1-4 by December 31, 2017 and Allen Fossil Plant ("Allen") Units 1-3. The Allen units will be retired upon a completion of a natural gas-fired plant at the Allen location, but no later than December 31, 2018. TVA estimates that the natural gas-fired plant will be completed in the spring of 2018. Depreciation rates are adjusted to reflect current assumptions so that the units will be fully depreciated by the applicable idle dates. As a result of TVA's decision to idle or retire units, TVA recognized $104 million , $ 139 million and $ 383 million in accelerated depreciation expense related to the units during the years ended September 30, 2017 , 2016 , and 2015 , respectively. Accelerated depreciation is based on the rate in effect at the time the decision is made to idle or retire a unit. Capital Lease Agreements. Property, plant, and equipment also includes assets recorded under capital lease agreements. These primarily consist of a natural gas lateral pipeline, power production facilities, water treatment assets, and land of $ 161 million and $ 163 million at September 30, 2017 and 2016 , respectively. Amortization expense related to capital leases is included in Depreciation and amortization in TVA’s statement of operations, excluding leases and other financing obligations where regulatory accounting is applied. See Note 7 — Other Non-Current Regulatory Assets — Deferred Capital Leases and Other Financing Obligations. On April 4, 2016, TVA entered into a letter agreement with Choctaw Generation Limited Partnership, LLLP (“CGLP”) for the reimbursement of certain capital costs and ongoing operating and maintenance costs related to assets recently constructed at the Red Hills lignite-fired power facility. These capital additions were required to comply with new Mercury and Air Toxics Standards. As a result of the new agreement, TVA was required to reassess a related 1997 power purchase and operating agreement (“PPOA”) with CGLP that was previously classified as an executory contract. This reassessment determined that the PPOA contained a capital lease and resulted in TVA recording a capital lease asset at the estimated fair value of $ 76 million with an offsetting capital lease liability included in Accounts payable and accrued liabilities and Other long-term liabilities. Allowance for Funds Used During Construction. TVA may capitalize interest on eligible projects as allowance for funds used during construction ("AFUDC") , based on the average interest rate of TVA’s outstanding debt. The allowance is applicable to construction in progress related to eligible projects with (1) an expected total project cost of $ 1.0 billion or more, and (2) an estimated construction period of at least three years in duration. No AFUDC was capitalized for the year ended September 30, 2017 , subsequent to the completion of Watts Bar Unit 2, which went into service in October 2016. TVA capitalized $235 million and $ 214 million of AFUDC for the years ended September 30, 2016 , and 2015, respectively, related to the Watts Bar Unit 2 project. Software Costs. TVA capitalizes certain costs incurred in connection with developing or obtaining internal-use software. Capitalized software costs are included in Property, plant, and equipment on the consolidated balance sheets and are generally amortized over seven years . At September 30, 2017 , and 2016 , unamortized computer software costs totaled $ 42 million and $ 27 million , respectively. Amortization expense related to capitalized computer software costs was $ 26 million , $43 million , and $38 million for 2017 , 2016 , and 2015 , respectively. Software costs that do not meet capitalization criteria are expensed as incurred. Impairment of Assets. TVA evaluates long-lived assets for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. For long-lived assets, TVA bases its evaluation on impairment indicators such as the nature of the assets, the future economic benefit of the assets, any historical or future profitability measurements, its regulatory approval and ability to set rates at levels that allow for recoverability of the assets, and other external market conditions or factors that may be present. If such impairment indicators are present or other factors exist that indicate that the carrying amount of an asset may not be recoverable, TVA determines whether an impairment has occurred based on an estimate of undiscounted cash flows attributable to the asset as compared with the carrying value of the asset. If an impairment has occurred, the amount of the impairment recognized is measured as the excess of the asset’s carrying value over its fair value. Additionally, TVA regularly evaluates construction projects. If the project is canceled or deemed to have no future economic benefit, the project is written off as an asset impairment or, upon TVA Board approval, reclassified as a regulatory asset. Decommissioning Costs TVA recognizes legal obligations associated with the future retirement of certain tangible long-lived assets. These obligations relate to fossil fuel-fired generating plants, nuclear generating plants, hydroelectric generating plants/dams, transmission structures, and other property-related assets. These other property-related assets include, but are not limited to, easements and coal rights. Activities involved with retiring these assets could include decontamination and demolition of structures, removal and disposal of wastes, and site restoration. Revisions to the estimates of asset retirement obligations ("AROs") are made whenever factors indicate that the timing or amounts of estimated cash flows have changed materially. Any accretion or depreciation expense related to these liabilities and assets is charged to a regulatory asset. See Note 7 — Nuclear Decommissioning Costs and Non-Nuclear Decommissioning Costs and Note 12 . Blended Low-Enriched Uranium Program Under the blended low-enriched uranium ("BLEU") program, TVA, the U.S. Department of Energy ("DOE") , and certain nuclear fuel contractors have entered into agreements providing for the DOE's surplus of enriched uranium to be blended with other uranium down to a level that allows the blended uranium to be fabricated into fuel that can be used in nuclear power plants. Under the terms of an interagency agreement between TVA and the DOE, in exchange for supplying highly enriched uranium materials to the appropriate third-party fuel processors for processing into usable BLEU fuel for TVA, the DOE participates to a degree in the savings generated by TVA’s use of this blended nuclear fuel. TVA accrues an obligation with each BLEU reload batch related to the portion of the ultimate future payments estimated to be attributable to the BLEU fuel currently in use. TVA estimated DOE's portion of the cost savings from the program to be $ 165 million . The last of the BLEU fuel was loaded in 2017. At September 30, 2017 , TVA had paid out approximately $ 164 million for this program, and the obligation recorded was $ 1 million . Investment Funds Investment funds consist primarily of trust funds designated to fund decommissioning requirements (see Note 21 — Contingencies — Decommissioning Costs ), the Supplemental Executive Retirement Plan ("SERP") (see Note 20 — Overview of Plans and Benefits — Supplemental Executive Retirement Plan ), and the Deferred Compensation Plan ("DCP") . The Nuclear Decommissioning Trust ("NDT") holds funds primarily for the ultimate decommissioning of TVA's nuclear power plants. The Asset Retirement Trust ("ART") holds funds primarily for the costs related to the future closure and retirement of TVA's other long-lived assets. The NDT, ART, SERP, and DCP funds are invested in portfolios of securities generally designed to achieve a return in line with overall equity and debt market performance. The NDT, ART, SERP, and DCP funds are all classified as trading. Energy Prepayment Obligations In 2004, TVA and its largest customer, Memphis Light, Gas and Water Division ("MLGW") , entered into an energy prepayment agreement under which MLGW prepaid TVA $ 1.5 billion for the future costs of electricity to be delivered by TVA to MLGW over a period of 180 months . TVA accounted for the prepayment as unearned revenue and is reporting the obligation to deliver power under this arrangement as Energy prepayment obligations and Current portion of energy prepayment obligations on the September 30, 2017 and 2016 Consolidated Balance Sheets. TVA expects to recognize approximately $ 100 million of noncash revenue in each year of the arrangement as electricity is delivered to MLGW based on the ratio of units of kilowatt hours delivered to total units of kilowatt hours under contract. At September 30, 2017 , approximately $ 1.4 billion had been recognized as noncash revenue on a cumulative basis during the life of the agreement, $ 100 million of which was recognized as noncash revenue during each of 2017 , 2016 , and 2015 . Discounts to account for the time value of money, which are recorded as a reduction to electricity sales, amounted to $ 46 million for each of the years ended September 30, 2017 , 2016 , and 2015 . Insurance Although TVA uses private companies to administer its healthcare plans for eligible active and retired employees not covered by Medicare, TVA does not purchase health insurance. Third-party actuarial specialists assist TVA in determining certain liabilities for self-insured claims. TVA recovers the costs of claims through power rates and through adjustments to the participants’ contributions to their benefit plans. These liabilities are included in Other liabilities on the balance sheets. TVA sponsors an Owner Controlled Insurance Program which provides workers' compensation and liability insurance for a select group of contractors performing maintenance, modifications, outage, and new construction activities at TVA facilities. The Federal Employees' Compensation Act ("FECA") governs liability to employees for service-connected injuries. TVA purchases excess workers' compensation insurance above a self-insured retention. In addition to excess workers' compensation insurance, TVA purchases the following types of insurance: • Nuclear liability insurance; nuclear property, decommissioning, and decontamination insurance; and nuclear accidental outage insurance. See Note 21 — Contingencies — Nuclear Insurance . • Excess liability insurance for aviation, auto, marine, and general liability exposures. • Property insurance for certain conventional (non-nuclear) assets. The insurance policies are subject to the terms and conditions of the specific policy, including deductibles or self-insured retentions. To the extent insurance would not provide either a partial or total recovery of the costs associated with a loss, TVA would have to recover any such costs through other means, including through power rates. Research and Development Costs Research and development costs are expensed when incurred. TVA’s research programs include those related to power delivery technologies, emerging technologies (clean energy, renewables, distributed resources, and energy efficiency), technologies related to generation (fossil fuel, nuclear, and hydroelectric), and environmental technologies. Tax Equivalents The TVA Act requires TVA to make payments to states and counties in which TVA conducts its power operations and in which TVA has acquired power properties previously subject to state and local taxation. The total amount of these payments is five percent of gross revenues from sales of power during the preceding year, excluding sales or deliveries to other federal agencies and off-system sales with other utilities, with a provision for minimum payments under certain circumstances. TVA calculates tax equivalent expense by subtracting the prior year fuel cost-related tax equivalent regulatory asset or liability from the payments made to the states and counties during the current year and adding back the current year fuel cost-related tax equivalent regulatory asset or liability. Fuel cost-related tax equivalent expense is recognized in the same accounting period in which the fuel cost-related revenue is recognized. Maintenance Costs TVA records maintenance costs and repairs related to its property, plant, and equipment in the consolidated statements of operations as they are incurred except for the recording of certain regulatory assets for retirement and removal costs.

Impact of New Accounting Standa

Impact of New Accounting Standards and Interpretations	12 Months Ended
	Sep. 30, 2017
Accounting Changes and Error Corrections [Abstract]
Impact of New Accounting Standards and Interpretations	The following are accounting standard updates issued by the Financial Accounting Standards Board ("FASB") that TVA adopted during 2017 . Consolidation Description This guidance amends the consolidation analysis for VIEs as well as voting interest entities. The standard reduces the number of consolidation models through the elimination of the indefinite deferral for certain entities that was previously allowed and places more emphasis on risk of loss when determining a controlling financial interest. This guidance allows for either a full retrospective or a modified retrospective application. Effective Date for TVA October 1, 2016 Effect on the Financial Statements or Other Significant Matters The adoption of the standard did not materially impact TVA's financial condition, results of operations, or cash flows. Business Combinations Description This guidance clarifies the definition of a business by providing guidelines to determine when a set of assets and activities constitutes a business. The standard says that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of assets and activities is not a business. If this characteristic is not met, the amendments in this update (1) require that to be considered a business, a set must include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create output and (2) remove the evaluation of whether a market participant could replace missing elements. The amendments provide a framework to assist entities in evaluating whether both an input and a substantive process are present. The framework includes two sets of criteria to consider that depend on whether a set of assets and activities has outputs. Although outputs are not required for a set of assets and activities to be a business, outputs generally are a key element of a business; therefore, the FASB has developed more stringent criteria for sets of assets and activities without outputs. This standard also updates the definition of the term output so that the term is consistent with how outputs are described in the new Revenue Recognition guidance. Effective Date for TVA July 1, 2017 Effect on the Financial Statements or Other Significant Matters As a result of adopting the standard, TVA accounted for purchase of the equity interests in two special purpose entities in 2017 as asset acquisitions, and not business combinations, because the entities did not meet the definition of a business under the new accounting standard. Going Concern Description This amendment requires an entity’s management to assess an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued or available to be issued. When management identifies conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern, management should consider whether its plans that are intended to mitigate those relevant conditions or events will alleviate the substantial doubt. If the substantial doubt can be alleviated as a result of consideration of management’s plan, the entity should disclose certain information. If substantial doubt is not alleviated after consideration of management’s plan, an entity should indicate in the footnotes that there is substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the final statements are issued or available to be issued. This assessment must be evaluated every reporting period including interim periods. Effective Date for TVA September 30, 2017 Effect on the Financial Statements or Other Significant Matters As a result of adopting the standard, management has assessed TVA’s ability to continue as a going concern within one year after the financial statements are available to be issued, and there were no conditions or events that raise substantial doubt about TVA’s ability to continue as a going concern. No additional disclosures are necessary. The following accounting standards have been issued, but as of September 30, 2017 , were not effective and had not been adopted by TVA. Derivatives and Hedging - Contingent Put and Call Options in Debt Instruments Description This guidance clarifies the requirements for assessing whether contingent call or put options that can accelerate the payment of principal on debt instruments are clearly and closely related to their debt hosts. An entity performing the assessment under the amendments in this update is required to assess the embedded call or put options solely in accordance with a four-step decision sequence. When the standard becomes effective, it will include interim periods within that fiscal year, and will be required to be applied using a modified retrospective transition. Effective Date for TVA The new standard was effective for TVA’s interim and annual reporting periods beginning October 1, 2017. While early adoption was permitted, TVA did not adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA has two issues of Putable Automatic Rate Reset Securities (“PARRS”) outstanding. After a fixed-rate period of five years, the coupon rate on the PARRS may automatically be reset downward under certain market conditions on an annual basis. The coupon rate reset on the PARRS is based on a calculation. If the coupon rate is going to be reset, holders may request, for a limited period of time, redemption of the PARRS at par value, with repayment of principal on the reset date. This put option is otherwise not available. For both series of PARRS, the coupon rate will reset downward on the reset date if the rate calculated is below the then-current coupon rate on the PARRS. TVA plans to assess whether these contingent put options that can accelerate the payment of principal on the PARRS continue to be clearly and closely related to their debt hosts under the new guidance. TVA plans to assess the put options in accordance with the four-step decision sequence clarified in the guidance. While a preliminary assessment indicates that TVA does not believe the new guidance will impact its current assessment, TVA will continue to evaluate the potential impact of the new guidance on its consolidated balance sheet. Inventory Valuation Description This guidance changes the model used for the subsequent measurement of inventory from the previous lower of cost or market model to the lower of cost or net realizable value. The guidance applies only to inventory valued using methods other than last-in, first out or the retail inventory method (for example, first-in, first-out or average cost). This amendment is intended to simplify the subsequent measurement of inventory. When the standard becomes effective, it includes interim periods within the fiscal year that begins on that date and is required to be applied prospectively. Effective Date for TVA The new standard was effective for TVA’s interim and annual reporting periods beginning October 1, 2017. While early adoption was permitted, TVA did not adopt the standard early. Effect on the Financial Statements or Other Significant Matters The adoption of this standard will not have a material impact on TVA’s financial condition, results of operations, and cash flows. Defined Benefit Costs Description This guidance changes how information about defined benefit costs for pension plans and other post-retirement benefit plans is presented in employer financial statements. The guidance requires employers that present a measure of operating income in their statement of income to include only the service cost component of net periodic pension cost and net periodic postretirement benefit cost in operating expenses (together with other employee compensation costs). The other components of net benefit cost, including amortization of prior service cost/credit and settlement and curtailment effects, are to be included in nonoperating expenses. Additionally, the guidance stipulates that only the service cost component of net benefit cost is eligible for capitalization in assets. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA has evaluated the impact of adopting this guidance, and if the guidance had been effective for TVA for the years ended September 30, 2017, 2016, and 2015, TVA would have reclassified $758 million, $179 million, and $185 million, respectively, of net periodic benefit costs from Operating and maintenance expense to Other income (expense), net on the consolidated statements of operations. Financial Instruments Description This guidance applies to the recognition and measurement of financial assets and liabilities. The standard requires all equity investments to be measured at fair value with changes in the fair value recognized through net income (other than those accounted for under the equity method of accounting or those that result in consolidation of the investee). The standard also amends presentation requirements related to certain changes in the fair value of a liability and eliminates certain disclosure requirements of significant assumptions for financial instruments measured at amortized cost on the balance sheet. Public entities must apply the amendments by means of a cumulative-effect adjustment to the balance sheet as of the beginning of the fiscal year of adoption. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. Early adoption is not permitted unless specific early adoption guidance is applied. TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA currently measures all of its equity investments (other than those that result in the consolidation of the investee) at fair value, with changes in the fair value recognized through net income. The TVA Board has authorized the use of regulatory accounting for changes in fair value of certain equity investments, and as a result, those changes in fair value are deferred as regulatory assets or liabilities. TVA currently discloses significant assumptions around its estimates of fair value for financial instruments carried at amortized cost on its consolidated balance sheet. TVA is currently evaluating the potential impact of the changes in this new guidance on its consolidated financial statements and related disclosures. Revenue Recognition Description In 2014, the FASB issued new guidance related to revenue from contracts with customers. The guidance, including subsequent amendments, replaces the existing accounting standard and industry specific guidance for revenue recognition with a five-step model for recognizing and measuring revenue from contracts with customers. The underlying principle of the guidance is to recognize revenue to depict the transfer of goods or services to customers at the amount expected to be collected. The objective of the new standard is to provide a single, comprehensive revenue recognition model for all contracts with customers to improve comparability within industries and across industries. The new standard also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenue and the related cash flows arising from contracts with customers. At adoption, companies must also select a transition method to be applied either retrospectively to each prior reporting period presented or retrospectively with a cumulative effect adjustment to retained earnings at the date of initial adoption. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. While early adoption is permitted, TVA will not adopt the standard early. Effect on the Financial Statements or Other Significant Matters While TVA expects most of its revenue to be included in the scope of the new guidance, it has not completed its evaluation of all such arrangements. TVA’s current efforts in evaluating the impact of the standard are focused on scoping of revenue streams and evaluating contracts with LPCs, which represent the majority of TVA's revenues. TVA is also conducting ongoing evaluations of how the new guidance impacts other transactions, including sales to directly served industrial customers, sales to federal agencies, purchase power agreements, fuel cost adjustments, and other revenue streams. In addition, the power and utilities industry is currently addressing certain industry-specific issues which have not yet been finalized. As the ultimate impact of the new standard has not yet been determined, TVA has not yet elected its transition method. Statement of Cash Flows Description This guidance requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This guidance does not provide a definition of restricted cash or restricted cash equivalents. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA does not expect the adoption of this standard to have a material impact on TVA’s financial condition, results of operations, or cash flows. Derivatives and Hedging - Improvements to Accounting for Hedging Activities Description This guidance better aligns an entity's risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. To meet that objective, the amendments expand and refine hedge accounting for both nonfinancial and financial risk components and align the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2019. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA does not expect the adoption of this standard to have a material impact on TVA’s financial condition, results of operations, or cash flows. Lease Accounting Description This guidance changes the provisions of recognition in both the lessee and lessor accounting models. The standard requires entities that lease assets — referred to as "lessees" — to recognize on the balance sheet the assets and liabilities for the rights and obligations created by leases with terms of more than 12 months. The recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a finance (similar to current capital leases) or operating lease. However, unlike current lease accounting rules — which require only capital leases to be recognized on the balance sheet — the new standard will require both types of leases to be recognized on the balance sheet. Operating leases will result in straight-line expense, while finance leases will result in recognition of interest on the lease liability separate from amortization expense. The accounting for the owner of the assets leased by the lessee — also known as lessor accounting — will remain largely unchanged from current lease accounting rules. The standard allows for certain practical expedients to be elected related to lease term determination, separation of lease and non-lease elements, reassessment of existing leases, and short-term leases. When the standard becomes effective, it will include interim periods within that fiscal year and will be required to be applied using a modified retrospective transition. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2019. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA is currently evaluating the potential impact of these changes on its consolidated financial statements and related disclosures. TVA expects the new standard to impact financial position as adoption is expected to increase the amount of assets and liabilities recognized on TVA’s consolidated balance sheets. TVA expects the new standard to have no material impact on results of operations or cash flows. TVA plans to elect certain of the practical expedients included in the new standard. TVA is also continuing to monitor unresolved industry implementation issues, including items related to renewables and purchased power agreements, easements, and rights-of-way, and will analyze the related impacts to lease accounting.

Accounts Receivable, Net

Accounts Receivable, Net	12 Months Ended
	Sep. 30, 2017
Accounts Receivable, Net [Abstract]
Accounts Receivable, Net	Accounts receivable primarily consist of amounts due from customers for power sales. The table below summarizes the types and amounts of TVA’s accounts receivable: Accounts Receivable, Net At September 30 2017 2016 Power receivables $ 1,441 $ 1,637 Other receivables 129 111 Allowance for uncollectible accounts (1 ) (1 ) Accounts receivable, net $ 1,569 $ 1,747

Inventories, Net

Inventories, Net	12 Months Ended
	Sep. 30, 2017
Inventory, Net [Abstract]
Inventories, Net	The table below summarizes the types and amounts of TVA’s inventories: Inventories, Net At September 30 2017 2016 Materials and supplies inventory $ 734 $ 673 Fuel inventory 355 345 RECs/emission allowance inventory, net 15 14 Allowance for inventory obsolescence (39 ) (39 ) Inventories, net $ 1,065 $ 993

Net Completed Plant

Net Completed Plant	12 Months Ended
	Sep. 30, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Net Completed Plant	Net completed plant consisted of the following: Net Completed Plant At September 30 2017 2016 Cost Accumulated Depreciation Net Cost Accumulated Depreciation Net Coal-fired $ 15,937 $ 10,791 $ 5,146 $ 15,587 $ 10,473 $ 5,114 Gas and oil-fired 4,995 1,359 3,636 3,918 1,267 2,651 Nuclear 25,010 10,834 14,176 19,280 10,422 8,858 Transmission 7,264 3,039 4,225 7,061 2,975 4,086 Hydroelectric 3,015 967 2,048 2,891 932 1,959 Other electrical plant 1,756 1,008 748 1,857 1,126 731 57,977 27,998 29,979 50,594 27,195 23,399 Multipurpose dams 928 387 541 928 379 549 Other stewardship 42 19 23 42 18 24 970 406 564 970 397 573 Total $ 58,947 $ 28,404 $ 30,543 $ 51,564 $ 27,592 $ 23,972

Other Long-Term Assets

Other Long-Term Assets	12 Months Ended
	Sep. 30, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Other Long-Term Assets	Other Long-Term Assets The table below summarizes the types and amounts of TVA’s other long-term assets: Other Long-Term Assets At September 30 2017 2016 EnergyRight ® receivables $ 100 $ 112 Loans and other long-term receivables, net 115 136 Commodity contract derivative assets 2 3 Prepaid capacity payments 34 42 Other 72 93 Total other long-term assets $ 323 $ 386 In association with the EnergyRight ® Solutions program, LPCs offer financing to end-use customers for the purchase of energy-efficient equipment. Depending on the nature of the energy-efficiency project, loans may have a maximum term of five years or ten years . TVA purchases the resulting loans receivable from its LPCs. The loans receivable are then transferred to a third-party bank with which TVA has agreed to repay in full any loan receivable that has been in default for 180 days or more or that TVA has determined is uncollectible. Given this continuing involvement, TVA accounts for the transfer of the loans receivable as secured borrowings. The current and long-term portions of the loans receivable are reported in Accounts receivable, net and Other long-term assets, respectively, on TVA’s consolidated balance sheets. As of September 30, 2017 and September 30, 2016 , the carrying amount of the loans receivable, net of discount, reported in Accounts receivable, net was approximately $ 25 million and $ 29 million , respectively. See Note 11 for information regarding the associated financing obligation.

Regulatory Assets and Liabiliti

Regulatory Assets and Liabilities	12 Months Ended
	Sep. 30, 2017
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets and Liabilities	Regulatory assets generally represent incurred costs that have been deferred because such costs are probable of future recovery in customer rates. Regulatory liabilities generally represent obligations to make refunds to customers for previous collections for costs that are not likely to be incurred or deferral of gains that will be credited to customers in future periods. Components of regulatory assets and regulatory liabilities are summarized in the table below. Regulatory Assets and Liabilities At September 30 2017 2016 Current regulatory assets Deferred nuclear generating units $ 237 $ 237 Unrealized losses on interest rate derivatives 93 — Unrealized losses on commodity derivatives 68 122 Environmental agreements 2 34 Environmental cleanup costs – Kingston ash spill 44 42 Fuel cost adjustment receivable 1 98 Other current regulatory assets 2 3 Total current regulatory assets 447 536 Non-current regulatory assets Deferred pension costs and other post-retirement benefits costs 4,009 5,385 Unrealized losses on interest rate derivatives 982 1,547 Gallatin coal combustion residual facilities 899 — Nuclear decommissioning costs 823 938 Environmental cleanup costs - Kingston ash spill 263 299 Non-nuclear decommissioning costs 703 819 Deferred nuclear generating units 759 850 Environmental agreements 13 18 Unrealized losses on commodity derivatives 9 56 Other non-current regulatory assets 238 252 Total non-current regulatory assets 8,698 10,164 Total regulatory assets $ 9,145 $ 10,700 Current regulatory liabilities Fuel cost adjustment tax equivalents $ 153 $ 148 Fuel cost adjustment 2 — Unrealized gains on commodity derivatives 8 6 Total current regulatory liabilities 163 154 Non-current regulatory liabilities Deferred other post-retirement benefits cost 23 — Unrealized gains on commodity derivatives 2 3 Total non-current regulatory liabilities 25 3 Total regulatory liabilities $ 188 $ 157 Deferred Pension Costs and Other Post-retirement Benefit Costs . TVA measures its benefit obligations related to pension and other post-retirement benefit ("OPEB") costs at each year-end balance sheet date. TVA recognizes the funded status of the plans on TVA's consolidated balance sheets which in an unregulated environment would result in a corresponding offset to accumulated other comprehensive income (loss) ("AOCI") . “Incurred cost” is a cost arising from cash paid out or an obligation to pay for an acquired asset or service, and a loss from any cause that has been sustained and for which payment has been or must be made. In the cases of pension and OPEB costs, the unfunded obligation represents a projected liability to the employee for services rendered, and thus it meets the definition of an incurred cost. Therefore, amounts that otherwise would be charged to AOCI for these costs are recorded as a regulatory asset or liability since TVA has historically recovered pension and OPEB expense in rates. Through historical and current year expense included in ratemaking, the TVA Board has demonstrated the ability and intent to include pension and OPEB costs in allowable costs and in rates for ratemaking purposes. As a result, it is probable that future revenue will result from inclusion of the pension and OPEB regulatory assets in allowable costs for ratemaking purposes. These regulatory assets are classified as long-term, which is consistent with the pension and post-retirement liabilities, and are not amortized to the consolidated statements of operations over a specified recovery period. They are adjusted either upward or downward each year in conjunction with the adjustments to the unfunded pension liability, as calculated by the actuaries. Ultimately this regulatory asset will be recognized in the consolidated statements of operations in the form of pension expense as the actuarial liability is eliminated in future periods. See Note 20 — Obligations and Funded Status . Additionally on October 1, 2014, TVA began recognizing pension costs as regulatory assets to the extent that the amount calculated under GAAP as pension expense differs from the amount TVA contributes to the pension plan. Unrealized Losses on Interest Rate Derivatives . TVA uses regulatory accounting treatment to defer the unrealized gains and losses on certain interest rate derivative contracts. When amounts in these contracts are realized, the resulting gains or losses are included in the ratemaking formula. The unrealized losses on these interest rate derivatives are recorded on TVA’s consolidated balance sheets as current and non-current regulatory assets, and the related realized gains or losses, if any, are recorded in TVA’s consolidated statements of operations. Unrealized gains and losses on interest rate derivatives with a maturity of less than one year are included as a current regulatory asset or liability on TVA's consolidated balance sheets. Gallatin Coal Combustion Residual Facilities. In August 2017, TVA began using regulatory accounting treatment to defer expected future costs related to Gallatin Fossil Plant ("Gallatin") coal combustion residuals ("CCR") . The TVA Board approved a plan to amortize these costs over the anticipated duration of the Gallatin CCR project (excluding post-closure care), beginning October 1, 2018 as amounts are included in rates or paid out. Accordingly, there are no amounts included as a current regulatory asset on TVA's consolidated balance sheets. See Note 8 . Unrealized Gains (Losses) on Commodity Derivatives. Unrealized gains (losses) on coal purchase contracts, included as part of unrealized gains (losses) on commodity derivatives, relate to the mark-to-market ("MtM") valuation of coal purchase contracts. These contracts qualify as derivative contracts but do not qualify for cash flow hedge accounting treatment. As a result, TVA recognizes the changes in the market value of these derivative contracts as a regulatory liability or asset. This treatment reflects TVA’s ability and intent to recover the cost of these commodity contracts on a settlement basis for ratemaking purposes through the fuel cost adjustment. TVA recognizes the actual cost of fuel received under these contracts in fuel expense at the time the fuel is used to generate electricity. These contracts expire at various times through 2019. Unrealized gains and losses on contracts with a maturity of less than one year are included as a current regulatory asset or liability on TVA's consolidated balance sheets. See Note 15 . Deferred gains and losses relating to TVA’s Financial Trading Program ("FTP") represent net unrealized gains and losses on swaps, which are also included as part of unrealized gains (losses) on commodity derivatives. Although currently suspended, the FTP was used to reduce TVA’s economic risk exposure associated with purchases and sales of commodities used in electricity generation, purchases, and sales. TVA defers all FTP MtM unrealized gains or losses as regulatory liabilities or assets, respectively, and records realized gains or losses in fuel and purchased power expense to match the delivery period of the underlying commodity product. Net unrealized losses at September 30, 2017 , and September 30, 2016 , were approximately $ 5 million and $ 39 million , respectively. This accounting treatment reflects TVA’s ability and intent to recover the cost of these commodity contracts in future periods through the fuel cost adjustment. The current regulatory asset/liability for net unrealized gains and losses, included as part of the commodity derivatives, represents deferred gains and losses from contracts with a maturity of less than one year. Deferred Nuclear Generating Units . In November 2013, the TVA Board approved the treatment of all amounts currently included in Construction in progress related to Bellefonte Nuclear Plant ("Bellefonte") as a regulatory asset. Additionally, the TVA Board approved combining (1) the amounts related to Bellefonte previously included in Construction in progress, (2) the $ 619 million in Regulatory asset-Construction costs, and (3) the remaining amounts included in Regulatory asset-Deferred nuclear generating units into a single regulatory asset titled Deferred nuclear generating units. Furthermore, in August 2016 the TVA Board approved the recognition of a regulatory asset for (1) all costs attributable to (a) the expected disposition of Bellefonte assets, including preparing or preserving the Bellefonte site, and (b) associated liabilities directly related to those assets, (2) any related future operating and project costs until the assets are sold, (3) the amount by which the book value of Bellefonte exceeds its fair market value less cost to sell, if any, (4) any subsequent gains and losses resulting from the disposition or impairment of Bellefonte, and (5) any costs attributable to the steam generators for Bellefonte until TVA disposes of the generators. Deferred costs related to Bellefonte totaled $ 1.0 billion at September 30, 2017 . Such amounts have been classified as a Regulatory asset in the September 30, 2017 Consolidated Balance Sheet. The TVA Board approved the recovery of this asset in future rates at an amount of $ 237 million per year until fully recovered. The amount to be amortized over the next year is included as a current regulatory asset on TVA's consolidated balance sheets. On November 14, 2016, following a public auction, TVA entered into a contract to sell substantially all of the Bellefonte Nuclear Plant ("Bellefonte") site for $ 111 million . The net book value of the Bellefonte assets to be sold and the related asset retirement costs are collectively $ 121 million and are included in Regulatory asset — Deferred nuclear generating units on TVA’s Consolidated Balance Sheet at September 30, 2017 , as approved by the TVA Board. TVA received $ 22 million on November 14, 2016, which is recorded as a long-term liability on TVA’s Consolidated Balance Sheet at September 30, 2017 , with the remaining $ 89 million due at closing. The buyer has up to two years from November 14, 2016, to close on the property. The closing is subject to, among other conditions, a determination by TVA's Chief Executive Officer that potential environmental impacts have been appropriately addressed or are acceptable. Proceeds received from the sale will be recorded as a reduction to the regulatory asset upon closing and will reduce amounts collected in future rates. Environmental Agreements. In conjunction with the Environmental Agreements (see Note 21 — Legal Proceedings — Environmental Agreements ), TVA recorded certain liabilities totaling $ 360 million ($ 290 million investment in energy efficiency projects, demand response projects, renewable energy projects, and other TVA projects; $ 60 million to be provided to Alabama, Kentucky, North Carolina, and Tennessee to fund environmental projects with preference for projects in the Tennessee River watershed; and $ 10 million in civil penalties). The TVA Board determined that these costs would be collected in customer rates in the future, and, accordingly, the amounts were deferred as a regulatory asset. Through the end of 2017 , $ 275 million has been paid with respect to environmental projects, $ 60 million has been paid to Alabama, Kentucky, North Carolina, and Tennessee, and $ 10 million has been paid with respect to civil penalties. The remaining deferred amounts will be charged to expense and recovered in rates over future periods as payments are made through 2027. Environmental Cleanup Costs – Kingston Ash Spill . In August 2009, TVA began using regulatory accounting treatment to defer all actual costs incurred and expected future costs related to the Kingston Fossil Plant ("Kingston") ash spill. The TVA Board approved a plan to amortize these costs over 15 years beginning October 1, 2009. Insurance proceeds are recorded as reductions to the regulatory asset and will reduce amounts collected in future rates. Amounts included as a current regulatory asset on TVA's consolidated balance sheets represent the amount to be amortized in the next 12 months . Fuel Cost Adjustment Receivable. The fuel cost adjustment provides a mechanism to alter rates monthly to reflect changing fuel and purchased power costs, including realized gains and losses relating to transactions under TVA’s FTP. There is typically a lag between the occurrence of a change in fuel and purchased power costs and the reflection of the change in fuel rates. Balances in the fuel cost adjustment regulatory accounts represent over-collected or under-collected revenues that offset fuel and purchased power costs, and the fuel rate is designed to recover or refund the balance in less than one year. Nuclear Decommissioning Costs. Nuclear decommissioning costs include: (1) certain deferred charges related to the future closure and decommissioning of TVA’s nuclear generating units under the NRC requirements, (2) recognition of changes in the liability, (3) recognition of changes in the value of TVA's NDT, and (4) certain other deferred charges under the accounting rules for AROs. These future costs will be funded through a combination of the NDT, future earnings on the NDT, and, if necessary, additional TVA cash contributions to the NDT and future earnings thereon. See Note 1 — Investment Funds. There is not a specified recovery period; therefore, the regulatory asset is classified as long-term consistent with the NDT investments and ARO liability. Non-Nuclear Decommissioning Costs. Non-nuclear decommissioning costs include: (1) certain deferred charges related to the future closure and decommissioning of TVA’s non-nuclear long-lived assets, (2) recognition of changes in the liability, (3) recognition of changes in the value of TVA’s ART, and (4) certain other deferred charges under the accounting rules for AROs. TVA has established the ART to more effectively segregate, manage, and invest funds to help meet future non-nuclear AROs. The funds from the ART may be used, among other things, to pay the costs related to the future closure and retirement of non-nuclear long-lived assets under various legal requirements. These future costs can be funded through a combination of investment funds already set aside in the ART, future earnings on those investment funds, and future cash contributions to the ART and future earnings thereon. For 2018, TVA will recover in rates a portion of its estimated current year non-nuclear decommissioning costs and contributions to the ART. Deferred charges will be recovered in rates based on an analysis of the expected expenditures, contributions, and investment earnings required to recover the decommissioning costs. Other Non-Current Regulatory Assets. Other non-current regulatory assets consist of the following: Deferred Capital Leases and Other Financing Obligations . Deferred capital lease and other financing asset costs represent the difference between the FERC's Uniform System of Accounts Prescribed for Public Utilities and Licensees Subject to the Provisions of the Federal Power Act ("Uniform System of Accounts") model balances and the balances under GAAP guidance. Under the Uniform System of Accounts, TVA recognizes the initial capital lease and other financing asset and liability at inception of the lease or other obligation; however, the annual expense under the Uniform System of Accounts is equal to the annual lease or other financing obligation payments, which differs from GAAP treatment. This practice results in TVA’s asset balances being higher than they otherwise would have been under GAAP, with the difference representing a regulatory asset related to each capital lease or other financing obligation. These costs will be amortized over the respective lease or other financing obligation terms as lease or other financing obligation payments are made. Debt Reacquisition Costs . Reacquisition expenses, call premiums, and other related costs, such as unamortized debt issue costs associated with redeemed Bond issues, are deferred and amortized (accreted) on a straight-line basis over the weighted average life of TVA’s debt portfolio. Nuclear Training Costs . As a result of refurbishing and restarting Browns Ferry Unit 1 in 2007 and the construction and startup of Watts Bar Unit 2, nuclear training costs associated with these units have been deferred as a regulatory asset and will be amortized over a cost recovery period equivalent to the expected useful life of the operating nuclear units. Retirement Removal Costs . Retirement removal costs, net of salvage, that are not legally required are recognized as a regulatory asset. Prior to 2017, net removal costs were amortized over a recovery period consistent with the depreciable lives of related assets under the most recent depreciation study. In 2017 and thereafter, net removal costs are amortized over a one-year period subsequent to completion of the removal activities. Fuel Cost Adjustment Tax Equivalents. The fuel cost adjustment includes a provision related to the current funding of the future payments TVA will make. As TVA records the fuel cost adjustment, the percent of the calculation that relates to a future asset or liability for tax equivalent payments is recorded as a current regulatory asset or liability and paid or refunded in the following year. Accelerated Amortization of Certain Regulatory Assets. In August 2017, the TVA Board authorized management to accelerate amortization of certain regulatory assets to the extent that actual net income in 2018 exceeds the budgeted amount, up to the aggregate amount of those certain regulatory assets. Assets included in this Board action include: deferred nuclear generating units, environmental cleanup costs related to the Kingston ash spill, and nuclear training costs related to the refurbishing and restarting of Browns Ferry Unit 1 and the construction and startup of Watts Bar Unit 2. The amount of additional amortization expense, if any, will be determined at the end of the year upon calculation of the excess (if any) and recorded as an adjustment as of the end of the year.

Asset Acquisitions and Business

Asset Acquisitions and Business Combinations (Notes)	12 Months Ended
	Sep. 30, 2017
Business Combinations and Settlement of Preexisting Relationships [Abstract]
Asset Acquisitions and Business Combinations Disclosure	Asset Acquisition On September 20, 2017, TVA acquired 100 percent of the equity interests in two SPEs designed to administer rent payments TVA makes under certain of its lease/leaseback arrangements. Each entity holds residual interests in four of TVA's peaking combustion turbine units ("CTs") . TVA acquired these entities in order to reacquire the residual interests in eight CTs it had previously granted in the lease/leaseback arrangements. TVA acquired the entities for total cash consideration of $36 million . The fair value of the assets acquired consisted of $110 million of reacquired rights, and the fair value of liabilities assumed consisted of $74 million in notes payable. Reacquired rights are an intangible asset included in TVA's Completed plant balance and are amortized over the estimated useful life of the underlying CTs. Notes payable assumed in the transaction are included in TVA's Long-term debt and require TVA to make semi-annual payments through May 2020. TVA recognized less than $1 million of amortization expense, related to reacquired rights, within TVA’s consolidated statements of operations. Transaction costs were not material. TVA determined that its lease/leaseback obligations were preexisting relationships that were effectively settled in the asset acquisitions. TVA settled the preexisting relationships separately from the asset acquisitions, resulting in a loss on extinguishment of the obligations of $3 million . The carrying value of lease/leaseback obligations effectively settled was $71 million , including accrued interest, and the reacquisition price was $74 million , paid in cash, at the acquisition date. Business Combination On July 20, 2016, TVA acquired 100 percent of the equity interests in Johnsonville Generation, LLC and Gallatin Generation, LLC, two special purpose entities ("SPEs") designed to administer rent payments TVA makes under certain of its lease/leaseback arrangements. The SPEs also each hold residual interests in four of TVA's peaking combustion turbine units. TVA acquired these businesses in order to exercise its rights of first refusal under certain of its lease/leaseback arrangements and to reacquire the residual interests in eight combustion turbine units it had previously granted in the lease/leaseback arrangements. TVA acquired the entities for total cash consideration of $ 33 million . The fair value of the assets acquired consisted of $ 111 million of reacquired rights, and the fair value of liabilities assumed consisted of $ 78 million in notes payable. Reacquired rights are an intangible asset included in TVA's completed plant balance and are amortized over the estimated useful life of the underlying combustion turbine units. Notes payable assumed in the transaction are included in TVA's long-term debt and are subject to semi-annual payments through March 2019. The entities acquired by TVA had $ 1 million of amortization expense, related to reacquired rights, and this expense is included within TVA’s consolidated statements of operations. Transaction costs were expensed as incurred and were not material. TVA determined that its lease/leaseback obligations were preexisting relationships that were effectively settled in the business combinations. TVA settled the preexisting relationships separately from the business combinations, resulting in a loss on extinguishment of the obligations of $ 6 million . The carrying value of lease/leaseback obligations effectively settled was $ 72 million , including accrued interest, and the reacquisition price was $ 78 million , paid in cash, at the acquisition date.

Variable Interest Entities

Variable Interest Entities	12 Months Ended
	Sep. 30, 2017
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Variable Interest Entities	A VIE is an entity that either (i) has insufficient equity to permit the entity to finance its activities without additional subordinated financial support or (ii) has equity investors who lack the characteristics of owning a controlling financial interest. When TVA determines that it has a variable interest in a VIE, a qualitative evaluation is performed to assess which interest holders have the power to direct the activities that most significantly impact the economic performance of the entity and have the obligation to absorb losses or receive benefits that could be significant to the entity. The evaluation considers the purpose and design of the business, the risks that the business was designed to create and pass along to other entities, the activities of the business that can be directed and which party can direct them, and the expected relative impact of those activities on the economic performance of the business through its life. TVA has the power to direct the activities of an entity when it has the ability to make key operating and financing decisions, including, but not limited to, capital investment and the issuance of debt. Based on the evaluation of these criteria, TVA has determined it is the primary beneficiary of certain entities and as such is required to account for the VIEs on a consolidated basis. John Sevier VIEs In 2012, TVA entered into a $ 1.0 billion construction management agreement and lease financing arrangement with John Sevier Combined Cycle Generation LLC ("JSCCG") for the completion and lease by TVA of the John Sevier Combined Cycle Facility ("John Sevier CCF") . JSCCG is a special single-purpose limited liability company formed in January 2012 to finance the John Sevier CCF through a $ 900 million secured note issuance (the “JSCCG notes”) and the issuance of $ 100 million of membership interests subject to mandatory redemption. The membership interests were purchased by John Sevier Holdco LLC ("Holdco") . Holdco is a special single-purpose entity, also formed in January 2012, established to acquire and hold the membership interests in JSCCG. A non-controlling interest in Holdco is held by a third party through nominal membership interests, to which none of the income, expenses, and cash flows are allocated. The membership interests held by Holdco in JSCCG were purchased with proceeds from the issuance of $ 100 million of secured notes (the “Holdco notes") and are subject to mandatory redemption pursuant to a schedule of amortizing, semi-annual payments due each January 15 and July 15, with a final payment due in January 2042. The payment dates for the mandatorily redeemable membership interests are the same as those of the Holdco notes. The sale of the JSCCG notes, the membership interests in JSCCG, and the Holdco notes closed in January 2012. The JSCCG notes are secured by TVA’s lease payments, and the Holdco notes are secured by Holdco's investment in, and amounts receivable from, JSCCG. TVA’s lease payments to JSCCG are equal to and payable on the same dates as JSCCG’s and Holdco’s semi-annual debt service payments. In addition to the lease payments, TVA pays administrative and miscellaneous expenses incurred by JSCCG and Holdco. Certain agreements related to this transaction contain default and acceleration provisions. Due to its participation in the design, business conduct, and credit and financial support of JSCCG and Holdco, TVA has determined that it has a variable interest in each of these entities. Based on its analysis, TVA has concluded that it is the primary beneficiary of JSCCG and Holdco and, as such, is required to account for the VIEs on a consolidated basis. Holdco's membership interests in JSCCG are eliminated in consolidation. Southaven VIE In 2013, TVA entered into a $ 400 million lease financing arrangement with Southaven Combined Cycle Generation LLC ("SCCG") for the lease by TVA of the Southaven Combined Cycle Facility ("Southaven CCF") . SCCG is a special single-purpose limited liability company formed in June 2013 to finance the Southaven CCF through a $ 360 million secured notes issuance (the “SCCG notes”) and the issuance of $ 40 million of membership interests subject to mandatory redemption. The membership interests were purchased by Southaven Holdco LLC ("SHLLC") . SHLLC is a special single-purpose entity, also formed in June 2013, established to acquire and hold the membership interests in SCCG. A non-controlling interest in SHLLC is held by a third party through nominal membership interests, to which none of the income, expenses, and cash flows of SHLLC are allocated. The membership interests held by SHLLC were purchased with proceeds from the issuance of $ 40 million of secured notes (the "SHLLC notes") and are subject to mandatory redemption pursuant to a schedule of amortizing, semi-annual payments due each February 15 and August 15, with a final payment due on August 15, 2033. The payment dates for the mandatorily redeemable membership interests are the same as those of the SHLLC notes, and the payment amounts are sufficient to provide returns on, as well as returns of, capital until the investment has been repaid to SHLLC in full. The rate of return on investment to SHLLC is 7.0 percent , which is reflected as interest expense in the consolidated statements of operations. SHLLC is required to pay a pre-determined portion of the return on investment to Seven States Southaven, LLC ("SSSL") on each lease payment date as agreed in SHLLC's formation documents (the "Seven States Return"). The current and long-term portions of the Membership interests of VIE subject to mandatory redemption are included in Accounts payable and accrued liabilities and Other long-term liabilities, respectively. The payment dates for the mandatorily redeemable membership interests are the same as those of the SHLLC notes. The SCCG notes are secured by TVA’s lease payments, and the SHLLC notes are secured by SHLLC’s investment in, and amounts receivable from, SCCG. TVA’s lease payments to SCCG are payable on the same dates as SCCG’s and SHLLC’s semi-annual debt service payments and are equal to the sum of (i) the amount of SCCG’s semi-annual debt service payments, (ii) the amount of SHLLC’s semi-annual debt service payments, and (iii) the amount of the Seven States Return. In addition to the lease payments, TVA pays administrative and miscellaneous expenses incurred by SCCG and SHLLC. Certain agreements related to this transaction contain default and acceleration provisions. In the event that TVA were to choose to exercise an early buy out feature of the Southaven facility lease, in part or in whole, TVA must pay to SCCG amounts sufficient for SCCG to repay or partially repay on a pro rata basis the membership interests held by SHLLC, including any outstanding investment amount plus accrued but unpaid return. TVA also has the right, at any time and without any early redemption of the other portions of the Southaven facility lease payments due to SCCG, to fully repay SHLLC's investment, upon which repayment SHLLC will transfer the membership interests to a designee of TVA. TVA participated in the design, business conduct, and financial support of SCCG and has determined that it has a direct variable interest in SCCG resulting from risk associated with the value of the Southaven CCF at the end of the lease term. Based on its analysis, TVA has determined that it is the primary beneficiary of SCCG and, as such, is required to account for the VIE on a consolidated basis. Impact on Consolidated Financial Statements The financial statement items attributable to carrying amounts and classifications of JSCCG, Holdco, and SCCG as of September 30, 2017 and 2016 , as reflected in the Consolidated Balance Sheets, are as follows: Summary of Impact of VIEs on Consolidated Balance Sheets At September 30 2017 2016 Current liabilities Accrued interest $ 11 $ 11 Accounts payable and accrued liabilities 2 2 Current maturities of long-term debt of variable interest entities 36 35 Total current liabilities 49 48 Other liabilities Other long-term liabilities 30 33 Long-term debt, net Long-term debt of variable interest entities, net 1,164 1,199 Total liabilities $ 1,243 $ 1,280 Interest expense of $ 59 million , $ 61 million , and $ 63 million related to debt of VIEs and membership interests of variable interest entity subject to mandatory redemption is included in the Consolidated Statements of Operations for the years ended September 30, 2017 , 2016 , and 2015 , respectively. Creditors of the VIEs do not have any recourse to the general credit of TVA. TVA does not have any obligations to provide financial support to the VIEs other than as prescribed in the terms of the agreements related to these transactions.

Other Long-Term Liabilities

Other Long-Term Liabilities	12 Months Ended
	Sep. 30, 2017
Other Liabilities, Noncurrent [Abstract]
Other Long-Term Liabilities	Other Long-Term Liabilities Other long-term liabilities consist primarily of liabilities related to certain derivative agreements as well as liabilities for environmental remediation liabilities, and liabilities under agreements related to compliance with certain environmental regulations. See Note 8 and Note 21 — Legal Proceedings — Environmental Agreements . The table below summarizes the types and amounts of Other long-term liabilities: Other Long-Term Liabilities At September 30 2017 2016 Interest rate swap liabilities $ 1,418 $ 1,938 Gallatin coal combustion residual facilities liability 880 — Capital lease obligations 182 177 Currency swap liabilities 92 162 EnergyRight® financing obligation 115 130 Environmental agreements liability 13 18 Membership interests of VIE subject to mandatory redemption 30 33 Commodity contract derivative liabilities 9 49 Other 316 266 Total other long-term liabilities $ 3,055 $ 2,773 EnergyRight ® Financing Obligation . TVA purchases certain loans receivable from its LPCs in association with the EnergyRight ® Solutions program. The current and long-term portions of the resulting financing obligation are reported in Accounts payable and accrued liabilities and Other long-term liabilities, respectively, on TVA’s consolidated balance sheets. As of September 30, 2017 and 2016 , the carrying amount of the financing obligation reported in Accounts payable and accrued liabilities was approximately $ 29 million and $ 33 million , respectively. See Note 6 for information regarding the associated loans receivable.

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Sep. 30, 2017
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations	During the year ended September 30, 2017 , TVA's total ARO liability increased $ 252 million . To estimate its decommissioning obligation related to its nuclear generating stations, TVA uses a probability-weighted, discounted cash flow model which, on a unit-by-unit basis, considers multiple outcome scenarios that include significant estimations and assumptions. Those assumptions include (1) estimates of the cost of decommissioning, (2) the method of decommissioning and the timing of the related cash flows, (3) the license period of the nuclear plant, considering the probability of license extensions, (4) cost escalation factors, and (5) the credit adjusted risk free rate to measure the obligation at the present value of the future estimated costs. TVA has ascribed probabilities to two different decommissioning methods related to its nuclear decommissioning obligation estimate: the DECON method and the SAFSTOR method. The DECON method requires radioactive contamination to be removed from a site and safely disposed of or decontaminated to a level that permits the site to be released for unrestricted use shortly after it ceases operation. The SAFSTOR method allows nuclear facilities to be placed and maintained in a condition that allows the facilities to be safely stored and subsequently decontaminated to levels that permit release for unrestricted use. TVA bases its nuclear decommissioning estimates on site-specific cost studies. The most recent study was approved and implemented in September 2017. An increase of $250 million was recorded to the nuclear AROs as a result of the updates. Site-specific cost studies are updated for each of TVA’s nuclear units at least every five years . On May 23, 2016, Watts Bar Unit 2 achieved initial criticality. As a result, TVA revised its decommissioning liability estimate for Watts Bar and recorded an increase of $198 million . During 2017, TVA recorded adjustments to non-nuclear ARO liabilities as a result of projects maturing and estimates being refined. This resulted in an increase of $161 million to the non-nuclear AROs. This amount was offset by a decrease of $188 million to non-nuclear AROs due to the reversal of certain Gallatin AROs given that the retirement obligations for the Gallatin ash ponds are now recorded as part of environmental remediation obligations. See Note 8 . During 2016, TVA performed reassessments of its AROs for its non-nuclear plants and other buildings. The reassessments consisted of detailed studies of various TVA sites conducted to identify and update benchmarks and standards used in estimating decommissioning costs. Additionally, TVA management updated its non-nuclear plant closure method assumption from a maintain-in-place method to a plant demolition method. TVA's reassessments and change in its closure method assumption resulted in a net increase of $ 32 million to TVA's liability for existing non-nuclear AROs during the year ended September 30, 2016. Also during 2016, TVA recorded a decrease of $ 54 million to its non-nuclear AROs as a result of changes in estimates related to active decommissioning projects and recorded $15 million of new AROs related to coal ash areas. Further adjustments to TVA's non-nuclear ARO liabilities may be required as projects mature and estimates are refined. Additionally, during the years ended September 30, 2017 and 2016 , both the nuclear and non-nuclear liabilities were increased by periodic accretion, partially offset by settlement projects that were conducted during these periods. The nuclear and non-nuclear accretion amounts were deferred as regulatory assets. During 2017 , 2016 , and 2015, $144 million , $ 144 million , and $ 44 million , respectively, of the related regulatory assets were amortized into expense as these amounts were collected in rates. See Note 7 . TVA maintains investment trusts to help fund its decommissioning obligations. See Note 16 and Note 21 — Contingencies — Decommissioning Costs for a discussion of the trusts' objectives and the current balances of the trusts. Asset Retirement Obligation Activity Nuclear Non-Nuclear Total Balance at September 30, 2015 $ 2,187 $ 1,656 $ 3,843 Settlements — (133 ) (133 ) Change in estimate 198 (22 ) 176 Additional obligations — 15 15 Accretion (recorded to regulatory asset) 107 44 151 Balance at September 30, 2016 $ 2,492 $ 1,560 $ 4,052 (1) Settlements — (123 ) (123 ) Change in estimate 250 161 411 Additional obligations — 1 1 Reclassification of Gallatin projects (2) — (188 ) (188 ) Accretion (recorded to regulatory asset) 117 34 151 Balance at September 30, 2017 $ 2,859 $ 1,445 $ 4,304 (1) Notes (1) The current portions of the ARO liability in the amounts of $ 128 million a nd $ 212 million as of September 30, 2017 and 2016 , respectively, are included in Accounts payable and accrued liabilities. (2) See Note 8 for additional information.

Debt and Other Obligations

Debt and Other Obligations	12 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt and Other Obligations	General The TVA Act authorizes TVA to issue Bonds in an amount not to exceed $ 30.0 billion at any time. At September 30, 2017 , TVA had only two types of Bonds outstanding: power bonds and discount notes. Power bonds have maturities between one and 50 years , and discount notes have maturities of less than one year . Power bonds and discount notes are both issued pursuant to Section 15d of the TVA Act and pursuant to the Basic Tennessee Valley Authority Power Bond Resolution adopted by the TVA Board on October 6, 1960, as amended on September 28, 1976, October 17, 1989, and March 25, 1992 (the "Basic Resolution") . Bonds are not obligations of the United States, and the United States does not guarantee the payments of principal or interest on Bonds. Power bonds and discount notes rank on parity and have first priority of payment from net power proceeds, which are defined as the remainder of TVA’s gross power revenues after deducting the costs of operating, maintaining, and administering its power properties and tax equivalent payments, but before deducting depreciation accruals or other charges representing the amortization of capital expenditures, plus the net proceeds from the sale or other disposition of any power facility or interest therein. TVA considers its scheduled rent payments under its leaseback transactions, as well as its scheduled payments under its lease financing arrangements involving John Sevier CCF and Southaven CCF, as costs of operating, maintaining, and administering its power properties. Costs of operating, maintaining, and administering TVA's power properties have priority over TVA’s payments on the Bonds. Once net power proceeds have been applied to payments on power bonds and discount notes as well as any other Bonds that TVA may issue in the future that rank on parity with or subordinate to power bonds and discount notes, Section 2.3 of the Basic Resolution provides that the remaining net power proceeds shall be used only for (1) minimum payments into the U.S. Treasury required by the TVA Act as repayment of, and as a return on, the Power Program Appropriation Investment, (2) investment in power assets, (3) additional reductions of TVA’s capital obligations, and (4) other lawful purposes related to TVA’s power program. The TVA Act and the Basic Resolution each contain two bond tests: the rate test and the bondholder protection test. Under the rate test, TVA must charge rates for power which will produce gross revenues sufficient to provide funds for, among other things, debt service on outstanding Bonds. As of September 30, 2017 , TVA was in compliance with the rate test. See Note 1 — General. Under the bondholder protection test, TVA must, in successive five -year periods, use an amount of net power proceeds at least equal to the sum of (1) the depreciation accruals and other charges representing the amortization of capital expenditures and (2) the net proceeds from any disposition of power facilities for either the reduction of its capital obligations (including Bonds and the Power Program Appropriation Investment) or investment in power assets. TVA met the bondholder protection test for the five -year period ended September 30, 2015, and must next meet the bondholder protection test for the five -year period ending September 30, 2020 . Secured Debt of VIEs On August 9, 2013, SCCG issued secured notes totaling $ 360 million that bear interest at a rate of 3.846 percent . The SCCG notes require amortizing semi-annual payments on each February 15 and August 15, and mature on August 15, 2033. Also on August 9, 2013, SCCG issued $ 40 million of membership interests subject to mandatory redemption. The proceeds from the secured notes issuance and the issuance of the membership interests was paid to TVA in accordance with the terms of the Southaven head lease. See Note 10 — Southaven VIE . TVA used the proceeds from the transaction primarily to fund the acquisition of the Southaven CCF from SSSL. On January 17, 2012, JSCCG issued secured notes totaling $ 900 million in aggregate principal amount that bear interest at a rate of 4.626 percent . Also on January 17, 2012, Holdco issued secured notes totaling $ 100 million that bear interest at a rate of 7.1 percent . The JSCCG notes and the Holdco notes require amortizing semi-annual payments on each January 15 and July 15, and mature on January 15, 2042. The Holdco notes require a $ 10 million balloon payment upon maturity. See Note 10 — John Sevier VIEs . TVA used the proceeds from the transaction to meet its requirements under the TVA Act. Secured debt of VIEs, including current maturities, outstanding at September 30, 2017 and 2016 totaled approximately $ 1.2 billion each year. Secured Notes On July 20, 2016, TVA acquired two entities, in a business combination, designed to administer rent payments TVA makes under certain of its lease/leaseback arrangements. See Note 9 . On September 27, 2000, the entities issued secured notes totaling $255 million that had an interest rate of 7.299 percent and required amortizing semi-annual payments on each March 15 and September 15 with a maturity date of March 15, 2019. In 2016, TVA assumed these secured notes in the acquisition at a fair value of $78 million . The secured notes of the entities, including current maturities, outstanding at September 30, 2017 , and 2016, totaled approximately $ 48 million and $75 million , respectively, and are included in Notes payable in TVA’s consolidated balance sheets. On September 20, 2017, TVA acquired two entities, in an asset acquisition, designed to administer rent payments TVA makes under certain of its lease/leaseback arrangements. On November 14, 2001, the entities issued secured notes totaling $272 million that had an interest rate of 5.572 percent and required amortizing semi-annual payments on each May 1 and November 1 with a maturity date of May 1, 2020. In 2017, TVA assumed these secured notes in the acquisition at a fair value of $74 million . The secured notes of the entities, including current maturities, outstanding at September 30, 2017, totaled approximately $74 million , and are included in Notes payable in TVA’s consolidated balance sheets. See Note 9 . Short-Term Debt The following table provides information regarding TVA's short-term borrowings: Short-term Borrowings At September 30 2017 2016 2015 Amount outstanding - discount notes $ 1,998 $ 1,407 $ 1,034 Weighted average interest rate - discount notes 1.000 % 0.203 % 0.055 % Put and Call Options Bond issues of $ 408 million held by the public are redeemable in whole or in part, at TVA’s option, on call dates ranging from the present to 2020 and at call prices of 100 percent the principal amount. Ten Bond issues totaling $ 268 million , with maturity dates ranging from 2025 to 2043 , include a “survivor’s option,” which allows for right of redemption upon the death of a beneficial owner in certain specified circumstances. The amount of these Bonds classified as short-term was $46 million , with the remaining balance of $ 222 million classified as long-term, as of September 30, 2017 . These bonds were classified as long-term as of September 30, 2016 . Additionally, TVA has two issues of PARRS outstanding. After a fixed-rate period of five years , the coupon rate on the PARRS may automatically be reset downward under certain market conditions on an annual basis. The coupon rate reset on the PARRS is based on a calculation. For both series of PARRS, the coupon rate will reset downward on the reset date if the rate calculated is below the then-current coupon rate on the Bond. The calculation dates, potential reset dates, and terms of the calculation are different for each series. The coupon rate on the 1998 Series D PARRS may be reset on June 1 (annually) if the sum of the five -day average of the 30-Year Constant Maturity Treasury ("CMT") rate for the week ending the last Friday in April, plus 94 basis points, is below the then-current coupon rate. The coupon rate on the 1999 Series A PARRS may be reset on May 1 (annually) if the sum of the five -day average of the 30-Year CMT rate for the week ending the last Friday in March, plus 84 basis points, is below the then-current coupon rate. The coupon rates may only be reset downward, but investors may request to redeem their Bonds at par value in conjunction with a coupon rate reset for a limited period of time prior to the reset dates under certain circumstances. The coupon rate for the 1998 Series D PARRS, which mature in June 2028, has been reset seven times, from an initial rate of 6.750 percent to the current rate of 3.550 percent. In connection with these resets, $ 301 million of the Bonds have been redeemed, so $ 274 million of the Bonds were outstanding at September 30, 2017 . The coupon rate for the 1999 Series A PARRS, which mature in May 2029, has been reset six times, from an initial rate of 6.50 percent to the current rate of 3.360 percent. In connection with these resets, $ 293 million of the Bonds have been redeemed, so $ 232 million of the Bonds were outstanding at September 30, 2017 . Due to the contingent nature of the put option on the PARRS, TVA determines whether the PARRS should be classified as long-term debt or current maturities of long-term debt by calculating the expected reset rate for the Bonds on the calculation dates, described above. If the expected reset rate is less than the then-current coupon rate on the PARRS, the PARRS are included in current maturities. Otherwise, the PARRS are included in long-term debt. Debt Securities Activity The table below summarizes the long-term debt securities activity for the period from October 1, 2015 , to September 30, 2017 . Debt Securities Activity For the years ended September 30 2017 2016 Issues 2017 Series A (1) $ 1,000 $ — Discount on debt issues (1 ) — Total $ 999 $ — Acquisitions Notes payable (2) $ 74 $ 78 Redemptions/Maturities (3) Variable interest entities $ 35 $ 33 Notes payable 27 3 electronotes ® 5 47 2009 Series A — 2 2009 Series B 28 27 2001 Series D 525 — 2007 Series A 1,000 — Total $ 1,620 $ 112 Notes (1) The 2017 Series A bonds were issued at 99.9 percent of par. (2) The related leaseback obligations of $70 million previously reported in Other liabilities in TVA's consolidated balance sheets were extinguished in the fourth quarters of both 2017 and 2016 as a result of each year's acquisition. See Note 9 for additional information. (3) All redemptions were at 100 percent of par. Debt Outstanding Total debt outstanding at September 30, 2017 , and 2016 , consisted of the following: Short-Term Debt At September 30 CUSIP or Other Identifier Maturity Call/(Put) Date Coupon Rate 2017 2016 Short-term debt, net of discounts $ 1,998 $ 1,407 Current maturities of long-term debt of variable interest entities issued at par 36 35 Current maturities of notes payable 53 27 Current maturities of power bonds issued at par 880591EF5 12/15/2017 3.770% 1 1 880591EF5 6/15/2018 3.770% 28 27 88059TEL1 11/15/2017 2.650% 1 1 88059TEL1 5/15/2018 2.650% 2 2 880591DS8 12/15/2016 4.875% — 524 880591EA6 7/18/2017 5.500% — 1,000 880591CU4 12/15/2017 6.250% 650 — 880591EC2 4/1/2018 4.500% 1,000 — 88059TFS5 10/15/2017 4.125% 46 — Total current maturities of power bonds issued at par 1,728 1,555 Total current debt outstanding, net $ 3,815 $ 3,024 Long-Term Debt (1) At September 30 CUSIP or Other Identifier Maturity Coupon Rate Call Date 2017 Par 2016 Par Stock Exchange Listings electronotes ®(2) 5/15/2020 - 2/15/2043 2.375% - 3.625% 2/15/2015 - 2/15/2018 $ 226 $ 278 None 880591CU4 12/15/2017 6.250% — 650 New York 880591EC2 4/1/2018 4.500% — 1,000 New York, Luxembourg 880591EQ1 10/15/2018 1.750% 1,000 1,000 New York 880591EL2 2/15/2021 3.875% 1,500 1,500 New York 880591DC3 6/7/2021 5.805% (3) 268 260 New York, Luxembourg 880591EN8 8/15/2022 1.875% 1,000 1,000 New York 880591ER9 9/15/2024 2.875% 1,000 1,000 New York 880591CJ9 11/1/2025 6.750% 1,350 1,350 New York, Hong Kong, Luxembourg, Singapore 880591EU26 2/1/2027 2.875% 1,000 — New York 880591300 (4) 6/1/2028 3.550% 273 273 New York 880591409 (4) 5/1/2029 3.360% 232 232 New York 880591DM1 5/1/2030 7.125% 1,000 1,000 New York, Luxembourg 880591DP4 6/7/2032 6.587% (3) 335 324 New York, Luxembourg 880591DV1 7/15/2033 4.700% 472 472 New York, Luxembourg 880591EF5 6/15/2034 3.770% 303 332 None 880591DX7 6/15/2035 4.650% 436 436 New York 880591CK6 4/1/2036 5.980% 121 121 New York 880591CS9 4/1/2036 5.880% 1,500 1,500 New York 880591CP5 1/15/2038 6.150% 1,000 1,000 New York 880591ED0 6/15/2038 5.500% 500 500 New York 880591EH1 9/15/2039 5.250% 2,000 2,000 New York 880591EP3 12/15/2042 3.500% 1,000 1,000 New York 880591DU3 6/7/2043 4.962% (3) 201 195 New York, Luxembourg 880591CF7 7/15/2045 6.235% 7/15/2020 140 140 New York 880591EB4 1/15/2048 4.875% 500 500 New York, Luxembourg 880591DZ2 4/1/2056 5.375% 1,000 1,000 New York 880591EJ7 9/15/2060 4.625% 1,000 1,000 New York 880591ES7 9/15/2065 4.250% 1,000 1,000 New York Subtotal 20,357 21,063 Unamortized discounts, premiums, issue costs, and other (152 ) (162 ) Total long-term outstanding power bonds, net 20,205 20,901 Long-term debt of variable interest entities, net 1,164 1,199 Long-term notes payable 69 48 Total long-term debt, net $ 21,438 $ 22,148 Notes (1) Includes net exchange gain from currency transactions of $ 125 million at September 30, 2017 and $ 150 million at September 30, 2016 . (2) Includes one electronotes ® issue with partial maturities of principal for each required annual payment. (3) The coupon rate represents TVA’s effective interest rate. (4) TVA PARRS, CUSIP numbers 880591300 and 880591409, may be redeemed under certain conditions. See Put and Call Options above. Maturities Due in the Year Ending September 30 2018 2019 2020 2021 2022 Thereafter Total Long-term power bonds, long-term debt of variable interest entities, and notes payable including current maturities (1) $ 1,817 $ 1,116 $ 93 $ 1,901 $ 1,071 $ 17,545 $ 23,543 Short-term debt, net of discounts 1,998 — — — — — 1,998 Note (1) Long-term power bonds does not include noncash items of foreign currency exchange gain of $ 125 million , unamortized debt issue costs of $59 million , and net discount on sale of Bonds of $ 93 million . Long-term debt of variable interest entities does not include noncash item of unamortized debt issue costs of $ 11 million . Credit Facility Agreements TVA and the U.S. Treasury, pursuant to the TVA Act, have entered into a memorandum of understanding under which the U.S. Treasury provides TVA with a $ 150 million credit facility. This credit facility was renewed for 2018 with a maturity date of September 30, 2018. Access to this credit facility or other similar financing arrangements with the U.S. Treasury has been available to TVA since the 1960s. TVA can borrow under the U.S. Treasury credit facility only if it cannot issue Bonds in the market on reasonable terms, and TVA considers the U.S. Treasury credit facility a secondary source of liquidity. The interest rate on any borrowing under this facility is based on the average rate on outstanding marketable obligations of the United States with maturities from date of issue of one year or less. There were no outstanding borrowings under the facility at September 30, 2017 . The availability of this credit facility may be impacted by how the U.S. government addresses the possibility of approaching its debt limit. TVA also has funding available under the four long-term revolving credit facilities totaling $2.7 billion : a $150 million credit facility that matures on December 12, 2019, a $500 million credit facility that matures on February 1, 2021, a $1.0 billion credit facility that matures on June 2, 2020, and a $1.0 billion credit facility that matures on September 30, 2020. The interest rate on any borrowing under these facilities varies based on market factors and the rating of TVA's senior unsecured, long-term, non-credit-enhanced debt. TVA is required to pay an unused facility fee on the portion of the total $2.7 billion that TVA has not borrowed or committed under letters of credit. This fee, along with letter of credit fees, may fluctuate depending on the rating of TVA's senior unsecured, long-term, non-credit-enhanced debt. At September 30, 2017 , and September 30, 2016 , there were $1.2 billion and $1.4 billion , respectively, of letters of credit outstanding under the facilities, and there were no borrowings outstanding. See Note 15 — Other Derivative Instruments — Collateral . The following table provides additional information regarding TVA's funding available under the four long-term revolving credit facilities: Summary of Long-Term Credit Facilities At September 30, 2017 Maturity Date Facility Limit Letters of Credit Outstanding Cash Borrowings Availability December 2019 $ 150 $ 38 $ — $ 112 February 2021 500 500 — — June 2020 1,000 278 — 722 September 2020 1,000 335 — 665 Total $ 2,650 $ 1,151 $ — $ 1,499 Lease/Leasebacks Prior to 2004, TVA received approximately $ 945 million in proceeds by entering into lease/leaseback transactions for 24 new peaking CTs. TVA also received approximately $ 389 million in proceeds by entering into lease/leaseback transactions for qualified technological equipment and software ("QTE") in 2003. Due to TVA's continuing involvement in the operation and maintenance of the leased units and equipment and its control over the distribution of power produced by the combustion turbine facilities during the leaseback term, TVA accounted for the lease proceeds as financing obligations. On September 20, 2017, TVA acquired 100 percent of the equity interests in two SPEs created for the purpose of facilitating a portion of the leaseback arrangements. As a result of the acquisition, TVA effectively settled $ 70 million of its leaseback obligations related to eight CTs. On July 20, 2016, TVA acquired 100 percent of the equity interests in two SPEs created for the purpose of facilitating lease/leaseback arrangements. As a result of the acquisition, TVA effectively settled $ 70 million of its leaseback obligations related to eight CTs. See Note 9 . At September 30, 2017 , and 2016 , the outstanding le aseback obligations related to the remaining CTs and QTE were $338 million and $467 million , respectively.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	12 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	. Accumulated Other Comprehensive Income (Loss) AOCI represents market valuation adjustments related to TVA’s currency swaps. The currency swaps are cash flow hedges and are the only derivatives in TVA’s portfolio that have been designated and qualify for hedge accounting treatment. TVA records exchange rate gains and losses on its foreign currency-denominated debt in net income and marks its currency swap assets and liabilities to fair value through other comprehensive income (loss) ("OCI") . TVA then reclassifies an amount out of AOCI into net income, offsetting the exchange gain/loss recorded on the debt. For the years ended September 30, 2017 and 2016, TVA reclassified $26 million of gains and $ 129 million of losses, respectively, related to its cash flow hedges from AOCI to Interest expense. See Note 15 . TVA records certain assets and liabilities that result from the regulated ratemaking process that would not be recorded under GAAP for non-regulated entities. As such, certain items that would generally be reported in AOCI or that would impact the consolidated statements of operations are recorded as regulatory assets or regulatory liabilities. See Note 7 for a schedule of regulatory assets and liabilities. See Note 15 for a discussion of the recognition in AOCI of gains and losses associated with certain derivative contracts. See Note 16 for a discussion of the recognition of certain investment fund gains and losses as regulatory assets and liabilities. See Note 20 for a discussion of the regulatory accounting related to components of TVA’s benefit plans.

Risk Management Activities and

Risk Management Activities and Derivative Transactions	12 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Risk Management Activities and Derivative Transactions	TVA is exposed to various risks. These include risks related to commodity prices, investment prices, interest rates, currency exchange rates, and inflation as well as counterparty credit and performance risks. To help manage certain of these risks, TVA has entered into various derivative transactions, principally commodity option contracts, forward contracts, swaps, swaptions, futures, and options on futures. Other than certain derivative instruments in its trust investment funds, it is TVA’s policy to enter into these derivative transactions solely for hedging purposes and not for speculative purposes. TVA has suspended its FTP and no longer uses financial instruments to hedge risks related to commodity prices; however, TVA plans to continue to manage fuel price volatility through other methods and to periodically reevaluate its suspended FTP program for future use of financial instruments. Overview of Accounting Treatment TVA recognizes certain of its derivative instruments as either assets or liabilities on its consolidated balance sheets at fair value. The accounting for changes in the fair value of these instruments depends on (1) whether TVA uses regulatory accounting to defer the derivative gains and losses, (2) whether the derivative instrument has been designated and qualifies for hedge accounting treatment, and (3) if so, the type of hedge relationship (for example, cash flow hedge). The following tables summarize the accounting treatment that certain of TVA's financial derivative transactions receive: Summary of Derivative Instruments That Receive Hedge Accounting Treatment (part 1) Amount of Mark-to-Market Gain (Loss) Recognized in Other Comprehensive Income (Loss) For the years ended September 30 Derivatives in Cash Flow Hedging Relationship Objective of Hedge Transaction Accounting for Derivative Hedging Instrument 2017 2016 Currency swaps To protect against changes in cash flows caused by changes in foreign currency exchange rates (exchange rate risk) Unrealized gains and losses are recorded in AOCI and reclassified to interest expense to the extent they are offset by gains and losses on the hedged transaction $ 59 $ (139 ) Summary of Derivative Instruments That Receive Hedge Accounting Treatment (part 2) (1) Amount of Gain (Loss) Reclassified from OCI to Interest Expense For the years ended September 30 Derivatives in Cash Flow Hedging Relationship 2017 2016 Currency swaps $ 26 $ (129 ) Note (1) There were no ineffective portions or amounts excluded from effectiveness testing for any of the periods presented. Based on forecasted foreign currency exchange rates, TVA expects to reclassify approximately $18 million of losses from AOCI to interest expense within the next twelve months to offset amounts anticipated to be recorded in interest expense related to exchange gain on the debt. Summary of Derivative Instruments That Do Not Receive Hedge Accounting Treatment Amount of Gain (Loss) Recognized in Income on Derivatives (1) For the years ended September 30 Derivative Type Objective of Derivative Accounting for Derivative Instrument 2017 2016 Interest rate swaps To fix short-term debt variable rate to a fixed rate (interest rate risk) Mark-to-Market gains and losses are recorded as regulatory assets or liabilities. Realized gains and losses are recognized in interest expense when incurred during the settlement period. $ (101 ) $ (109 ) Commodity derivatives under FTP To protect against fluctuations in market prices of purchased commodities (price risk) Mark-to-Market gains and losses are recorded as regulatory assets or liabilities. Realized gains and losses are recognized in fuel expense or purchased power expense when the related commodity is used in production. (36 ) (94 ) Note (1) All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in income but instead are deferred as regulatory assets and liabilities. As such, there was no related gain (loss) recognized in income for these unrealized gains (losses) for the years ended September 30, 2017 and 2016 . Fair Values of TVA Derivatives At September 30 2017 2016 Derivatives That Receive Hedge Accounting Treatment: Balance Balance Sheet Presentation Balance Balance Sheet Presentation Currency swaps £200 million Sterling $ (67 ) Accounts payable and accrued liabilities $(5); Other long-term liabilities $(62) $ (82 ) Other long-term liabilities £250 million Sterling (15 ) Accounts payable and accrued liabilities $(4); Other long-term liabilities $(11) (41 ) Other long-term liabilities £150 million Sterling (21 ) Accounts payable and accrued liabilities $(2); Other long-term liabilities $(19) (39 ) Other long-term liabilities Derivatives That Do Not Receive Hedge Accounting Treatment: Balance Balance Sheet Presentation Balance Balance Sheet Presentation Interest rate swaps $1.0 billion notional $ (1,093 ) Accounts payable and accrued liabilities $(66); Other long-term liabilities $(1,027) $ (1,387 ) Other long-term liabilities $476 million notional (410 ) Accounts payable and accrued liabilities $(25); Other long-term liabilities $(385) (539 ) Other long-term liabilities $42 million notional (8 ) Accounts payable and accrued liabilities $(2); Other long-term liabilities $(6) (12 ) Other long-term liabilities Commodity contract derivatives (60 ) Other current assets $8; Other long-term assets $2; Other long-term liabilities $(9); Accounts payable and accrued liabilities $(61) (125 ) Other current assets $6; Other long-term assets $3; Other long-term liabilities $(49); Accounts payable and accrued liabilities $(85) FTP Derivatives under FTP (1) (5 ) Other current assets $(4); Accounts payable and accrued liabilities $(1) (39 ) Other current assets $(30); Other long-term liabilities $(2); Accounts payable and accrued liabilities $(7) Note (1) Fair values of certain derivatives under the FTP that were in net liability positions totaling $ 4 million and $ 30 million at September 30, 2017 and 2016 , respectively, are recorded in TVA's margin cash accounts in Other current assets. These derivatives are transacted with futures commission merchants, and cash deposits have been posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full. Cash Flow Hedging Strategy for Currency Swaps To protect against exchange rate risk related to three British pound sterling denominated Bond transactions, TVA entered into foreign currency hedges at the time the Bond transactions occurred. TVA had the following currency swaps outstanding at September 30, 2017 : Currency Swaps Outstanding At September 30, 2017 Effective Date of Currency Swap Contract Associated TVA Bond Issues Currency Exposure Expiration Date of Swap Overall Effective Cost to TVA 1999 £200 million 2021 5.81% 2001 £250 million 2032 6.59% 2003 £150 million 2043 4.96% When the dollar strengthens against the British pound sterling, the exchange gain on the Bond liability is offset by an equal amount of loss on the swap contract that is reclassified out of AOCI. Conversely, the exchange loss on the Bond liability is offset by an equal amount of gain on the swap contract that is reclassified out of AOCI. All such exchange gains or losses on the Bond liability are included in Long-term debt, net. The offsetting exchange losses or gains on the swap contracts are recognized in AOCI. If any gain (loss) were to be incurred as a result of the early termination of the foreign currency swap contract, the resulting income (expense) would be amortized over the remaining life of the associated Bond as a component of Interest expense. The values of the currency swap liabilities are included in Accounts payable and accrued liabilities and Other long-term liabilities on the consolidated balance sheets. Derivatives Not Receiving Hedge Accounting Treatment Interest Rate Derivatives . Generally TVA uses interest rate swaps to fix variable short-term debt to a fixed rate, and TVA uses regulatory accounting treatment to defer the MtM gains and losses on its interest rate swaps. The net deferred unrealized gains and losses are classified as regulatory assets or liabilities on TVA's consolidated balance sheets and are included in the ratemaking formula when gains or losses are realized. The values of these derivatives are included in Accounts payable and accrued liabilities and Other long-term liabilities on the consolidated balance sheets, and realized gains and losses, if any, are included in TVA's consolidated statements of operations. For the years ended September 30, 2017 and 2016 , the changes in market value of the interest rate derivatives resulted in deferred unrealized gains of $ 472 million and unrealized losses of $ 311 million , respectively. Commodity Derivatives . TVA enters into certain derivative contracts for coal and natural gas that require physical delivery of the contracted quantity of the commodity. TVA marks to market all such contracts and defers the fair values as regulatory assets or liabilities on a gross basis. At September 30, 2017 , TVA's coal contract derivatives had terms of up to two years and TVA's natural gas contract derivatives had terms of up to four years. Commodity Contract Derivatives At September 30 2017 2016 Number of Contracts Notional Amount Fair Value (MtM) Number of Contracts Notional Amount Fair Value ( MtM ) Coal contract derivatives 20 17 million tons $ (67 ) 20 20 million tons $ (127 ) Natural gas contract derivatives 53 271 million mmBtu $ 7 39 148 million mmBtu $ 2 Derivatives Under FTP. While TVA has suspended its FTP and no longer uses financial instruments to hedge risks related to commodity prices, certain natural gas swaps with a maturity of one year or less remain as part of the suspended FTP. Under the FTP, TVA was authorized to purchase and sell futures, swaps, options, and combinations of these instruments (as long as they were standard in the industry) to hedge TVA’s exposure to (1) the price of natural gas, fuel oil, electricity, coal, emission allowances, nuclear fuel, and other commodities included in TVA’s fuel cost adjustment calculation, (2) the price of construction materials, and (3) contracts for goods priced in or indexed to foreign currencies. The combined transaction limit for the fuel cost adjustment and construction material transactions was $ 130 million (based on one-day value at risk). In addition, the maximum hedge volume for the construction material transactions was 75 percent of the underlying net notional volume of the material that TVA anticipated using in approved TVA projects, and the market value of all outstanding hedging transactions involving construction materials was limited to $ 100 million at the execution of any new transaction. The portfolio value at risk limit for the foreign currency transactions was $ 5 million and was separate and distinct from the $ 130 million transaction limit discussed above. TVA's policy prohibits trading financial instruments under the FTP for speculative purposes. Derivatives under Financial Trading Program (1) At September 30 2017 2016 Notional Amount (in mmBtu) Fair Value (MtM) (in millions) Notional Amount (in mmBtu) Fair Value (MtM) (in millions) Natural gas Swap contracts 2,800,000 $ (5 ) 21,052,500 $ (39 ) Note (1) Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net value of contractual amounts. TVA defers all FTP unrealized gains (losses) as regulatory liabilities (assets) and records only realized gains or losses to match the delivery period of the underlying commodity. In addition to the open commodity derivatives disclosed above, TVA had closed derivative contracts with market values of $ (3) million and $ (5) million at September 30, 2017 and 2016 , respectively. TVA experienced the following unrealized and realized gains and losses related to the FTP at the dates and during the periods, as applicable, set forth in the tables below: Financial Trading Program Unrealized Gains (Losses) At September 30 FTP unrealized gains (losses) deferred as regulatory liabilities (assets) 2017 2016 Natural gas $ (5 ) $ (39 ) Financial Trading Program Realized Gains (Losses) At September 30 Decrease (increase) in fuel expense 2017 2016 Natural gas $ (29 ) $ (75 ) Decrease (increase) in purchased power expense Natural gas $ (7 ) $ (19 ) Offsetting of Derivative Assets and Liabilities The amounts of TVA's derivative instruments as reported in the consolidated balance sheets as of September 30, 2017 , and September 30, 2016 , are shown in the table below. At September 30, 2017 Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Balance Sheet (1) Net Amounts of Assets/Liabilities Presented in the Balance Sheet (2) Assets Commodity derivatives not subject to master netting or similar arrangement $ 10 $ — $ 10 Liabilities Currency swap(s) (3) $ 103 $ — $ 103 Interest rate swaps (3) 1,511 — 1,511 Commodity derivatives under FTP 5 (4 ) 1 Total derivatives subject to master netting or similar arrangement 1,619 (4 ) 1,615 Total derivatives not subject to master netting or similar arrangement 70 — 70 Total liabilities $ 1,689 $ (4 ) $ 1,685 At September 30, 2016 Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Balance Sheet (1) Net Amounts of Assets/Liabilities Presented in the Balance Sheet (2) Assets Commodity derivatives under FTP subject to master netting or similar agreement $ 6 $ (6 ) $ — Commodity derivatives not subject to master netting or similar arrangement 9 — 9 Total assets $ 15 $ (6 ) $ 9 Liabilities Currency swap(s) (3) $ 162 $ — $ 162 Interest rate swaps (3) 1,938 — 1,938 Commodity derivatives under FTP 45 (36 ) 9 Total derivatives subject to master netting or similar arrangement 2,145 (36 ) 2,109 Total derivatives not subject to master netting or similar arrangement 134 — 134 Total liabilities $ 2,279 $ (36 ) $ 2,243 Notes (1) Amounts primarily include counterparty netting of derivative contracts, margin account deposits for futures commission merchants transactions, and cash collateral received or paid in accordance with the accounting guidance for derivatives and hedging transactions. (2) There are no derivative contracts subject to a master netting arrangement or similar agreement which are not offset in the balance sheets. (3) Letters of credit of approximately $ 1.2 billion and $ 1.4 billion were posted as collateral at September 30, 2017 and 2016 , respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives. Other Derivative Instruments Investment Fund Derivatives . Investment funds consist primarily of funds held in the NDT, ART, SERP, and DCP. All securities in the trusts are classified as trading. See Note 16 — Investments Funds for a discussion of the trusts' objectives and the types of investments that they hold. The NDT and ART may invest in derivative instruments which may include swaps, futures, options, forwards, and other instruments. At September 30, 2017 and 2016, the NDT held investments in forward contracts to purchase debt securities. The fair values of these derivatives were in asset positions totaling $19 million at September 30, 2017 , and asset positions totaling $15 million at September 30, 2016 . Collateral . TVA's interest rate swaps and currency swaps contain contract provisions that require a party to post collateral (in a form such as cash or a letter of credit) when the party's liability balance under the agreement exceeds a certain threshold. At September 30, 2017 , the aggregate fair value of all derivative instruments with credit-risk related contingent features that were in a liability position was $1.6 billion . TVA's collateral obligations at September 30, 2017 , under these arrangements, were approximately $1.1 billion , for which TVA had posted approximately $1.2 billion in letters of credit. These letters of credit reduce the available balance under the related credit facilities. TVA's assessment of the risk of its nonperformance includes a reduction in its exposure under the contract as a result of this posted collateral. For all of its derivative instruments with credit-risk related contingent features: • If TVA remains a majority-owned U.S. government entity but Standard & Poor's Financial Services, LLC ("S&P") or Moody's Investors Service, Inc. ("Moody's") downgrades TVA's credit rating to AA or Aa2, respectively, TVA's collateral obligations would likely increase by $22 million ; and • If TVA ceases to be majority-owned by the U.S. government, TVA's credit rating would likely be downgraded and TVA would be required to post additional collateral. Counterparty Risk TVA may be exposed to certain risks when a counterparty has the potential to fail to meet its obligations in accordance with agreed terms. These risks may be related to credit, operational, or nonperformance matters. To mitigate certain counterparty risk, TVA analyzes the counterparty’s financial condition prior to entering into an agreement, establishes credit limits, monitors the appropriateness of those limits, as well as any changes in the creditworthiness of the counterparty, on an ongoing basis, and when required, employs credit mitigation measures, such as collateral or prepayment arrangements and master purchase and sale agreements, to mitigate credit risk. Customers . TVA is exposed to counterparty credit risk associated with trade accounts receivable from delivered power sales to LPCs, and from industries and federal agencies directly served, all located in the Tennessee Valley region. Of the $1.4 billion and $1.6 billion of receivables from power sales outstanding at September 30, 2017 and 2016 , respectively, nearly all were rated investment grade. TVA is also exposed to risk from exchange power arrangements with a small number of investor-owned regional utilities related to either delivered power or the replacement of open positions of longer-term purchased power or fuel agreements. TVA believes its policies and procedures for counterparty performance risk reviews have generally protected TVA against significant exposure related to market and economic conditions. See Note 1 — Allowance for Uncollectible Accounts and Note 3 . TVA had revenue from six LPCs that accounted for 33 percent of total operating revenue for the years ended both September 30, 2017 and September 30, 2016 . Suppliers . If one of TVA's fuel or purchased power suppliers fails to perform under the terms of its contract with TVA, TVA might lose the mone y that it paid to the supplier under the contract and have to purchase replacement fuel or power on the spot market, perhaps at a significantly higher price than TVA was entitled to pay under the contract. In addition, TVA might not be able to acquire replacement fuel or power in a timely manner and thus might be unable to satisfy its own obligations to deliver power. Nuclear fuel requirements, including uranium mining and milling, conversion services, enrichment services, and fabrication services, are met from various suppliers, depending on the type of service. TVA purchases the majority of its natural gas requirements from a variety of suppliers under short-term contracts. To help ensure a reliable supply of coal, TVA had coal contracts with multiple suppliers at September 30, 2017 . The contracted su pply of coal is sourced from multiple geographic regions of the United States and is to be delivered via various transportation methods (i.e., barge, rail, and truck). Emerging technologies, environmental regulations, and low natural gas prices have contributed to weak demand for coal. As a result, coal suppliers are facing increased financial pressure, which has led to relatively poor credit ratings and bankruptcies. Continued difficulties by coal suppliers could result in consolidations, additional bankruptcies, restructurings, contract renegotiations, or other scenarios. Under these scenarios and TVA’s potential available responses, TVA does not anticipate a significant financial impact in obtaining continued fuel supply for its coal-fired generation. On March 29, 2017, one of TVA’s suppliers filed for protection under Chapter 11 of the United States Bankruptcy Code. Certain subsidiaries of the parent company have entered into contracts to supply goods and services to TVA, including contracts for the enrichment and fabrication of nuclear fuel and the manufacture of a steam generator, as well as several ongoing agreements for maintenance and outage support at its nuclear and coal-fired plants. TVA is assessing potential performance impacts, including procurement of parts and services as well as outage schedules. The suppliers are currently performing under the TVA contracts; however, if any supplier is unable to perform under TVA’s existing contracts and TVA is unable to obtain similar services at similar terms from other vendors, TVA could experience delays, disruptions, ad ditional costs, or other operational outcomes which TVA cannot predict at this time, but which could be material. TVA has a power purchase agreement that expires on March 31, 2032, with a supplier of electricity for 440 megawatts ("MW") of summer net capability from a lignite-fired generating plant. TVA has determined that the supplier has the equivalent of a non-investment grade credit rating; therefore, the supplier has provided credit assurance to TVA under the terms of the agreement. Derivative Counterparties . TVA has entered into physical and financial contracts that qualify as derivatives for hedging purposes, and TVA's NDT fund and qualified defined benefit pension plan have entered into derivative contracts for investment purposes. If a counterparty to one of TVA's hedging transactions defaults, TVA might incur substantial costs in connection with entering into a replacement hedging transaction. If a counterparty to the derivative contracts into which the NDT fund and the qualified pension plan have entered for investment purposes defaults, the value of the investment could decline significantly or perhaps become worthless. TVA has concentrations of credit risk from the banking and coal industries because multiple companies in these industries serve as counterparties to TVA in various derivative transactions. At September 30, 2017 , all of TVA's commodity derivatives under the FTP, currency swaps, and interest rate swaps as well as all of the derivatives in the NDT were with banking counterparties whose Moody's credit ratings were A3 or higher. TVA classifies qualified forward coal and natural gas contracts as derivatives. See Derivatives Not Receiving Hedge Accounting Treatment above. At September 30, 2017 , the coal contracts were with counterparties whose Moody's credit rating, or TVA’s internal analysis when such information was unavailable, ranged from Ca to Ba2 . At September 30, 2017 , the natural gas contracts were with counterparties whose ratings ranged from B1 to A2 . See Suppliers above for discussion of challenges facing the coal industry. TVA's total value for derivative contracts with coal and natural gas counterparties in an asset position as of September 30, 2017 , was approximately $10 million . TVA currently utilizes two futures commission merchants ("FCMs") to clear commodity contracts, including futures, options, and similar financial derivatives. These transactions are executed under the FTP by the FCMs on exchanges on behalf of TVA. TVA maintains margin cash accounts with the FCMs. TVA makes deposits to the margin cash accounts to adequately cover any net liability positions on its derivatives transacted with the FCMs. See the note to the Fair Values of TVA Derivatives table above.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair value is determined based on the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the asset or liability's principal market, or in the absence of a principal market, the most advantageous market for the asset or liability in an orderly transaction between market participants. TVA uses market or observable inputs as the preferred source of values, followed by assumptions based on hypothetical transactions in the absence of market inputs. Valuation Techniques The measurement of fair value results in classification into a hierarchy by the inputs used to determine the fair value as follows: Level 1 — Unadjusted quoted prices in active markets accessible by the reporting entity for identical assets or liabilities. Active markets are those in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing. Level 2 — Pricing inputs other than quoted market prices included in Level 1 that are based on observable market data and that are directly or indirectly observable for substantially the full term of the asset or liability. These include quoted market prices for similar assets or liabilities, quoted market prices for identical or similar assets in markets that are not active, adjusted quoted market prices, inputs from observable data such as interest rate and yield curves, volatilities and default rates observable at commonly quoted intervals, and inputs derived from observable market data by correlation or other means. Level 3 — Pricing inputs that are unobservable, or less observable, from objective sources. Unobservable inputs are only to be used to the extent observable inputs are not available. These inputs maintain the concept of an exit price from the perspective of a market participant and should reflect assumptions of other market participants. An entity should consider all market participant assumptions that are available without unreasonable cost and effort. These are given the lowest priority and are generally used in internally developed methodologies to generate management's best estimate of the fair value when no observable market data is available. A financial instrument's level within the fair value hierarchy (where Level 1 is the highest and Level 3 is the lowest) is based on the lowest level of input significant to the fair value measurement. The following sections describe the valuation methodologies TVA uses to measure different financial instruments at fair value. Except for gains and losses on SERP and DCP assets, all changes in fair value of these assets and liabilities have been recorded as changes in regulatory assets, regulatory liabilities, or AOCI on TVA's consolidated balance sheets and consolidated statements of comprehensive income (loss). Except for gains and losses on SERP and DCP assets, there has been no impact to the consolidated statements of operations or the consolidated statements of cash flows related to these fair value measurements. Investment Funds At September 30, 2017 , Investment funds were composed of $2.6 billion of securities classified as trading and measured at fair value. Trading securities are held in the NDT, ART, SERP, and DCP. The NDT holds funds for the ultimate decommissioning of TVA's nuclear power plants. The ART holds funds primarily for the costs related to the future closure and retirement of TVA's other long-lived assets. The balances in the NDT and ART were $1.9 billion and $632 million , respectively, at September 30, 2017 . TVA established a SERP to provide benefits to selected employees of TVA which are comparable to those provided by competing organizations. The DCP is designed to provide participants with the ability to defer compensation until employment with TVA ends. NDT and SERP funds are invested in portfolios of securities generally designed to achieve a return in line with overall equity market performance, and ART and DCP funds are invested in portfolios of securities generally designed to achieve a return in line with overall debt and equity market performance. The NDT, ART, SERP, and DCP are composed of multiple types of investments and are managed by external institutional investment managers. Most U.S. and international equities, U.S. Treasury inflation-protected securities, real estate investment trust securities, and cash securities and certain derivative instruments are measured based on quoted exchange prices in active markets and are classified as Level 1 valuations. Fixed-income investments, high-yield fixed-income investments, currencies, and most derivative instruments are non-exchange traded and are classified as Level 2 valuations. These measurements are based on market and income approaches with observable market inputs. Private equity limited partnerships and private real estate investments may include holdings of investments in private real estate, venture capital, buyout, mezzanine or subordinated debt, restructuring or distressed debt, and special situations through funds managed by third-party investment managers. These investments generally involve a three to four year period where the investor contributes capital, followed by a period of distribution, typically over several years. The investment period is generally, at a minimum, 10 years or longer. The NDT had unfunded commitments related to private equity limited partnerships of $50 million and unfunded commitments related to private real estate of $5 million at September 30, 2017 . These investments have no redemption or limited redemption options and may also impose restrictions on the NDT’s ability to liquidate its investments. There are no readily available quoted exchange prices for these investments. The fair value of the investments is based on TVA’s ownership percentage of the fair value of the underlying investments as provided by the investment managers. These investments are typically valued on a quarterly basis. TVA’s private equity limited partnerships and private real estate investments are valued at net asset values ("NAV") as a practical expedient for fair value. TVA classifies its interest in these types of investments as investments measured at net asset value in the fair value hierarchy. Commingled funds represent investment funds comprising multiple individual financial instruments. The commingled funds held by the NDT, ART, SERP, and DCP consist of either a single class of securities, such as equity, debt, or foreign currency securities, or multiple classes of securities. All underlying positions in these commingled funds are either exchange traded or measured using observable inputs for similar instruments. The fair value of commingled funds is based on NAV per fund share (the unit of account), derived from the prices of the underlying securities in the funds. These commingled funds can be redeemed at the measurement date NAV and are classified as Commingled funds measured at net asset value in the fair value hierarchy. Realized and unrealized gains and losses on trading securities are recognized in current earnings and are based on average cost. The gains and losses of the NDT and ART are subsequently reclassified to a regulatory asset or liability account in accordance with TVA's regulatory accounting policy. See Note 1 — Cost-Based Regulation . TVA recorded unrealized gains and losses related to its trading securities held during each period as follows: Unrealized Investment Gains (Losses) At September 30 Financial Statement Presentation 2017 2016 SERP Other income (expense) $ 4 $ 2 DCP Other income (expense) 2 1 NDT Regulatory asset 92 89 ART Regulatory asset 43 29 Currency and Interest Rate Derivatives See Note 15 — Cash Flow Hedging Strategy for Currency Swaps and Derivatives Not Receiving Hedge Accounting Treatment for a discussion of the nature, purpose, and contingent features of TVA's currency swaps and interest rate swaps. These swaps are classified as Level 2 valuations and are valued based on income approaches using observable market inputs for similar instruments. Commodity Contract Derivatives and Commodity Derivatives Under FTP Commodity Contract Derivatives. Most of these contracts are valued based on market approaches which utilize short- and mid-term market-quoted prices from an external industry brokerage service. A small number of these contracts are valued based on a pricing model using long-term price estimates from TVA's coal price forecast. To value the volume option component of applicable coal contracts, TVA uses a Black-Scholes pricing model which includes inputs from the forecast, contract-specific terms, and other market inputs. These contracts are classified as Level 3 valuations. Commodity Derivatives Under FTP. These contracts are valued based on market approaches which utilize Chicago Mercantile Exchange ("CME") quoted prices and other observable inputs. Swap contracts are valued using a pricing model based on CME inputs and are subject to nonperformance risk outside of the exit price. These contracts are classified as Level 2 valuations. See Note 15 — Derivatives Not Receiving Hedge Accounting Treatment — Commodity Derivatives and — Derivatives Under FTP for a discussion of the nature and purpose of coal contracts and derivatives under TVA's FTP. Nonperformance Risk The assessment of nonperformance risk, which includes credit risk, considers changes in current market conditions, readily available information on nonperformance risk, letters of credit, collateral, other arrangements available, and the nature of master netting arrangements. TVA is a counterparty to currency swaps, interest rate swaps, commodity contracts, and other derivatives which subject TVA to nonperformance risk. Nonperformance risk on the majority of investments and certain exchange-traded instruments held by TVA is incorporated into the exit price that is derived from quoted market data that is used to mark the investment to market. Nonperformance risk for most of TVA's derivative instruments is an adjustment to the initial asset/liability fair value. TVA adjusts for nonperformance risk, both of TVA (for liabilities) and the counterparty (for assets), by applying credit valuation adjustments ("CVAs") . TVA determines an appropriate CVA for each applicable financial instrument based on the term of the instrument and TVA's or the counterparty's credit rating as obtained from Moody's. For companies that do not have an observable credit rating, TVA uses internal analysis to assign a comparable rating to the counterparty. TVA discounts each financial instrument using the historical default rate (as reported by Moody’s for CY 1983 to CY 2016) for companies with a similar credit rating over a time period consistent with the remaining term of the contract. The application of CVAs resulted in a less than $ 1 million decrease in the fair value of assets and a $ 1 million decrease in the fair value of liabilities at September 30, 2017 . Fair Value Measurements The following tables set forth by level, within the fair value hierarchy, TVA's financial assets and liabilities that were measured at fair value on a recurring basis at September 30, 2017 , and 2016 . Financial assets and liabilities have been classified in their entirety based on the lowest level of input that is significant to the fair value measurement. TVA's assessment of the significance of a particular input to the fair value measurement requires judgment and may affect the determination of the fair value of the assets and liabilities and their classification in the fair value hierarchy levels. Fair Value Measurements At September 30, 2017 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets Investments Equity securities $ 226 $ — $ — $ 226 Government debt securities 100 42 — 142 Corporate debt securities — 373 — 373 Mortgage and asset-backed securities — 49 — 49 Institutional mutual funds 94 — — 94 Forward debt securities contracts — 19 — 19 Private equity funds measured at net asset value (1) — — — 136 Private real estate funds measured at net asset value (1) — — — 113 Commingled funds measured at net asset value (1) — — — 1,451 Total investments 420 483 — 2,603 Commodity contract derivatives — 8 2 10 Total $ 420 $ 491 $ 2 $ 2,613 Quoted Prices in Active Markets for Identical Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Liabilities Currency swap(s) (2) $ — $ 103 $ — $ 103 Interest rate swaps — 1,511 — 1,511 Commodity contract derivatives — 1 69 70 Commodity derivatives under FTP (2) Swap contracts — 1 — 1 Total $ — $ 1,616 $ 69 $ 1,685 Notes (1) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (2) Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities . Fair Value Measurements At September 30, 2016 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets Investments Equity securities $ 196 $ — $ — $ 196 Government debt securities 88 36 — 124 Corporate debt securities — 393 — 393 Mortgage and asset-backed securities — 50 — 50 Institutional mutual funds 92 — — 92 Forward debt securities contracts — 15 — 15 Private equity funds measured at net asset value (1) — — — 132 Private real estate funds measured at net asset value (1) — — — 113 Commingled funds measured at net asset value (1) — — — 1,142 Total investments 376 494 — 2,257 Commodity contract derivatives — 5 4 9 Total $ 376 $ 499 $ 4 $ 2,266 Quoted Prices in Active Markets for Identical Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Liabilities Currency swap(s) (2) $ — $ 162 $ — $ 162 Interest rate swaps — 1,938 — 1,938 Commodity contract derivatives — 3 131 134 Commodity derivatives under FTP (2) Swap contracts — 9 — 9 Total $ — $ 2,112 $ 131 $ 2,243 Notes (1) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (2) Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of any cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities . TVA uses internal valuation specialists for the calculation of its commodity contract derivatives fair value measurements classified as Level 3. Analytical testing is performed on the change in fair value measurements each period to ensure the valuation is reasonable based on changes in general market assumptions. Significant changes to the estimated data used for unobservable inputs, in isolation or combination, may result in significant variations to the fair value measurement reported. The following table presents a reconciliation of all commodity contract derivatives measured at fair value on a recurring basis using significant unobservable inputs (Level 3): Fair Value Measurements Using Significant Unobservable Inputs Commodity Contract Derivatives Balance at October 1, 2015 $ (98 ) Purchases — Issuances — Sales — Settlements — Change in net unrealized gains (losses) deferred as regulatory assets and liabilities (29 ) Balance at September 30, 2016 (127 ) Purchases — Issuances — Sales — Settlements — Change in net unrealized gains (losses) deferred as regulatory assets and liabilities 60 Balance at September 30, 2017 $ (67 ) The following table presents quantitative information related to the significant unobservable inputs used in the measurement of fair value of TVA's assets and liabilities classified as Level 3 in the fair value hierarchy: Quantitative Information about Level 3 Fair Value Measurements Fair Value at September 30 2017 Valuation Technique(s) Unobservable Inputs Range Assets Commodity contract derivatives $ 2 Pricing model Coal supply and demand 0.6 - 0.7 billion tons/year Long-term market prices $11.40 - $112.23/ton Liabilities Commodity contract derivatives $ 69 Pricing model Coal supply and demand 0.6 - 0.7 billion tons/year Long-term market prices $11.40 - $112.23/ton Quantitative Information about Level 3 Fair Value Measurements Fair Value at September 30 2016 Valuation Technique(s) Unobservable Inputs Range Assets Commodity contract derivatives $ 4 Pricing model Coal supply and demand 0.7 - 0.8 billion tons/year Long-term market prices $11.80 - $85.02/ton Liabilities Commodity contract derivatives $ 131 Pricing model Coal supply and demand 0.7 - 0.8 billion tons/year Long-term market prices $11.80 - $85.02/ton Other Financial Instruments Not Recorded at Fair Value TVA uses the methods and assumptions described below to estimate the fair values of each significant class of financial instrument. The fair value of the financial instruments held at September 30, 2017 , and 2016 , may not be representative of the actual gains or losses that will be recorded when these instruments mature or are called or presented for early redemption. The estimated values of TVA's financial instruments not recorded at fair value at September 30, 2017 , and 2016 , were as follows: Estimated Values of Financial Instruments Not Recorded at Fair Value At September 30, 2017 At September 30, 2016 Valuation Classification Carrying Amount Fair Value Carrying Amount Fair Value EnergyRight ® receivables (including current portion) Level 2 $ 125 $ 127 $ 141 $ 144 Loans and other long-term receivables, net (including current portion) Level 2 $ 118 $ 107 $ 141 $ 130 EnergyRight ® financing obligation (including current portion) Level 2 $ 144 $ 161 $ 163 $ 183 Unfunded loan commitments Level 2 $ — $ 18 $ — $ 17 Membership interests of variable interest entity subject to mandatory redemption (including current portion) Level 2 $ 32 $ 41 $ 35 $ 46 Long-term outstanding power bonds (including current maturities), net Level 2 $ 21,933 $ 26,857 $ 22,456 $ 28,620 Long-term debt of variable interest entities (including current maturities), net Level 2 $ 1,200 $ 1,356 $ 1,234 $ 1,468 Long-term notes payable (including current maturities) Level 2 $ 122 $ 121 $ 75 $ 75 Due to the short-term maturity of Cash and cash equivalents, Restricted cash and investments, and Short-term debt, net (each considered a Level 1 valuation classification), the carrying amounts of these instruments approximate their fair values. The fair value for loans and other long-term receivables is estimated by determining the present value of future cash flows using a discount rate equal to lending rates for similar loans made to borrowers with similar credit ratings and for similar remaining maturities, where applicable. The fair value of long-term debt traded in the public market is determined by multiplying the par value of the debt by the indicative market price at the balance sheet date. The fair value of other long-term debt and membership interests of variable interest entities subject to mandatory redemption is estimated by determining the present value of future cash flows using current market rates for similar obligations, giving effect to credit ratings and remaining maturities.

Proprietary Capital

Proprietary Capital	12 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Proprietary Capital	Appropriation Investment TVA’s power program and stewardship (nonpower) programs were originally funded primarily by appropriations from Congress. In 1959, Congress passed an amendment to the TVA Act that required TVA’s power program to be self-financing from power revenues and proceeds from power program financings. While TVA’s power program did not directly receive appropriated funds after it became self-financing, TVA continued to receive appropriations for certain multipurpose and other nonpower mission-related activities as well as for its stewardship activities. TVA has not received any appropriations from Congress for any activities since 1999, and since that time, TVA has funded stewardship program activities primarily with power revenues. The 1959 amendment to the TVA Act also required TVA, beginning in 1961, to make annual payments to the U.S. Treasury from net power proceeds as a repayment of and as a return on the Power Program Appropriation Investment until a total of $ 1.0 billion of the Power Program Appropriation Investment has been repaid in accordance with the 1959 amendment. TVA fulfilled its requirement to repay $ 1.0 billion of the Power Program Appropriation Investment in 2014. The TVA Act requires TVA to continue making payments to the U.S. Treasury as a return on the remaining $ 258 million of the Power Program Appropriation Investment. The table below summarizes TVA's activities related to appropriated funds and retained earnings. Summary of Proprietary Capital Activity At or for the years ended September 30 2017 2016 Power Program Nonpower Programs Power Program Nonpower Programs Appropriation Investment $ 258 $ 4,351 $ 258 $ 4,351 Retained Earnings Balance at beginning of year 7,594 (3,771 ) 6,357 (3,761 ) Net income (expense) for year 693 (8 ) 1,243 (10 ) Return on power program appropriation investment (5 ) — (6 ) — Balance at end of year 8,282 (3,779 ) 7,594 (3,771 ) Net proprietary capital at September 30 $ 8,540 $ 572 $ 7,852 $ 580 Payments to the U.S. Treasury TVA paid the U.S. Treasury $ 5 million in 2017 , $ 6 million in 2016 , and $ 5 million in 2015 as a return on the Power Program Appropriation Investment. The amount of the return on the Power Program Appropriation Investment is based on the Power Program Appropriation Investment balance at the beginning of that year and the computed average interest rate payable by the U.S. Treasury on its total marketable public obligations at the same date. The interest rates payable by TVA on the Power Program Appropriation Investment were 2.00 percent , 2.04 percent , and 2.04 percent for 2017 , 2016 , and 2015 , respectively. Accumulated Other Comprehensive Income (Loss) The items included in AOCI consist of market valuation adjustments for certain derivative instruments. See Note 15 . TVA records exchange rate gains and losses on debt in net income and marks its currency swap assets and liabilities to market through OCI. TVA had unrealized gains (losses) of $ 59 million and $ (139) million in 2017 and 2016 , respectively, on the mark-to-market of currency swaps. TVA then reclassifies an amount out of AOCI into net income, offsetting the gain/loss from recording the exchange gain/loss on the debt. The amounts reclassified from OCI into net income resulted in increases (decreases) to net income of $ 26 million , $ (129) million , and $ (65) million in 2017 , 2016 , and 2015 , respectively. These reclassifications, coupled with the recording of the exchange gain/loss on the debt, did not have an impact on net income in 2017 , 2016 , and 2015 . Based on forecasted foreign currency exchange rates, TVA expects to reclassify approximately $18 million of losses from AOCI to interest expense within the next twelve months to offset amounts anticipated to be recorded in interest expense related to exchange gain on the debt.

Other Income (Expense), Net

Other Income (Expense), Net	12 Months Ended
	Sep. 30, 2017
Other Income and Expenses [Abstract]
Other Income (Expense), Net	Income and expenses not related to TVA’s operating activities are summarized in the following table: Other Income (Expense), Net For the years ended September 30 2017 2016 2015 Interest income $ 23 $ 24 $ 24 External services 14 12 12 Gains (losses) on investments 9 7 (1 ) Miscellaneous 10 — (6 ) Total other income (expense), net $ 56 $ 43 $ 29

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	12 Months Ended
	Sep. 30, 2017
Supplemental Cash Flow Information [Abstract]
Supplemental Cash Flow Information	Interest paid was $1.3 billion for 2017 , 2016 , and 2015 . These amounts differ from interest expense due to the timing of payments and interest capitalized of $235 million in 2016 and $214 million in 2015 as a part of major capital expenditures. Construction in progress and Nuclear fuel expenditures included in Accounts payable and accrued liabilities at September 30, 2017 , 2016 , and 2015 were $425 million , $526 million , and $530 million , respectively, and are excluded from the Statements of Consolidated Cash Flows for the years ended September 30, 2017 , 2016 , and 2015 as non-cash investing activities. Excluded from the Statement of Consolidated Cash Flows for the years ended September 30, 2017 , 2016 , and 2015 as non-cash financing activities were capital lease obligations incurred related to purchase power assets of $10 million , $81 million , and less than $1 million , respectively. Also excluded from the Statement of Consolidated Cash Flows for the years ended September 30, 2017 and 2016 were $74 million and $78 million , respectively, of notes payable related to TVA's acquisition of equity interests in certain SPEs. See Note 9 . Cash flows from futures contracts, forward contracts, option contracts, and swap contracts that are accounted for as hedges are classified in the same category as the item being hedged or on a basis consistent with the nature of the instrument.

Benefit Plans

Benefit Plans	12 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
Benefit Plans	TVA sponsors a qualified defined benefit plan (“pension plan”) that covers most of its full-time employees hired prior to July 1, 2014, a qualified defined contribution plan (“401(k) plan”) that covers most of its full-time employees, two unfunded post-retirement health care plans that provide for non-vested contributions toward the cost of eligible retirees' medical coverage, other postemployment benefits such as workers' compensation, and the SERP. The pension plan and the 401(k) plan are administered by a separate legal entity, the TVA Retirement System (“TVARS”), which is governed by its own board of directors (the "TVARS Board"). Overview of Plans and Benefits Retirement Plans. The participants in the pension plan receive either a traditional final average pay pension or a cash balance pension. The traditional pension benefit is based on the participant’s creditable service, average monthly salary for their highest three consecutive years of eligible compensation, and a pension factor based on the participant’s age and years of service, less a Social Security offset. The cash balance pension benefit is based on pay and interest credits accumulated in the participant’s account and the participant’s age. Participants in the pension plan are also eligible to receive 401(k) plan matching contributions and may also be eligible to make after-tax contributions of up to $ 10,000 per year to TVARS, which at the election of the participant are invested in either the fixed fund, which receives a fixed interest rate set forth in the plan, or the variable fund, which receives a rate of return based on an S&P 500 index fund. Participants in the pension plan may also become eligible for a supplemental pension benefit based on age and years of service at retirement, which is provided to help offset the cost of retiree medical insurance. Employees first hired on or after July 1, 2014, are participants in the 401(k) plan only and receive both nonelective and matching contributions to their accounts in the 401(k) plan. On August 8, 2016, the TVARS Board approved amendments to the pension plan and the 401(k) plan, and these amendments were also approved by the TVA Board on August 25, 2016. The amendments, which became effective on October 1, 2016, changed future retirement benefits for employees and retirees and made certain other changes regarding TVA's minimum funding requirements to the pension plan and plan governance. With respect to current cash balance participants in the pension plan, these amendments shift future benefit accruals from the cash balance pension to the 401(k) plan based on hire date and years of service as of Octobe r 1, 2016. For cash balance participants first hired on or after January 1, 1996, and having more than 10 years of service as of October 1, 2016, participants will begin receiving nonelective contributions to their accounts in the 401(k) plan and reduced pay credits to their cash balance accounts in the pension plan. For cash balance participants first hired on or after January 1, 1996, and having less than 10 years of service as of October 1, 2016, participants will begin receiving nonelective contributions and higher matching contributions to their accounts in the 401(k) plan and will no longer receive pay credits to their cash balance accounts; however, their cash balance accounts will continue to receive interest credits. The amendments also made the following additional benefit changes: reducing the future cash balance interest crediting rate and the fixed fund interest rate with a floor and ceiling based on the assumed rate of investment return on TVARS assets; closing the fixed and variable funds to new contributions from pension plan participants first hired on or after January 1, 1996; reducing the rate of future cost-of-living-adjustments (“COLAs”) while increasing the maximum eligible COLA; vesting COLAs; increasing the eligibility age for COLAs for pension plan participants under age 50 ; restricting COLAs to pension amounts based on compensation up to Executive Level IV; eliminating future COLAs to SERP participants with less than 10 years of service; and capping the maximum supplemental benefit amounts. The amendments also changed the annual minimum contribution required by TVA to the pension plan to the greater of (a) the minimum contribution calculated by TVARS’s actuary according to the TVARS Rules and Regulations, or (b) $300 million , for a period of 20 years (from 2017 through 2036) or, if earlier, through the fiscal year in which the plan reaches and remains at a 100 percent funded status under the actuarial rules applicable to TVARS. 401(k) Plan Contributions . TVA made non-elective and matching contributions to the 401(k) plan of approximately $ 80 million during 2017 , $ 38 million during 2016 , and $ 36 million during 2015 . Supplemental Executive Retirement Plan. TVA has established a SERP for certain executives in critical positions to provide supplemental pension benefits tied to compensation that exceeds limits imposed by IRS rules applicable to the qualified defined benefit pension plan. Other Post-Retirement Benefits. TVA sponsors two unfunded post-retirement benefit plans that provide for non-vested contributions toward the cost of certain eligible retirees’ medical coverage. The first plan covers only certain retirees and surviving dependents who do not qualify for TVARS benefits, including the supplemental pension benefit. The second plan is designed to place a limit on the out-of-pocket amount certain eligible retirees pay for medical coverage and provides a credit based on years of TVA service and monthly base pension amount, reduced by any TVARS supplemental pension benefits or any TVA contribution from the first plan, described above. Effective January 2017, all Medicare-eligible retirees and spouses were provided Medicare coverage through a private exchange. Transition to the exchange does not affect any supplemental benefits for eligible retirees, and the credit will continue to be calculated in the same manner as before. Other Post-Employment Benefits. TVA employees injured in work-related incidents are covered by the workers’ compensation program for federal employees administered through the Department of Labor by the Office of Workers’ Compensation Programs in accordance with the provisions of Federal Employees' Compensation Act ("FECA") . FECA provides compensation and medical benefits to federal employees for permanent and temporary disability due to employment-related injury or disease. Accounting Mechanisms Regulatory Accounting. TVA has classified all amounts related to unrecognized prior service costs, net actuarial gains or losses, and the funded status as regulatory assets or liabilities as such amounts are probable of collection in future rates. Additionally, on October 1, 2014, TVA began recognizing pension costs as regulatory assets or liabilities to the extent that the amount calculated under GAAP as pension expense differs from the amount TVA contributes to the pension plan. Cost Method. TVA uses the projected unit credit cost method to determine the service cost and the projected benefit obligation for retirement, termination, and ancillary benefits. Under this method, a “projected accrued benefit” is calculated at the beginning of the year and at the end of the year for each benefit that may be payable in the future. The “projected accrued benefit” is based on the plan’s accrual formula and upon service at the beginning or end of the year, but it uses final average compensation, social security benefits, and other relevant factors projected to the age at which the employee is assumed to leave active service. The projected benefit obligation is the actuarial present value of the “projected accrued benefits” at the beginning of the year for employed participants and is the actuarial present value of all benefits for other participants. The service cost is the actuarial present value of the difference between the “projected accrued benefits” at the beginning and end of the year. Amortization of Net Gain or Loss. TVA utilizes the corridor approach for gain/loss amortization. Differences between actuarial assumptions and actual plan results are deferred and amortized into periodic cost only when the accumulated differences exceed 10 percent of the greater of the projected benefit obligation or the market-related value of plan assets. If necessary, the excess is amortized over the average remaining service period of participating employees expected to receive benefits under the plan, which is currently 10 years and will continue to decline since no new participants will be added to the plan. Amortization of Prior Service Cost/(Credit). Amortization of net prior service cost/(credit) resulting from a plan change is included as a component of period expense in the year first recognized and every year thereafter until it is fully amortized. The increase or decrease in the benefit obligation due to the plan change is amortized over the average remaining service period of participating employees expected to receive benefits under the plan. The pension and post-retirement plans have prior service credits related to plan changes made in 2009, 2010, and 2016 with remaining amortization periods of three to ten years. However, when a plan change reduces the benefit obligation, existing positive prior service costs are reduced or eliminated starting with the earliest established before a new prior service credit base is established. Asset Method. TVA recognizes the impact of asset performance on pension expense over a three -year phase-in period through a “market-related” value of assets calculation. Since the “market-related” value of assets recognizes investment gains and losses over a three -year period, the future value of assets will be impacted as previously deferred gains or losses are recognized. The “market-related” value is used in calculating expected return on plan assets and net gain or loss for pension cost determination. Obligations and Funded Status The changes in plan obligations, assets, and funded status for the years ended September 30, 2017 and 2016 , were as follows: Obligations and Funded Status For the years ended September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Change in benefit obligation Benefit obligation at beginning of year $ 13,083 $ 12,824 $ 571 $ 657 Service cost 60 133 18 16 Interest cost 464 564 21 29 Plan participants’ contributions 9 25 — — Collections (1) — — 47 92 Actuarial (gain) loss (286 ) 1,188 (80 ) 68 Plan change — (960 ) — (158 ) Net transfers (to) from variable fund/401(k) plan (12 ) 7 — — Expenses paid (5 ) (6 ) — — Benefits paid (712 ) (692 ) (83 ) (133 ) Benefit obligation at end of year 12,601 13,083 494 571 Change in plan assets Fair value of net plan assets at beginning of year 7,145 6,797 — — Actual return on plan assets 759 733 — — Plan participants’ contributions 9 25 — — Collections (1) — — 47 92 Net transfers (to) from variable fund/401(k) plan (12 ) 7 — — Employer contributions (2) 805 281 36 41 Expenses paid (5 ) (6 ) — — Benefits paid (712 ) (692 ) (83 ) (133 ) Fair value of net plan assets at end of year 7,989 7,145 — — Funded status $ (4,612 ) $ (5,938 ) $ (494 ) $ (571 ) Notes (1) Collections include retiree contributions as well as federal reinsurance payments and provider discounts and rebates. (2) Other Post-Retirement Benefits Employer contributions are reduced by federal reinsurance payments and provider discounts and rebates. The pension actuarial gain for 2017 primarily reflects the impact of the increase in the discount rate from 3.65 percent to 3.85 percent , which decreased the liability by $292 million . In addition, gains of $117 million were due to mortality assumption changes. These gains were partially offset by a $119 million loss related to a change in the assumption of participants’ benefit payment elections, based on recent plan experience. The 2016 pension plan change was a result of the amendments to the TVA qualified defined benefit pension plan described above, which reduced the projected benefit obligation by $ 960 million and established an additional unrecognized prior service credit at September 30, 2016 to be amortized for approximately 11 years as a component of net periodic pension benefit cost. The $ 1.2 billion pension actuarial loss for 2016 was primarily due to the decrease in the discount rate from 4.50 percent to 3.65 percent, which increased the projected benefit obligation by $ 1.4 billion . This loss was partially offset by assumption changes for the COLA of $ 168 million and for mortality of $ 133 million to better reflect anticipated future plan experience. The other post-retirement actuarial gain for 2017 was primarily due to lower per capita costs, which decreased the liability by $ 66 million . In addition, gains of $ 18 million were due to an increase in the discount rate from 3.70 percent to 3.95 percent , and gains of $ 6 million resulted from the updated mortality assumption. These gains were slightly offset by a change in the pre-Medicare trend rate, primarily driven by recent increases in prescription drug costs. The 2016 post-retirement plan change was a result of transitioning all Medicare eligible retirees and spouses to a private exchange effective January 2017, which reduced the projected benefit obligation by $ 158 million . The other post-retirement actuarial loss for 2016 was primarily due to the decrease in the discount rate from 4.65 percent to 3.70 percent, which increased the liability by $ 91 million . The loss was partially offset by a gain of $ 17 million due to demographic experience related to updated per capita costs and retiree contributions, and a gain of $ 7 million related to assumption changes for mortality to better reflect anticipated future plan experience. Amounts related to these benefit plans recognized on TVA's consolidated balance sheets consist of regulatory assets that have not been recognized as components of net periodic benefit cost at September 30, 2017 and 2016 , and the funded status of TVA’s benefit plans, which are included in Accounts payable and accrued liabilities and Post-retirement and post-employment benefit obligations: Amounts Recognized on TVA's Consolidated Balance Sheets At September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Regulatory assets (liabilities) $ 4,009 $ 5,336 $ (23 ) $ 49 Accounts payable and accrued liabilities (4 ) (5 ) (33 ) (35 ) Pension and post-retirement benefit obligations (1) (4,608 ) (5,933 ) (461 ) (536 ) Note (1) The table above excludes $408 million and $460 million of post-employment benefit costs that are recorded in Post-retirement and post-employment benefit obligations on the Consolidated Balance Sheets at September 30, 2017 and 2016 , respectively. Unrecognized amounts included in regulatory assets or liabilities yet to be recognized as components of accrued benefit cost at September 30 consisted of: Post-Retirement Benefit Costs Deferred as Regulatory Assets At September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Unrecognized prior service credit $ (918 ) $ (1,017 ) $ (163 ) $ (185 ) Unrecognized net loss 4,885 5,946 140 234 Amount capitalized due to actions of regulator 42 407 — — Total regulatory assets $ 4,009 $ 5,336 $ (23 ) $ 49 The projected benefit obligation, accumulated benefit obligation, and fair value of plan assets for the pension plan at September 30, 2017 , and 2016 , were as follows: Projected Benefit Obligations and Accumulated Benefit Obligations in Excess of Plan Assets At September 30 2017 2016 Projected benefit obligation $ 12,601 $ 13,083 Accumulated benefit obligation 12,461 12,912 Fair value of net plan assets 7,989 7,145 The components of net periodic benefit cost and other amounts recognized as changes in regulatory assets for the years ended September 30, 2017 , and 2016 , were as follows: Components of Net Periodic Benefit Cost For the years ended September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2015 2017 2016 2015 Service cost $ 60 $ 133 $ 130 $ 18 $ 16 $ 16 Interest cost 464 564 540 21 29 29 Expected return on plan assets (457 ) (446 ) (437 ) — — — Amortization of prior service credit (99 ) (23 ) (21 ) (22 ) (6 ) (6 ) Recognized net actuarial loss 472 310 299 14 7 9 Curtailment — (78 ) — — — — Total net periodic benefit cost as actuarially determined 440 460 511 31 46 48 Amount expensed (capitalized) due to actions of regulator 365 (179 ) (228 ) — — — Total net period benefit cost $ 805 $ 281 $ 283 $ 31 $ 46 $ 48 The amounts in the regulatory asset that are expected to be recognized as components of net periodic benefit cost during the next fiscal year are as follows: Expected Amortization of Regulatory Assets in 2018 At September 30, 2017 Pension Benefits Other Post-Retirement Benefits Total Prior service credit $ (98 ) $ (22 ) $ (120 ) Net actuarial loss 412 7 419 The amount in the components of net periodic benefit cost expected to be capitalized due to actions of the regulator in the next fiscal year is $ 57 million . Plan Assumptions TVA’s reported costs of providing the plan benefits are impacted by numerous factors including the provisions of the plans, changing employee demographics, and various assumptions, the most significant of which are noted below. Actuarial Assumptions Utilized to Determine Benefit Obligations at September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Discount rate 3.85 % 3.65 % 3.95 % 3.70 % Rate of compensation increase 5.43 % 5.55 % N/A N/A Pre-Medicare eligible Initial health care cost trend rate N/A N/A 6.50 % 6.50 % Ultimate health care cost trend rate N/A N/A 5.00 % 5.00 % Year ultimate trend rate is reached N/A N/A 2024 2019 Post-Medicare eligible Initial health care cost trend rate N/A N/A — % — % Ultimate health care cost trend rate N/A N/A 4.00 % 4.00 % Year ultimate trend rate is reached N/A N/A 2021 2021 Actuarial Assumptions Utilized to Determine Net Periodic Benefit Cost for the Years Ended September 30 (1) Pension Benefits Other Post-Retirement Benefits 2017 2016 2015 2017 2016 2015 Discount rate 3.65 % 4.50 % 4.45 % 3.70 % 4.65 % 4.50 % Expected return on plan assets 7.00 % 7.00 % 7.00 % N/A N/A N/A Rate of compensation increase 5.55 % 5.70 % 5.70 % N/A N/A N/A Pre-Medicare eligible Initial health care cost trend rate N/A N/A N/A 6.50 % 7.00 % 7.50 % Ultimate health care cost trend rate N/A N/A N/A 5.00 % 5.00 % 5.00 % Year ultimate trend rate is reached N/A N/A N/A 2019 2019 2019 Post-Medicare eligible Initial health care cost trend rate N/A N/A N/A — % 7.00 % 7.50 % Ultimate health care cost trend rate N/A N/A N/A 4.00 % 5.00 % 5.00 % Year ultimate trend rate is reached N/A N/A N/A 2021 2019 2019 Note (1) The actuarial assumptions used to determine the benefit obligations at September 30 of each year are subsequently used to determine net periodic benefit cost for the following year. Discount Rate. In selecting the assumed discount rate, TVA reviews market yields on high-quality corporate debt and long-term obligations of the U.S. Treasury and endeavors to match, through the use of a hypothetical bond portfolio, instrument maturities with the maturities of its pension obligations in accordance with the prevailing accounting standards. The selected bond portfolio is derived from a universe of high quality corporate bonds of Aa-rated quality or higher. After the bond portfolio is selected, a single interest rate is determined that equates the present value of the plan's projected benefit payments discounted at this rate with the market value of the bonds selected. Based on recent market trends and economic conditions, TVA increased its discount rate used to determine the pension benefit obligation and other post-retirement benefit obligation. Rate of Return . The qualified defined benefit pension plan is the only plan that is funded with qualified plan assets. In determining the expected long-term rate of return on pension plan assets, TVA uses a process that incorporates actual historical asset class returns and an assessment of expected future performance and takes into consideration external actuarial advice, the current outlook on capital markets, the asset allocation policy, and the anticipated impact of active management. Asset allocations are periodically updated using the pension plan asset/liability studies and are part of the determination of the estimates of long-term rates of return. The current asset allocation policy approved by the TVARS Board diversifies plan assets across multiple asset classes so as to minimize the risk of large losses. The asset allocation policy is designed to be dynamic in nature and responsive to changes in the funded status of TVARS. Changes in the expected return rates are based on annual studies performed by third party professional investment consultants. In 2017, upon review of the changes in the plan’s asset target allocation mix, capital market outlooks, and the most recent studies, TVA management adopted a 6.75 percent expected long-term return on plan assets which will be used to calculate the 2018 net periodic pension cost. Compensation Increases . Assumptions related to compensation increases are based on the results obtained from an actual company experience study performed during the most recent five years for plan participants. TVA obtained an updated study in 2013 and determined that future compensation would likely increase at rates between 3.50 percent and 13.00 percent per year, depending upon the employee's age. The average assumed compensation increase used to determine benefit obligations is based upon the current active participants. Mortality. The mortality assumption is comprised of a base table that represents the current future expectation of life expectancy adjusted by an improvement scale to project future improvements in life expectancy. TVA's mortality assumptions are based upon actuarial projections in combination with studies of the actual mortality experience of TVA's pension and post-retirement plan participants while taking into consideration the published Society of Actuaries ("SOA") mortality table and projection scale at September 30. Based upon an updated review of mortality improvements, TVA adopted a modified version of the SOA MP-2016 improvement scale and maintained its adjusted version of the SOA RP-2014 mortality table to measure the pension and post-retirement benefit obligation at September 30, 2017 . The following mortality assumptions were used to determine the benefit obligations for the pension and other post-retirement benefit plans at September 30, 2017, 2016, and 2015. Assumptions used to determine year-end benefit obligations are the assumptions used to determine the subsequent year’s net periodic benefit costs. Mortality Assumptions At September 30 2017 2016 2015 Mortality table RP-2014 table (adjusted) RP-2014 table (adjusted) RP-2014 table (adjusted) Improvement scale MP-2016 (modified) RP-2015 scale (modified) MP-2014 (modified) Health Care Cost Trends. TVA reviews actual recent cost trends and projected future trends in establishing health care cost trend rates. In 2017, TVA reset the current trend rate assumption used to determine the pre-Medicare eligible post-retirement obligation to 6.50 percent with the assumption to gradually decrease each successive year until it reaches a 5.00 percent annual increase in health care costs in 2024 and beyond. TVA maintained the post-Medicare eligible health care cost trend assumption at 0.00 percent through 2020 at which point it increases to 4.00 percent in 2021 and beyond as a result of the move of Medicare eligible retirees to a private exchange beginning January 2017. Cost of Living Adjustment. COLAs are an increase in the benefits for eligible retirees to help maintain the purchasing power of benefits as consumer prices increase. Eligible retirees receive a COLA on the base pension portion of the monthly pension benefit equal to the percentage change in the Consumer Price Index for All Urban Consumers (“CPI-U”) in January following any year in which the 12-month average CPI-U exceeded by as much as one percent the 12-month average of the CPI-U for the preceding year in which a COLA was given. Increases in the COLA will be the percent increase in CPI-U over the preceding year less 0.25 percent, with a 6.00 percent cap for any one year. TVA's COLA assumption is derived from long-term expectations of the expected future rate of inflation, based upon capital market assumptions, economic forecasts, and the Federal Reserve policy. TVA’s 2017 COLA assumption remained at 2.00 percent for CY 2018 and thereafter to measure the benefit obligations. TVA’s 2016 COLA assumption was 1.25 percent for CY 2017 and 2.00 percent for CY 2018 and thereafter. The COLA assumption used to determine the benefit obligations at September 30 are used to determine the net periodic benefit costs for the following fiscal year adjusted for the actual COLA determined for the following calendar year. The actual 2017, 2016, and 2015 COLAs were 0.99 percent , 0.00 percent , and 1.68 percent , respectively. The assumed COLAs for 2018 and beyond are based on the underlying CPI assumption of 2.25 percent , less 0.25 percent . The actual 2018 COLA will not be determined until December 2017. Sensitivity of Costs to Changes in Assumptions. The following chart reflects the sensitivity of pension cost to changes in certain actuarial assumptions: Sensitivity to Certain Changes in Pension Assumptions At September 30, 2017 Actuarial Assumption Change in Assumption Impact on 2017 Pension Cost Impact on 2017 Projected Benefit Obligation Discount rate (0.25 )% $ 17 $ 367 Rate of return on plan assets (0.25 )% 16 N/A Each fluctuation above assumes that the other components of the calculation are held constant and excludes any impact for unamortized actuarial gains or losses. The following chart reflects the sensitivity of post-retirement benefit cost to changes in the health care trend rate: Sensitivity to Changes in Assumed Health Care Cost Trend Rates At September 30, 2017 1% Increase 1% Decrease Effect on total of service and interest cost components for the year $ 5 $ (5 ) Effect on end-of-year accumulated post-retirement benefit obligation 70 (67 ) Each fluctuation above assumes that the other components of the calculation are held constant and excludes any impact for unamortized actuarial gains or losses. Plan Investments The qualified defined benefit pension plan (the "Plan") , which includes the Original Benefit Structure and the Cash Balance Benefit Structure, is the only plan that includes qualified plan assets. The TVARS Board’s current asset allocation policy for the investment of qualified pension plan assets has targets of 47 percent equity including global public and private equity investments, 30 percent fixed income securities, and 23 percent real assets including public and private real assets. TVARS has a long-term investment plan that contains a dynamic de-risking strategy which will allocate investments to assets that better match the liability, such as long duration fixed income securities, over time as improved funding status targets are met. Pursuant to the TVARS Rules and Regulations, any proposed changes in asset allocation that would change the system’s assumed rate of investment return are subject to TVA’s review and veto. As set forth above, the qualified pension plan assets are invested across global public equity, private equity, safety oriented fixed income, opportunistic fixed income, public real assets, and private real assets. The TVARS asset allocation policy includes permissible deviations from target allocations. The TVARS Board can take action, as appropriate, to rebalance the system’s assets consistent with the asset allocation policy. At September 30, 2017 and 2016 , the asset holdings of the system included the following: Asset Holdings of TVARS At September 30 Plan Assets at September 30 Asset Category Target Allocation 2017 2016 Global public equity 39 % 44 % 44 % Private equity 8 % 5 % 4 % Safety oriented fixed income 15 % 21 % 18 % Opportunistic fixed income 15 % 10 % 10 % Public real assets 15 % 13 % 15 % Private real assets 8 % 7 % 9 % Total 100 % 100 % 100 % Fair Value Measurements The following table provides the fair value measurement amounts for assets held by TVARS at September 30, 2017 : TVA Retirement System At September 30, 2017 Total (1) (2) Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Equity securities $ 1,771 $ 1,770 $ — $ 1 Preferred securities 14 3 11 — Debt securities Corporate debt securities 1,100 — 1,088 12 Residential mortgage-backed securities 325 — 317 8 Debt securities issued by U.S. Treasury and other U.S. government agencies 193 193 — — Debt securities issued by foreign governments 331 — 307 24 Asset-backed securities 146 — 109 37 Debt securities issued by state/local governments 19 — 17 2 Commercial mortgage-backed securities 68 — 62 6 Commingled funds measured at net asset value (3) Equity 1,134 — — — Debt 709 — — — Commodities 224 — — — Institutional mutual funds 155 155 — — Cash equivalents and other short-term investments 916 — 916 — Certificates of deposit 6 — 6 — Private equity measured at net asset value (3) 500 — — — Private real estate measured at net asset value (3) 533 — — — Securities lending collateral 369 — 369 — Derivatives Futures 18 18 — — Swaps 1 — 1 — Foreign currency forward receivable 4 — 4 — Total assets $ 8,536 $ 2,139 $ 3,207 $ 90 Liabilities Futures $ 3 $ 2 $ — $ 1 Foreign currency forward payable 6 — 6 — Credit default swaps 1 — — 1 Total liabilities $ 10 $ 2 $ 6 $ 2 Notes (1) Excludes approximately $ 168 million in net payables associated with security purchases and sales and various other payables. (2) Excludes a $ 369 million payable for collateral on loaned securities in connection with TVARS’s participation in securities lending programs. (3) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The following table provides the fair value measurement amounts for assets held by TVARS at September 30, 2016 : TVA Retirement System At September 30, 2016 Total (1) (2) Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Equity securities $ 1,847 $ 1,846 $ — $ 1 Preferred securities 20 3 17 — Debt securities Corporate debt securities 1,145 — 1,135 10 Residential mortgage-backed securities 181 — 165 16 Debt securities issued by U.S. Treasury and other U.S. government agencies 117 117 — — Debt securities issued by foreign governments 332 — 299 33 Asset-backed securities 118 — 87 31 Debt securities issued by state/local governments 16 — 16 — Commercial mortgage-backed securities 44 — 38 6 Commingled funds measured at net asset value (3) Equity 682 — — — Debt 653 — — — Commodities 302 — — — Blended 225 — — — Institutional mutual funds 10 10 — — Cash equivalents and other short-term investments 621 41 580 — Certificates of deposit 16 — 16 — Private equity measured at net asset value (3) 385 — — — Private real estate measured at net asset value (3) 568 — — — Securities lending collateral 3 — 3 — Derivatives Futures 2 2 — — Swaps 1 — 1 — Foreign currency forward receivable 5 — 5 — Total assets $ 7,293 $ 2,019 $ 2,362 $ 97 Liabilities Futures $ 2 $ 2 $ — $ — Foreign currency forward payable 9 — 9 — Written options 1 — 1 — Interest rate swaps 3 — 3 — Credit default swaps 1 — 1 — Total liabilities $ 16 $ 2 $ 14 $ — Notes (1) Excludes approximately $ 129 million in net payables associated with security purchases and sales and various other payables. (2) Excludes a $ 3 million payable for collateral on loaned securities in connection with TVARS’s participation in securities lending programs. (3) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The following table provides a reconciliation of beginning and ending balances of pension plan assets measured at fair value on a recurring basis where the determination of fair value includes significant unobservable inputs (Level 3): Fair Value Measuremen

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments Power Purchase Obligations. TVA has contracted with various independent power producers and LPCs for additional capacity to be made available to TVA. Several of these agreements have contractual minimum payments and are accounted for as either capital or operating leases. In total, these agreements provide 2,230 MW of summer net capability. The remaining terms of the agreements range up to fifteen years years. Additionally, TVA has contracted with regional transmission organizations to reserve 1,450 MW of transmission service to support purchases from the market and wind power purchase agreements. The remaining terms of these agreements range up to fifteen years. TVA incurred $ 178 million , $ 218 million , and $ 218 million of expense under these power purchase and transmission service agreements during 2017 , 2016 , and 2015 , respectively. Lease-related costs under TVA’s power purchase agreements not accounted for as capital leases are included in TVA's consolidated statements of operations as purchased power expense and are expensed as incurred. Under federal law, TVA is obligated to purchase power from qualifying facilities (cogenerators and small power producers). As of September 30, 2017 , there was a combined qualifying facility capacity of 258 MW from 30 different generation sources, from which TVA purchased power under this law. Membership Interests of VIE Subject to Mandatory Redemption . At September 30, 2017 , TVA had outstanding membership interests subject to mandatory redemption (including current portion) of $ 32 million issued by one of its VIEs of which it is the primary beneficiary. See Note 10 for additional information. At September 30, 2017 , the mandatory redemptions for each of the next five years are shown below: 2018 2019 2020 2021 2022 Membership interests of variable interest entity subject to mandatory redemption $ 2 $ 2 $ 3 $ 3 $ 3 Leases . TVA leases certain property, plant, and equipment under agreements with terms ranging from one to 38 years. TVA’s rental expense for operating leases, including power purchase agreement operating leases, was $ 90 million in 2017 , $ 86 million in 2016 , and $ 88 million in 2015 . At September 30, 2017 , the future minimum lease payments under operating leases, including purchased power agreements that are accounted for as operating leases, are shown below. Operating Leases Minimum payments due in years ending September 30 2018 $ 67 2019 60 2020 59 2021 59 2022 45 Thereafter 13 Total $ 303 At September 30, 2017 , the future minimum lease payments under capital leases are shown below. Capital Leases Minimum payments due in years ending September 30 2018 $ 52 2019 51 2020 51 2021 51 2022 51 Thereafter 519 Minimum annual payments 775 Less: amount representing interest (588 ) Total $ 187 Leasebacks . At September 30, 2017 , and 2016 , the outstanding leaseback obligations related to CTs and QTE were $ 338 million and $467 million , respectively. See Note 13 — Lease/Leasebacks . At September 30, 2017 , the future minimum payments under leaseback obligations are shown below. Lease/Leasebacks Minimum payments due in years ending September 30 2018 $ 50 2019 49 2020 50 2021 207 2022 25 Thereafter — Total $ 381 Unfunded Loan Commitments . At September 30, 2017 , TVA's commitments under unfunded loan commitments for each of the next five years are shown below: Unfunded Loan Commitments Payments due in the years ending September 30 2018 2019 2020 2021 2022 Unfunded loan commitments $ 12 $ — $ — $ — $ — In addition to the commitments above, TVA has contractual obligations in the form of revenue discounts related to energy prepayments. See Note 1 — Energy Prepayment Obligations. Energy Prepayment Obligations Payments due in the years ending September 30 2018 2019 2020 2021 2022 Thereafter Total Energy prepayment obligations $ 100 $ 10 $ — $ — $ — $ — $ 110 Interest payments relating to energy prepayment obligations 46 4 — — — — 50 Total $ 146 $ 14 $ — $ — $ — $ — $ 160 Contingencies Nuclear Insurance . Section 170 of the Atomic Energy Act, commonly known as the Price-Anderson Act, provides a layered framework of protection to compensate for liability claims of members of the public for personal injury and property damages arising from a nuclear event in the United States. For the first layer, all of the NRC nuclear plant licensees, including TVA, purchase $ 450 million of nuclear liability insurance from American Nuclear Insurers for each plant with an operating license. Funds for the second layer, the Secondary Financial Program, would come from an assessment of up to $ 127 million from the licensees of each of the 102 NRC licensed reactors in the United States. The assessment for any nuclear accident would be limited to $ 19 million per year per unit. American Nuclear Insurers, under a contract with the NRC, administers the Secondary Financial Program. With its seven licensed units, TVA could be required to pay a maximum of $ 891 million per nuclear incident, but it would have to pay no more than $ 133 million per incident in any one year. When the contributions of the nuclear plant licensees are added to the insurance proceeds of $ 450 million , over $ 13.0 billion , including a five percent surcharge for legal expenses, would be available. Under the Price-Anderson Act, if the first two layers are exhausted, the U.S. Congress is required to take action to provide additional funds to cover the additional losses. Federal law requires that each NRC power reactor licensee obtain property insurance from private sources to cover the cost of stabilizing or shutting down a reactor after an accident. TVA carries property, decommissioning, and decontamination insurance from Nuclear Electric Insurance Limited ("NEIL") , totaling $5.1 billion for its licensed nuclear plants with up to $2.1 billion available for a loss at any one site. Some of this insurance may require the payment of retrospective premiums up to a maximum of approximately $126 million . TVA purchases accidental outage (business interruption) insurance for TVA’s nuclear sites from NEIL. In the event that an accident covered by this policy takes a nuclear unit offline or keeps a nuclear unit offline, NEIL will pay TVA, after a waiting period, an indemnity (a set dollar amount per week) up to a maximum indemnity of $ 490 million per unit. This insurance policy may require the payment of retrospective premiums up to a maximum of approximately $ 43 million . Decommissioning Costs. TVA recognizes legal obligations associated with the future retirement of certain tangible long-lived assets related primarily to nuclear generating plants, coal-fired generating plants, hydroelectric generating plants/dams, transmission structures, and other property-related assets. See Note 12 . Nuclear Decommissioning . Provision for decommissioning costs of nuclear generating units is based on options prescribed by the NRC procedures to dismantle and decontaminate the facilities to meet the NRC criteria for license termination. At September 30, 2017 , the estimated future decommissioning cost of $2.9 billion was included in AROs. The actual decommissioning costs may vary from the derived estimates because of, among other things, changes in current assumptions, such as the assumed dates of decommissioning, changes in regulatory requirements, changes in technology, and changes in the cost of labor, materials, and equipment. Utilities that own and operate nuclear plants are required to use different procedures in calculating nuclear decommissioning costs under GAAP than those that are used in calculating nuclear decommissioning costs when reporting to the NRC. The two sets of procedures produce different estimates for the costs of decommissioning primarily because of differences in the underlying assumptions. TVA maintains a NDT to provide funding for the ultimate decommissioning of its nuclear power plants. See Note 16 . TVA monitors the value of its NDT and believes that, over the long term and before cessation of nuclear plant operations and commencement of decommissioning activities, adequate funds from investments and additional contributions, if necessary, will be available to support decommissioning. TVA’s operating nuclear power units are licensed through 2033 - 2055, depending on the unit. It may be possible to extend the operating life of some of the units with approval from the NRC. See Note 7 — Nuclear Decommissioning Costs and Note 12 . Non-Nuclear Decommissioning . The estimated future non-nuclear decommissioning ARO was $1.4 billion at September 30, 2017 . This decommissioning cost estimate involves estimating the amount and timing of future expenditures and making judgments concerning whether or not such costs are considered a legal obligation. Estimating the amount and timing of future expenditures includes, among other things, making projections of the timing and duration of the asset retirement process and how costs will escalate with inflation. The actual decommissioning costs may vary from the derived estimates because of changes in current assumptions, such as the assumed dates of decommissioning, changes in regulatory requirements, changes in technology, and changes in the cost of labor, materials, and equipment. TVA maintains an ART to help fund the ultimate decommissioning of its non-nuclear power assets. See Note 16 . Estimates involved in determining if additional funding will be made to the ART include inflation rate, rate of return projections on the fund investments, and the planned use of other sources to fund decommissioning costs. See Note 7 — Non-Nuclear Decommissioning Costs and Note 12 . Environmental Matters. TVA’s power generation activities, like those across the utility industry and in other industrial sectors, are subject to federal, state, and local environmental laws and regulations. Major areas of regulation affecting TVA’s activities include air quality control, water quality control, and management and disposal of solid and hazardous wastes. In the future, regulations in all of these areas are expected to become more stringent. Regulations are also expected to apply to new emissions and sources, with a particular emphasis on climate change, renewable generation, and energy efficiency. TVA has incurred, and expects to continue to incur, substantial capital and operating and maintenance costs to comply with evolving environmental requirements primarily associated with, but not limited to, the operation of TVA’s coal-fired generating units. Environmental requirements placed on the operation of TVA’s coal-fired and other generating units will likely continue to become more restrictive over time. Litigation over emissions or discharges from coal-fired generating units is also occurring, including litigation against TVA. Failure to comply with environmental and safety laws can result in TVA being subject to enforcement actions, which can lead to the imposition of significant civil liability, including fines and penalties, criminal sanctions, and/or the shutting down of non-compliant facilities . From the 1970s to 2017 , TVA spent approximately $ 6.7 billion to reduce emissions from its power plants, including $ 206 million , $ 259 million , and $ 315 million in 2017 , 2016 , and 2015 , respectively, on clean air controls. TVA estimates that compliance with existing and future Clean Air Act ("CAA") requirements (excluding greenhouse gas ("GHG") requirements) could lead to costs of $ 200 million from 2018 to 2022, which include future clean air controls, existing controls capital projects, and air operations and maintenance projects. The majority of the $ 200 million is expected to be spent by 2018 on new controls at Gallatin and Shawnee Fossil Plants. TVA also estimates additional expenditures of approximately $1.1 billion from 2018 to 2022 relating to TVA’s CCR conversion program, not including costs related to any new requirements related to the Gallatin lawsuits, as well as expenditures of approximately $ 500 million from 2018 to 2026 relating to compliance with Clean Water Act requirements. Future costs could differ from these estimates if new environmental laws or regulations become applicable to TVA or the facilities it operates, or if existing environmental laws or regulations are revised or reinterpreted. There could also be costs that cannot reasonably be predicted at this time, due to uncertainty of actions, that could increase these estimates. Liability for releases and cleanup of hazardous substances is primarily regulated by the federal Comprehensive Environmental Response, Compensation, and Liability Act ("CERCLA") , and other federal and parallel state statutes. In a manner similar to many other industries and power systems, TVA has generated or used hazardous substances over the years. TVA operations at some facilities have resulted in contamination that TVA is addressing. At September 30, 2017 , and September 30, 2016 , TVA’s estimated liability for cleanup and similar environmental work for those sites for which sufficient information is available to develop a cost estimate was approximately $7 million and $23 million , respectively, on a non-discounted basis, and was included in Accounts payable and accrued liabilities and Other long-term liabilities on the Consolidated Balance Sheets. Legal Proceedings From time to time, TVA is party to or otherwise involved in lawsuits, claims, proceedings, investigations, and other legal matters ("Legal Proceedings") that have arisen in the ordinary course of conducting TVA's activities, as a result of a catastrophic event or otherwise. General. At September 30, 2017 , TVA had accrued $ 22 million with respect to Legal Proceedings. Of the accrued amount, $ 13 million is included in Other long-term liabilities and $ 9 million is included in Accounts payable and accrued liabilities. No assurance can be given that TVA will not be subject to significant additional claims and liabilities. If actual liabilities significantly exceed the estimates made, TVA's results of operations, liquidity, and financial condition could be materially adversely affected. Environmental Agreements . In April 2011, TVA entered into two substantively similar agreements, one with the EPA and the other with Alabama, Kentucky, North Carolina, Tennessee, and three environmental advocacy groups: the Sierra Club, the National Parks Conservation Association, and Our Children's Earth Foundation (collectively, the "Environmental Agreements”). They became effective in June 2011. Under the Environmental Agreements, TVA committed to (1) retire on a phased schedule 18 coal-fired units with a combined summer net dependable capability of 2,200 MW, (2) control, convert, or retire additional coal-fired units with a combined summer net dependable capability of 3,500 MW, (3) comply with annual, declining emission caps for SO 2 and NO x , (4) invest $ 290 million in certain TVA environmental projects (of which TVA had spent approximately $ 275 million as of September 30, 2017 ), (5) provide $ 60 million to Alabama, Kentucky, North Carolina, and Tennessee to fund environmental projects, and (6) pay civil penalties of $ 10 million . In exchange for these commitments, most past claims against TVA based on alleged New Source Review and associated violations were waived and cannot be brought against TVA. Future claims, including those for sulfuric acid mist and GHG emissions, can still be brought against TVA, and claims for increases in particulates can also be pursued at many of TVA’s coal-fired units. Additionally, the Environmental Agreements do not address compliance with new laws and regulations or the cost associated with such compliance. The liabilities related to the Environmental Agreements are included in Accounts payable and accrued liabilities and Other long-term liabilities on the September 30, 2017 Consolidated Balance Sheet. In conjunction with the approval of the Environmental Agreements, the TVA Board determined that it was appropriate to record TVA's obligations under the Environmental Agreements as regulatory assets, and they are included as such on the September 30, 2017 Consolidated Balance Sheet and will be recovered in rates in future periods. Case Involving Tennessee Valley Authority Retirement System . In March 2010, eight current and former participants in and beneficiaries of TVARS filed suit in the U.S. District Court for the Middle District of Tennessee challenging the TVARS Board's 2009 decision to amend the TVARS Rules and Regulations (“Rules”) in exchange for a $1.0 billion contribution from TVA. The changes approved by the TVARS Board (1) suspended the TVA contribution requirements for 2010 through 2013, (2) reduced the calculation for COLAs for CY 2010 through CY 2013, (3) reduced the interest crediting rate for the fixed fund accounts, and (4) increased the eligibility age to receive COLAs from age 55 to 60 . The plaintiffs alleged that these changes violated their constitutional rights (due process, equal protection, and property rights), violated the Administrative Procedure Act, and violated the substantive and procedural components of an anti-cutback provision in the Rules. TVA and the plaintiffs filed cross motions for summary judgment. In August 2015, the court granted TVA’s motion for summary judgment and dismissed the case with prejudice. In September 2015, the plaintiffs appealed this decision to the Sixth Circuit. On August 12, 2016, the Sixth Circuit held that the plaintiffs’ rights were not violated because COLAs are not vested benefits. A few other issues were remanded to the district court for further proceedings. On March 2, 2017, the district court granted TVA's motion for a judgment on the administrative record and dismissed all the remaining claims in this case. On March 31, 2017, the plaintiffs appealed the district court's decision to the Sixth Circuit. Cases Involving Gallatin Fossil Plant CCR Facilities. TVA is a party in two lawsuits relating to alleged releases of waste materials from the CCR facilities at Gallatin. See Note 8 — Background — Lawsuit Brought by TSRA and TCWN and Lawsuit Brought by TDEC . Administrative Proceeding Regarding Browns Ferry Nuclear Plant Extended Power Uprate. In September 2016, the Bellefonte Efficiency and Sustainability Team and Mothers Against Tennessee River Radiation requested a hearing and sought to intervene in TVA’s license amendment request for extended power uprates at Browns Ferry Nuclear Plant. The petitioners contend that TVA's application did not correctly report the potential risk from operating at increased power levels. TVA and the NRC staff filed answers opposing the petition to intervene in October 2016. The Atomic Safety and Licensing Board ("ASLB") rejected the petition to intervene in November 2016. In April 2017, the NRC affirmed the ASLB's decision to deny the petition, which terminated the administrative proceeding. Petitions to Intervene in the Proceeding Involving the Early Site Permit Application for Small Modular Reactors at TVA's Clinch River Site . Three environmental groups the Southern Alliance for Clean Energy ("SACE"), Tennessee Environmental Council ("TEC"), and Blue Ridge Environmental Defense League ("BREDL") filed petitions to intervene in the proceeding regarding the Early Site Permit Application that TVA submitted for review by the NRC in May 2016 relating to the potential future construction and operation of two or more small modular reactor units at TVA’s Clinch River site in Oak Ridge, Tennessee. On October 10, 2017, the Atomic Safety and Licensing Board issued a decision admitting two contentions proffered jointly by SACE and TEC and dismissing a third. The decision also denied admission of BREDL’s one proffered contention. Bull Run Fossil Plant Clean Air Act Permit . In September 2015, the Sierra Club and Environmental Integrity Project filed a petition with the EPA requesting that the EPA object to the CAA renewal permit issued by TDEC to TVA for operations at Bull Run. The petitioners alleged that the permit contained impermissibly lax monitoring requirements for opacity. In February 2016, the petitioners sued the EPA for not responding to the petition in a timely manner. In August 2016, the United States District Court for the District of Columbia entered a consent decree requiring the EPA to respond to the petition by November 10, 2016. On November 10, 2016, the EPA granted the petition and ordered TDEC to revise the permit to assure compliance with the opacity limits. TDEC revised the permit in accordance with the EPA's order and released it for public comment on February 7, 2017. No comments were received and TDEC issued a final permit, which became effective on April 10, 2017. The final permit provides that compliance with the particulate matter ("PM") limit is deemed sufficient to demonstrate compliance with the opacity limit. TVA plans to continue to meet the PM limit in the permit. Gallatin Fossil Plant Clean Air Act Permit . In August 2016, the Sierra Club filed a petition with the EPA requesting that the EPA object to the CAA renewal permit issued by TDEC to TVA for operations at Gallatin. The petition alleges that the permit (1) contains compliance evaluation requirements for opacity, particulate matter, and fugitive dust that are not as stringent as required, (2) includes allowances for startup, shutdown, and malfunctions that are inconsistent with the CAA, (3) fails to include reporting requirements to ensure compliance with the Environmental Agreements, and (4) contains impermissibly high SO 2 emission limits. The EPA has not yet acted on the petition. On May 15, 2017, the Sierra Club filed a lawsuit in the United States District Court for the District of Columbia seeking to compel the EPA to act on the petition. TDEC issued a public notice on July 3, 2017, proposing to revise the CAA renewal permit. The period to submit comments on this draft permit closed on August 9, 2017. TDEC will review the comments and provide any revisions to the EPA, which has 45 days to review the revisions before the permit becomes final. Gallatin can still operate under the existing permit while the renewal permit is being revised.

Related Parties

Related Parties	12 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Related Parties	TVA is a wholly-owned corporate agency of the federal government, and because of this relationship, TVA’s revenues and expenses are included as part of the federal budget as a revolving fund. TVA’s purpose and responsibilities as an agency are described under the “Other Agencies” section of the federal budget. TVA currently receives no appropriations from Congress and funds its business using power system revenues, power financings, and other revenues. TVA is a source of cash to the federal government. TVA will indefinitely continue to pay a return on the outstanding $ 258 million power program appropriation investment. See Note 17 — Appropriation Investment. TVA also has access to a financing arrangement with the U.S. Treasury pursuant to the TVA Act. TVA and the U.S. Treasury entered into a memorandum of understanding under which the U.S. Treasury provides TVA with a $ 150 million credit facility. This credit facility was renewed and has a maturity date of September 30, 2018. Access to this credit facility or other similar financing arrangements has been available to TVA since the 1960s. See Note 13 — Credit Facility Agreements. In the normal course of business, TVA contracts with other federal agencies for sales of electricity and other services. Transactions with agencies of the federal government were as follows: Related Party Transactions For the years ended, or at, September 30 2017 2016 2015 Revenue from sales of electricity $ 126 $ 126 $ 130 Other income 136 161 167 Expenditures Operating expenses 216 216 227 Additions to property, plant, and equipment 16 32 37 Cash and cash equivalents 46 54 45 Accounts receivable, net 119 129 106 Accounts payable and accrued liabilities 71 77 98 Long-term power bonds, net 1 4 5 Return on Power Program Appropriation Investment 5 6 5

Unaudited Quarterly Financial I

Unaudited Quarterly Financial Information	12 Months Ended
	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Information	A summary of the unaudited quarterly results of operations for the years 2017 and 2016 follows. This summary should be read in conjunction with the audited consolidated financial statements appearing herein. Results for interim periods may fluctuate as a result of seasonal weather conditions, changes in rates, and other factors. Unaudited Quarterly Financial Information 2017 First Second Third Fourth Total Operating revenues $ 2,546 $ 2,547 $ 2,571 $ 3,075 $ 10,739 Operating expenses 2,117 2,013 2,010 2,624 8,764 Operating income 429 534 561 451 1,975 Net income (loss) 102 211 233 139 685 Unaudited Quarterly Financial Information 2016 First Second Third Fourth Total Operating revenues $ 2,280 $ 2,571 $ 2,479 $ 3,286 $ 10,616 Operating expenses 2,052 1,982 1,913 2,343 8,290 Operating income 228 589 566 943 2,326 Net income (loss) (37 ) 318 291 661 1,233

Gallatin coal combustion residu

Gallatin coal combustion residual facilities (Notes)	12 Months Ended
	Sep. 30, 2017
Gallatin coal combustion residual facilities [Abstract]
Gallatin coal combustion residual facilities [Text Block]	Gallatin Coal Combustion Residual Facilities Background TVA is planning to close wet CCR impoundments in accordance with federal and applicable state requirements when (1) coal-fired plants are converted to dry CCR processes and dry storage landfills become operational or (2) plant operations cease. Closure project schedules and costs are driven by the selected closure technology. The impoundments at Gallatin are pending additional studies to determine the final closure methodology and schedule. While plans are currently being formulated for the CCR closure methodology for Gallatin, TVA is involved in two lawsuits relating to alleged discharges of pollutants from the CCR facilities at Gallatin. Lawsuit Brought by TDEC . In January 2015, the Tennessee Department of Environment and Conservation (“TDEC”) filed a lawsuit against TVA in the Chancery Court for Davidson County, Tennessee. The lawsuit alleges that pollutants have been discharged into waters of the State from CCR facilities at Gallatin in violation of the Tennessee Water Quality Control Act and the Tennessee Solid Waste Disposal Act. TDEC seeks injunctive relief, which could include an order requiring TVA to relocate the CCR facilities. TDEC also requested civil penalties of up to $ 17,000 per day for each day TVA is found to have violated the statutes. In February 2015, the court issued an order allowing Tennessee Scenic Rivers Association ("TSRA") and Tennessee Clean Water Network ("TCWN") to intervene in the case, and in January 2016, the court ordered TVA, among other things, to develop and submit to TDEC an environmental investigation plan and an environmental assessment report (the “January 2016 Order”). On August 4, 2017, TDEC filed an amended complaint adding new facts, claims, and causes of action. Consequently, on August 10, 2017, TVA removed the case from state court to federal court. The case is now in the United States District Court for the Middle District of Tennessee. The plaintiffs have filed motions requesting that the case be remanded to state court. Briefing on the motions is expected to be completed in November 2017. Lawsuit Brought by TSRA and TCWN . In April 2015, TSRA and the TCWN filed a lawsuit against TVA in the United States District Court for the Middle District of Tennessee alleging that pollutants have been discharged into the Cumberland River from CCR facilities at Gallatin in violation of the Clean Water Act (“CWA”). The plaintiffs are seeking injunctive relief, including an order requiring TVA to relocate the CCR facilities, civil penalties of up to $ 37,500 per violation per day, and attorneys’ fees. Trial in this action began on January 30, 2017, and concluded February 2, 2017. On August 4, 2017, the court issued a decision largely in favor of the plaintiffs (the “August 2017 Order”), finding that TVA had discharged pollutants into the Cumberland River in the past and that the discharge was likely ongoing. The court ordered TVA to excavate the CCR materials and move them to a lined facility. The court further required TVA to file within 30 days a draft timetable for excavating and removing the material. The court did not assess any monetary penalties against TVA for the CWA violations, citing the fact that its order to relocate the CCR material would cause TVA to incur significant costs. On September 5, 2017, TVA submitted the required draft timetable, which assumes that a new lined facility can be permitted and built on the Gallatin site. The process of obtaining the necessary permits, constructing the facility, and moving all of the CCR materials is estimated to take approximately 24 years. Under current regulations, TVA would be required to monitor the existing facilities and the new facility for thirty years after closure. The estimated cost of the potential Gallatin CCR project is approximately $ 900 million . At September 30, 2017, related liabilities of $ 880 million and $ 19 million were recorded in Other long-term liabilities and Accounts payable and accrued liabilities, respectively. Prior to the court’s decision, TVA had anticipated spending approximately $ 200 million to cap and close the existing CCR facilities. On October 2, 2017, TVA appealed the court’s decision to the United States Court of Appeals for the Sixth Circuit ("Sixth Circuit"). Financial Impact In August 2017, TVA began using regulatory accounting treatment to defer expected future costs of compliance with orders or settlements related to lawsuits involving the Gallatin CCR facilities. The TVA Board approved a plan to amortize these costs over the anticipated duration of the Gallatin CCR project (excluding post-closure care), beginning October 1, 2018 as amounts are included in rates or paid out. TVA has estimated these costs to be approximately $ 900 million . These costs include, among other things, environmental studies concerning the existing and new facilities, the licensing activities for the new facility, design and construction of the new facility, relocating the material from the existing facilities to the new facility, closing the existing facilities, monitoring activities, and an amount of additional costs reflecting the expected impacts of inflation given the anticipated duration of the project. The costs do not include such items as any additional order or penalty arising from the TDEC lawsuit, which cannot be reasonably estimated at this time. TVA has not discounted this environmental obligation to a present value amount. TVA also committed in its draft timetable to complete capital projects related to construction of a permanent bottom ash dewatering facility and wastewater process ponds. These capital projects, which are not included in the estimate for cleanup costs above, are estimated to cost approximately $ 91 million and be completed over 3 years . It is reasonably possible that TVA will not be able to obtain the necessary permits to build the facility on the Gallatin site and will be required to move the CCR materials offsite. Offsite relocation would materially increase both the cost and the time to comply with the August 2017 Order. TVA has estimated that if it is required to relocate the materials to a facility off the Gallatin site, TVA may incur up to $ 2.0 billion in expenses, plus an amount of additional costs reflecting the expected impacts of inflation given the extended duration of an offsite relocation project. The process of obtaining the necessary permits for offsite disposal, locating or constructing an offsite facility, and moving all of the CCR materials offsite is estimated to take approximately 40 years. TVA would also be required to monitor the existing facilities and the offsite facility for 30 years after the facilities are closed, based on current regulations. The ultimate cost of the removal project will depend on actual timing and results of ongoing litigation, environmental studies, licensing, permitting, site subsurface conditions, contractor availability, weather, equipment, available material resources, and other contingency factors. These contingency factors could cause the project cost estimate to change materially in the near term. TVA updates its estimate for project costs as changes in these factors are determined to be probable of occurring.

Summary of Significant Accoun30

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
General	General The Tennessee Valley Authority ("TVA") is a corporate agency and instrumentality of the United States that was created in 1933 by legislation enacted by the United States ("U.S.") Congress in response to a request by President Franklin D. Roosevelt. TVA was created to, among other things, improve navigation on the Tennessee River, reduce the damage from destructive flood waters within the Tennessee River system and downstream on the lower Ohio and Mississippi Rivers, further the economic development of TVA's service area in the southeastern United States, and sell the electricity generated at the facilities TVA operates. Today, TVA operates the nation's largest public power system and supplies power in most of Tennessee, northern Alabama, northeastern Mississippi, and southwestern Kentucky and in portions of northern Georgia, western North Carolina, and southwestern Virginia to a population of over nine million people. TVA also manages the Tennessee River, its tributaries, and certain shorelines to provide, among other things, year-round navigation, flood damage reduction, and affordable and reliable electricity. Consistent with these primary purposes, TVA also manages the river system and public lands to provide recreational opportunities, adequate water supply, improved water quality, cultural and natural resource protection, and economic development. The power program has historically been separate and distinct from the stewardship programs. It is required to be self-supporting from power revenues and proceeds from power financings, such as proceeds from the issuance of bonds, notes, or other evidences of indebtedness ("Bonds") . Although TVA does not currently receive congressional appropriations, it is required to make annual payments to the United States Department of the Treasury ("U.S. Treasury") as a return on the government's appropriation investment in TVA's power facilities (the "Power Program Appropriation Investment") . In the 1998 Energy and Water Development Appropriations Act, Congress directed TVA to fund essential stewardship activities related to its management of the Tennessee River system and nonpower or stewardship properties with power revenues in the event that there were insufficient appropriations or other available funds to pay for such activities in any fiscal year. Congress has not provided any appropriations to TVA to fund such activities since 1999. Consequently, during 2000, TVA began paying for essential stewardship activities primarily with power revenues, with the remainder funded with user fees and other forms of revenues derived in connection with those activities. The activities related to stewardship properties do not meet the criteria of an operating segment under accounting principles generally accepted in the United States of America ("GAAP") . Accordingly, these assets and properties are included as part of the power program, TVA's only operating segment. Power rates are established by the TVA Board of Directors (the "TVA Board") as authorized by the Tennessee Valley Authority Act of 1933, as amended, 16 U.S.C. §§ 831-831ee (the “TVA Act”). The TVA Act requires TVA to charge rates for power that will produce gross revenues sufficient to provide funds for operation, maintenance, and administration of its power system; payments to states and counties in lieu of taxes ("tax equivalents") ; debt service on outstanding indebtedness; payments to the U.S. Treasury in repayment of and as a return on the Power Program Appropriation Investment; and such additional margin as the TVA Board may consider desirable for investment in power system assets, retirement of outstanding Bonds in advance of maturity, additional reduction of the Power Program Appropriation Investment, and other purposes connected with TVA's power business. In setting TVA's rates, the TVA Board is charged by the TVA Act to have due regard for the primary objectives of the TVA Act, including the objective that power shall be sold at rates as low as are feasible. Rates set by the TVA Board are not subject to review or approval by any state or other federal regulatory body.
Fiscal Year	Fiscal Year TVA's fiscal year ends September 30. Years ( 2017 , 2016 , etc.) refer to TVA's fiscal years unless they are preceded by “CY,” in which case the references are to calendar years.
Cost-Based Regulation	Cost-Based Regulation Since the TVA Board is authorized by the TVA Act to set rates for power sold to its customers, TVA is self-regulated. Additionally, TVA's regulated rates are designed to recover its costs. Based on current projections, TVA believes that rates, set at levels that will recover TVA's costs, can be charged and collected. As a result of these factors, TVA records certain assets and liabilities that result from the regulated ratemaking process that would not be recorded under GAAP for non-regulated entities. Regulatory assets generally represent incurred costs that have been deferred because such costs are probable of future recovery in customer rates. Regulatory liabilities generally represent obligations to make refunds to customers for previous collections for costs that are not likely to be incurred or deferral of gains that will be credited to customers in future periods. TVA assesses whether the regulatory assets are probable of future recovery by considering factors such as applicable regulatory changes, potential legislation, and changes in technology. Based on these assessments, TVA believes the existing regulatory assets are probable of recovery. This determination reflects the current regulatory and political environment and is subject to change in the future. If future recovery of regulatory assets ceases to be probable, or any of the other factors described above cease to be applicable, TVA would no longer be considered to be a regulated entity and would be required to write off these costs. All regulatory asset write offs would be required to be recognized in earnings in the period in which future recovery ceases to be probable.
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements, which have been prepared in accordance with GAAP, include the accounts of TVA, wholly-owned direct subsidiaries, and variable interest entities ("VIE") of which TVA is the primary beneficiary. See Note 9 and Note 10 . Intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements requires TVA to estimate the effects of various matters that are inherently uncertain as of the date of the consolidated financial statements. Although the consolidated financial statements are prepared in conformity with GAAP, TVA is required to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities, and the amounts of revenues and expenses reported during the reporting period. Each of these estimates varies in regard to the level of judgment involved and its potential impact on TVA's financial results. Estimates are considered critical either when a different estimate could have reasonably been used, or where changes in the estimate are reasonably likely to occur from period to period, and such use or change would materially impact TVA's financial condition, results of operations, or cash flows.
Reclassifications	Reclassifications Amounts previously presented in Cash flows from operating activities as Insurance recoveries of $ 7 million and $ 63 million for the years ended September 30, 2016 and 2015 , respectively, are currently reported in Other, net.
Cash and Cash Equivalents and Restricted Cash and Investments	Cash and Cash Equivalents Cash includes cash on hand and non-interest bearing cash and deposit accounts. All highly liquid investments with original maturities of three months or less are considered cash equivalents.
Allowance for Uncollectible Accounts	Allowance for Uncollectible Accounts The allowance for uncollectible accounts reflects TVA's estimate of probable losses inherent in its accounts and loans receivable balances. TVA determines the allowance based on known accounts, historical experience, and other currently available information including events such as customer bankruptcy and/or a customer failing to fulfill payment arrangements after 90 days . It also reflects TVA's corporate credit department's assessment of the financial condition of customers and the credit quality of the receivables. The allowance for uncollectible accounts was less than $ 1 million and $ 1 million at September 30, 2017 , and 2016 , for accounts receivable. Additionally, loans receivable of $118 million and $ 141 million at September 30, 2017 , and 2016 , respectively, are included in Accounts receivable, net and Other long-term assets, for the current and long-term portions, respectively, and are reported net of allowances for uncollectible accounts of less than $ 1 million and $ 8 million at September 30, 2017 , and 2016 , respectively.
Energy Prepayment Obligations	Energy Prepayment Obligations In 2004, TVA and its largest customer, Memphis Light, Gas and Water Division ("MLGW") , entered into an energy prepayment agreement under which MLGW prepaid TVA $ 1.5 billion for the future costs of electricity to be delivered by TVA to MLGW over a period of 180 months . TVA accounted for the prepayment as unearned revenue and is reporting the obligation to deliver power under this arrangement as Energy prepayment obligations and Current portion of energy prepayment obligations on the September 30, 2017 and 2016 Consolidated Balance Sheets. TVA expects to recognize approximately $ 100 million of noncash revenue in each year of the arrangement as electricity is delivered to MLGW based on the ratio of units of kilowatt hours delivered to total units of kilowatt hours under contract. At September 30, 2017 , approximately $ 1.4 billion had been recognized as noncash revenue on a cumulative basis during the life of the agreement, $ 100 million of which was recognized as noncash revenue during each of 2017 , 2016 , and 2015 . Discounts to account for the time value of money, which are recorded as a reduction to electricity sales, amounted to $ 46 million for each of the years ended September 30, 2017 , 2016 , and 2015 .
Pre-Commercial Plant Operations	Pre-Commercial Plant Operations As part of the process of completing the construction of a generating unit, the electricity produced is used to serve the demands of the electric system. TVA estimates revenue from such pre-commercial generation based on the guidance provided by Federal Energy Regulatory Commission ("FERC") regulations. Watts Bar Nuclear Plant ("Watts Bar") Unit 2 commenced pre-commercial plant operations on June 3, 2016, and commercial operations began on October 19, 2016. In addition, the Paradise Combined Cycle Plant commenced pre-commercial plant operations on October 10, 2016, and commercial operations began on April 7, 2017. The Allen Combined Cycle Plant began pre-commercial operations on September 9, 2017. Estimated revenue of $ 22 million and $ 18 million related to these projects was capitalized to offset project costs for the years ended September 30, 2017 , and 2016, respectively. TVA also capitalized related fuel costs for these three construction projects of approximately $ 14 million and $ 6 million during the years ended September 30, 2017 , and 2016, respectively.
Inventories	Inventories Certain Fuel, Materials, and Supplies . Materials and supplies inventories are valued using an average unit cost method. A new average cost is computed after each inventory purchase transaction, and inventory issuances are priced at the latest moving weighted average unit cost. Coal, fuel oil, and natural gas inventories are valued using an average cost method. A new weighted average cost is computed monthly, and monthly issues are priced accordingly. Allowance for Inventory Obsolescence . TVA reviews material and supplies inventories by category and usage on a periodic basis. Each category is assigned a probability of becoming obsolete based on the type of material and historical usage data. Based on the estimated value of the inventory, TVA adjusts its allowance for inventory obsolescence. Emission Allowances . TVA has emission allowances for sulfur dioxide ("SO 2 ") and nitrogen oxides ("NO x ") which are accounted for as inventory. The average cost of allowances used each month is charged to operating expense based on tons of SO 2 and NO x emitted during the respective compliance periods. Allowances granted to TVA by the Environmental Protection Agency ("EPA") are recorded at zero cost. Renewable Energy Credits. TVA accounts for Renewable Energy Credits ("RECs") using the specific identification cost method. RECs that are acquired through power purchases are recorded as inventory and charged to purchased power expense when the RECs are subsequently used or sold. TVA assigns a value to the RECs at the inception of power purchase arrangement using a relative fair value approach. RECs created through TVA-owned asset generation are recorded at zero cost.
Property, Plant, and Equipment, and Depreciation	Property, Plant, and Equipment, and Depreciation Property, Plant, and Equipment. Additions to plant are recorded at cost, which includes direct and indirect costs and may include allowance for funds used during construction ("AFUDC") , if eligible. The cost of current repairs and minor replacements is charged to operating expense. Nuclear fuel inventories, which are included in Property, plant, and equipment, are valued using the average cost method for raw materials and the specific identification method for nuclear fuel in a reactor. Amortization of nuclear fuel in a reactor is calculated on a units-of-production basis and is included in fuel expense. Depreciation. TVA accounts for depreciation of its properties using the composite depreciation convention of accounting. Accordingly, the original cost of property retired is charged to accumulated depreciation. Depreciation is generally computed on a straight-line basis over the estimated service lives of the various classes of assets. Depreciation rates are determined based on an external depreciation study. TVA concluded and implemented a new depreciation study effective October 1, 2016, resulting in approximately $ 224 million less depreciation expense in 2017 compared to the prior year. This study will be updated at least every five years. Depreciation expense for the years ended September 30, 2017 , 2016 , and 2015 was $ 1.3 billion , $ 1.4 billion , and $ 1.7 billion , respectively. Depreciation expense expressed as a percentage of the average annual depreciable completed plant was 2.49 percent for 2017 , 2.97 percent for 2016 , and 3.71 percent for 2015 . Average depreciation rates by asset class are as follows: Property, Plant, and Equipment Depreciation Rates At September 30 (percent) 2017 2016 2015 Asset Class Nuclear 2.66 2.37 2.81 Coal-fired 2.33 3.50 5.50 Hydroelectric 1.58 1.29 1.30 Gas and oil-fired 3.27 3.09 3.18 Transmission 1.34 2.80 2.78 Other 6.12 8.97 8.65 Nuclear . In September 2015, the Nuclear Regulatory Commission ("NRC") approved renewed licenses for Sequoyah Nuclear Plant ("Sequoyah") Units 1 and 2, which allow both units to operate for an additional 20 years, and TVA adjusted prospectively the Sequoyah depreciation rate. Coal-Fired . In April 2011, TVA entered into two substantively similar agreements, one with the EPA and the other with Alabama, Kentucky, North Carolina, Tennessee, and three environmental advocacy groups (collectively, the "Environmental Agreements”). See Note 21 — Legal Proceedings — Environmental Agreements . Under the Environmental Agreements, TVA committed to retire 18 coal-fired units on a phased schedule, among other things. Since its November 2013 meeting, the TVA Board has approved the retirement of certain coal-fired units. Units subsequently retired include: Widows Creek Fossil Plant ("Widows Creek") Units 7 and 8 on September 30, 2015; Colbert Fossil Plant ("Colbert") Units 1-5 on April 16, 2016; and Paradise Fossil Plant ("Paradise") Units 1 and 2 on April 15, 2017. Other pending TVA Board actions at September 30, 2017 are the retirement of Johnsonville Fossil Plant ("Johnsonville") Units 1-4 by December 31, 2017 and Allen Fossil Plant ("Allen") Units 1-3. The Allen units will be retired upon a completion of a natural gas-fired plant at the Allen location, but no later than December 31, 2018. TVA estimates that the natural gas-fired plant will be completed in the spring of 2018. Depreciation rates are adjusted to reflect current assumptions so that the units will be fully depreciated by the applicable idle dates. As a result of TVA's decision to idle or retire units, TVA recognized $104 million , $ 139 million and $ 383 million in accelerated depreciation expense related to the units during the years ended September 30, 2017 , 2016 , and 2015 , respectively. Accelerated depreciation is based on the rate in effect at the time the decision is made to idle or retire a unit. Capital Lease Agreements. Property, plant, and equipment also includes assets recorded under capital lease agreements. These primarily consist of a natural gas lateral pipeline, power production facilities, water treatment assets, and land of $ 161 million and $ 163 million at September 30, 2017 and 2016 , respectively. Amortization expense related to capital leases is included in Depreciation and amortization in TVA’s statement of operations, excluding leases and other financing obligations where regulatory accounting is applied. See Note 7 — Other Non-Current Regulatory Assets — Deferred Capital Leases and Other Financing Obligations. On April 4, 2016, TVA entered into a letter agreement with Choctaw Generation Limited Partnership, LLLP (“CGLP”) for the reimbursement of certain capital costs and ongoing operating and maintenance costs related to assets recently constructed at the Red Hills lignite-fired power facility. These capital additions were required to comply with new Mercury and Air Toxics Standards. As a result of the new agreement, TVA was required to reassess a related 1997 power purchase and operating agreement (“PPOA”) with CGLP that was previously classified as an executory contract. This reassessment determined that the PPOA contained a capital lease and resulted in TVA recording a capital lease asset at the estimated fair value of $ 76 million with an offsetting capital lease liability included in Accounts payable and accrued liabilities and Other long-term liabilities. Allowance for Funds Used During Construction. TVA may capitalize interest on eligible projects as allowance for funds used during construction ("AFUDC") , based on the average interest rate of TVA’s outstanding debt. The allowance is applicable to construction in progress related to eligible projects with (1) an expected total project cost of $ 1.0 billion or more, and (2) an estimated construction period of at least three years in duration. No AFUDC was capitalized for the year ended September 30, 2017 , subsequent to the completion of Watts Bar Unit 2, which went into service in October 2016. TVA capitalized $235 million and $ 214 million of AFUDC for the years ended September 30, 2016 , and 2015, respectively, related to the Watts Bar Unit 2 project. Software Costs. TVA capitalizes certain costs incurred in connection with developing or obtaining internal-use software. Capitalized software costs are included in Property, plant, and equipment on the consolidated balance sheets and are generally amortized over seven years . At September 30, 2017 , and 2016 , unamortized computer software costs totaled $ 42 million and $ 27 million , respectively. Amortization expense related to capitalized computer software costs was $ 26 million , $43 million , and $38 million for 2017 , 2016 , and 2015 , respectively. Software costs that do not meet capitalization criteria are expensed as incurred. Impairment of Assets. TVA evaluates long-lived assets for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. For long-lived assets, TVA bases its evaluation on impairment indicators such as the nature of the assets, the future economic benefit of the assets, any historical or future profitability measurements, its regulatory approval and ability to set rates at levels that allow for recoverability of the assets, and other external market conditions or factors that may be present. If such impairment indicators are present or other factors exist that indicate that the carrying amount of an asset may not be recoverable, TVA determines whether an impairment has occurred based on an estimate of undiscounted cash flows attributable to the asset as compared with the carrying value of the asset. If an impairment has occurred, the amount of the impairment recognized is measured as the excess of the asset’s carrying value over its fair value. Additionally, TVA regularly evaluates construction projects. If the project is canceled or deemed to have no future economic benefit, the project is written off as an asset impairment or, upon TVA Board approval, reclassified as a regulatory asset.
Decommissioning Costs	Decommissioning Costs TVA recognizes legal obligations associated with the future retirement of certain tangible long-lived assets. These obligations relate to fossil fuel-fired generating plants, nuclear generating plants, hydroelectric generating plants/dams, transmission structures, and other property-related assets. These other property-related assets include, but are not limited to, easements and coal rights. Activities involved with retiring these assets could include decontamination and demolition of structures, removal and disposal of wastes, and site restoration. Revisions to the estimates of asset retirement obligations ("AROs") are made whenever factors indicate that the timing or amounts of estimated cash flows have changed materially. Any accretion or depreciation expense related to these liabilities and assets is charged to a regulatory asset. See Note 7 — Nuclear Decommissioning Costs and Non-Nuclear Decommissioning Costs and Note 12 . To estimate its decommissioning obligation related to its nuclear generating stations, TVA uses a probability-weighted, discounted cash flow model which, on a unit-by-unit basis, considers multiple outcome scenarios that include significant estimations and assumptions. Those assumptions include (1) estimates of the cost of decommissioning, (2) the method of decommissioning and the timing of the related cash flows, (3) the license period of the nuclear plant, considering the probability of license extensions, (4) cost escalation factors, and (5) the credit adjusted risk free rate to measure the obligation at the present value of the future estimated costs. TVA has ascribed probabilities to two different decommissioning methods related to its nuclear decommissioning obligation estimate: the DECON method and the SAFSTOR method. The DECON method requires radioactive contamination to be removed from a site and safely disposed of or decontaminated to a level that permits the site to be released for unrestricted use shortly after it ceases operation. The SAFSTOR method allows nuclear facilities to be placed and maintained in a condition that allows the facilities to be safely stored and subsequently decontaminated to levels that permit release for unrestricted use.
Blended Low-Enriched Uranium Program	Blended Low-Enriched Uranium Program Under the blended low-enriched uranium ("BLEU") program, TVA, the U.S. Department of Energy ("DOE") , and certain nuclear fuel contractors have entered into agreements providing for the DOE's surplus of enriched uranium to be blended with other uranium down to a level that allows the blended uranium to be fabricated into fuel that can be used in nuclear power plants. Under the terms of an interagency agreement between TVA and the DOE, in exchange for supplying highly enriched uranium materials to the appropriate third-party fuel processors for processing into usable BLEU fuel for TVA, the DOE participates to a degree in the savings generated by TVA’s use of this blended nuclear fuel. TVA accrues an obligation with each BLEU reload batch related to the portion of the ultimate future payments estimated to be attributable to the BLEU fuel currently in use. TVA estimated DOE's portion of the cost savings from the program to be $ 165 million . The last of the BLEU fuel was loaded in 2017. At September 30, 2017 , TVA had paid out approximately $ 164 million for this program, and the obligation recorded was $ 1 million .
Investment Funds	Investment Funds Investment funds consist primarily of trust funds designated to fund decommissioning requirements (see Note 21 — Contingencies — Decommissioning Costs ), the Supplemental Executive Retirement Plan ("SERP") (see Note 20 — Overview of Plans and Benefits — Supplemental Executive Retirement Plan ), and the Deferred Compensation Plan ("DCP") . The Nuclear Decommissioning Trust ("NDT") holds funds primarily for the ultimate decommissioning of TVA's nuclear power plants. The Asset Retirement Trust ("ART") holds funds primarily for the costs related to the future closure and retirement of TVA's other long-lived assets. The NDT, ART, SERP, and DCP funds are invested in portfolios of securities generally designed to achieve a return in line with overall equity and debt market performance. The NDT, ART, SERP, and DCP funds are all classified as trading.
Insurance	Insurance Although TVA uses private companies to administer its healthcare plans for eligible active and retired employees not covered by Medicare, TVA does not purchase health insurance. Third-party actuarial specialists assist TVA in determining certain liabilities for self-insured claims. TVA recovers the costs of claims through power rates and through adjustments to the participants’ contributions to their benefit plans. These liabilities are included in Other liabilities on the balance sheets. TVA sponsors an Owner Controlled Insurance Program which provides workers' compensation and liability insurance for a select group of contractors performing maintenance, modifications, outage, and new construction activities at TVA facilities. The Federal Employees' Compensation Act ("FECA") governs liability to employees for service-connected injuries. TVA purchases excess workers' compensation insurance above a self-insured retention. In addition to excess workers' compensation insurance, TVA purchases the following types of insurance: • Nuclear liability insurance; nuclear property, decommissioning, and decontamination insurance; and nuclear accidental outage insurance. See Note 21 — Contingencies — Nuclear Insurance . • Excess liability insurance for aviation, auto, marine, and general liability exposures. • Property insurance for certain conventional (non-nuclear) assets. The insurance policies are subject to the terms and conditions of the specific policy, including deductibles or self-insured retentions. To the extent insurance would not provide either a partial or total recovery of the costs associated with a loss, TVA would have to recover any such costs through other means, including through power rates.
Research and Development Costs	Research and Development Costs Research and development costs are expensed when incurred. TVA’s research programs include those related to power delivery technologies, emerging technologies (clean energy, renewables, distributed resources, and energy efficiency), technologies related to generation (fossil fuel, nuclear, and hydroelectric), and environmental technologies.
Tax Equivalents	Tax Equivalents The TVA Act requires TVA to make payments to states and counties in which TVA conducts its power operations and in which TVA has acquired power properties previously subject to state and local taxation. The total amount of these payments is five percent of gross revenues from sales of power during the preceding year, excluding sales or deliveries to other federal agencies and off-system sales with other utilities, with a provision for minimum payments under certain circumstances. TVA calculates tax equivalent expense by subtracting the prior year fuel cost-related tax equivalent regulatory asset or liability from the payments made to the states and counties during the current year and adding back the current year fuel cost-related tax equivalent regulatory asset or liability. Fuel cost-related tax equivalent expense is recognized in the same accounting period in which the fuel cost-related revenue is recognized.
Maintenance Costs	Maintenance Costs TVA records maintenance costs and repairs related to its property, plant, and equipment in the consolidated statements of operations as they are incurred except for the recording of certain regulatory assets for retirement and removal costs.

Variable Interest Entities Vari

Variable Interest Entities Variable Interest Entities (Policies)	12 Months Ended
	Sep. 30, 2017
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Variable Interest Entity Policy	A VIE is an entity that either (i) has insufficient equity to permit the entity to finance its activities without additional subordinated financial support or (ii) has equity investors who lack the characteristics of owning a controlling financial interest. When TVA determines that it has a variable interest in a VIE, a qualitative evaluation is performed to assess which interest holders have the power to direct the activities that most significantly impact the economic performance of the entity and have the obligation to absorb losses or receive benefits that could be significant to the entity. The evaluation considers the purpose and design of the business, the risks that the business was designed to create and pass along to other entities, the activities of the business that can be directed and which party can direct them, and the expected relative impact of those activities on the economic performance of the business through its life. TVA has the power to direct the activities of an entity when it has the ability to make key operating and financing decisions, including, but not limited to, capital investment and the issuance of debt. Based on the evaluation of these criteria, TVA has determined it is the primary beneficiary of certain entities and as such is required to account for the VIEs on a consolidated basis.

Asset Retirement Obligations As

Asset Retirement Obligations Asset Retirement Obligations (Policies)	12 Months Ended
	Sep. 30, 2017
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations, Policy	Decommissioning Costs TVA recognizes legal obligations associated with the future retirement of certain tangible long-lived assets. These obligations relate to fossil fuel-fired generating plants, nuclear generating plants, hydroelectric generating plants/dams, transmission structures, and other property-related assets. These other property-related assets include, but are not limited to, easements and coal rights. Activities involved with retiring these assets could include decontamination and demolition of structures, removal and disposal of wastes, and site restoration. Revisions to the estimates of asset retirement obligations ("AROs") are made whenever factors indicate that the timing or amounts of estimated cash flows have changed materially. Any accretion or depreciation expense related to these liabilities and assets is charged to a regulatory asset. See Note 7 — Nuclear Decommissioning Costs and Non-Nuclear Decommissioning Costs and Note 12 . To estimate its decommissioning obligation related to its nuclear generating stations, TVA uses a probability-weighted, discounted cash flow model which, on a unit-by-unit basis, considers multiple outcome scenarios that include significant estimations and assumptions. Those assumptions include (1) estimates of the cost of decommissioning, (2) the method of decommissioning and the timing of the related cash flows, (3) the license period of the nuclear plant, considering the probability of license extensions, (4) cost escalation factors, and (5) the credit adjusted risk free rate to measure the obligation at the present value of the future estimated costs. TVA has ascribed probabilities to two different decommissioning methods related to its nuclear decommissioning obligation estimate: the DECON method and the SAFSTOR method. The DECON method requires radioactive contamination to be removed from a site and safely disposed of or decontaminated to a level that permits the site to be released for unrestricted use shortly after it ceases operation. The SAFSTOR method allows nuclear facilities to be placed and maintained in a condition that allows the facilities to be safely stored and subsequently decontaminated to levels that permit release for unrestricted use.

Benefit Plans Benefit Plans (Po

Benefit Plans Benefit Plans (Policies)	12 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
Benefit Plans	Accounting Mechanisms Regulatory Accounting. TVA has classified all amounts related to unrecognized prior service costs, net actuarial gains or losses, and the funded status as regulatory assets or liabilities as such amounts are probable of collection in future rates. Additionally, on October 1, 2014, TVA began recognizing pension costs as regulatory assets or liabilities to the extent that the amount calculated under GAAP as pension expense differs from the amount TVA contributes to the pension plan. Cost Method. TVA uses the projected unit credit cost method to determine the service cost and the projected benefit obligation for retirement, termination, and ancillary benefits. Under this method, a “projected accrued benefit” is calculated at the beginning of the year and at the end of the year for each benefit that may be payable in the future. The “projected accrued benefit” is based on the plan’s accrual formula and upon service at the beginning or end of the year, but it uses final average compensation, social security benefits, and other relevant factors projected to the age at which the employee is assumed to leave active service. The projected benefit obligation is the actuarial present value of the “projected accrued benefits” at the beginning of the year for employed participants and is the actuarial present value of all benefits for other participants. The service cost is the actuarial present value of the difference between the “projected accrued benefits” at the beginning and end of the year. Amortization of Net Gain or Loss. TVA utilizes the corridor approach for gain/loss amortization. Differences between actuarial assumptions and actual plan results are deferred and amortized into periodic cost only when the accumulated differences exceed 10 percent of the greater of the projected benefit obligation or the market-related value of plan assets. If necessary, the excess is amortized over the average remaining service period of participating employees expected to receive benefits under the plan, which is currently 10 years and will continue to decline since no new participants will be added to the plan. Amortization of Prior Service Cost/(Credit). Amortization of net prior service cost/(credit) resulting from a plan change is included as a component of period expense in the year first recognized and every year thereafter until it is fully amortized. The increase or decrease in the benefit obligation due to the plan change is amortized over the average remaining service period of participating employees expected to receive benefits under the plan. The pension and post-retirement plans have prior service credits related to plan changes made in 2009, 2010, and 2016 with remaining amortization periods of three to ten years. However, when a plan change reduces the benefit obligation, existing positive prior service costs are reduced or eliminated starting with the earliest established before a new prior service credit base is established. Asset Method. TVA recognizes the impact of asset performance on pension expense over a three -year phase-in period through a “market-related” value of assets calculation. Since the “market-related” value of assets recognizes investment gains and losses over a three -year period, the future value of assets will be impacted as previously deferred gains or losses are recognized. The “market-related” value is used in calculating expected return on plan assets and net gain or loss for pension cost determination.

Gallatin coal combustion resi34

Gallatin coal combustion residual facilities Regulatory Asset Treatment (Policies)	12 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Regulatory Asset Accounting Treatment [Table Text Block]	In August 2017, TVA began using regulatory accounting treatment to defer expected future costs of compliance with orders or settlements related to lawsuits involving the Gallatin CCR facilities. The TVA Board approved a plan to amortize these costs over the anticipated duration of the Gallatin CCR project (excluding post-closure care), beginning October 1, 2018 as amounts are included in rates or paid out.

Summary of Significant Accoun35

Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Property, Plant, and Equipment Depreciation Rates	Average depreciation rates by asset class are as follows: Property, Plant, and Equipment Depreciation Rates At September 30 (percent) 2017 2016 2015 Asset Class Nuclear 2.66 2.37 2.81 Coal-fired 2.33 3.50 5.50 Hydroelectric 1.58 1.29 1.30 Gas and oil-fired 3.27 3.09 3.18 Transmission 1.34 2.80 2.78 Other 6.12 8.97 8.65

Impact of New Accounting Stan36

Impact of New Accounting Standards and Interpretations Impact of New Accounting Standards and Interpretations (Tables)	12 Months Ended
	Sep. 30, 2017
Accounting Changes and Error Corrections [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The following are accounting standard updates issued by the Financial Accounting Standards Board ("FASB") that TVA adopted during 2017 . Consolidation Description This guidance amends the consolidation analysis for VIEs as well as voting interest entities. The standard reduces the number of consolidation models through the elimination of the indefinite deferral for certain entities that was previously allowed and places more emphasis on risk of loss when determining a controlling financial interest. This guidance allows for either a full retrospective or a modified retrospective application. Effective Date for TVA October 1, 2016 Effect on the Financial Statements or Other Significant Matters The adoption of the standard did not materially impact TVA's financial condition, results of operations, or cash flows. Business Combinations Description This guidance clarifies the definition of a business by providing guidelines to determine when a set of assets and activities constitutes a business. The standard says that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of assets and activities is not a business. If this characteristic is not met, the amendments in this update (1) require that to be considered a business, a set must include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create output and (2) remove the evaluation of whether a market participant could replace missing elements. The amendments provide a framework to assist entities in evaluating whether both an input and a substantive process are present. The framework includes two sets of criteria to consider that depend on whether a set of assets and activities has outputs. Although outputs are not required for a set of assets and activities to be a business, outputs generally are a key element of a business; therefore, the FASB has developed more stringent criteria for sets of assets and activities without outputs. This standard also updates the definition of the term output so that the term is consistent with how outputs are described in the new Revenue Recognition guidance. Effective Date for TVA July 1, 2017 Effect on the Financial Statements or Other Significant Matters As a result of adopting the standard, TVA accounted for purchase of the equity interests in two special purpose entities in 2017 as asset acquisitions, and not business combinations, because the entities did not meet the definition of a business under the new accounting standard. Going Concern Description This amendment requires an entity’s management to assess an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued or available to be issued. When management identifies conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern, management should consider whether its plans that are intended to mitigate those relevant conditions or events will alleviate the substantial doubt. If the substantial doubt can be alleviated as a result of consideration of management’s plan, the entity should disclose certain information. If substantial doubt is not alleviated after consideration of management’s plan, an entity should indicate in the footnotes that there is substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the final statements are issued or available to be issued. This assessment must be evaluated every reporting period including interim periods. Effective Date for TVA September 30, 2017 Effect on the Financial Statements or Other Significant Matters As a result of adopting the standard, management has assessed TVA’s ability to continue as a going concern within one year after the financial statements are available to be issued, and there were no conditions or events that raise substantial doubt about TVA’s ability to continue as a going concern. No additional disclosures are necessary. The following accounting standards have been issued, but as of September 30, 2017 , were not effective and had not been adopted by TVA. Derivatives and Hedging - Contingent Put and Call Options in Debt Instruments Description This guidance clarifies the requirements for assessing whether contingent call or put options that can accelerate the payment of principal on debt instruments are clearly and closely related to their debt hosts. An entity performing the assessment under the amendments in this update is required to assess the embedded call or put options solely in accordance with a four-step decision sequence. When the standard becomes effective, it will include interim periods within that fiscal year, and will be required to be applied using a modified retrospective transition. Effective Date for TVA The new standard was effective for TVA’s interim and annual reporting periods beginning October 1, 2017. While early adoption was permitted, TVA did not adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA has two issues of Putable Automatic Rate Reset Securities (“PARRS”) outstanding. After a fixed-rate period of five years, the coupon rate on the PARRS may automatically be reset downward under certain market conditions on an annual basis. The coupon rate reset on the PARRS is based on a calculation. If the coupon rate is going to be reset, holders may request, for a limited period of time, redemption of the PARRS at par value, with repayment of principal on the reset date. This put option is otherwise not available. For both series of PARRS, the coupon rate will reset downward on the reset date if the rate calculated is below the then-current coupon rate on the PARRS. TVA plans to assess whether these contingent put options that can accelerate the payment of principal on the PARRS continue to be clearly and closely related to their debt hosts under the new guidance. TVA plans to assess the put options in accordance with the four-step decision sequence clarified in the guidance. While a preliminary assessment indicates that TVA does not believe the new guidance will impact its current assessment, TVA will continue to evaluate the potential impact of the new guidance on its consolidated balance sheet. Inventory Valuation Description This guidance changes the model used for the subsequent measurement of inventory from the previous lower of cost or market model to the lower of cost or net realizable value. The guidance applies only to inventory valued using methods other than last-in, first out or the retail inventory method (for example, first-in, first-out or average cost). This amendment is intended to simplify the subsequent measurement of inventory. When the standard becomes effective, it includes interim periods within the fiscal year that begins on that date and is required to be applied prospectively. Effective Date for TVA The new standard was effective for TVA’s interim and annual reporting periods beginning October 1, 2017. While early adoption was permitted, TVA did not adopt the standard early. Effect on the Financial Statements or Other Significant Matters The adoption of this standard will not have a material impact on TVA’s financial condition, results of operations, and cash flows. Defined Benefit Costs Description This guidance changes how information about defined benefit costs for pension plans and other post-retirement benefit plans is presented in employer financial statements. The guidance requires employers that present a measure of operating income in their statement of income to include only the service cost component of net periodic pension cost and net periodic postretirement benefit cost in operating expenses (together with other employee compensation costs). The other components of net benefit cost, including amortization of prior service cost/credit and settlement and curtailment effects, are to be included in nonoperating expenses. Additionally, the guidance stipulates that only the service cost component of net benefit cost is eligible for capitalization in assets. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA has evaluated the impact of adopting this guidance, and if the guidance had been effective for TVA for the years ended September 30, 2017, 2016, and 2015, TVA would have reclassified $758 million, $179 million, and $185 million, respectively, of net periodic benefit costs from Operating and maintenance expense to Other income (expense), net on the consolidated statements of operations. Financial Instruments Description This guidance applies to the recognition and measurement of financial assets and liabilities. The standard requires all equity investments to be measured at fair value with changes in the fair value recognized through net income (other than those accounted for under the equity method of accounting or those that result in consolidation of the investee). The standard also amends presentation requirements related to certain changes in the fair value of a liability and eliminates certain disclosure requirements of significant assumptions for financial instruments measured at amortized cost on the balance sheet. Public entities must apply the amendments by means of a cumulative-effect adjustment to the balance sheet as of the beginning of the fiscal year of adoption. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. Early adoption is not permitted unless specific early adoption guidance is applied. TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA currently measures all of its equity investments (other than those that result in the consolidation of the investee) at fair value, with changes in the fair value recognized through net income. The TVA Board has authorized the use of regulatory accounting for changes in fair value of certain equity investments, and as a result, those changes in fair value are deferred as regulatory assets or liabilities. TVA currently discloses significant assumptions around its estimates of fair value for financial instruments carried at amortized cost on its consolidated balance sheet. TVA is currently evaluating the potential impact of the changes in this new guidance on its consolidated financial statements and related disclosures. Revenue Recognition Description In 2014, the FASB issued new guidance related to revenue from contracts with customers. The guidance, including subsequent amendments, replaces the existing accounting standard and industry specific guidance for revenue recognition with a five-step model for recognizing and measuring revenue from contracts with customers. The underlying principle of the guidance is to recognize revenue to depict the transfer of goods or services to customers at the amount expected to be collected. The objective of the new standard is to provide a single, comprehensive revenue recognition model for all contracts with customers to improve comparability within industries and across industries. The new standard also requires enhanced disclosures regarding the nature, amount, timing, and uncertainty of revenue and the related cash flows arising from contracts with customers. At adoption, companies must also select a transition method to be applied either retrospectively to each prior reporting period presented or retrospectively with a cumulative effect adjustment to retained earnings at the date of initial adoption. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. While early adoption is permitted, TVA will not adopt the standard early. Effect on the Financial Statements or Other Significant Matters While TVA expects most of its revenue to be included in the scope of the new guidance, it has not completed its evaluation of all such arrangements. TVA’s current efforts in evaluating the impact of the standard are focused on scoping of revenue streams and evaluating contracts with LPCs, which represent the majority of TVA's revenues. TVA is also conducting ongoing evaluations of how the new guidance impacts other transactions, including sales to directly served industrial customers, sales to federal agencies, purchase power agreements, fuel cost adjustments, and other revenue streams. In addition, the power and utilities industry is currently addressing certain industry-specific issues which have not yet been finalized. As the ultimate impact of the new standard has not yet been determined, TVA has not yet elected its transition method. Statement of Cash Flows Description This guidance requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This guidance does not provide a definition of restricted cash or restricted cash equivalents. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2018. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA does not expect the adoption of this standard to have a material impact on TVA’s financial condition, results of operations, or cash flows. Derivatives and Hedging - Improvements to Accounting for Hedging Activities Description This guidance better aligns an entity's risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. To meet that objective, the amendments expand and refine hedge accounting for both nonfinancial and financial risk components and align the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2019. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA does not expect the adoption of this standard to have a material impact on TVA’s financial condition, results of operations, or cash flows. Lease Accounting Description This guidance changes the provisions of recognition in both the lessee and lessor accounting models. The standard requires entities that lease assets — referred to as "lessees" — to recognize on the balance sheet the assets and liabilities for the rights and obligations created by leases with terms of more than 12 months. The recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a finance (similar to current capital leases) or operating lease. However, unlike current lease accounting rules — which require only capital leases to be recognized on the balance sheet — the new standard will require both types of leases to be recognized on the balance sheet. Operating leases will result in straight-line expense, while finance leases will result in recognition of interest on the lease liability separate from amortization expense. The accounting for the owner of the assets leased by the lessee — also known as lessor accounting — will remain largely unchanged from current lease accounting rules. The standard allows for certain practical expedients to be elected related to lease term determination, separation of lease and non-lease elements, reassessment of existing leases, and short-term leases. When the standard becomes effective, it will include interim periods within that fiscal year and will be required to be applied using a modified retrospective transition. Effective Date for TVA The new standard is effective for TVA’s interim and annual reporting periods beginning October 1, 2019. While early adoption is permitted, TVA does not currently plan to adopt the standard early. Effect on the Financial Statements or Other Significant Matters TVA is currently evaluating the potential impact of these changes on its consolidated financial statements and related disclosures. TVA expects the new standard to impact financial position as adoption is expected to increase the amount of assets and liabilities recognized on TVA’s consolidated balance sheets. TVA expects the new standard to have no material impact on results of operations or cash flows. TVA plans to elect certain of the practical expedients included in the new standard. TVA is also continuing to monitor unresolved industry implementation issues, including items related to renewables and purchased power agreements, easements, and rights-of-way, and will analyze the related impacts to lease accounting.

Accounts Receivable, Net Accoun

Accounts Receivable, Net Accounts Receivable, Net (Tables)	12 Months Ended
	Sep. 30, 2017
Accounts Receivable, Net [Abstract]
Accounts Receivable, Net	The table below summarizes the types and amounts of TVA’s accounts receivable: Accounts Receivable, Net At September 30 2017 2016 Power receivables $ 1,441 $ 1,637 Other receivables 129 111 Allowance for uncollectible accounts (1 ) (1 ) Accounts receivable, net $ 1,569 $ 1,747

Inventories, Net Inventories, N

Inventories, Net Inventories, Net (Tables)	12 Months Ended
	Sep. 30, 2017
Inventory, Net [Abstract]
Inventories, Net	The table below summarizes the types and amounts of TVA’s inventories: Inventories, Net At September 30 2017 2016 Materials and supplies inventory $ 734 $ 673 Fuel inventory 355 345 RECs/emission allowance inventory, net 15 14 Allowance for inventory obsolescence (39 ) (39 ) Inventories, net $ 1,065 $ 993

Net Completed Plant Net Complet

Net Completed Plant Net Completed Plant (Tables)	12 Months Ended
	Sep. 30, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Net Completed Plant	Net completed plant consisted of the following: Net Completed Plant At September 30 2017 2016 Cost Accumulated Depreciation Net Cost Accumulated Depreciation Net Coal-fired $ 15,937 $ 10,791 $ 5,146 $ 15,587 $ 10,473 $ 5,114 Gas and oil-fired 4,995 1,359 3,636 3,918 1,267 2,651 Nuclear 25,010 10,834 14,176 19,280 10,422 8,858 Transmission 7,264 3,039 4,225 7,061 2,975 4,086 Hydroelectric 3,015 967 2,048 2,891 932 1,959 Other electrical plant 1,756 1,008 748 1,857 1,126 731 57,977 27,998 29,979 50,594 27,195 23,399 Multipurpose dams 928 387 541 928 379 549 Other stewardship 42 19 23 42 18 24 970 406 564 970 397 573 Total $ 58,947 $ 28,404 $ 30,543 $ 51,564 $ 27,592 $ 23,972

Other Long-Term Assets Other Lo

Other Long-Term Assets Other Long-Term Assets (Tables)	12 Months Ended
	Sep. 30, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Other Long-Term Assets	The table below summarizes the types and amounts of TVA’s other long-term assets: Other Long-Term Assets At September 30 2017 2016 EnergyRight ® receivables $ 100 $ 112 Loans and other long-term receivables, net 115 136 Commodity contract derivative assets 2 3 Prepaid capacity payments 34 42 Other 72 93 Total other long-term assets $ 323 $ 386

Regulatory Assets and Liabili41

Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Sep. 30, 2017
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets and Liabilities	Components of regulatory assets and regulatory liabilities are summarized in the table below. Regulatory Assets and Liabilities At September 30 2017 2016 Current regulatory assets Deferred nuclear generating units $ 237 $ 237 Unrealized losses on interest rate derivatives 93 — Unrealized losses on commodity derivatives 68 122 Environmental agreements 2 34 Environmental cleanup costs – Kingston ash spill 44 42 Fuel cost adjustment receivable 1 98 Other current regulatory assets 2 3 Total current regulatory assets 447 536 Non-current regulatory assets Deferred pension costs and other post-retirement benefits costs 4,009 5,385 Unrealized losses on interest rate derivatives 982 1,547 Gallatin coal combustion residual facilities 899 — Nuclear decommissioning costs 823 938 Environmental cleanup costs - Kingston ash spill 263 299 Non-nuclear decommissioning costs 703 819 Deferred nuclear generating units 759 850 Environmental agreements 13 18 Unrealized losses on commodity derivatives 9 56 Other non-current regulatory assets 238 252 Total non-current regulatory assets 8,698 10,164 Total regulatory assets $ 9,145 $ 10,700 Current regulatory liabilities Fuel cost adjustment tax equivalents $ 153 $ 148 Fuel cost adjustment 2 — Unrealized gains on commodity derivatives 8 6 Total current regulatory liabilities 163 154 Non-current regulatory liabilities Deferred other post-retirement benefits cost 23 — Unrealized gains on commodity derivatives 2 3 Total non-current regulatory liabilities 25 3 Total regulatory liabilities $ 188 $ 157

Variable Interest Entities Va42

Variable Interest Entities Variable Interest Entities (Tables)	12 Months Ended
	Sep. 30, 2017
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Summary of Impact of VIEs on Consolidated Balance Sheets	The financial statement items attributable to carrying amounts and classifications of JSCCG, Holdco, and SCCG as of September 30, 2017 and 2016 , as reflected in the Consolidated Balance Sheets, are as follows: Summary of Impact of VIEs on Consolidated Balance Sheets At September 30 2017 2016 Current liabilities Accrued interest $ 11 $ 11 Accounts payable and accrued liabilities 2 2 Current maturities of long-term debt of variable interest entities 36 35 Total current liabilities 49 48 Other liabilities Other long-term liabilities 30 33 Long-term debt, net Long-term debt of variable interest entities, net 1,164 1,199 Total liabilities $ 1,243 $ 1,280

Other Long-Term Liabilities Oth

Other Long-Term Liabilities Other Long-Term Liabilities (Tables)	12 Months Ended
	Sep. 30, 2017
Other Liabilities, Noncurrent [Abstract]
Other Long-Term Liabilities	The table below summarizes the types and amounts of Other long-term liabilities: Other Long-Term Liabilities At September 30 2017 2016 Interest rate swap liabilities $ 1,418 $ 1,938 Gallatin coal combustion residual facilities liability 880 — Capital lease obligations 182 177 Currency swap liabilities 92 162 EnergyRight® financing obligation 115 130 Environmental agreements liability 13 18 Membership interests of VIE subject to mandatory redemption 30 33 Commodity contract derivative liabilities 9 49 Other 316 266 Total other long-term liabilities $ 3,055 $ 2,773

Asset Retirement Obligations 44

Asset Retirement Obligations Asset Retirement Obligations (Tables)	12 Months Ended
	Sep. 30, 2017
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligation Activity	Asset Retirement Obligation Activity Nuclear Non-Nuclear Total Balance at September 30, 2015 $ 2,187 $ 1,656 $ 3,843 Settlements — (133 ) (133 ) Change in estimate 198 (22 ) 176 Additional obligations — 15 15 Accretion (recorded to regulatory asset) 107 44 151 Balance at September 30, 2016 $ 2,492 $ 1,560 $ 4,052 (1) Settlements — (123 ) (123 ) Change in estimate 250 161 411 Additional obligations — 1 1 Reclassification of Gallatin projects (2) — (188 ) (188 ) Accretion (recorded to regulatory asset) 117 34 151 Balance at September 30, 2017 $ 2,859 $ 1,445 $ 4,304 (1) Notes (1) The current portions of the ARO liability in the amounts of $ 128 million a nd $ 212 million as of September 30, 2017 and 2016 , respectively, are included in Accounts payable and accrued liabilities. (2) See Note 8 for additional information

Debt and Other Obligations Debt

Debt and Other Obligations Debt and Other Obligations (Tables)	12 Months Ended
	Sep. 30, 2017
Debt and Other Obligations [Abstract]
Schedule of short-term borrowings	The following table provides information regarding TVA's short-term borrowings: Short-term Borrowings At September 30 2017 2016 2015 Amount outstanding - discount notes $ 1,998 $ 1,407 $ 1,034 Weighted average interest rate - discount notes 1.000 % 0.203 % 0.055 %
Debt Securities Activity	The table below summarizes the long-term debt securities activity for the period from October 1, 2015 , to September 30, 2017 . Debt Securities Activity For the years ended September 30 2017 2016 Issues 2017 Series A (1) $ 1,000 $ — Discount on debt issues (1 ) — Total $ 999 $ — Acquisitions Notes payable (2) $ 74 $ 78 Redemptions/Maturities (3) Variable interest entities $ 35 $ 33 Notes payable 27 3 electronotes ® 5 47 2009 Series A — 2 2009 Series B 28 27 2001 Series D 525 — 2007 Series A 1,000 — Total $ 1,620 $ 112 Notes (1) The 2017 Series A bonds were issued at 99.9 percent of par. (2) The related leaseback obligations of $70 million previously reported in Other liabilities in TVA's consolidated balance sheets were extinguished in the fourth quarters of both 2017 and 2016 as a result of each year's acquisition. See Note 9 for additional information. (3) All redemptions were at 100 percent of par.
Debt Outstanding	Total debt outstanding at September 30, 2017 , and 2016 , consisted of the following: Short-Term Debt At September 30 CUSIP or Other Identifier Maturity Call/(Put) Date Coupon Rate 2017 2016 Short-term debt, net of discounts $ 1,998 $ 1,407 Current maturities of long-term debt of variable interest entities issued at par 36 35 Current maturities of notes payable 53 27 Current maturities of power bonds issued at par 880591EF5 12/15/2017 3.770% 1 1 880591EF5 6/15/2018 3.770% 28 27 88059TEL1 11/15/2017 2.650% 1 1 88059TEL1 5/15/2018 2.650% 2 2 880591DS8 12/15/2016 4.875% — 524 880591EA6 7/18/2017 5.500% — 1,000 880591CU4 12/15/2017 6.250% 650 — 880591EC2 4/1/2018 4.500% 1,000 — 88059TFS5 10/15/2017 4.125% 46 — Total current maturities of power bonds issued at par 1,728 1,555 Total current debt outstanding, net $ 3,815 $ 3,024 Long-Term Debt (1) At September 30 CUSIP or Other Identifier Maturity Coupon Rate Call Date 2017 Par 2016 Par Stock Exchange Listings electronotes ®(2) 5/15/2020 - 2/15/2043 2.375% - 3.625% 2/15/2015 - 2/15/2018 $ 226 $ 278 None 880591CU4 12/15/2017 6.250% — 650 New York 880591EC2 4/1/2018 4.500% — 1,000 New York, Luxembourg 880591EQ1 10/15/2018 1.750% 1,000 1,000 New York 880591EL2 2/15/2021 3.875% 1,500 1,500 New York 880591DC3 6/7/2021 5.805% (3) 268 260 New York, Luxembourg 880591EN8 8/15/2022 1.875% 1,000 1,000 New York 880591ER9 9/15/2024 2.875% 1,000 1,000 New York 880591CJ9 11/1/2025 6.750% 1,350 1,350 New York, Hong Kong, Luxembourg, Singapore 880591EU26 2/1/2027 2.875% 1,000 — New York 880591300 (4) 6/1/2028 3.550% 273 273 New York 880591409 (4) 5/1/2029 3.360% 232 232 New York 880591DM1 5/1/2030 7.125% 1,000 1,000 New York, Luxembourg 880591DP4 6/7/2032 6.587% (3) 335 324 New York, Luxembourg 880591DV1 7/15/2033 4.700% 472 472 New York, Luxembourg 880591EF5 6/15/2034 3.770% 303 332 None 880591DX7 6/15/2035 4.650% 436 436 New York 880591CK6 4/1/2036 5.980% 121 121 New York 880591CS9 4/1/2036 5.880% 1,500 1,500 New York 880591CP5 1/15/2038 6.150% 1,000 1,000 New York 880591ED0 6/15/2038 5.500% 500 500 New York 880591EH1 9/15/2039 5.250% 2,000 2,000 New York 880591EP3 12/15/2042 3.500% 1,000 1,000 New York 880591DU3 6/7/2043 4.962% (3) 201 195 New York, Luxembourg 880591CF7 7/15/2045 6.235% 7/15/2020 140 140 New York 880591EB4 1/15/2048 4.875% 500 500 New York, Luxembourg 880591DZ2 4/1/2056 5.375% 1,000 1,000 New York 880591EJ7 9/15/2060 4.625% 1,000 1,000 New York 880591ES7 9/15/2065 4.250% 1,000 1,000 New York Subtotal 20,357 21,063 Unamortized discounts, premiums, issue costs, and other (152 ) (162 ) Total long-term outstanding power bonds, net 20,205 20,901 Long-term debt of variable interest entities, net 1,164 1,199 Long-term notes payable 69 48 Total long-term debt, net $ 21,438 $ 22,148 Notes (1) Includes net exchange gain from currency transactions of $ 125 million at September 30, 2017 and $ 150 million at September 30, 2016 . (2) Includes one electronotes ® issue with partial maturities of principal for each required annual payment. (3) The coupon rate represents TVA’s effective interest rate. (4) TVA PARRS, CUSIP numbers 880591300 and 880591409, may be redeemed under certain conditions. See Put and Call Options above.
Maturities Due in the Year Ending September 30	Maturities Due in the Year Ending September 30 2018 2019 2020 2021 2022 Thereafter Total Long-term power bonds, long-term debt of variable interest entities, and notes payable including current maturities (1) $ 1,817 $ 1,116 $ 93 $ 1,901 $ 1,071 $ 17,545 $ 23,543 Short-term debt, net of discounts 1,998 — — — — — 1,998 Note (1) Long-term power bonds does not include noncash items of foreign currency exchange gain of $ 125 million , unamortized debt issue costs of $59 million , and net discount on sale of Bonds of $ 93 million . Long-term debt of variable interest entities does not include noncash item of unamortized debt issue costs of $ 11 million .
Summary of Long-Term Credit Facilities	The following table provides additional information regarding TVA's funding available under the four long-term revolving credit facilities: Summary of Long-Term Credit Facilities At September 30, 2017 Maturity Date Facility Limit Letters of Credit Outstanding Cash Borrowings Availability December 2019 $ 150 $ 38 $ — $ 112 February 2021 500 500 — — June 2020 1,000 278 — 722 September 2020 1,000 335 — 665 Total $ 2,650 $ 1,151 $ — $ 1,499

Risk Management Activities an46

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions (Tables)	12 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Summary of Derivative Instruments That Receive Hedge Accounting Treatment	The following tables summarize the accounting treatment that certain of TVA's financial derivative transactions receive: Summary of Derivative Instruments That Receive Hedge Accounting Treatment (part 1) Amount of Mark-to-Market Gain (Loss) Recognized in Other Comprehensive Income (Loss) For the years ended September 30 Derivatives in Cash Flow Hedging Relationship Objective of Hedge Transaction Accounting for Derivative Hedging Instrument 2017 2016 Currency swaps To protect against changes in cash flows caused by changes in foreign currency exchange rates (exchange rate risk) Unrealized gains and losses are recorded in AOCI and reclassified to interest expense to the extent they are offset by gains and losses on the hedged transaction $ 59 $ (139 ) Summary of Derivative Instruments That Receive Hedge Accounting Treatment (part 2) (1) Amount of Gain (Loss) Reclassified from OCI to Interest Expense For the years ended September 30 Derivatives in Cash Flow Hedging Relationship 2017 2016 Currency swaps $ 26 $ (129 ) Note (1) There were no ineffective portions or amounts excluded from effectiveness testing for any of the periods presented. Based on forecasted foreign currency exchange rates, TVA expects to reclassify approximately $18 million of losses from AOCI to interest expense within the next twelve months to offset amounts anticipated to be recorded in interest expense related to exchange gain on the debt.
Summary of Derivative Instruments That Do Not Receive Hedge Accounting Treatment	Summary of Derivative Instruments That Do Not Receive Hedge Accounting Treatment Amount of Gain (Loss) Recognized in Income on Derivatives (1) For the years ended September 30 Derivative Type Objective of Derivative Accounting for Derivative Instrument 2017 2016 Interest rate swaps To fix short-term debt variable rate to a fixed rate (interest rate risk) Mark-to-Market gains and losses are recorded as regulatory assets or liabilities. Realized gains and losses are recognized in interest expense when incurred during the settlement period. $ (101 ) $ (109 ) Commodity derivatives under FTP To protect against fluctuations in market prices of purchased commodities (price risk) Mark-to-Market gains and losses are recorded as regulatory assets or liabilities. Realized gains and losses are recognized in fuel expense or purchased power expense when the related commodity is used in production. (36 ) (94 ) Note (1) All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in income but instead are deferred as regulatory assets and liabilities. As such, there was no related gain (loss) recognized in income for these unrealized gains (losses) for the years ended September 30, 2017 and 2016 . Fair Values of TVA Derivatives At September 30 2017 2016 Derivatives That Receive Hedge Accounting Treatment: Balance Balance Sheet Presentation Balance Balance Sheet Presentation Currency swaps £200 million Sterling $ (67 ) Accounts payable and accrued liabilities $(5); Other long-term liabilities $(62) $ (82 ) Other long-term liabilities £250 million Sterling (15 ) Accounts payable and accrued liabilities $(4); Other long-term liabilities $(11) (41 ) Other long-term liabilities £150 million Sterling (21 ) Accounts payable and accrued liabilities $(2); Other long-term liabilities $(19) (39 ) Other long-term liabilities Derivatives That Do Not Receive Hedge Accounting Treatment: Balance Balance Sheet Presentation Balance Balance Sheet Presentation Interest rate swaps $1.0 billion notional $ (1,093 ) Accounts payable and accrued liabilities $(66); Other long-term liabilities $(1,027) $ (1,387 ) Other long-term liabilities $476 million notional (410 ) Accounts payable and accrued liabilities $(25); Other long-term liabilities $(385) (539 ) Other long-term liabilities $42 million notional (8 ) Accounts payable and accrued liabilities $(2); Other long-term liabilities $(6) (12 ) Other long-term liabilities Commodity contract derivatives (60 ) Other current assets $8; Other long-term assets $2; Other long-term liabilities $(9); Accounts payable and accrued liabilities $(61) (125 ) Other current assets $6; Other long-term assets $3; Other long-term liabilities $(49); Accounts payable and accrued liabilities $(85) FTP Derivatives under FTP (1) (5 ) Other current assets $(4); Accounts payable and accrued liabilities $(1) (39 ) Other current assets $(30); Other long-term liabilities $(2); Accounts payable and accrued liabilities $(7)
Fair Values of TVA Derivatives	Fair Values of TVA Derivatives At September 30 2017 2016 Derivatives That Receive Hedge Accounting Treatment: Balance Balance Sheet Presentation Balance Balance Sheet Presentation Currency swaps £200 million Sterling $ (67 ) Accounts payable and accrued liabilities $(5); Other long-term liabilities $(62) $ (82 ) Other long-term liabilities £250 million Sterling (15 ) Accounts payable and accrued liabilities $(4); Other long-term liabilities $(11) (41 ) Other long-term liabilities £150 million Sterling (21 ) Accounts payable and accrued liabilities $(2); Other long-term liabilities $(19) (39 ) Other long-term liabilities Derivatives That Do Not Receive Hedge Accounting Treatment: Balance Balance Sheet Presentation Balance Balance Sheet Presentation Interest rate swaps $1.0 billion notional $ (1,093 ) Accounts payable and accrued liabilities $(66); Other long-term liabilities $(1,027) $ (1,387 ) Other long-term liabilities $476 million notional (410 ) Accounts payable and accrued liabilities $(25); Other long-term liabilities $(385) (539 ) Other long-term liabilities $42 million notional (8 ) Accounts payable and accrued liabilities $(2); Other long-term liabilities $(6) (12 ) Other long-term liabilities Commodity contract derivatives (60 ) Other current assets $8; Other long-term assets $2; Other long-term liabilities $(9); Accounts payable and accrued liabilities $(61) (125 ) Other current assets $6; Other long-term assets $3; Other long-term liabilities $(49); Accounts payable and accrued liabilities $(85) FTP Derivatives under FTP (1) (5 ) Other current assets $(4); Accounts payable and accrued liabilities $(1) (39 ) Other current assets $(30); Other long-term liabilities $(2); Accounts payable and accrued liabilities $(7) Note (1) Fair values of certain derivatives under the FTP that were in net liability positions totaling $ 4 million and $ 30 million at September 30, 2017 and 2016 , respectively, are recorded in TVA's margin cash accounts in Other current assets. These derivatives are transacted with futures commission merchants, and cash deposits have been posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full.
Currency Swaps Outstanding	TVA had the following currency swaps outstanding at September 30, 2017 : Currency Swaps Outstanding At September 30, 2017 Effective Date of Currency Swap Contract Associated TVA Bond Issues Currency Exposure Expiration Date of Swap Overall Effective Cost to TVA 1999 £200 million 2021 5.81% 2001 £250 million 2032 6.59% 2003 £150 million 2043 4.96%
Commodity Contract Derivatives	Commodity Contract Derivatives At September 30 2017 2016 Number of Contracts Notional Amount Fair Value (MtM) Number of Contracts Notional Amount Fair Value ( MtM ) Coal contract derivatives 20 17 million tons $ (67 ) 20 20 million tons $ (127 ) Natural gas contract derivatives 53 271 million mmBtu $ 7 39 148 million mmBtu $ 2
Derivatives Under Financial Trading Program	Derivatives under Financial Trading Program (1) At September 30 2017 2016 Notional Amount (in mmBtu) Fair Value (MtM) (in millions) Notional Amount (in mmBtu) Fair Value (MtM) (in millions) Natural gas Swap contracts 2,800,000 $ (5 ) 21,052,500 $ (39 ) Note (1) Fair value amounts presented are based on the net commodity position with the counterparty. Notional amounts disclosed represent the net value of contractual amounts.
Financial Trading Program Unrealized Gains (Losses)	Financial Trading Program Unrealized Gains (Losses) At September 30 FTP unrealized gains (losses) deferred as regulatory liabilities (assets) 2017 2016 Natural gas $ (5 ) $ (39 )
Financial Trading Program Realized Gains (Losses)	Financial Trading Program Realized Gains (Losses) At September 30 Decrease (increase) in fuel expense 2017 2016 Natural gas $ (29 ) $ (75 ) Decrease (increase) in purchased power expense Natural gas $ (7 ) $ (19 )
Offsetting Assets and Liabilities	The amounts of TVA's derivative instruments as reported in the consolidated balance sheets as of September 30, 2017 , and September 30, 2016 , are shown in the table below. At September 30, 2017 Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Balance Sheet (1) Net Amounts of Assets/Liabilities Presented in the Balance Sheet (2) Assets Commodity derivatives not subject to master netting or similar arrangement $ 10 $ — $ 10 Liabilities Currency swap(s) (3) $ 103 $ — $ 103 Interest rate swaps (3) 1,511 — 1,511 Commodity derivatives under FTP 5 (4 ) 1 Total derivatives subject to master netting or similar arrangement 1,619 (4 ) 1,615 Total derivatives not subject to master netting or similar arrangement 70 — 70 Total liabilities $ 1,689 $ (4 ) $ 1,685 At September 30, 2016 Gross Amounts of Recognized Assets/Liabilities Gross Amounts Offset in the Balance Sheet (1) Net Amounts of Assets/Liabilities Presented in the Balance Sheet (2) Assets Commodity derivatives under FTP subject to master netting or similar agreement $ 6 $ (6 ) $ — Commodity derivatives not subject to master netting or similar arrangement 9 — 9 Total assets $ 15 $ (6 ) $ 9 Liabilities Currency swap(s) (3) $ 162 $ — $ 162 Interest rate swaps (3) 1,938 — 1,938 Commodity derivatives under FTP 45 (36 ) 9 Total derivatives subject to master netting or similar arrangement 2,145 (36 ) 2,109 Total derivatives not subject to master netting or similar arrangement 134 — 134 Total liabilities $ 2,279 $ (36 ) $ 2,243 Notes (1) Amounts primarily include counterparty netting of derivative contracts, margin account deposits for futures commission merchants transactions, and cash collateral received or paid in accordance with the accounting guidance for derivatives and hedging transactions. (2) There are no derivative contracts subject to a master netting arrangement or similar agreement which are not offset in the balance sheets. (3) Letters of credit of approximately $ 1.2 billion and $ 1.4 billion were posted as collateral at September 30, 2017 and 2016 , respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives.

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Measurements - (Tables)	12 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Valuation Techniques	The measurement of fair value results in classification into a hierarchy by the inputs used to determine the fair value as follows: Level 1 — Unadjusted quoted prices in active markets accessible by the reporting entity for identical assets or liabilities. Active markets are those in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing. Level 2 — Pricing inputs other than quoted market prices included in Level 1 that are based on observable market data and that are directly or indirectly observable for substantially the full term of the asset or liability. These include quoted market prices for similar assets or liabilities, quoted market prices for identical or similar assets in markets that are not active, adjusted quoted market prices, inputs from observable data such as interest rate and yield curves, volatilities and default rates observable at commonly quoted intervals, and inputs derived from observable market data by correlation or other means. Level 3 — Pricing inputs that are unobservable, or less observable, from objective sources. Unobservable inputs are only to be used to the extent observable inputs are not available. These inputs maintain the concept of an exit price from the perspective of a market participant and should reflect assumptions of other market participants. An entity should consider all market participant assumptions that are available without unreasonable cost and effort. These are given the lowest priority and are generally used in internally developed methodologies to generate management's best estimate of the fair value when no observable market data is available.
Unrealized Investment Gains (Losses)	TVA recorded unrealized gains and losses related to its trading securities held during each period as follows: Unrealized Investment Gains (Losses) At September 30 Financial Statement Presentation 2017 2016 SERP Other income (expense) $ 4 $ 2 DCP Other income (expense) 2 1 NDT Regulatory asset 92 89 ART Regulatory asset 43 29
Fair Value Measurements	Fair Value Measurements At September 30, 2017 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets Investments Equity securities $ 226 $ — $ — $ 226 Government debt securities 100 42 — 142 Corporate debt securities — 373 — 373 Mortgage and asset-backed securities — 49 — 49 Institutional mutual funds 94 — — 94 Forward debt securities contracts — 19 — 19 Private equity funds measured at net asset value (1) — — — 136 Private real estate funds measured at net asset value (1) — — — 113 Commingled funds measured at net asset value (1) — — — 1,451 Total investments 420 483 — 2,603 Commodity contract derivatives — 8 2 10 Total $ 420 $ 491 $ 2 $ 2,613 Quoted Prices in Active Markets for Identical Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Liabilities Currency swap(s) (2) $ — $ 103 $ — $ 103 Interest rate swaps — 1,511 — 1,511 Commodity contract derivatives — 1 69 70 Commodity derivatives under FTP (2) Swap contracts — 1 — 1 Total $ — $ 1,616 $ 69 $ 1,685 Notes (1) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (2) Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities . Fair Value Measurements At September 30, 2016 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Assets Investments Equity securities $ 196 $ — $ — $ 196 Government debt securities 88 36 — 124 Corporate debt securities — 393 — 393 Mortgage and asset-backed securities — 50 — 50 Institutional mutual funds 92 — — 92 Forward debt securities contracts — 15 — 15 Private equity funds measured at net asset value (1) — — — 132 Private real estate funds measured at net asset value (1) — — — 113 Commingled funds measured at net asset value (1) — — — 1,142 Total investments 376 494 — 2,257 Commodity contract derivatives — 5 4 9 Total $ 376 $ 499 $ 4 $ 2,266 Quoted Prices in Active Markets for Identical Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Liabilities Currency swap(s) (2) $ — $ 162 $ — $ 162 Interest rate swaps — 1,938 — 1,938 Commodity contract derivatives — 3 131 134 Commodity derivatives under FTP (2) Swap contracts — 9 — 9 Total $ — $ 2,112 $ 131 $ 2,243 Notes (1) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. (2) Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of any cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities .
Fair Value Measurements Using Significant Unobservable Inputs	The following table presents a reconciliation of all commodity contract derivatives measured at fair value on a recurring basis using significant unobservable inputs (Level 3): Fair Value Measurements Using Significant Unobservable Inputs Commodity Contract Derivatives Balance at October 1, 2015 $ (98 ) Purchases — Issuances — Sales — Settlements — Change in net unrealized gains (losses) deferred as regulatory assets and liabilities (29 ) Balance at September 30, 2016 (127 ) Purchases — Issuances — Sales — Settlements — Change in net unrealized gains (losses) deferred as regulatory assets and liabilities 60 Balance at September 30, 2017 $ (67 )
Quantitative Information about Level 3 Fair Value Measurements	The following table presents quantitative information related to the significant unobservable inputs used in the measurement of fair value of TVA's assets and liabilities classified as Level 3 in the fair value hierarchy: Quantitative Information about Level 3 Fair Value Measurements Fair Value at September 30 2017 Valuation Technique(s) Unobservable Inputs Range Assets Commodity contract derivatives $ 2 Pricing model Coal supply and demand 0.6 - 0.7 billion tons/year Long-term market prices $11.40 - $112.23/ton Liabilities Commodity contract derivatives $ 69 Pricing model Coal supply and demand 0.6 - 0.7 billion tons/year Long-term market prices $11.40 - $112.23/ton Quantitative Information about Level 3 Fair Value Measurements Fair Value at September 30 2016 Valuation Technique(s) Unobservable Inputs Range Assets Commodity contract derivatives $ 4 Pricing model Coal supply and demand 0.7 - 0.8 billion tons/year Long-term market prices $11.80 - $85.02/ton Liabilities Commodity contract derivatives $ 131 Pricing model Coal supply and demand 0.7 - 0.8 billion tons/year Long-term market prices $11.80 - $85.02/ton
Estimated Values of Financial Instruments Not Recorded at Fair Value	The estimated values of TVA's financial instruments not recorded at fair value at September 30, 2017 , and 2016 , were as follows: Estimated Values of Financial Instruments Not Recorded at Fair Value At September 30, 2017 At September 30, 2016 Valuation Classification Carrying Amount Fair Value Carrying Amount Fair Value EnergyRight ® receivables (including current portion) Level 2 $ 125 $ 127 $ 141 $ 144 Loans and other long-term receivables, net (including current portion) Level 2 $ 118 $ 107 $ 141 $ 130 EnergyRight ® financing obligation (including current portion) Level 2 $ 144 $ 161 $ 163 $ 183 Unfunded loan commitments Level 2 $ — $ 18 $ — $ 17 Membership interests of variable interest entity subject to mandatory redemption (including current portion) Level 2 $ 32 $ 41 $ 35 $ 46 Long-term outstanding power bonds (including current maturities), net Level 2 $ 21,933 $ 26,857 $ 22,456 $ 28,620 Long-term debt of variable interest entities (including current maturities), net Level 2 $ 1,200 $ 1,356 $ 1,234 $ 1,468 Long-term notes payable (including current maturities) Level 2 $ 122 $ 121 $ 75 $ 75

Proprietary Capital Proprietary

Proprietary Capital Proprietary Capital (Tables)	12 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Summary of Proprietary Capital Activity	The table below summarizes TVA's activities related to appropriated funds and retained earnings. Summary of Proprietary Capital Activity At or for the years ended September 30 2017 2016 Power Program Nonpower Programs Power Program Nonpower Programs Appropriation Investment $ 258 $ 4,351 $ 258 $ 4,351 Retained Earnings Balance at beginning of year 7,594 (3,771 ) 6,357 (3,761 ) Net income (expense) for year 693 (8 ) 1,243 (10 ) Return on power program appropriation investment (5 ) — (6 ) — Balance at end of year 8,282 (3,779 ) 7,594 (3,771 ) Net proprietary capital at September 30 $ 8,540 $ 572 $ 7,852 $ 580

Other Income (Expense), Net Oth

Other Income (Expense), Net Other Income (Expense), Net (Tables)	12 Months Ended
	Sep. 30, 2017
Other Income and Expenses [Abstract]
Other Income (Expense), Net	Income and expenses not related to TVA’s operating activities are summarized in the following table: Other Income (Expense), Net For the years ended September 30 2017 2016 2015 Interest income $ 23 $ 24 $ 24 External services 14 12 12 Gains (losses) on investments 9 7 (1 ) Miscellaneous 10 — (6 ) Total other income (expense), net $ 56 $ 43 $ 29

Benefit Plans Benefit Plans (Ta

Benefit Plans Benefit Plans (Tables)	12 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
Obligations and Funded Status	The changes in plan obligations, assets, and funded status for the years ended September 30, 2017 and 2016 , were as follows: Obligations and Funded Status For the years ended September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Change in benefit obligation Benefit obligation at beginning of year $ 13,083 $ 12,824 $ 571 $ 657 Service cost 60 133 18 16 Interest cost 464 564 21 29 Plan participants’ contributions 9 25 — — Collections (1) — — 47 92 Actuarial (gain) loss (286 ) 1,188 (80 ) 68 Plan change — (960 ) — (158 ) Net transfers (to) from variable fund/401(k) plan (12 ) 7 — — Expenses paid (5 ) (6 ) — — Benefits paid (712 ) (692 ) (83 ) (133 ) Benefit obligation at end of year 12,601 13,083 494 571 Change in plan assets Fair value of net plan assets at beginning of year 7,145 6,797 — — Actual return on plan assets 759 733 — — Plan participants’ contributions 9 25 — — Collections (1) — — 47 92 Net transfers (to) from variable fund/401(k) plan (12 ) 7 — — Employer contributions (2) 805 281 36 41 Expenses paid (5 ) (6 ) — — Benefits paid (712 ) (692 ) (83 ) (133 ) Fair value of net plan assets at end of year 7,989 7,145 — — Funded status $ (4,612 ) $ (5,938 ) $ (494 ) $ (571 ) Notes (1) Collections include retiree contributions as well as federal reinsurance payments and provider discounts and rebates. (2) Other Post-Retirement Benefits Employer contributions are reduced by federal reinsurance payments and provider discounts and rebates.
Amounts Recognized on TVA's Consolidated Balance Sheets	Amounts related to these benefit plans recognized on TVA's consolidated balance sheets consist of regulatory assets that have not been recognized as components of net periodic benefit cost at September 30, 2017 and 2016 , and the funded status of TVA’s benefit plans, which are included in Accounts payable and accrued liabilities and Post-retirement and post-employment benefit obligations: Amounts Recognized on TVA's Consolidated Balance Sheets At September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Regulatory assets (liabilities) $ 4,009 $ 5,336 $ (23 ) $ 49 Accounts payable and accrued liabilities (4 ) (5 ) (33 ) (35 ) Pension and post-retirement benefit obligations (1) (4,608 ) (5,933 ) (461 ) (536 ) Note (1) The table above excludes $408 million and $460 million of post-employment benefit costs that are recorded in Post-retirement and post-employment benefit obligations on the Consolidated Balance Sheets at September 30, 2017 and 2016 , respectively.
Post-Retirement Benefit Costs Deferred as Regulatory Assets	Unrecognized amounts included in regulatory assets or liabilities yet to be recognized as components of accrued benefit cost at September 30 consisted of: Post-Retirement Benefit Costs Deferred as Regulatory Assets At September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Unrecognized prior service credit $ (918 ) $ (1,017 ) $ (163 ) $ (185 ) Unrecognized net loss 4,885 5,946 140 234 Amount capitalized due to actions of regulator 42 407 — — Total regulatory assets $ 4,009 $ 5,336 $ (23 ) $ 49
Projected Benefit Obligations and Accumulated Benefit Obligations in Excess of Plan Assets	The projected benefit obligation, accumulated benefit obligation, and fair value of plan assets for the pension plan at September 30, 2017 , and 2016 , were as follows: Projected Benefit Obligations and Accumulated Benefit Obligations in Excess of Plan Assets At September 30 2017 2016 Projected benefit obligation $ 12,601 $ 13,083 Accumulated benefit obligation 12,461 12,912 Fair value of net plan assets 7,989 7,145
Components of Net Periodic Benefit Cost	The components of net periodic benefit cost and other amounts recognized as changes in regulatory assets for the years ended September 30, 2017 , and 2016 , were as follows: Components of Net Periodic Benefit Cost For the years ended September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2015 2017 2016 2015 Service cost $ 60 $ 133 $ 130 $ 18 $ 16 $ 16 Interest cost 464 564 540 21 29 29 Expected return on plan assets (457 ) (446 ) (437 ) — — — Amortization of prior service credit (99 ) (23 ) (21 ) (22 ) (6 ) (6 ) Recognized net actuarial loss 472 310 299 14 7 9 Curtailment — (78 ) — — — — Total net periodic benefit cost as actuarially determined 440 460 511 31 46 48 Amount expensed (capitalized) due to actions of regulator 365 (179 ) (228 ) — — — Total net period benefit cost $ 805 $ 281 $ 283 $ 31 $ 46 $ 48
Expected Amortization of Regulatory Assets in Next Fiscal Year	The amounts in the regulatory asset that are expected to be recognized as components of net periodic benefit cost during the next fiscal year are as follows: Expected Amortization of Regulatory Assets in 2018 At September 30, 2017 Pension Benefits Other Post-Retirement Benefits Total Prior service credit $ (98 ) $ (22 ) $ (120 ) Net actuarial loss 412 7 419
Actuarial Assumptions	TVA’s reported costs of providing the plan benefits are impacted by numerous factors including the provisions of the plans, changing employee demographics, and various assumptions, the most significant of which are noted below. Actuarial Assumptions Utilized to Determine Benefit Obligations at September 30 Pension Benefits Other Post-Retirement Benefits 2017 2016 2017 2016 Discount rate 3.85 % 3.65 % 3.95 % 3.70 % Rate of compensation increase 5.43 % 5.55 % N/A N/A Pre-Medicare eligible Initial health care cost trend rate N/A N/A 6.50 % 6.50 % Ultimate health care cost trend rate N/A N/A 5.00 % 5.00 % Year ultimate trend rate is reached N/A N/A 2024 2019 Post-Medicare eligible Initial health care cost trend rate N/A N/A — % — % Ultimate health care cost trend rate N/A N/A 4.00 % 4.00 % Year ultimate trend rate is reached N/A N/A 2021 2021 Actuarial Assumptions Utilized to Determine Net Periodic Benefit Cost for the Years Ended September 30 (1) Pension Benefits Other Post-Retirement Benefits 2017 2016 2015 2017 2016 2015 Discount rate 3.65 % 4.50 % 4.45 % 3.70 % 4.65 % 4.50 % Expected return on plan assets 7.00 % 7.00 % 7.00 % N/A N/A N/A Rate of compensation increase 5.55 % 5.70 % 5.70 % N/A N/A N/A Pre-Medicare eligible Initial health care cost trend rate N/A N/A N/A 6.50 % 7.00 % 7.50 % Ultimate health care cost trend rate N/A N/A N/A 5.00 % 5.00 % 5.00 % Year ultimate trend rate is reached N/A N/A N/A 2019 2019 2019 Post-Medicare eligible Initial health care cost trend rate N/A N/A N/A — % 7.00 % 7.50 % Ultimate health care cost trend rate N/A N/A N/A 4.00 % 5.00 % 5.00 % Year ultimate trend rate is reached N/A N/A N/A 2021 2019 2019 Note (1) The actuarial assumptions used to determine the benefit obligations at September 30 of each year are subsequently used to determine net periodic benefit cost for the following year.
Sensitivity to Certain Changes in Pension Assumptions	The following chart reflects the sensitivity of pension cost to changes in certain actuarial assumptions: Sensitivity to Certain Changes in Pension Assumptions At September 30, 2017 Actuarial Assumption Change in Assumption Impact on 2017 Pension Cost Impact on 2017 Projected Benefit Obligation Discount rate (0.25 )% $ 17 $ 367 Rate of return on plan assets (0.25 )% 16 N/A
Sensitivity to Changes in Assumed Health Care Cost Trend Rates	The following chart reflects the sensitivity of post-retirement benefit cost to changes in the health care trend rate: Sensitivity to Changes in Assumed Health Care Cost Trend Rates At September 30, 2017 1% Increase 1% Decrease Effect on total of service and interest cost components for the year $ 5 $ (5 ) Effect on end-of-year accumulated post-retirement benefit obligation 70 (67 )
Asset Holdings and Fair Value Measurements	s consistent with the asset allocation policy. At September 30, 2017 and 2016 , the asset holdings of the system included the following: Asset Holdings of TVARS At September 30 Plan Assets at September 30 Asset Category Target Allocation 2017 2016 Global public equity 39 % 44 % 44 % Private equity 8 % 5 % 4 % Safety oriented fixed income 15 % 21 % 18 % Opportunistic fixed income 15 % 10 % 10 % Public real assets 15 % 13 % 15 % Private real assets 8 % 7 % 9 % Total 100 % 100 % 100 % Fair Value Measurements The following table provides the fair value measurement amounts for assets held by TVARS at September 30, 2017 : TVA Retirement System At September 30, 2017 Total (1) (2) Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Equity securities $ 1,771 $ 1,770 $ — $ 1 Preferred securities 14 3 11 — Debt securities Corporate debt securities 1,100 — 1,088 12 Residential mortgage-backed securities 325 — 317 8 Debt securities issued by U.S. Treasury and other U.S. government agencies 193 193 — — Debt securities issued by foreign governments 331 — 307 24 Asset-backed securities 146 — 109 37 Debt securities issued by state/local governments 19 — 17 2 Commercial mortgage-backed securities 68 — 62 6 Commingled funds measured at net asset value (3) Equity 1,134 — — — Debt 709 — — — Commodities 224 — — — Institutional mutual funds 155 155 — — Cash equivalents and other short-term investments 916 — 916 — Certificates of deposit 6 — 6 — Private equity measured at net asset value (3) 500 — — — Private real estate measured at net asset value (3) 533 — — — Securities lending collateral 369 — 369 — Derivatives Futures 18 18 — — Swaps 1 — 1 — Foreign currency forward receivable 4 — 4 — Total assets $ 8,536 $ 2,139 $ 3,207 $ 90 Liabilities Futures $ 3 $ 2 $ — $ 1 Foreign currency forward payable 6 — 6 — Credit default swaps 1 — — 1 Total liabilities $ 10 $ 2 $ 6 $ 2 Notes (1) Excludes approximately $ 168 million in net payables associated with security purchases and sales and various other payables. (2) Excludes a $ 369 million payable for collateral on loaned securities in connection with TVARS’s participation in securities lending programs. (3) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The following table provides the fair value measurement amounts for assets held by TVARS at September 30, 2016 : TVA Retirement System At September 30, 2016 Total (1) (2) Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Equity securities $ 1,847 $ 1,846 $ — $ 1 Preferred securities 20 3 17 — Debt securities Corporate debt securities 1,145 — 1,135 10 Residential mortgage-backed securities 181 — 165 16 Debt securities issued by U.S. Treasury and other U.S. government agencies 117 117 — — Debt securities issued by foreign governments 332 — 299 33 Asset-backed securities 118 — 87 31 Debt securities issued by state/local governments 16 — 16 — Commercial mortgage-backed securities 44 — 38 6 Commingled funds measured at net asset value (3) Equity 682 — — — Debt 653 — — — Commodities 302 — — — Blended 225 — — — Institutional mutual funds 10 10 — — Cash equivalents and other short-term investments 621 41 580 — Certificates of deposit 16 — 16 — Private equity measured at net asset value (3) 385 — — — Private real estate measured at net asset value (3) 568 — — — Securities lending collateral 3 — 3 — Derivatives Futures 2 2 — — Swaps 1 — 1 — Foreign currency forward receivable 5 — 5 — Total assets $ 7,293 $ 2,019 $ 2,362 $ 97 Liabilities Futures $ 2 $ 2 $ — $ — Foreign currency forward payable 9 — 9 — Written options 1 — 1 — Interest rate swaps 3 — 3 — Credit default swaps 1 — 1 — Total liabilities $ 16 $ 2 $ 14 $ — Notes (1) Excludes approximately $ 129 million in net payables associated with security purchases and sales and various other payables. (2) Excludes a $ 3 million payable for collateral on loaned securities in connection with TVARS’s participation in securities lending programs. (3) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have
Fair Value Measurements Using Significant Unobservable Inputs	The following table provides a reconciliation of beginning and ending balances of pension plan assets measured at fair value on a recurring basis where the determination of fair value includes significant unobservable inputs (Level 3): Fair Value Measurements Using Significant Unobservable Inputs Fair Value Measurements Using Significant Unobservable Inputs (Level 3) Balance at October 1, 2015 $ 91 Net realized/unrealized gains (losses) 18 Purchases, sales, issuances, and settlements (net) (12 ) Transfers in and/or out of Level 3 — Balance at September 30, 2016 97 Net realized/unrealized gains (losses) 2 Purchases, sales, issuances, and settlements (net) (6 ) Transfers in and/or out of Level 3 (5 ) Balance at September 30, 2017 $ 88
Estimated Future Benefit Payments	Cash Flows Estimated Future Benefit Payments. The following table sets forth the estimated future benefit payments under the benefit plans. Estimated Future Benefits Payments At September 30, 2017 Pension Benefits (1) Other Post-Retirement Benefits 2018 $ 768 $ 33 2019 769 31 2020 773 29 2021 774 27 2022 774 25 2023 - 2027 3,821 115 Note (1) Participants are assumed to receive the Fixed Fund in a lump sum in lieu of available annuity options allowed for certain grandfathered participants resulting in higher estimated pension benefits payments.
Amounts recognized on Consolidated Balance Sheets	Amounts related to other post-employment benefit obligations are recognized on TVA's consolidated balance sheets. The current portion which represents unpaid losses and administrative fees due are in Accounts payable and accrued liabilities. The long-term portion is recognized in Post-retirement and post-employment benefit obligations. Amounts Recognized on TVA's Consolidated Balance Sheets At September 30 2017 2016 Accounts payable and accrued liabilities $ 39 $ 41 Post-employment benefit obligations 408 460

Commitments and Contingencies C

Commitments and Contingencies Commitments and Contingencies (Tables)	12 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of lease/leaseback future minimum payments [Table Text Block]	At September 30, 2017 , the future minimum payments under leaseback obligations are shown below. Lease/Leasebacks Minimum payments due in years ending September 30 2018 $ 50 2019 49 2020 50 2021 207 2022 25 Thereafter — Total $ 381
Schedule of future minimum payments for membership interests subject to mandatory redemption [Table Text Block]	At September 30, 2017 , the mandatory redemptions for each of the next five years are shown below: 2018 2019 2020 2021 2022 Membership interests of variable interest entity subject to mandatory redemption $ 2 $ 2 $ 3 $ 3 $ 3
Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block]	At September 30, 2017 , the future minimum lease payments under operating leases, including purchased power agreements that are accounted for as operating leases, are shown below. Operating Leases Minimum payments due in years ending September 30 2018 $ 67 2019 60 2020 59 2021 59 2022 45 Thereafter 13 Total $ 303
Energy Prepayment Obligations	Energy Prepayment Obligations Payments due in the years ending September 30 2018 2019 2020 2021 2022 Thereafter Total Energy prepayment obligations $ 100 $ 10 $ — $ — $ — $ — $ 110 Interest payments relating to energy prepayment obligations 46 4 — — — — 50 Total $ 146 $ 14 $ — $ — $ — $ — $ 160
Unfunded loan commitments	At September 30, 2017 , TVA's commitments under unfunded loan commitments for each of the next five years are shown below: Unfunded Loan Commitments Payments due in the years ending September 30 2018 2019 2020 2021 2022 Unfunded loan commitments $ 12 $ — $ — $ — $ —
Schedule of Future Minimum Lease Payments for Capital Leases	At September 30, 2017 , the future minimum lease payments under capital leases are shown below. Capital Leases Minimum payments due in years ending September 30 2018 $ 52 2019 51 2020 51 2021 51 2022 51 Thereafter 519 Minimum annual payments 775 Less: amount representing interest (588 ) Total $ 187

Related Parties Related Parties

Related Parties Related Parties (Tables)	12 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Transactions with agencies of the federal government were as follows: Related Party Transactions For the years ended, or at, September 30 2017 2016 2015 Revenue from sales of electricity $ 126 $ 126 $ 130 Other income 136 161 167 Expenditures Operating expenses 216 216 227 Additions to property, plant, and equipment 16 32 37 Cash and cash equivalents 46 54 45 Accounts receivable, net 119 129 106 Accounts payable and accrued liabilities 71 77 98 Long-term power bonds, net 1 4 5 Return on Power Program Appropriation Investment 5 6 5

Unaudited Quarterly Financial53

Unaudited Quarterly Financial Information Unaudited Quarterly Financial Information (Tables)	12 Months Ended
	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Information	Unaudited Quarterly Financial Information 2017 First Second Third Fourth Total Operating revenues $ 2,546 $ 2,547 $ 2,571 $ 3,075 $ 10,739 Operating expenses 2,117 2,013 2,010 2,624 8,764 Operating income 429 534 561 451 1,975 Net income (loss) 102 211 233 139 685 Unaudited Quarterly Financial Information 2016 First Second Third Fourth Total Operating revenues $ 2,280 $ 2,571 $ 2,479 $ 3,286 $ 10,616 Operating expenses 2,052 1,982 1,913 2,343 8,290 Operating income 228 589 566 943 2,326 Net income (loss) (37 ) 318 291 661 1,233

Summary of Significant Accoun54

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - General (Details) People in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)PeopleCustomers	Sep. 30, 2016USD ($)
Population of Service Area
Loans and other long-term receivables, net (including current portion)	$ 107	$ 130
Population of TVA's service area (number of people) | People	9
Maximum original maturity	3 months
Allowance for uncollectible accounts - loans	$ 1	8
Number of customers for whom unbilled revenues are estimated | Customers	6
Revenue Capitalized During Pre-Commercial Operations	$ 18	22
Fuel Cost Capitalized During Pre-Commercial Operations	14	$ 6
Recorded cost for emission allowances granted by the Environmental Protection Agency	0
Payments attributable to blended low-enriched uranium fuel currently in use	164
BLEU fuel obligation	$ 1

Summary of Significant Accoun55

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Reclassificatons (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Reclassifications
Reclassification from Insurance Recoveries to Other, net	$ 7	$ 63

Summary of Significant Accoun56

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Allowance for Uncollectible Accounts
Revenue Capitalized During Pre-Commercial Operations	$ 18	$ 22
Period of time for customers to fulfill payment arrangements	90 days
Allowance for uncollectible accounts - receivables	$ 1	1
Loans receivable	107	130
Allowance for uncollectible accounts - loans	1	8
Fuel Cost Capitalized During Pre-Commercial Operations	$ 14	$ 6

Summary of Significant Accoun57

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Property, Plant, and Equipment, and Depreciation (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Property, Plant, and Equipment, and Depreciation
Change in depreciation due to new study	$ 224,000,000
Loans and other long-term receivables, net (including current portion)	107,000,000	$ 130,000,000
Depreciation	$ 1,300,000,000	$ 1,400,000,000	$ 1,700,000,000
Composite depreciation rate for completed plant	2.49%	2.97%	3.71%
Accelerated depreciation	$ 104,000,000	$ 139,000,000	$ 383,000,000
CGLP Capital lease liability		76,000,000
AFUDC minimum total project cost	1,000,000,000
Allowance for funds used during construction	$ 0	235,000,000	214,000,000
Capitalized software amortization period	7 years
Unamortized computer software costs	$ 42,000,000	27,000,000
Amortization expense of capitalized computer software costs	$ 26,000,000	$ 43,000,000	$ 38,000,000
Nuclear
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.66%	2.37%	2.81%
Coal-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	2.33%	3.50%	5.50%
Hydroelectric
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.58%	1.29%	1.30%
Gas and oil-fired
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	3.27%	3.09%	3.18%
Transmission
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	1.34%	2.80%	2.78%
Other
Property, Plant, and Equipment, and Depreciation
Composite depreciation rate for completed plant	6.12%	8.97%	8.65%
Electricity Generation Plant, Non-Nuclear
Property, Plant, and Equipment, and Depreciation
Capital Leases, Net	$ 161,000,000	$ 163,000,000

Summary of Significant Accoun58

Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Energy Prepayment Obligations and Discounts on Sales (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2004
Energy Prepayment Obligations and Discounts on Sales
MLGW prepayment						$ 1,500
MLGW prepayment period						180 months
MLGW
Energy Prepayment Obligations and Discounts on Sales
Deferred revenue expected recognition each year			$ 100
Deferred revenue, revenue recognized	$ 100	$ 100	100
Total
Energy Prepayment Obligations and Discounts on Sales
Discounts reducing electricity sales			$ 46	$ 46	$ 46

Impact of New Accounting Stan59

Impact of New Accounting Standards and Interpretations Impact of New Accounting Standards and Interpretations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from Operating and maintenance expense to Other income	$ 758	$ 179	$ 185

Accounts Receivable, Net Acco60

Accounts Receivable, Net Accounts Receivable, Net (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Accounts Receivable, Net
Power receivables	$ 1,441	$ 1,637
Other receivables	129	111
Allowance for uncollectible accounts	(1)	(1)
Accounts receivable, net	$ 1,569	$ 1,747

Inventories, Net Inventories,61

Inventories, Net Inventories, Net (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Inventories, Net
Materials and supplies inventory	$ 734	$ 673
Fuel inventory	355	345
RECs/emission allowance inventory, net	15	14
Allowance for inventory obsolescence	(39)	(39)
Inventories, net	$ 1,065	$ 993

Net Completed Plant Net Compl62

Net Completed Plant Net Completed Plant (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Completed Plant
Completed plant cost	$ 58,947	$ 51,564
Accumulated depreciation	28,404	27,592
Net completed plant	30,543	23,972
Coal-fired
Completed Plant
Completed plant cost	15,937	15,587
Accumulated depreciation	10,791	10,473
Net completed plant	5,146	5,114
Gas and oil-fired
Completed Plant
Completed plant cost	4,995	3,918
Accumulated depreciation	1,359	1,267
Net completed plant	3,636	2,651
Nuclear
Completed Plant
Completed plant cost	25,010	19,280
Accumulated depreciation	10,834	10,422
Net completed plant	14,176	8,858
Transmission
Completed Plant
Completed plant cost	7,264	7,061
Accumulated depreciation	3,039	2,975
Net completed plant	4,225	4,086
Hydroelectric
Completed Plant
Completed plant cost	3,015	2,891
Accumulated depreciation	967	932
Net completed plant	2,048	1,959
Other electrical plant
Completed Plant
Completed plant cost	1,756	1,857
Accumulated depreciation	1,008	1,126
Net completed plant	748	731
Electric Subtotal
Completed Plant
Completed plant cost	57,977	50,594
Accumulated depreciation	27,998	27,195
Net completed plant	29,979	23,399
Multipurpose dams
Completed Plant
Completed plant cost	928	928
Accumulated depreciation	387	379
Net completed plant	541	549
Other stewardship
Completed Plant
Completed plant cost	42	42
Accumulated depreciation	19	18
Net completed plant	23	24
Other Subtotal
Completed Plant
Completed plant cost	970	970
Accumulated depreciation	406	397
Net completed plant	$ 564	$ 573

Other Long-Term Assets Other 63

Other Long-Term Assets Other Long-Term Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Other Long-Term Assets
EnergyRight receivables	$ 127	$ 144
Total other long-term assets	$ 323	386
Energy Right
Other Long-Term Assets
Number of days in default	180 days
Other long-term assets
Other Long-Term Assets
EnergyRight receivables	$ 100	112
Loans and other long-term receivables, net	115	136
Commodity contract derivative assets	2	3
Prepaid capacity payments	34	42
Other	72	93
Accounts Receivable
Other Long-Term Assets
EnergyRight receivables	$ 25	$ 29
Minimum | Energy Right
Other Long-Term Assets
EnergyRight loan terms	5 years
Maximum | Energy Right
Other Long-Term Assets
EnergyRight loan terms	10 years

Regulatory Assets and Liabili64

Regulatory Assets and Liabilities Regulatory Assets and Liabilities - Table (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets and Liabilities
Current regulatory assets	$ 447	$ 536
Non-current regulatory assets	8,698	10,164
Regulatory assets	9,145	10,700
Current regulatory liabilities	163	154
Non-current regulatory liabilities	25	3
Regulatory liabilities	188	157
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	153	148
Fuel cost adjustment receivable
Regulatory Assets and Liabilities
Current regulatory liabilities	2	0
Deferred other post-retirement benefits cost
Regulatory Assets and Liabilities
Non-current regulatory liabilities	23	0
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory liabilities	8	6
Non-current regulatory liabilities	2	3
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	237	237
Non-current regulatory assets	759	850
Regulatory assets	1,000
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Current regulatory assets	93	0
Non-current regulatory assets	982	1,547
Gallatin coal combustion residual facilities
Regulatory Assets and Liabilities
Non-current regulatory assets	899	0
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	68	122
Non-current regulatory assets	9	56
Environmental agreements
Regulatory Assets and Liabilities
Current regulatory assets	2	34
Non-current regulatory assets	13	18
Environmental cleanup costs - Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	44	42
Non-current regulatory assets	263	299
Fuel cost adjustment receivable
Regulatory Assets and Liabilities
Current regulatory assets	1	98
Other current regulatory assets
Regulatory Assets and Liabilities
Current regulatory assets	2	3
Deferred pension costs and other post-retirement benefits costs
Regulatory Assets and Liabilities
Non-current regulatory assets	4,009	5,385
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	823	938
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	703	819
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 238	$ 252

Regulatory Assets and Liabili65

Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Nov. 14, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets and Liabilities
Current regulatory assets			$ 447	$ 536
Regulatory Assets			9,145	10,700
Yearly amortization of Deferred nuclear generating units			$ 237
Disposal Group, Including Discontinued Operation, Consideration	$ 111
Net Book Value and related ARO's of Assets Sold		$ 121
Proceeds from Sale of Other Property, Plant, and Equipment		22
Proceeds Due at Closing		$ 89
Max Years to Close	2 years
Period collected in rates			15 years
Deferred Nuclear Generating Units and Construction Costs
Regulatory Assets and Liabilities
Balance in Regulatory asset-Construction costs			$ 619
Environmental Agreements
Regulatory Assets and Liabilities
Initial regulatory asset			360
Investment in environmental agreement projects			290
Amount to be provided to fund environmental projects			60
Amount to be paid in civil penalties			$ 10
Environmental Cleanup Costs - Kingston Ash Spill
Regulatory Assets and Liabilities
Current regulatory asset amortization period			12 months
Energy Efficiency Projects | Environmental Agreements
Regulatory Assets and Liabilities
Regulatory asset amount expensed			$ 275
Paid to States | Environmental Agreements
Regulatory Assets and Liabilities
Regulatory asset amount expensed			60
Civil Penalties | Environmental Agreements
Regulatory Assets and Liabilities
Regulatory asset amount expensed			10
Derivatives Under FTP
Regulatory Assets and Liabilities
Net unrealized gains (losses)			(5)	(39)	[1]
Other current regulatory assets
Regulatory Assets and Liabilities
Current regulatory assets			2	3
Deferred Nuclear Generating Units and Construction Costs
Regulatory Assets and Liabilities
Current regulatory assets			237	$ 237
Regulatory Assets			$ 1,000
[1]	Fair values of certain derivatives under the FTP that were in net liability positions totaling $4 million and $30 million at September 30, 2017 and 2016, respectively, are recorded in TVA's margin cash accounts in Other current assets. These derivatives are transacted with futures commission merchants, and cash deposits have been posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full.

Asset Acquisitions and Busine66

Asset Acquisitions and Business Combinations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 20, 2017	Jul. 20, 2016
Business Combinations and Settlement of Preexisting Relationships [Abstract]
Percentage equity interests acquired			100.00%	100.00%
Total cash consideration	$ 36	$ 33
Fair value of assets acquired			$ 110	$ 111
Fair value of liabilities assumed		78	74	78
Amortization expense of SPE's acquired	1	1
Loss on extinguishment	$ 3	$ 6
Lease/leaseback obligations settled			71	72
Reacquisition price			$ 74	$ 78

Variable Interest Entities Va67

Variable Interest Entities Variable Interest Entities (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2013	Aug. 09, 2013	Sep. 30, 2012	Jan. 17, 2012
Liabilities
Total liabilities	$ 1,243	$ 1,280		$ 400		$ 1,000
Current maturities of long-term debt of variable interest entities issued at par	36	35
Long-term debt of variable interest entities, net	1,164	1,199
VIE Financing
Face Amount				$ 40
Rate of Return SHLLC				7.00%
Accrued interest	11	11
Interest expense	59	61	$ 63
JSCCG
VIE Financing
Face Amount						900	$ 900
Holdco
VIE Financing
Face Amount						$ 100	$ 100
SCCG
VIE Financing
Face Amount				$ 360	$ 360
Accounts payable and accrued liabilities
Liabilities
Total liabilities	2	2
Total current liabilities
Liabilities
Total liabilities	49	48
Other long-term liabilities
Liabilities
Total liabilities	$ 30	$ 33

Other Long-Term Liabilities O68

Other Long-Term Liabilities Other Long-Term Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017		Sep. 30, 2016
Other Long-Term Liabilities
Interest rate swap liabilities	$ 1,511		$ 1,938
Capital lease obligations	302		409
Currency swap liabilities	103	[1]	162	[2]
EnergyRight financing obligation	161		183
Non-current regulatory liabilities	25		3
Total other long-term liabilities	3,055		2,773
Other long-term liabilities
Other Long-Term Liabilities
Interest rate swap liabilities	1,418		1,938
Gallatin coal combustion residual facilities	880		0
Capital lease obligations	182		177
Currency swap liabilities	92		162
EnergyRight financing obligation	115		130
Environmental agreements liability	13		18
Membership interests of VIE subject to mandatory redemption	30		33
Commodity contract derivative liabilities	9		49
Other	316		266
Accounts payable and accrued liabilities
Other Long-Term Liabilities
Gallatin coal combustion residual facilities	19
EnergyRight financing obligation	$ 29		$ 33
[1]	Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities.
[2]	Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of any cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities.

Asset Retirement Obligations 69

Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Asset Retirement Obligations
Increase in ARO liability		$ 252
Change in estimate	[1]	411	$ 176
Additional obligations	[1]	1	15
Reclassification of Gallatin projects		(188)
Amortization of regulatory asset		144	144	$ 44
Balance	[1]	4,304	4,052	3,843
Settlements	[1]	(123)	(133)
Accretion (recorded as regulatory asset)	[1]	151	151
Non-nuclear - Closure Method Revision
Asset Retirement Obligations
Change in estimate			32
Non-nuclear - Active Projects Revisions
Asset Retirement Obligations
Change in estimate			(54)
Nuclear
Asset Retirement Obligations
Revision of estimate (other non nuclear)		(250)
Change in estimate			198
Additional obligations		0	0
Reclassification of Gallatin projects		0
Balance		2,859	2,492	2,187
Settlements		0	0
Accretion (recorded as regulatory asset)		117	107
Non-nuclear
Asset Retirement Obligations
Revision of estimate (other non nuclear)		(188)
Change in estimate		161	(22)
Additional obligations		1	15
Reclassification of Gallatin projects		(188)
Balance		1,445	1,560	$ 1,656
Settlements		(123)	(133)
Accretion (recorded as regulatory asset)		34	44
Accounts payable and accrued liabilities
Asset Retirement Obligations
Current portion of ARO		$ 128	$ 212
[1]	(1) The current portions of the ARO liability in the amounts of $128 million and $212 million as of September 30, 2017 and 2016, respectively, are included in Accounts payable and accrued liabilities.

Debt and Other Obligations De70

Debt and Other Obligations Debt and Other Obligations - General (Details) $ in Billions	Sep. 30, 2017USD ($)
Debt Instrument
Debt ceiling	$ 30

Debt and Other Obligations De71

Debt and Other Obligations Debt and Other Obligations - Secured Debt of VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2013	Aug. 09, 2013	Sep. 30, 2012	Jan. 17, 2012
Debt Instrument
Face Amount			$ 40
Membership interests of VIE subject to mandatory redemption (including current portion)	$ 41	$ 46		$ 40
Long-term debt of variable interest entities (including current maturities)	1,356	1,468
SCCG
Debt Instrument
Face Amount			$ 360	$ 360
Interest rate				3.846%
JSCCG
Debt Instrument
Face Amount					$ 900	$ 900
Interest rate						4.626%
Holdco
Debt Instrument
Face Amount					$ 100	$ 100
Interest rate						7.10%
Holdco balloon payment upon maturity						$ 10
Secured debt of VIEs
Debt Instrument
Membership interests of VIE subject to mandatory redemption (including current portion)	32	35
Long-term debt of variable interest entities (including current maturities)	$ 1,200	$ 1,234

Debt and Other Obligations De72

Debt and Other Obligations Debt and Other Obligations - Secured Notes of SPEs (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 20, 2017	Sep. 30, 2016	Jul. 20, 2016	Sep. 30, 2013	Nov. 14, 2001	Sep. 27, 2000
Secured notes
Secured notes					$ 40
Fair value of liabilities assumed		$ 74	$ 78	$ 78
Notes Payable	$ 121		75
Special purpose entity
Secured notes
Secured notes						$ 272	$ 255
Interest rate						5.572%	7.299%
7/20/16 Business Combination
Secured notes
Notes Payable	48		$ 75
9/20/17 Business Combination
Secured notes
Notes Payable	$ 74

Debt and Other Obligations De73

Debt and Other Obligations Debt and Other Obligations - Short-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Short-term Debt [Abstract]
Short-term debt, net of discounts	$ 1,998	$ 1,407	$ 1,034
Weighted average interest rate - discount notes	1.00%	0.203%	0.055%

Debt and Other Obligations De74

Debt and Other Obligations Debt and Other Obligations - Put and Call Options (Details) $ in Millions	Sep. 30, 2017USD ($)
Debt Instrument
Amount of redeemable bond issues outstanding	$ 408
Call price	100.00%
Bond issues with survivor's option	268
Bonds outstanding with survivor's option	$ 222
PARRS 1998 Series D Bond
Debt Instrument
Amount of redeemable bond issues outstanding	$ 274
PARRS interest rate prior to rate reset	6.75%
PARRS interest rate after rate reset	3.55%
Amount of bonds redeemed	$ 301
PARRS 1999 Series A Bond
Debt Instrument
Amount of redeemable bond issues outstanding	$ 232
PARRS interest rate prior to rate reset	6.50%
PARRS interest rate after rate reset	3.36%
Amount of bonds redeemed	$ 293

Debt and Other Obligations De75

Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 20, 2017USD ($)	Feb. 07, 2017USD ($)	Jul. 20, 2016USD ($)	Sep. 30, 2003Units
Debt Instrument
Number of Businesses Acquired	2
Leaseback transaction, number of units | Units							24
Issues of power bonds	$ 999	$ 0	$ 973
Discount on debt issues		0			$ (1)
Acquisition of notes payable	74	78
Redemptions/Maturities of variable interest entities	35	33	32
Redemptions/Maturities of power bonds	1,558	76	$ 1,180
Leaseback obligation settled as a result of acquisition				$ 70		$ 70
Total
Debt Instrument
Proceeds from (Repayments of) Other Debt	$ 1,620	$ 112
Percent of par value	100.00%	100.00%
Debt of variable interest entities
Debt Instrument
Redemptions/Maturities of variable interest entities	$ 35	$ 33
Notes Payable
Debt Instrument
Redemptions/Maturities of notes payable	27	3
electronotes
Debt Instrument
Redemptions/Maturities of power bonds	5	47
2009 Series A
Debt Instrument
Redemptions/Maturities of power bonds	0	2
2009 Series B
Debt Instrument
Redemptions/Maturities of power bonds	28	27
2001 Series D
Debt Instrument
Redemptions/Maturities of power bonds	525	0
2007 Series A
Debt Instrument
Redemptions/Maturities of power bonds	1,000	0
Notes payable
Debt Instrument
Acquisition of notes payable	74	78
Total
Debt Instrument
Proceeds from (Repayments of) Other Debt	999	0
2017 Series A
Debt Instrument
Issues of power bonds	$ 1,000	$ 0
Percent of par value	99.90%

Debt and Other Obligations De76

Debt and Other Obligations Debt and Other Obligations - Debt Outstanding (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2015
Short-term debt
Short-term debt, net of discounts		$ 1,998	$ 1,407		$ 1,034
Current maturities of long-term debt of variable interest entities issued at par		36	35
Current maturities of notes payable		53	27
Total Current maturities of power bonds issued at par		1,728	1,555
Bonds outstanding with survivor's option		222
Current maturities of power bonds		1,728	1,555
Total current debt outstanding, net		3,815	3,024
Long-term debt
Long-term power bonds, net		20,205	20,901
Long-term power bonds	[1]	20,357	21,063
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		(152)	162
Long-term debt of variable interest entities, net		1,164	1,199
Long-term notes payable		69	48
Total long-term debt, net		21,438	22,148
Foreign Currency Transaction Gain (Loss), before Tax		$ 125	150
880591DX7
Short-term debt
Coupon rate	[2]	4.65%
Debt Instrument, Maturity Date		Jun. 15, 2035
Long-term debt
Long-term power bonds, net		$ 436	436
880591EF5
Short-term debt
Coupon rate		3.77%
Debt Instrument, Maturity Date		Jun. 15, 2034
Long-term debt
Long-term power bonds, net		$ 303	332
880591DC3
Short-term debt
Coupon rate		5.805%
Debt Instrument, Maturity Date		Jun. 7, 2021
Long-term debt
Long-term power bonds, net		$ 268	260
880591EL2
Short-term debt
Coupon rate		3.875%
Debt Instrument, Maturity Date		Feb. 15, 2021
Long-term debt
Long-term power bonds, net		$ 1,500	1,500
880591EQ1
Short-term debt
Coupon rate		1.75%
Debt Instrument, Maturity Date		Oct. 15, 2018
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591EC2
Short-term debt
Coupon rate		4.50%
Debt Instrument, Maturity Date		Apr. 1, 2018
Long-term debt
Long-term power bonds, net		$ 0	1,000	[3]
electronotes
Long-term debt
Maturity date - earliest		May 15, 2020
Maturity date - latest		Feb. 15, 2043
Call date - earliest		Feb. 15, 2015
Call date - latest		Feb. 15, 2018
Long-term power bonds, net		$ 226	278
880591CU4
Short-term debt
Coupon rate		6.25%
Debt Instrument, Maturity Date		Dec. 15, 2017
Long-term debt
Long-term power bonds, net		$ 0	650
880591EN8
Short-term debt
Coupon rate		1.875%
Debt Instrument, Maturity Date		Aug. 15, 2022
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591ER9
Short-term debt
Coupon rate		2.875%
Debt Instrument, Maturity Date		Sep. 15, 2024
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591CJ9
Short-term debt
Coupon rate		6.75%
Debt Instrument, Maturity Date		Nov. 1, 2025
Long-term debt
Long-term power bonds, net		$ 1,350	1,350
880591EU26
Short-term debt
Coupon rate	[2]	2.875%
Debt Instrument, Maturity Date		Feb. 1, 2027
Long-term debt
Long-term power bonds, net		$ 1,000	0
880,591,300
Short-term debt
Coupon rate		3.55%
Debt Instrument, Maturity Date		Jun. 1, 2028
Long-term debt
Long-term power bonds, net		$ 273	273
880,591,409
Short-term debt
Coupon rate		3.36%
Debt Instrument, Maturity Date		May 1, 2029
Long-term debt
Long-term power bonds, net		$ 232	232
880591DM1
Short-term debt
Coupon rate		7.125%
Debt Instrument, Maturity Date		May 1, 2030
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591DV1
Short-term debt
Coupon rate		4.70%
Debt Instrument, Maturity Date		Jul. 15, 2033
Long-term debt
Long-term power bonds, net		$ 472	472	[4]
880591DP4
Short-term debt
Coupon rate		6.587%
Debt Instrument, Maturity Date		Jun. 7, 2032
Long-term debt
Long-term power bonds, net		$ 335	324	[4]
880591CK6
Short-term debt
Coupon rate		5.98%
Debt Instrument, Maturity Date		Apr. 1, 2036
Long-term debt
Long-term power bonds, net		$ 121	121
880591CS9
Short-term debt
Coupon rate		5.88%
Debt Instrument, Maturity Date		Apr. 1, 2036
Long-term debt
Long-term power bonds, net		$ 1,500	1,500	[3]
880591CP5
Short-term debt
Coupon rate		6.15%
Debt Instrument, Maturity Date		Jan. 15, 2038
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591ED0
Short-term debt
Coupon rate		5.50%
Debt Instrument, Maturity Date		Jun. 15, 2038
Long-term debt
Long-term power bonds, net		$ 500	500
880591EH1
Short-term debt
Coupon rate		5.25%
Debt Instrument, Maturity Date		Sep. 15, 2039
Long-term debt
Long-term power bonds, net		$ 2,000	2,000
880591EP3
Short-term debt
Coupon rate		3.50%
Debt Instrument, Maturity Date		Dec. 15, 2042
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591DU3
Short-term debt
Coupon rate		4.962%
Debt Instrument, Maturity Date		Jun. 7, 2043
Long-term debt
Long-term power bonds, net		$ 201	195
880591CF7
Short-term debt
Coupon rate		6.235%
Debt Instrument, Maturity Date		Jul. 15, 2045
Long-term debt
Long-term power bonds, net		$ 140	140
880591EB4
Short-term debt
Coupon rate		4.875%
Debt Instrument, Maturity Date		Jan. 15, 2048
Long-term debt
Long-term power bonds, net		$ 500	500
880591DZ2
Short-term debt
Coupon rate	[2]	5.375%
Debt Instrument, Maturity Date		Apr. 1, 2056
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591EJ7
Short-term debt
Coupon rate		4.625%
Debt Instrument, Maturity Date		Sep. 15, 2060
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591ES7
Short-term debt
Coupon rate		4.25%
Debt Instrument, Maturity Date		Sep. 15, 2065
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591EF5 (6/15/2018)
Short-term debt
Coupon rate		3.77%
Total Current maturities of power bonds issued at par		$ 28	27
Debt Instrument, Maturity Date		Jun. 15, 2018
88059TFS5
Short-term debt
Coupon rate		4.125%
Debt Instrument, Maturity Date		Oct. 15, 2017
Bonds outstanding with survivor's option		$ 46	0
880591EF5
Short-term debt
Coupon rate		3.77%
Total Current maturities of power bonds issued at par		$ 1	1
Debt Instrument, Maturity Date		Dec. 15, 2017
88059TEL1
Short-term debt
Coupon rate		2.65%
Total Current maturities of power bonds issued at par		$ 1	1
Debt Instrument, Maturity Date		Nov. 15, 2017
88059TEL1 (5/15/2018)
Short-term debt
Coupon rate		2.65%
Total Current maturities of power bonds issued at par		$ 2	2
Debt Instrument, Maturity Date		May 15, 2018
880591DS8
Short-term debt
Coupon rate		4.875%
Total Current maturities of power bonds issued at par		$ 0	524
Debt Instrument, Maturity Date		Dec. 15, 2016
880591EA6
Short-term debt
Coupon rate		5.50%
Total Current maturities of power bonds issued at par		$ 0	1,000
Debt Instrument, Maturity Date		Jul. 18, 2017
880591CU4
Short-term debt
Coupon rate		6.25%
Total Current maturities of power bonds issued at par		$ 650	0
Debt Instrument, Maturity Date		Dec. 15, 2017
880591EC2
Short-term debt
Coupon rate		4.50%
Total Current maturities of power bonds issued at par		$ 1,000	$ 0
Debt Instrument, Maturity Date		Apr. 1, 2018
Minimum | electronotes
Short-term debt
Coupon rate		2.375%
Maximum | electronotes
Short-term debt
Coupon rate		3.625%
[1]	Includes net exchange gain from currency transactions of $125 million at September 30, 2017 and $150 million at September 30, 2016.
[2]	The coupon rate represents TVA’s effective interest rate.
[3]	(2) Includes one electronotes® issue with partial maturities of principal for each required annual payment.
[4]	TVA PARRS, CUSIP numbers 880591300 and 880591409, may be redeemed under certain conditions. See Put and Call Options above.

Debt and Other Obligations De77

Debt and Other Obligations Debt and Other Obligations - Maturities Due (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Debt Instrument
2,018	$ 1,817
2,019	1,116
2,020	93
2,021	1,901
2,022	1,071
Thereafter	17,545
Total	23,543
Short-term debt, net of discounts	1,998	$ 1,407	$ 1,034
Short-term debt, net of discounts total	1,998
Foreign Currency Transaction Gain (Loss), before Tax	125	$ 150
Net discount on sale of Bonds	93
Power bonds
Debt Instrument
Debt issuance costs	59
Other long-term debt
Debt Instrument
Debt issuance costs	$ 11

Debt and Other Obligations De78

Debt and Other Obligations Debt and Other Obligations - Credit Facility Agreements (Details)	Sep. 30, 2017USD ($)Credit_facilities	Sep. 30, 2016USD ($)
Credit Facility Agreements
Current borrowing capacity	$ 150,000,000
Line of Credit
Credit Facility Agreements
Current borrowing capacity	150,000,000
Credit facility agreements borrowings outstanding	0
Revolving Credit Facilities
Credit Facility Agreements
Current borrowing capacity	2,700,000,000
Credit facility agreements borrowings outstanding	$ 0	$ 0
Number of revolving credit facilities | Credit_facilities	4
Revolving Credit Facility 4	$ 150,000,000
Revolving credit facility 3	500,000,000
Revolving credit facility 1	1,000,000,000
Revolving Credit Facility 2	1,000,000,000
Long-term Line of Credit, Borrowings 4	0
Long-term Line of Credit, Borrowings 3	0
Long-term Line of Credit, Borrowings 1	0
Long-term Line of Credit, Borrowings 2	0
Line of Credit Facility, Remaining Borrowing Capacity 4	112,000,000
Line of Credit Facility, Remaining Borrowing Capacity 3	0
Line of Credit Facility, Remaining Borrowing Capacity 1	722,000,000
Line of Credit Facility, Remaining Borrowing Capacity 2	665,000,000
Line of Credit Facility, Remaining Borrowing Capacity	1,499,000,000
Letter of Credit
Credit Facility Agreements
Amount of letters of credit outstanding	1,200,000,000	$ 1,400,000,000
Letters of Credit Outstanding, Amount 4	38,000,000
Letters of Credit Outstanding, Amount 3	500,000,000
Letters of Credit Outstanding, Amount 1	278,000,000
Letter of Credit Outstanding, Amount 2	$ 335,000,000

Debt and Other Obligations De79

Debt and Other Obligations Debt and Other Obligations - Lease/Leasebacks (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2003	Sep. 30, 2017	Sep. 20, 2017	Sep. 30, 2016	Jul. 20, 2016
Lease/Leasebacks [Abstract]
Proceeds prior to 2004 for CTs	$ 945
Proceeds in 2003 for QTEs	$ 389
Percentage equity interests acquired			100.00%		100.00%
Leaseback obligation settled as a result of acquisition			$ 70		$ 70
CT and QTE outstanding leaseback obligation		$ 338		$ 467

Accumulated Other Comprehensi80

Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Equity [Abstract]
Reclassification on cash flow hedges from AOCI to interest expense	$ (26)	$ 129	$ 65

Risk Management Activities an81

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Receive Hedge Accounting Treatment (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Summary of Derivative Instruments That Receive Hedge Accounting Treatment
Net unrealized gain (loss) on future cash flow hedges	$ 59,000,000	$ (139,000,000)	$ (72,000,000)
Reclassification to earnings from cash flow hedges	(26,000,000)	129,000,000	$ 65,000,000
Ineffective portion excluded from testing	0	$ 0
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	$ 18,000,000

Risk Management Activities an82

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)		12 Months Ended
		Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Derivative
Unrealized gains/losses on derivatives		$ 0	$ 0
Change in Unrealized gains (losses) on Interest Rate Derivatives		472,000,000	(311,000,000)
FTP transaction limit		$ 130,000,000
Maximum hedge volume		75.00%
Market value limitation of outstanding construction materials hedging transactions		$ 100,000,000
Portfolio value at risk limit for foreign currency transactions		5,000,000
Interest Rate Swap
Derivative
Amount of gain (loss) recognized in income on derivatives		(101,000,000)	(109,000,000)
Fair value		(410,000,000)
Commodity Contract Derivatives
Derivative
Fair value		(60,000,000)	(125,000,000)
Commodity derivatives under FTP
Derivative
Amount of gain (loss) recognized in income on derivatives		(36,000,000)	$ (94,000,000)
Fair value	[1]	$ (5,000,000)
Coal Contract Derivatives
Derivative
Number of contracts		20	20
Notional amount		17,000,000	20,000,000
Fair value		$ (67,000,000)	$ (127,000,000)
Natural Gas Contract Derivatives
Derivative
Number of contracts		53	39
Notional amount		271,000,000	148,000,000
Fair value		$ 7,000,000	$ 2,000,000
Maximum | Coal Contract Derivatives
Derivative
Derivative, term		2 years
Maximum | Natural Gas Contract Derivatives
Derivative
Derivative, term		4 years
Natural Gas Swap
Derivative
Notional amount		2,800,000	21,052,500
Fair value		$ (5,000,000)	$ (39,000,000)
[1]	Fair values of certain derivatives under the FTP that were in net liability positions totaling $4 million and $30 million at September 30, 2017 and 2016, respectively, are recorded in TVA's margin cash accounts in Other current assets. These derivatives are transacted with futures commission merchants, and cash deposits have been posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full.

Risk Management Activities an83

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Mark-to-Market Values of TVA Derivatives (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Derivatives, Fair Value
Derivative Liability, Fair Value	$ 1,619		$ 2,145
200 million Sterling currency swap
Derivatives, Fair Value
Derivative Liability, Fair Value	103		162	[1]
Fair value	(67)
200 million Sterling currency swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(62)		(82)
200 million Sterling currency swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(5)
250 million Sterling currency swap
Derivatives, Fair Value
Fair value	(15)
250 million Sterling currency swap | Other long-term assets
Derivatives, Fair Value
Fair value			(41)
250 million Sterling currency swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(11)
250 million Sterling currency swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(4)
150 million Sterling currency swap
Derivatives, Fair Value
Fair value	(21)
150 million Sterling currency swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(19)		(39)
150 million Sterling currency swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(2)
$1.0 billion notional interest rate swap
Derivatives, Fair Value
Fair value	(1,093)
$1.0 billion notional interest rate swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(1,027)		(1,387)
$1.0 billion notional interest rate swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(66)
$476 million notional interest rate swap
Derivatives, Fair Value
Fair value	(410)
$476 million notional interest rate swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(385)		(539)
$476 million notional interest rate swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(25)
$42 million notional interest rate swap
Derivatives, Fair Value
Fair value	(8)
$42 million notional interest rate swap | Other long-term liabilities
Derivatives, Fair Value
Fair value	(6)		(12)
$42 million notional interest rate swap | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value	(2)
Commodity contract derivatives
Derivatives, Fair Value
Fair value	(60)		(125)
Commodity contract derivatives | Other long-term assets
Derivatives, Fair Value
Fair value	8		6
Commodity contract derivatives | Other long-term liabilities
Derivatives, Fair Value
Fair value	(9)		(49)
Commodity contract derivatives | Other current assets
Derivatives, Fair Value
Fair value		$ 2	3
Commodity contract derivatives | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value		(61)	(85)
Derivatives Under FTP
Derivatives, Fair Value
Fair value	5		39	[2]
Derivatives Under FTP | Other long-term liabilities
Derivatives, Fair Value
Fair value		0	(2)
Derivatives Under FTP | Other current assets
Derivatives, Fair Value
Derivative Liability, Fair Value	$ 4		30
Fair value		(4)	(30)
Derivatives Under FTP | Accounts payable and accrued liabilities
Derivatives, Fair Value
Fair value		$ (1)	$ (7)
[1]	Letters of credit of approximately $1.2 billion and $1.4 billion were posted as collateral at September 30, 2017 and 2016, respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives.
[2]	Fair values of certain derivatives under the FTP that were in net liability positions totaling $4 million and $30 million at September 30, 2017 and 2016, respectively, are recorded in TVA's margin cash accounts in Other current assets. These derivatives are transacted with futures commission merchants, and cash deposits have been posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full.

Risk Management Activities an84

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Currency Swaps Outstanding (Details) £ in Millions	12 Months Ended
	Sep. 30, 2017GBP (£)Bond_issues
Derivative
Number of British pound sterling denominated bond transactions | Bond_issues	3
1999 Currency Swap Contract
Derivative
Effective Date of Currency Swap Contract	1,999
Associated TVA bond issues currency exposure	£ 200
Expiration Date of Swap	2,021
Overall effective cost to TVA	5.81%
2001 Currency Swap Contract
Derivative
Effective Date of Currency Swap Contract	2,001
Associated TVA bond issues currency exposure	£ 250
Expiration Date of Swap	2,032
Overall effective cost to TVA	6.59%
2003 Currency Swap Contract
Derivative
Effective Date of Currency Swap Contract	2,003
Associated TVA bond issues currency exposure	£ 150
Expiration Date of Swap	2,043
Overall effective cost to TVA	4.96%

Risk Management Activities an85

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivatives Under Financial Trading Program (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Derivative
FTP transaction limit	$ 130
Natural Gas Swap
Derivative
Notional amount	2,800,000	21,052,500
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ (5)	$ (39)
Natural Gas Contract Derivatives
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	(5)	(39)
Decrease (increase) in fuel expense	(29)	(75)
Decrease (increase) in purchased power expense	(7)	(19)
Closed Derivative Contracts
Derivative
Unrealized gains (losses) deferred as regulatory liabilities (assets)	$ (3)	$ (5)

Risk Management Activities an86

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Counterparty Credit Risk (Details) $ in Millions		12 Months Ended
		Sep. 30, 2017USD ($)Customersmerchantsmegawatts	Sep. 30, 2016USD ($)Customers
Derivative
Receivables from power sales		$ 1,441	$ 1,637
Coal counterparties in asset position	[1]		$ 9
Number of active future commission merchants | merchants		2
Credit of Customers
Derivative
Number of customers that represent the percent of sales | Customers		6	6
Percent of total sales by customers		33.00%	33.00%
Power Purchase Agreement
Derivative
Megawatts | megawatts		440
Coal Contract Derivatives
Derivative
Coal counterparties in asset position		$ 10
[1]	There are no derivative contracts subject to a master netting arrangement or similar agreement which are not offset in the balance sheets.

Risk Management Activities an87

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions		Sep. 30, 2017	Sep. 30, 2016
Offsetting Assets [Line Items]
Gross Amounts Offset in the Balance Sheet	[1]		$ (6)
Net Amounts of Assets Presented in the Balance Sheet	[2]		9
Commodity derivatives not subject to master netting or similar arrangement	[2]		9
Total			15
Letter of Credit
Offsetting Assets [Line Items]
Amount of letters of credit outstanding		$ 1,200	1,400
Other Contract
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, subject to master netting or similar arrangements		10	6
Gross Amounts Offset in the Balance Sheet	[1]		(6)
Net Amounts of Assets Presented in the Balance Sheet	[2]	$ 10	$ 0
[1]	Amounts primarily include counterparty netting of derivative contracts, margin account deposits for futures commission merchants transactions, and cash collateral received or paid in accordance with the accounting guidance for derivatives and hedging transactions.
[2]	There are no derivative contracts subject to a master netting arrangement or similar agreement which are not offset in the balance sheets.

Risk Management Activities an88

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2017	Sep. 30, 2016
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements		$ 1,619	$ 2,145
Gross Amounts Offset in the Balance Sheet	[1]	4	36
Net Amounts of Liabilities Presented in the Balance Sheet	[2]	1,685	2,243
Total derivatives not subject to master netting or similar arrangement	[2]	70	134
Total		1,689	2,279
Currency Swap
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements		103	162	[3]
Gross Amounts Offset in the Balance Sheet		0	0	[1],[3]
Net Amounts of Liabilities Presented in the Balance Sheet	[2],[3]	103	162
Interest Rate Contract
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements		1,511	1,938	[3]
Gross Amounts Offset in the Balance Sheet		0	0	[1],[3]
Net Amounts of Liabilities Presented in the Balance Sheet	[2],[3]	1,511	1,938
Other Contract
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities, subject to master netting or similar arrangements		5	45
Gross Amounts Offset in the Balance Sheet		4	36	[1]
Net Amounts of Liabilities Presented in the Balance Sheet	[2]	1	9
Total derivatives subject to master netting or similar arrangement
Offsetting Liabilities [Line Items]
Net Amounts of Liabilities Presented in the Balance Sheet	[2]	1,615	2,109
Letter of Credit
Offsetting Liabilities [Line Items]
Amount of letters of credit outstanding		$ 1,200	$ 1,400
[1]	Amounts primarily include counterparty netting of derivative contracts, margin account deposits for futures commission merchants transactions, and cash collateral received or paid in accordance with the accounting guidance for derivatives and hedging transactions.
[2]	There are no derivative contracts subject to a master netting arrangement or similar agreement which are not offset in the balance sheets.
[3]	Letters of credit of approximately $1.2 billion and $1.4 billion were posted as collateral at September 30, 2017 and 2016, respectively, to partially secure the liability positions of one of the currency swaps and one of the interest rate swaps in accordance with the collateral requirements for these derivatives.

Risk Management Activities an89

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Other Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Derivative
Forward Contract Derivative Asset, at Fair Value	$ 19	$ 15
Fair Value, Inputs, Level 2
Derivative
Forward Contract Derivative Asset, at Fair Value	$ 19	$ 15

Risk Management Activities an90

Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Collateral (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Derivative
Likely collateral obligation increase if downgraded	$ 22
Collateral
Derivative
Aggregate fair value of derivative instruments with credit-risk related contingent features that were in a liability position	1,600
Collateral obligations	1,100
Letter of Credit
Derivative
Amount of letters of credit outstanding	$ 1,200	$ 1,400

Fair Value Measurements Fair 91

Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Investment Gains (Losses)
Investment fund securities classified as trading and measured at fair value	$ 2,600
Balance in the NDT	1,900
Balance in the ART	$ 632
Period of time where the investor contributes capital to an investment in a private partnership - minimum	three
Period of time where the investor contributes capital to an investment in a private partnership - maximum	four
Minimum investment period	10 years
Number of readily available quoted exchange prices for the investments	0
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	$ 4	$ 2
DCP
Investment Gains (Losses)
Unrealized gains (losses) on investments	2	1
NDT
Investment Gains (Losses)
Unrealized gains (losses) on investments	92	89
ART
Investment Gains (Losses)
Unrealized gains (losses) on investments	43	$ 29
Equity Funds [Member]
Investment Gains (Losses)
Commitments, Fair Value Disclosure	50
Real Estate Funds [Member]
Investment Gains (Losses)
Commitments, Fair Value Disclosure	$ 5

Fair Value Measurements Fair 92

Fair Value Measurements Fair Value Measurements - Nonperformance Risk (Details) $ in Millions	Sep. 30, 2017USD ($)
Nonperformance Risk
Derivative credit valuation adjustment, assets	$ 1
Derivative credit valuation adjustment, liabilities	$ 1

Fair Value Measurements Fair 93

Fair Value Measurements Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2017		Dec. 31, 2016	Sep. 30, 2016
Investments
Equity securities	$ 226			$ 196
Government debt securities	142			124
Corporate debt securities	373			393
Mortgage and asset-backed securities	49			50
Institutional mutual funds	94			92
Forward debt securities contracts - asset	19			15
Private equity funds measured at net asset value	136	[1]		132	[2]
Private real estate measured at net asset value	113	[1]		113	[2]
Commingled funds measured at net asset value	1,451	[1]		1,142	[2]
Total investments	2,603			2,257
Commodity contract derivatives	10			9
Total	2,613			2,266
Liabilities
Currency swaps	103	[3]		162	[4]
Interest rate swaps	1,511			1,938
Commodity contract derivatives	70			134
Swap contracts	1	[3]		9	[4]
Total	1,685			2,243
Fair Value, Inputs, Level 1
Investments
Equity securities	226			196
Government debt securities	100			88
Corporate debt securities	0			0
Mortgage and asset-backed securities	0			0
Institutional mutual funds	94			92
Forward debt securities contracts - asset	0			0
Private equity funds measured at net asset value	0	[1]		0	[2]
Private real estate measured at net asset value	0	[1]		0	[2]
Commingled funds measured at net asset value	0	[1]		0	[2]
Total investments	420			376
Commodity contract derivatives	0			0
Total	420			376
Liabilities
Currency swaps	0	[3]		0	[4]
Interest rate swaps	0			0
Commodity contract derivatives	0			0
Swap contracts	0	[3]		0	[4]
Total			$ 0	0
Fair Value, Inputs, Level 2
Investments
Equity securities	0			0
Government debt securities	42			36
Corporate debt securities	373			393
Mortgage and asset-backed securities	49			50
Institutional mutual funds	0			0
Forward debt securities contracts - asset	19			15
Private equity funds measured at net asset value	0	[1]		0	[2]
Private real estate measured at net asset value	0	[1]		0	[2]
Commingled funds measured at net asset value	0	[1]		0	[2]
Total investments	483			494
Commodity contract derivatives	8			5
Total	491			499
Liabilities
Currency swaps	103	[3]		162	[4]
Interest rate swaps	1,511			1,938
Commodity contract derivatives	1			3
Swap contracts	1	[3]		9	[4]
Total			$ 1,616	2,112
Fair Value, Inputs, Level 3
Investments
Equity securities	0			0
Government debt securities	0			0
Corporate debt securities	0			0
Mortgage and asset-backed securities	0			0
Institutional mutual funds	0			0
Forward debt securities contracts - asset	0			0
Private equity funds measured at net asset value	0	[1]		0	[2]
Private real estate measured at net asset value	0	[1]		0	[2]
Commingled funds measured at net asset value	0	[1]		0	[2]
Total investments	0			0
Commodity contract derivatives	2			4
Total	2			4
Liabilities
Currency swaps	0	[3]		0	[4]
Interest rate swaps	0			0
Commodity contract derivatives	69			131
Swap contracts	0	[3]		0	[4]
Total	$ 69			$ 131
[1]	Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets.
[2]	Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been categorized in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets.
[3]	Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities.
[4]	Due to the right of setoff and method of settlement, TVA elects to record commodity derivatives under the FTP based on its net commodity position with the counterparty or FCM. Deposits are made to TVA's margin cash accounts held with each FCM to offset any net liability positions in full for derivatives that are transacted with FCMs. TVA records currency swaps net of any cash collateral received from or paid to the counterparty, to the extent such amount is not recorded in Accounts payable and accrued liabilities. See Note 15 — Offsetting of Derivative Assets and Liabilities.

Fair Value Measurements Fair 94

Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) tons-per-year in Billions	12 Months Ended
	Sep. 30, 2017USD ($)tons-per-year	Sep. 30, 2016USD ($)tons-per-year	Sep. 30, 2015USD ($)
Fair Value Measurements
Commodity contract derivatives, assets	$ 10,000,000	$ 9,000,000
Commodity contract derivatives, liabilities	$ 70,000,000	$ 134,000,000
Minimum
Assets
Fair value measurements tons per year | tons-per-year	0.6	0.7
Price per ton	$ 11.40	$ 11.80
Liabilities
Fair value measurements tons per year | tons-per-year	0.6	0.7
Price per ton	$ 11.40	$ 11.80
Maximum
Assets
Fair value measurements tons per year | tons-per-year	0.7	0.8
Price per ton	$ 112.23	$ 85.02
Liabilities
Fair value measurements tons per year | tons-per-year	0.7	0.8
Price per ton	$ 112.23	$ 85.02
Commodity Contract Derivatives
Fair Value Measurements
Balance at beginning/end of period	(67,000,000)	(127,000,000)	$ (98,000,000)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Change in net unrealized gains (losses) deferred as regulatory assets and liabilities	60,000,000	(29,000,000)
Fair Value, Inputs, Level 3
Fair Value Measurements
Commodity contract derivatives, assets	2,000,000	4,000,000
Commodity contract derivatives, liabilities	$ 69,000,000	$ 131,000,000

Fair Value Measurements Fair 95

Fair Value Measurements Fair Value Measurements - Estimated Values of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Aug. 09, 2013
Estimated Values of Financial Instruments (Level 2 Valuation)
EnergyRight receivables (including current portion)	$ 127	$ 144
Loans and other long-term receivables, net (including current portion)	107	130
EnergyRight® financing obligation (including current portion)	161	183
Unfunded Loan Commitments	18	17
Membership interests of VIE subject to mandatory redemption (including current portion)	41	46	$ 40
Long-term outstanding power bonds (including current maturities), net	26,857	28,620
Long-term debt of variable interest entities (including current maturities)	1,356	1,468
Long-term notes payable (including current maturities)	121	75
Secured debt of VIEs
Estimated Values of Financial Instruments (Level 2 Valuation)
EnergyRight receivables (including current portion)	125	141
Loans and other long-term receivables, net (including current portion)	118	141
EnergyRight® financing obligation (including current portion)	144	163
Unfunded Loan Commitments	0	0
Membership interests of VIE subject to mandatory redemption (including current portion)	32	35
Long-term outstanding power bonds (including current maturities), net	21,933	22,456
Long-term debt of variable interest entities (including current maturities)	1,200	1,234
Long-term notes payable (including current maturities)	$ 122	$ 75

Proprietary Capital Proprieta96

Proprietary Capital Proprietary Capital (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Appropriation Investment
Amount of appropriation investment that was repaid	$ 1,000								$ 1,000
Remaining appropriation investment	258								258
Balance at beginning of year				$ 8,420				$ 7,203	8,420	$ 7,203	$ 6,104
Net income (loss)	139	$ 233	$ 211	102	$ 661	$ 291	$ 318	(37)	685	1,233	1,111
Return on power program appropriation investment									(5)	(6)	(5)
Balance at end of year	9,133				8,420				9,133	8,420	$ 7,203
Net proprietary capital at September 30	$ 9,133				$ 8,420				$ 9,133	$ 8,420
Computed average interest rate payable	2.00%				2.04%				2.00%	2.04%	2.04%
Nonpower Programs Appropriation Investment
Appropriation Investment
Balance at beginning of year				4,351					$ 4,351
Balance at end of year	$ 4,351				$ 4,351				4,351	$ 4,351
Power Program Appropriation Investment
Appropriation Investment
Balance at beginning of year				258				258	258	258	$ 258
Net income (loss)									0	0	0
Return on power program appropriation investment									0	0	0
Balance at end of year	258				258				258	258	258
Power Program Retained Earnings
Appropriation Investment
Balance at beginning of year				7,594				6,357	7,594	6,357	5,240
Net income (loss)									693	1,243	1,122
Return on power program appropriation investment									(5)	(6)	(5)
Balance at end of year	8,282				7,594				8,282	7,594	6,357
Net proprietary capital at September 30	8,540				7,852				8,540	7,852
Nonpower Programs Appropriation Investment, Net
Appropriation Investment
Balance at beginning of year				580				590	580	590	601
Net income (loss)									(8)	(10)	(11)
Return on power program appropriation investment									0	0	0
Balance at end of year	572				580				572	580	590
Nonpower Programs Retained Earnings
Appropriation Investment
Balance at beginning of year				(3,771)				$ (3,761)	(3,771)	(3,761)
Return on power program appropriation investment				$ 0						0
Balance at end of year	(3,779)				(3,771)				(3,779)	(3,771)	(3,761)
Net proprietary capital at September 30	$ 572				$ 580				572	580
Affiliated Entity
Appropriation Investment
Return on power program appropriation investment									$ (5)	$ (6)	$ (5)

Proprietary Capital Proprieta97

Proprietary Capital Proprietary Capital - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss)
Net effect on earnings	$ 0	$ 0	$ 0
Net unrealized gain (loss) on future cash flow hedges	59	(139)	(72)
Reclassification to earnings from cash flow hedges	26	$ (129)	$ (65)
Reclassification to earnings from cash flow hedges in the next twelve months	$ (18)

Other Income (Expense), Net O98

Other Income (Expense), Net Other Income (Expense), Net (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Other Income (Expense), Net
Interest income	$ 23	$ 24	$ 24
External services	14	12	12
Gains (losses) on investments	9	7	(1)
Miscellaneous		0	(6)
Miscellaneous	10
Total other income (expense), net	$ 56	$ 43	$ 29

Supplemental Cash Flow Inform99

Supplemental Cash Flow Information Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Supplemental Cash Flow Information
Interest paid	$ 1,300	$ 1,300	$ 1,300
Interest capitalized				$ 235	$ 214
Capital lease obligations incurred			10	81	1
Acquisition of notes payable			74	78
Accounts payable and accrued liabilities
Supplemental Cash Flow Information
Construction in progress and Nuclear fuel expenditures			$ 425	$ 526	$ 530

Benefit Plans Components of Ben

Benefit Plans Components of Benefit Plans (Details)	12 Months Ended			240 Months Ended
	Sep. 30, 2017USD ($)Years	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2036USD ($)	Oct. 01, 2016Years
Defined Benefit Plan Disclosure
Fixed and variable fund annual maximum contribution	$ 10,000
Age Threshold for Changes to COLA Changes for Cash Balance Plan | Years					50
Defined contribution plan contribution amount	$ 80,000,000	$ 38,000,000	$ 36,000,000
Market-related value phase-in period number of years | Years	3
Minimum
Defined Benefit Plan Disclosure
Threshold for Deferral of Actuarial Gain/Loss Under Corridor Approach	10.00%
Maximum
Defined Benefit Plan Disclosure
Years of Service Threshold for Changes to Cash Balance Plan	10 years
Scenario, Forecast
Defined Benefit Plan Disclosure
Minimum Funding for TVA Pension Plan Under 2016 Amendments				$ 300,000,000
Funded Status Threshold to Reduce Minimum Contribution Under 2016 Changes to TVA Pension Plan				100.00%

Benefit Plans Obligations and F

Benefit Plans Obligations and Funded Status (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Amortization Period for Change in Benefit Obligation Related to 2016 Amendments to TVA Pension Plan			11 years
Pension Benefits
Change in benefit obligation
Benefit obligation		$ 12,601	$ 13,083	$ 12,824
Service cost		60	133	130
Interest cost		464	564	540
Plan participants’ contributions		9	25
Actuarial loss (gain)		(286)	1,188
Plan change		0	960
Net transfers from variable fund/401(k) plan		(12)	7
Expenses paid		(5)	(6)
Benefits paid		712	692
Change in plan assets
Fair value of net plan assets		7,989	7,145	$ 6,797
Actual return on plan assets		759	733
Employer contributions		805	281
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		$ (4,612)	$ (5,938)
Discount rate		3.85%	3.65%	4.50%
Amount of defined benefit plan actuarial gain (loss) from change in COLA			$ 168
Amount of defined benefit plan actuarial gain (loss) from discount rate change		$ (292)	(1,400)
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption		(117)	(133)
Amount related to change in payment elections		119
Other Post-retirement Benefits
Change in benefit obligation
Benefit obligation		494	571	$ 657
Service cost		18	16	16
Interest cost		21	29	29
Collections	[1]	47	92
Actuarial loss (gain)		(80)	68
Plan change		0	158
Net transfers from variable fund/401(k) plan		0	0
Expenses paid		0	0
Benefits paid		83	133
Change in plan assets
Fair value of net plan assets		0	0	$ 0
Actual return on plan assets		0	0
Employer contributions	[2]	36	41
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status		$ (494)	$ (571)
Discount rate		3.95%	3.70%	4.65%
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions		$ (66)	$ 17
Amount of defined benefit plan actuarial gain (loss) from discount rate change		(18)	(91)
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption		$ (6)	$ (7)
[1]	Collections include retiree contributions as well as federal reinsurance payments and provider discounts and rebates.
[2]	Other Post-Retirement Benefits Employer contributions are reduced by federal reinsurance payments and provider discounts and rebates.

Benefit Plans Amounts Recognize

Benefit Plans Amounts Recognized on TVA's Consolidated Balance Sheets (Details) - USD ($) $ in Millions		Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure
Regulatory assets		$ 8,698	$ 10,164
Non-current regulatory liabilities		25	3
Accounts payable and accrued liabilities		(1,940)	(2,163)
Pension and post-retirement benefit obligations		(5,477)	(6,929)
Pension Benefits
Defined Benefit Plan Disclosure
Amount capitalized due to actions of regulator		42	407
Regulatory assets		4,009	5,336
Accounts payable and accrued liabilities		(4)	(5)
Pension and post-retirement benefit obligations	[1]	(4,608)	(5,933)
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Amount capitalized due to actions of regulator		0	0
Regulatory assets			49
Non-current regulatory liabilities		23
Accounts payable and accrued liabilities		(33)	(35)
Pension and post-retirement benefit obligations	[1]	(461)	(536)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent
Defined Benefit Plan Disclosure
Postemployment benefits liability, noncurrent		$ 408	$ 460
[1]	The table above excludes $408 million and $460 million of post-employment benefit costs that are recorded in Post-retirement and post-employment benefit obligations on the Consolidated Balance Sheets at September 30, 2017 and 2016, respectively.

Benefit Plans Postretirement Be

Benefit Plans Postretirement Benefit Costs Deferred as Regulatory Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure
Regulatory assets	$ 8,698	$ 10,164
Non-current regulatory liabilities	(25)	(3)
Pension Benefits
Defined Benefit Plan Disclosure
Unrecognized prior service cost (credit)	(918)	(1,017)
Unrecognized net loss	4,885	5,946
Amount capitalized due to actions of regulator	(42)	(407)
Regulatory assets	4,009	5,336
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Unrecognized prior service cost (credit)	(163)	(185)
Unrecognized net loss	140	234
Amount capitalized due to actions of regulator	0	0
Regulatory assets		$ 49
Non-current regulatory liabilities	$ (23)

Benefit Plans Projected Benefit

Benefit Plans Projected Benefit Obligations and Accumulated Benefit Obligations in Exess of Plan Assets (Details) - Pension Benefits - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure
Projected benefit obligation		$ 13,083
Accumulated benefit obligation	$ 12,461	12,912
Fair value of net plan assets		$ 7,145

Benefit Plans Components of Net

Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits
Defined Benefit Plan Disclosure
Service cost		$ 60	$ 133	$ 130
Interest cost		464	564	540
Expected return on plan assets		(457)	(446)	(437)
Amortization of prior service credit		(99)	(23)	(21)
Recognized net actuarial loss		472	310	299
Curtailments		0	(78)	0
Net periodic benefit cost as acutarially determined		440	460	511
Amount expensed (capitalized) due to actions of regulator		365	(179)	(228)
Total net period benefit cost		805	281	283
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Service cost		18	16	16
Interest cost		21	29	29
Expected return on plan assets		0	0	0
Amortization of prior service credit		(22)	(6)	(6)
Recognized net actuarial loss		14	7	9
Curtailments		0	0	0
Net periodic benefit cost as acutarially determined		31	46	48
Amount expensed (capitalized) due to actions of regulator		0	0	0
Total net period benefit cost		$ 31	$ 46	$ 48
Scenario, Forecast | Pension Benefits
Defined Benefit Plan Disclosure
Amount expensed (capitalized) due to actions of regulator	$ (57)

Benefit Plans Expected Amortiza

Benefit Plans Expected Amortization of Regulatory Assets in Next Fiscal Year (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)
Defined Benefit Plan Disclosure
Prior service cost (credit)	$ (120)
Net actuarial loss	419
Pension Benefits
Defined Benefit Plan Disclosure
Prior service cost (credit)	(98)
Net actuarial loss	412
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Prior service cost (credit)	(22)
Net actuarial loss	$ 7

Benefit Plans Actuarial Assumpt

Benefit Plans Actuarial Assumptions (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Oct. 01, 2023	Sep. 30, 2018	Sep. 30, 2014
Defined Benefit Plan Disclosure
Period during which actual company compensation experience study performed	5 years
Defined Benefit Plan, Cost of Living Adjustment Assumption	0.99%	0.00%	1.68%
Pension Benefits
Defined Benefit Plan Disclosure
Discount rate	3.65%	4.50%	4.45%
Discount rate	3.85%	3.65%	4.50%
Rate of compensation increase	5.00%	5.55%
Expected return on plan assets	6.75%	7.00%	7.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	5.55%	5.70%	5.70%
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Discount rate	3.70%	4.65%	4.50%
Discount rate	3.95%	3.70%	4.65%
Minimum
Defined Benefit Plan Disclosure
Rate of compensation increase	3.50%
Maximum
Defined Benefit Plan Disclosure
Rate of compensation increase	13.00%
Scenario, Forecast
Defined Benefit Plan Disclosure
Defined Benefit Plan, Cost of Living Adjustment Assumption					2.00%
Scenario, Plan [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Cost of Living Adjustment Assumption	1.25%
Pre-Medicare Eligible [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Initial health care cost trend rate	6.50%	6.50%	7.00%			7.50%
Ultimate health care cost trend rate	5.00%	5.00%	5.00%			5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2,024	2,019
Pre-Medicare Eligible [Member] | Scenario, Forecast | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Ultimate health care cost trend rate				5.00%
Post-Medicare Eligible [Member] [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Initial health care cost trend rate	0.00%	0.00%	7.00%			7.50%
Ultimate health care cost trend rate	4.00%	4.00%	5.00%			5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2,021	2,021

Benefit Plans Sensitivity to Ce

Benefit Plans Sensitivity to Certain Changes in Pension Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Oct. 01, 2023	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Discount rate
Defined Benefit Plan Disclosure
Change in Assumption	(0.25%)
Impact on Pension Cost	$ 17
Impact on Projected Benefit Obligation	$ 367
Rate of return on plan assets
Defined Benefit Plan Disclosure
Change in Assumption	(0.25%)
Impact on Pension Cost	$ 16
Pre-Medicare Eligible [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate	5.00%		5.00%	5.00%	5.00%
Scenario, Forecast | Pre-Medicare Eligible [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate		5.00%

Benefit Plans Sensitivity to Ch

Benefit Plans Sensitivity to Changes in Assumed Health Care Cost Trend Rates (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)
Maximum
Defined Benefit Plan Disclosure
Effect of one percentage point increase on total service and interest cost components	$ 5
Effect of one percentage point increase on end-of-year accumulated postretirement benefit obligation	70
Minimum
Defined Benefit Plan Disclosure
Effect of one percentage point decrease on total service and interest cost components	(5)
Effect of one percentage point decrease on end-of-year accumulated postretirement benefit obligation	$ (67)

Benefit Plans Asset Holdings (D

Benefit Plans Asset Holdings (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure
Target Allocation	100.00%
Plan Asset Allocations	100.00%	100.00%
Global public equity
Defined Benefit Plan Disclosure
New Policy Target Allocation	47.00%
Target Allocation	39.00%
Plan Asset Allocations	44.00%	44.00%
Fixed income
Defined Benefit Plan Disclosure
New Policy Target Allocation	30.00%
Public real assets
Defined Benefit Plan Disclosure
Target Allocation	15.00%
Plan Asset Allocations	13.00%	15.00%
Private equity
Defined Benefit Plan Disclosure
Target Allocation	8.00%
Plan Asset Allocations	5.00%	4.00%
Safety oriented fixed income
Defined Benefit Plan Disclosure
Target Allocation	15.00%
Plan Asset Allocations	21.00%	18.00%
Opportunistic fixed income
Defined Benefit Plan Disclosure
Target Allocation	15.00%
Plan Asset Allocations	10.00%	10.00%
Private real assets
Defined Benefit Plan Disclosure
New Policy Target Allocation	23.00%
Target Allocation	8.00%
Plan Asset Allocations	7.00%	9.00%

Benefit Plans Fair Value Measur

Benefit Plans Fair Value Measurements (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)Years	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)
Defined Benefit Plan Disclosure
Fair value of gross plan assets	$ 8,536	$ 7,293
Derivative liabilities	10	16
Net payables	168	129
Payables for collateral on loaned securities	$ 369	3
Voting percentage required to desolve partnership in private equity	80.00%
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	$ 2,139	2,019
Derivative liabilities	2	2
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	3,207	2,362
Derivative liabilities	6	14
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	90	97	$ 91
Derivative liabilities	2	0
Equity securities
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1,771	1,847
Equity securities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1,770	1,846
Equity securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Equity securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1	1
Preferred securities
Defined Benefit Plan Disclosure
Fair value of gross plan assets	14	20
Preferred securities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	3	3
Preferred securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	11	17
Preferred securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Corporate debt securities
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1,100	1,145
Corporate debt securities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Corporate debt securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1,088	1,135
Corporate debt securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	12	10
Residential mortgage-backed securities
Defined Benefit Plan Disclosure
Fair value of gross plan assets	325	181
Residential mortgage-backed securities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Residential mortgage-backed securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	317	165
Residential mortgage-backed securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	8	16
Debt securities issued by U.S. Treasury and other U.S. government agencies
Defined Benefit Plan Disclosure
Fair value of gross plan assets	193	117
Debt securities issued by U.S. Treasury and other U.S. government agencies | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	193	117
Debt securities issued by U.S. Treasury and other U.S. government agencies | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Debt securities issued by U.S. Treasury and other U.S. government agencies | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Asset-backed securities
Defined Benefit Plan Disclosure
Fair value of gross plan assets	146	118
Asset-backed securities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Asset-backed securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	109	87
Asset-backed securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	37	31
Debt securities issued by state/local governments
Defined Benefit Plan Disclosure
Fair value of gross plan assets	19	16
Debt securities issued by state/local governments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Debt securities issued by state/local governments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	17	16
Debt securities issued by state/local governments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	2	0
Debt securities issued by foreign governments
Defined Benefit Plan Disclosure
Fair value of gross plan assets	331	332
Debt securities issued by foreign governments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Debt securities issued by foreign governments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	307	299
Debt securities issued by foreign governments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	24	33
Commercial mortgage-backed securities
Defined Benefit Plan Disclosure
Fair value of gross plan assets	68	44
Commercial mortgage-backed securities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Commercial mortgage-backed securities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	62	38
Commercial mortgage-backed securities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	6	6
Equity security commingled funds
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1,134	682
Equity security commingled funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Equity security commingled funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Equity security commingled funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Debt security commingled funds
Defined Benefit Plan Disclosure
Fair value of gross plan assets	709	653
Debt security commingled funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Debt security commingled funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Debt security commingled funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Commodity commingled funds
Defined Benefit Plan Disclosure
Fair value of gross plan assets	224	302
Commodity commingled funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Commodity commingled funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Commodity commingled funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Blended security commingled funds
Defined Benefit Plan Disclosure
Fair value of gross plan assets		225
Blended security commingled funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets		0
Blended security commingled funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets		0
Blended security commingled funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets		0
Institutional mutual funds
Defined Benefit Plan Disclosure
Fair value of gross plan assets	155	10
Institutional mutual funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	155	10
Institutional mutual funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Institutional mutual funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Cash equivalents and other short-term investments
Defined Benefit Plan Disclosure
Fair value of gross plan assets	916	621
Cash equivalents and other short-term investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	41
Cash equivalents and other short-term investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	916	580
Cash equivalents and other short-term investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Certificates of deposit
Defined Benefit Plan Disclosure
Fair value of gross plan assets	6	16
Certificates of deposit | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Certificates of deposit | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	6	16
Certificates of deposit | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Private equity funds measured at net asset value
Defined Benefit Plan Disclosure
Fair value of gross plan assets	500	385
Private equity funds measured at net asset value | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Private equity funds measured at net asset value | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Private equity funds measured at net asset value | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Private real estate funds measured at net asset value
Defined Benefit Plan Disclosure
Fair value of gross plan assets	533	568
Private real estate funds measured at net asset value | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Private real estate funds measured at net asset value | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Private real estate funds measured at net asset value | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Securities lending commingled funds
Defined Benefit Plan Disclosure
Fair value of gross plan assets	369	3
Securities lending commingled funds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Securities lending commingled funds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	369	3
Securities lending commingled funds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Futures
Defined Benefit Plan Disclosure
Fair value of gross plan assets	18
Derivative liabilities	3	2
Futures | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	18
Derivative liabilities	2	2
Futures | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0
Derivative liabilities	0	0
Futures | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0
Derivative liabilities	1	0
Purchased options
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1
Purchased options | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0
Purchased options | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	1
Purchased options | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0
Interest Rate Swap
Defined Benefit Plan Disclosure
Derivative liabilities		3
Interest Rate Swap | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Derivative liabilities		0
Interest Rate Swap | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Derivative liabilities		3
Interest Rate Swap | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Derivative liabilities		0
Foreign currency forward
Defined Benefit Plan Disclosure
Fair value of gross plan assets	4	5
Derivative liabilities	6	9
Foreign currency forward | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Derivative liabilities	0	0
Foreign currency forward | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets	4	5
Derivative liabilities	6	9
Foreign currency forward | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets	0	0
Derivative liabilities	0	0
Written options
Defined Benefit Plan Disclosure
Fair value of gross plan assets		1
Derivative liabilities		1
Written options | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of gross plan assets		0
Derivative liabilities		0
Written options | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of gross plan assets		1
Derivative liabilities		1
Written options | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of gross plan assets		0
Derivative liabilities		0
Credit default swaps
Defined Benefit Plan Disclosure
Derivative liabilities	1	1
Credit default swaps | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Derivative liabilities	0	0
Credit default swaps | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Derivative liabilities	0	1
Credit default swaps | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Derivative liabilities	$ 1	$ 0
Minimum
Defined Benefit Plan Disclosure
Number of years partnerships in private equity generally continue | Years	10
Number of one year extensions for partnerships in private equity | Years	2
Maximum
Defined Benefit Plan Disclosure
Number of years partnerships in private equity generally continue | Years	12
Number of one year extensions for partnerships in private equity | Years	3

Benefit Plans Fair Value Mea112

Benefit Plans Fair Value Measurements Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of gross plan assets	$ 8,536	$ 7,293
Fair Value, Inputs, Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of gross plan assets	90	97	$ 91
Net realized/unrealized gains	2	18
Purchases, sales, issuances, and settlements, net	(6)	(12)
Transfers in and/or out of Level 3	(5)	$ 0
Fair value of net plan assets	$ 88

Benefit Plans Estimated Future

Benefit Plans Estimated Future Benefit Payments (Details) $ in Millions	Sep. 30, 2017USD ($)
Pension Benefits
Defined Benefit Plan Disclosure
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	$ 768
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	769
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	773
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	774
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	774
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	3,821
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	33
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	31
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	29
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	27
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	25
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	$ 115

Benefit Plans Contributions (De

Benefit Plans Contributions (Details) - USD ($)		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure
Other postretirement benefit contributions			$ 30,000,000	$ 47,000,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure
Employer contributions			5,000,000	6,000,000
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Employer contributions	[1]		36,000,000	41,000,000
Pension Benefits
Defined Benefit Plan Disclosure
Employer contributions			805,000,000	281,000,000
Other Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure
Employer contributions			800,000,000	275,000,000
Minimum | Other Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure
Employer contributions			$ 300,000,000	$ 209,000,000
Scenario, Forecast
Defined Benefit Plan Disclosure
Other postretirement benefit contributions		$ 33,000,000
Scenario, Forecast | Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure
Employer contributions		4,000,000
Scenario, Forecast | Other Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure
Employer contributions		$ 300,000,000
[1]	Other Post-Retirement Benefits Employer contributions are reduced by federal reinsurance payments and provider discounts and rebates.

Benefit Plans Other Postemploym

Benefit Plans Other Postemployment Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Other Post-Employment Benefits
Discount rate	2.33%	1.60%	2.05%
Period expense	$ (12)	$ (35)	$ (39)
Postemployment benefits liability	447	501	$ 511
Accounts Payable and Accrued Liabilities [Member]
Other Post-Employment Benefits
Postemployment Benefits Liability, Current	39	41
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent
Other Post-Employment Benefits
Postemployment benefits liability, noncurrent	$ 408	$ 460

Commitments and Contingencie116

Commitments and Contingencies Commitments and Contingencies - Table (Details) $ in Millions		Sep. 30, 2017USD ($)megawatts	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)
Obligations
Megawatts provided under transmission obligations | megawatts		1,450
Accrual for Environmental Loss Contingencies, Gross		$ 7	$ 23
Estimated future decommissioning cost	[1]	4,304	4,052	$ 3,843
2,018		52
2,019		51
2,020		51
2,021		51
2,022		51
Thereafter		519
Total		775
2,018		67
2,019		60
2,020		59
2,021		59
2,022		45
Thereafter		13
Total		303
CT and QTE outstanding leaseback obligation		338	$ 467
2,018		50
2,019		49
2,020		50
2,021		207
2,022		25
Thereafter		0
Total		$ 381
[1]	(1) The current portions of the ARO liability in the amounts of $128 million and $212 million as of September 30, 2017 and 2016, respectively, are included in Accounts payable and accrued liabilities.

Commitments and Contingencie117

Commitments and Contingencies Commitments and Contingencies - Energy Prepayment Obligations (Details) $ in Millions	Sep. 30, 2017USD ($)
Energy prepayment obligations
Obligations
2,018	$ 100
2,019	10
2,020	0
2,021	0
2,022	0
Thereafter	0
Total	110
Interest Payments relating to energy prepayment obligations
Obligations
2,018	46
2,019	4
2,020	0
2,021	0
2,022	0
Thereafter	0
Total	50
Total
Obligations
2,018	146
2,019	14
2,020	0
2,021	0
2,022	0
Thereafter	0
Total	$ 160

Commitments and Contingencie118

Commitments and Contingencies Commitments and Contingencies - Membership Interests of VIE Subject to Mandatory Redemption (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Aug. 09, 2013
Long-term Purchase Commitment
Membership interests of VIE subject to mandatory redemption (including current portion)	$ 41	$ 46	$ 40
2,018	2
Total	381
2,019	2
2,020	3
2,021	3
2,022	$ 3

Commitments and Contingencie119

Commitments and Contingencies Commitments and Contingencies - Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Leases
Operating lease expense	$ 90	$ 86	$ 88
2,018	50
CT and QTE outstanding leaseback obligation	338	$ 467
2,018	52
2,018	67
2,019	60
2,020	59
2,021	59
2,022	45
Thereafter	13
Total	303
2,019	51
2,020	51
2,021	51
2,022	51
Thereafter	519
Total	775
Less: amount representing interest	(588)
Total	187
2,019	49
2,020	50
2,021	207
2,022	25
Thereafter	0
Total	$ 381

Commitments and Contingencie120

Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)megawattsSuppliersMegawatts	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)
Obligations
Megawatts provided under power purchase obligations	2,230
Remaining terms of the agreements, high end of range	15 years
Megawatts provided under transmission obligations | megawatts	1,450
Power purchased under agreement | $	$ 178	$ 218	$ 218
Maximum term length for the purchase, storage, and transportation of natural gas	fifteen
Purchase Agreements Required by Federal Law
Obligations
Megawatts provided under power purchase obligations	258
Number of suppliers | Suppliers	30

Commitments and Contingencie121

Commitments and Contingencies Commitments and Contingencies - Contingencies (Details) $ in Millions		12 Months Ended
		Sep. 30, 2017USD ($)UnitsreactorsInsurance_layersProcedures	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)
Contingencies
Nuclear liability insurance		$ 450
Assessment from licensees for each licensed reactor		$ 127
Number of licensed reactors in US | reactors		102
Nuclear accident assessment limitation per year per unit		$ 19
Number of licensed nuclear units | Units		7
Maximum assessment per nuclear incident		$ 891
Maximum payment required per accident in any one year		133
Total amount of protection available		$ 13,000
Surcharge for legal expenses		5.00%
Number of layers until the U.S. Congress is required to take action | Insurance_layers		2
Amount of property, decommissioning, and decontamination insurance carried		$ 5,100
Amount of insurance available for loss at any one site		2,100
Maximum amount of retrospective premiums		126
Maximum idemnity if a covered accident tasks or keeps a nuclear unit offline		490
Maximum amount of retrospective premiums		43
Estimated future decommissioning cost	[1]	$ 4,304	$ 4,052	$ 3,843
Number of procedures for determining estimates for the costs of nuclear decommissioning | Procedures		2
Amount spent to reduce emissions since 1970		$ 6,700
Amount spent to reduce emissions		206	259	315
Possible additional future costs for compliance with Clean Air Act requirements		200
Possible additional future costs for compliance with CCR requirements		1,100
Possible additional future costs for compliance with Clean Water requirements.		500
Estimated liability for cleanup and similar environmental work on a non-discounted basis		7	23
Nuclear
Contingencies
Estimated future decommissioning cost		2,859	2,492	2,187
Non-nuclear
Contingencies
Estimated future decommissioning cost		$ 1,445	$ 1,560	$ 1,656
[1]	(1) The current portions of the ARO liability in the amounts of $128 million and $212 million as of September 30, 2017 and 2016, respectively, are included in Accounts payable and accrued liabilities.

Commitments and Contingencie122

Commitments and Contingencies Commitments and Contingencies - Legal Proceedings (Details) $ in Millions	12 Months Ended
	Sep. 30, 2017USD ($)PeopleUnitsmegawattsGroupsYearsAgreements
Legal Proceedings
Amount spent under environmental agreements	$ 275
General
Legal Proceedings
Legal loss contingency accrual	$ 22
Environmental Agreements
Legal Proceedings
Number of similar environmental agreements entered into | Agreements	2
Number of environmental agreements entered into with the EPA | Agreements	1
Number of environmental agreements entered into with environmental advocacy groups | Groups	3
Number of units to be idled | Units	18
Megawatts option 1 | megawatts	2,200
Megawatts option 2	3,500
Amount to be invested in certain environmental projects	$ 290
Amount to be provided to fund environmental projects	60
Amount to pay civil penalties	$ 10
Case Involving Tennessee Valley Authority Retirement System
Legal Proceedings
Number of participants that filed suit | People	8
Contribution related to TVARS case	$ 1,000
Retirement age of eligibility for cost of living adjustment before January 1, 2010 | Years	55
Retirement age of eligibility for cost of living adjustment after January 1, 2010 | Years	60
Other long-term liabilities | General
Legal Proceedings
Legal loss contingency accrual	$ 13
Accounts payable and accrued liabilities | General
Legal Proceedings
Legal loss contingency accrual	$ 9

Commitments and Contingencies U

Commitments and Contingencies Unfunded loan commitments (Details) $ in Millions	Sep. 30, 2017USD ($)
Other commitments - unfunded loan commitments [Abstract]
2,018	$ 12
2,019	0
2,020	0
2,021	0
2,022	$ 0

Related Parties Related Part124

Related Parties Related Parties (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Related Parties
Remaining appropriation investment	$ 258	$ 258
Current borrowing capacity	150
Return on power program appropriation investment	(5)	(6)	$ (5)
Related Party Transactions
Related Parties
Revenue from sales of electricity	126	126	130
Other income	136	161	167
Operating expenses	216	216	227
Additions to property, plant, and equipment	16	32	37
Cash and cash equivalents	46	54	45
Accounts receivable, net	119	129	106
Accounts payable and accrued liabilities	71	77	98
Long-term power bonds, net	1	4	5
Return on power program appropriation investment	$ (5)	$ (6)	$ (5)

Unaudited Quarterly Financia125

Unaudited Quarterly Financial Information Unaudited Quarterly Financial Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 3,075	$ 2,571	$ 2,547	$ 2,546	$ 3,286	$ 2,479	$ 2,571	$ 2,280	$ 10,739	$ 10,616	$ 11,003
Operating expenses	2,624	2,010	2,013	2,117	2,343	1,913	1,982	2,052	8,764	8,290	8,788
Operating income	451	561	534	429	943	566	589	228	1,975	2,326	2,215
Net income (loss)	$ 139	$ 233	$ 211	$ 102	$ 661	$ 291	$ 318	$ (37)	$ 685	$ 1,233	$ 1,111

Gallatin coal combustion res126

Gallatin coal combustion residual facilities (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Other Long-Term Liabilities
Gallatin coal combustion residual facilities estimated cost to cap and close	$ 200,000,000
Gallatin coal combustion residual facilities original estimated cost for dewatering facility	$ 91,000,000
Gallatin coal combustion residual facilities estimated timeframe to complete dewatering facility	3 years
Gallatin coal combustion residual facilities estimated cost relocate offsite	$ 2,000,000,000
TDEC civil penalties related to Gallatin	17,000
Plaintiff civil penalties related to Gallatin	37,500
Gallatin coal combustion residual facilities estimated cost relocate onsite	900,000,000
Other long-term liabilities
Other Long-Term Liabilities
Gallatin coal combustion residual facilities	880,000,000	$ 0
Accounts payable and accrued liabilities
Other Long-Term Liabilities
Gallatin coal combustion residual facilities	$ 19,000,000