Oglethorpe Power 10-Q 11 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 2017shares
Document and Entity Information Abstract
Entity Registrant Name	OGLETHORPE POWER CORP
Entity Central Index Key	788,816
Document Type	10-Q
Document Period End Date	Mar. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding	0
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Electric plant:
In service	$ 8,805,272	$ 8,786,839
Less: Accumulated provision for depreciation	(4,164,233)	(4,115,339)
Net in service	4,641,039	4,671,500
Nuclear fuel, at amortized cost	377,593	377,653
Construction work in progress	3,401,701	3,228,214
Total electric plant	8,420,333	8,277,367
Investments and funds:
Nuclear decommissioning trust fund	400,022	386,029
Investment in associated companies	73,503	72,783
Long-term investments	106,444	99,874
Restricted investments	184,132	221,122
Other	21,045	20,730
Total investments and funds	785,146	800,538
Current assets:
Cash and cash equivalents	278,551	366,290
Restricted short-term investments	247,009	247,006
Receivables	149,359	155,042
Inventories, at average cost	271,859	259,831
Prepayments and other current assets	24,181	32,919
Total current assets	970,959	1,061,088
Deferred charges:
Regulatory assets	564,314	545,387
Other	15,476	16,733
Total deferred charges	579,790	562,120
Total assets	10,756,228	10,701,113
Capitalization:
Patronage capital and membership fees	881,264	859,810
Accumulated other comprehensive margin	(409)	(370)
Total patronage capital and membership fees and accumulated other comprehensive margin	880,855	859,440
Long-term debt	7,876,337	7,892,836
Obligations under capital lease	92,096	92,096
Other	19,081	18,765
Total capitalization	8,868,369	8,863,137
Current liabilities:
Long-term debt and capital lease due within one year	153,305	316,861
Short-term borrowings	328,890	102,168
Accounts payable	93,300	73,801
Accrued interest	58,342	93,634
Member power bill prepayments, current	173,705	176,988
Other current liabilities	42,713	59,979
Total current liabilities	850,255	823,431
Deferred credits and other liabilities:
Asset retirement obligations	706,913	698,051
Member power bill prepayments, non-current	53,115	48,115
Contract retainage	40,374	40,008
Regulatory liabilities	205,603	197,748
Other	31,599	30,623
Total deferred credits and other liabilities	1,037,604	1,014,545
Total equity and liabilities	$ 10,756,228	$ 10,701,113

Consolidated Statements of Reve

Consolidated Statements of Revenues and Expenses - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating revenues:
Sales to Members	$ 354,144	$ 348,097
Sales to non-Members	26	64
Total operating revenues	354,170	348,161
Operating expenses:
Fuel	103,915	98,952
Production	101,088	103,471
Depreciation and amortization	55,863	53,486
Purchased power	14,976	13,143
Accretion	8,998	8,016
Total operating expenses	284,840	277,068
Operating margin	69,330	71,093
Other income:
Investment income	14,819	12,323
Other	640	2,301
Total other income	15,459	14,624
Interest charges:
Interest expense	93,285	88,517
Allowance for debt funds used during construction	(33,087)	(26,380)
Amortization of debt discount and expense	3,137	2,982
Net interest charges	63,335	65,119
Net margin	$ 21,454	$ 20,598

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Margin - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Consolidated Statements of Comprehensive Margin
Net margin	$ 21,454	$ 20,598
Other comprehensive margin:
Unrealized (loss) gain on available-for-sale securities	(39)	284
Total comprehensive margin	$ 21,415	$ 20,882

Consolidated Statements of Patr

Consolidated Statements of Patronage Capital and Membership Fees and Accumulated Other Comprehensive (Deficit) Margin - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Increase (Decrease) in Members' Capital
Balance	$ 859,440	$ 809,523
Components of comprehensive margin:
Net margin	21,454	20,598
Unrealized gain (loss) on available-for-sale securities	(39)	284
Balance	880,855	830,405
Patronage Capital and Membership Fees
Increase (Decrease) in Members' Capital
Balance	859,810	809,465
Components of comprehensive margin:
Net margin	21,454	20,598
Balance	881,264	830,063
Accumulated Other Comprehensive (Deficit) Margin
Increase (Decrease) in Members' Capital
Balance	(370)	58
Components of comprehensive margin:
Unrealized gain (loss) on available-for-sale securities	(39)	284
Balance	$ (409)	$ 342

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net margin	$ 21,454	$ 20,598
Adjustments to reconcile net margin to net cash provided by operating activities:
Depreciation and amortization, including nuclear fuel	92,049	87,413
Accretion cost	8,998	8,016
Amortization of deferred gains	(447)	(447)
Allowance for equity funds used during construction	(193)	(177)
Deferred outage costs	(19,385)	(24,869)
(Gain) loss on sale of investments	(16,045)	849
Regulatory deferral of costs associated with nuclear decommissioning	10,602	(5,814)
Other	(3,138)	(1,685)
Change in operating assets and liabilities:
Receivables	6,241	4,044
Inventories	(12,028)	5,253
Prepayments and other current assets	3,822	273
Accounts payable	(18,028)	(39,787)
Accrued interest	(35,292)	(5,528)
Accrued taxes	(12,909)	(13,160)
Other current liabilities	(7,059)	(9,777)
Member power bill prepayments	1,717	(6,779)
Total adjustments	(1,095)	(2,175)
Net cash provided by operating activities	20,359	18,423
Cash flows from investing activities:
Property additions	(171,806)	(259,447)
Activity in nuclear decommissioning trust fund - Purchases	(165,213)	(129,886)
Activity in nuclear decommissioning trust fund - Proceeds	163,635	128,179
Decrease (increase) in restricted cash and investments	36,990	(9,419)
(Increase) decrease in restricted short-term investments	(3)	2,663
Activity in other long-term investments - Purchases	(18,190)	(14,267)
Activity in other long-term investments - Proceeds	14,093	11,639
Other	(1,658)	3,147
Net cash used in investing activities	(142,152)	(267,391)
Cash flows from financing activities:
Long-term debt proceeds	4,517	7,998
Long-term debt payments	(201,398)	(36,677)
Increase in short-term borrowings, net	226,722	171,280
Other	4,213	4,422
Net cash provided by financing activities	34,054	147,023
Net decrease in cash and cash equivalents	(87,739)	(101,945)
Cash and cash equivalents at beginning of period	366,290	213,038
Cash and cash equivalents at end of period	278,551	111,093
Cash paid for -
Interest (net of amounts capitalized)	94,754	66,950
Supplemental disclosure of non-cash investing and financing activities:
Change in asset retirement obligations		(10,425)
Change in accrued property additions	40,259	(79,336)
Interest paid-in-kind	$ 13,909	$ 10,606

General

General	3 Months Ended
	Mar. 31, 2017
General
General	(A) General. The consolidated financial statements included in this report have been prepared by us purs to the rules and regulations of the Securities and Exchange Commission. In the opinion of management, the information furnished in this report reflects all adjustments (which include only normal recurring adjustments) and estimates necessary to fairly state, in all material respects, the results for the three-month periods ended March 31, 2017 and 2016. Certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles have been condensed or omitted pursuant to SEC rules and regulations, although we believe that the disclosures are adequate to make the information presented not misleading. These consolidated financial statements should be read in conjunction with the financial statements and the notes thereto included in our Annual Report on Form 10-K for the fiscal year ended December 31, 2016, as filed with the SEC. The results of operations for the three-month period ended March 31, 2017 are not necessarily indicative of results to be expected for the full year. As noted in our 2016 Form 10-K, our revenues consist primarily of sales to our 38 electric distribution cooperative members and, thus, the receivables on the consolidated balance sheets are principally from our members. (See "Notes to Consolidated Financial Statements" in our 2016 Form 10-K.)

Fair Value

Fair Value	3 Months Ended
	Mar. 31, 2017
Fair Value
Fair Value	(B) Fair Value. Authoritative guidance regarding fair value measurements for financial and non-financial assets and liabilities defines fair value, establishes a framework for measuring fair value in accordance with generally accepted accounting principles, and expands disclosures about fair value measurements. The guidance establishes a three-tier fair value hierarchy which prioritizes the inputs used in measuring fair value as follows: · Level 1. Quoted prices from active markets for identical assets or liabilities as of the reporting date. Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis. Quoted prices in active markets provide the most reliable evidence of fair value and are used to measure fair value whenever available. Level 1 primarily consists of financial instruments that are exchange-traded. · Level 2. Pricing inputs other than quoted prices in active markets included in Level 1, which are either directly or indirectly observable as of the reporting date. Level 2 includes financial instruments that are valued using models or other valuation methodologies. These models are primarily industry-standard models that consider various assumptions, including quoted forward prices for commodities, time value, volatility factors, and current market and contractual prices for the underlying instruments, as well as other relevant economic measures. Level 2 primarily consists of financial instruments that are non-exchange-traded but have significant observable inputs. · Level 3. Pricing inputs that include significant inputs which are generally less observable from objective sources. These inputs may include internally developed methodologies that result in management's best estimate of fair value. Level 3 financial instruments are those whose fair value is based on significant unobservable inputs. As required by the guidance, assets and liabilities measured at fair value are based on one or more of the following three valuation techniques: 1. Market approach . The market approach uses prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities (including a business) and deriving fair value based on these inputs. 2. Income approach . The income approach uses valuation techniques to convert future amounts (for example, cash flows or earnings) to a single present amount (discounted). The measurement is based on the value indicated by current market expectations about those future amounts. 3. Cost approach. The cost approach is based on the amount that currently would be required to replace the service capacity of an asset (often referred to as current replacement cost). This approach assumes that the fair value would not exceed what it would cost a market participant to acquire or construct a substitute asset or comparable utility, adjusted for obsolescence. The tables below detail assets and liabilities measured at fair value on a recurring basis at March 31, 2017 and December 31, 2016. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at Reporting Date Using March 31, Quoted Prices in Significant Other Significant ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Nuclear decommissioning trust funds: Domestic equity $ $ $ — $ — International equity trust — — Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — Mutual funds — — Municipal bonds — — Other — — Long-term investments: International equity trust — — Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — Mutual funds — — Natural gas swaps ) — ) — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at Reporting Date Using December 31, Quoted Prices in Significant Other Significant ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Nuclear decommissioning trust funds: Domestic equity $ $ $ — $ — International equity trust — — Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — Municipal bonds — Other — — Long-term investments: Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — International equity trust — — Mutual funds — — Other — — Natural gas swaps ) — ) — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The estimated fair values of our long-term debt, including current maturities at March 31, 2017 and December 31, 2016 were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2017 2016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Carrying Fair Carrying Fair ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Long-term debt $ $ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The estimated fair value of long-term debt is classified as Level 2 and is estimated based on observed or quoted market prices for the same or similar issues or on current rates offered to us for debt of similar maturities. The primary sources of our long-term debt consist of first mortgage bonds, pollution control revenue bonds and long-term debt issued by the Federal Financing Bank that is guaranteed by the Rural Utilities Service or the U.S. Department of Energy. We also have small amounts of long-term debt provided by National Rural Utilities Cooperative Finance Corporation (CFC) and by CoBank, ACB. The valuations for the first mortgage bonds and the pollution control revenue bonds were obtained from a third party data reporting service, and are based on secondary market trading of our debt. Valuations for debt issued by the Federal Financing Bank are based on U.S. Treasury rates as of March 31, 2017 plus an applicable spread, which reflects our borrowing rate for new loans of this type from the Federal Financing Bank. The rates on the CFC debt are fixed and the valuation is based on rate quotes provided by CFC. We use an interest rate quote sheet provided by CoBank for valuation of the CoBank debt, which reflects current rates for similar loans. For cash and cash equivalents, and receivables, the carrying amount approximates fair value because of the short-term maturity of those instruments. Restricted investments consist of funds on deposit with the Rural Utilities Service in the Cushion of Credit Account. The carrying amount approximates fair value because of the liquid nature of the deposits with the U.S. Treasury.

Derivative Instruments

Derivative Instruments	3 Months Ended
	Mar. 31, 2017
Derivative Instruments
Derivative Instruments	(C) Derivative Instruments. Our risk management and compliance committee provides general oversight over all risk management and compliance activities, including but not limited to, commodity trading, investment portfolio management and interest rate risk management. We use commodity trading derivatives to manage our exposure to fluctuations in the market price of natural gas. We do not apply hedge accounting for any of these derivatives, but apply regulatory accounting. Consistent with our rate-making, unrealized gains or losses on our natural gas swaps are reflected as regulatory assets or liabilities, as appropriate. We are exposed to credit risk as a result of entering into these hedging arrangements. Credit risk is the potential loss resulting from a counterparty's nonperformance under an agreement. We have established policies and procedures to manage credit risk through counterparty analysis, exposure calculation and monitoring, exposure limits, collateralization and certain other contractual provisions. It is possible that volatility in commodity prices could cause us to have credit risk exposures with one or more counterparties. If such counterparties fail to perform their obligations, we could suffer a financial loss. However, as of March 31, 2017 all of the counterparties with transaction amounts outstanding under our hedging programs are rated investment grade by the major rating agencies or have provided a guaranty from one of their affiliates that is rated investment grade. We have entered into International Swaps and Derivatives Association agreements with our natural gas hedge counterparties that mitigate credit exposure by creating contractual rights relating to creditworthiness, collateral, termination and netting (which, in certain cases, allows us to use the net value of affected transactions with the same counterparty in the event of default by the counterparty or early termination of the agreement). Additionally, we have implemented procedures to monitor the creditworthiness of our counterparties and to evaluate nonperformance in valuing counterparty positions. We have contracted with a third party to assist in monitoring certain of our counterparties' credit standing and condition. Net liability positions are generally not adjusted as we use derivative transactions as hedges and have the ability and intent to perform under each of our contracts. In the instance of net asset positions, we consider general market conditions and the observable financial health and outlook of specific counterparties, forward looking data such as credit default swaps, when available, and historical default probabilities from credit rating agencies in evaluating the potential impact of nonperformance risk to derivative positions. The contractual agreements contain provisions that could require us or the counterparty to post collateral or credit support. The amount of collateral or credit support that could be required is calculated as the difference between the aggregate fair value of the hedges and pre-established credit thresholds. The credit thresholds are contingent upon each party's credit ratings from the major credit rating agencies. The collateral and credit support requirements vary by contract and by counterparty. Gas hedges. Under the natural gas swap arrangements, we pay the counterparty a fixed price for specified natural gas quantities and receive a payment for such quantities based on a market price index. These payment obligations are netted, such that if the market price index is lower than the fixed price, we will make a net payment, and if the market price index is higher than the fixed price, we will receive a net payment. At March 31, 2017 and December 31, 2016, the estimated fair value of our natural gas contracts was a net asset of approximately $4,968,000 and $15,090,000, respectively. As of March 31, 2017 and December 31, 2016, neither we nor any counterparties were required to post credit support or collateral under the natural gas swap agreements. If the credit-risk-related contingent features underlying these agreements were triggered on March 31, 2017 due to our credit rating being downgraded below investment grade, we would have been required to post collateral or letters of credit of $1,837,000 with our counterparties. The following table reflects the volume activity of our natural gas derivatives as of March 31, 2017 that is expected to settle or mature each year: ​ ​ ​ ​ ​ Year Natural Gas Swaps (in millions) ​ ​ ​ ​ ​ 2017 2018 2019 2020 2021 2022 .02 ​ ​ ​ ​ ​ Total ​ ​ ​ ​ ​ Interest rate options. In fourth quarter of 2011, we purchased seventeen LIBOR swaptions at a cost of $100,000,000 with a total notional amount of approximately $2,200,000,000 to hedge the interest rates on a portion of the debt that we are incurring to finance the two additional nuclear units at Plant Vogtle. The last of these options, having a notional value of $80,169,000, expired without value at March 31, 2017. We are deferring the premiums paid to purchase these LIBOR swaptions, related carrying and other incidental costs in accordance with our rate-making treatment. The deferral will continue and costs will be amortized and collected in rates over the life of the associated debt that we hedged with the swaptions. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance Sheet Fair Value ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2017 2016 (dollars in thousands) Not designated as hedge: Assets: Natural gas swaps Other current assets $ $ Natural gas swaps Other deferred charges $ $ Liabilities: Natural gas swaps Other current liabilities $ $ Natural gas swaps Other deferred credits $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The following table presents the gross realized gains and (losses) on derivative instruments recognized in margin for the three months ended March 31, 2017 and 2016. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Statement of Three months Location 2017 2016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Not Designated as hedges: Natural Gas Swaps Fuel $ $ Natural Gas Swaps Fuel ) ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The following table presents the unrealized gains and (losses) on derivative instruments deferred on the balance sheet at March 31, 2017 and December 31, 2016. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance Sheet 2017 2016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Not designated as hedges: Natural gas swaps Regulatory asset $ ) $ ) Natural gas swaps Regulatory liability Interest rate options Regulatory asset — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total not designated as hedges $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The following table presents the gross amounts of derivatives and their related offset amounts as permitted by their respective master netting agreements. There were no obligations to return cash collateral as of March 31, 2017 or December 31, 2016. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Amounts Gross Net Amounts of ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) March 31, 2017 Natural gas swaps $ $ — $ December 31, 2016 Natural gas swaps $ $ — $ Interest rate options $ $ ) $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Investments in Debt and Equity

Investments in Debt and Equity Securities	3 Months Ended
	Mar. 31, 2017
Investments in Debt and Equity Securities
Investments in Debt and Equity Securities	(D) Investments in Debt and Equity Securities. Investment securities we hold are classified as available-for-sale. Available-for-sale securities are carried at market value with unrealized gains and losses, net of any tax effect, added to or deducted from other comprehensive margin, except that, in accordance with our rate-making treatment, unrealized gains and losses from investment securities held in the nuclear decommissioning funds are directly added to or deducted from the regulatory asset for asset retirement obligations. Realized gains and losses on the nuclear decommissioning funds are also recorded to the regulatory asset. All realized and unrealized gains and losses are determined using the specific identification method. As of March 31, 2017 approximately 66% of these gross unrealized losses had been unrealized for a duration of less than one year. The following tables summarize available-for-sale securities as of March 31, 2017 and December 31, 2016. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) March 31, 2017 Cost Gains Losses Fair ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Equity $ $ $ ) $ Debt ) Other — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) December 31, 2016 Cost Gains Losses Fair ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Equity $ $ $ ) $ Debt ) Other — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Recently Issued or Adopted Acco

Recently Issued or Adopted Accounting Pronouncements	3 Months Ended
	Mar. 31, 2017
Recently Issued or Adopted Accounting Pronouncements
Recently Issued or Adopted Accounting Pronouncements	(E) Recently Issued or Adopted Accounting Pronouncements. In May 2014, the Financial Accounting Standards Board (FASB) issued "Revenue from Contracts with Customers" (Topic 606). The new revenue standard requires that an entity recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. The standard was effective for the annual reporting period beginning after December 15, 2016 using either of the following transition methods: (i) a full retrospective approach reflecting the application of the standard in each prior reporting period with the option to elect certain practical expedients, or (ii) a modified retrospective approach with the cumulative effect of initially adopting the standard recognized at the date of adoption (which includes additional footnote disclosures). Early adoption was not permitted. In August 2015, the FASB issued an update to Topic 606 deferring the effective date by one year. The standard is effective for annual reporting periods beginning after December 15, 2017 and interim periods therein. The standard also permits early adoption of the standard, but not before the original effective date of December 15, 2016. While we expect that the majority of our revenues will be included in the scope of Topic 606, we have not fully completed our evaluation of the new revenue standard. Our evaluation process includes, but is not limited to, identifying contracts within the scope of Topic 606, reviewing and documenting our accounting for these contracts and assessing the applicability of the variable consideration guidance. A large majority of our revenues is derived from substantially identical wholesale power contracts that we have with each of our 38 members. We expect the pattern of revenue recognition pursuant to our wholesale power contracts will remain unchanged on an annual basis under the new revenue standard. However, we continue to evaluate the effects, if any, of Topic 606 on our interim period revenues as it relates to budget adjustments, which may be made during the year that affect our annual revenue requirement. We also continue to evaluate other revenue streams and the related contracts, as well as monitor issues specific to the power and utilities industry. As the ultimate impact of the new revenue standard has not yet been determined, we have not elected our transition method. In January 2016, the FASB issued "Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities." The amendments in this update address certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. The new standard is effective for us for annual reporting periods beginning after December 15, 2017, and interim periods therein. Certain provisions within this update can be adopted early. Certain provisions within this update should be applied by means of a cumulative effect adjustment to the balance sheet of the fiscal year of adoption and certain provisions should be applied prospectively. We are currently evaluating the future impact of this standard on our consolidated financial statements. In February 2016, the FASB issued "Leases (Topic 842)." The new leases standard requires a dual approach for lessee accounting under which a lessee would account for leases as finance leases or operating leases. Both finance leases and operating leases will result in the lessee recognizing a right-of-use (ROU) asset and a corresponding lease liability. For finance leases the lessee would recognize interest expense and amortization of the ROU asset and for operating leases the lessee would recognize a straight-line total lease expense. The new lease standard does not substantially change lessor accounting. The new leases standard is effective for us on a modified retrospective approach for annual reporting periods beginning after December 15, 2018, and interim periods therein. Early adoption is permitted. We are currently evaluating the future impact of this standard on our consolidated financial statements. In June 2016, the FASB issued "Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments." The amendments in this update replace the current incurred loss impairment methodology with a methodology that reflects expected credit losses. The new standard is effective for us prospectively for annual reporting periods beginning after December 15, 2019, and interim periods therein. The amendments in this update can be adopted earlier as of the fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. We are currently evaluating the future impact of this standard on our consolidated financial statements. In August 2016, the FASB issued "Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments." The amendments in this standard provide specific guidance on eight cash flow classification issues relating to how certain cash receipts and cash payments are presented and classified in the statement of cash flows, thereby reducing the current and potential future diversity in practice. The new standard is effective for us for annual reporting periods beginning after December 15, 2017, and interim periods therein. Early adoption is permitted, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. An entity that elects early adoption must adopt all of the amendments in the same period. The amendments should be applied using a retrospective transition method to each period presented. If it is impracticable to apply the amendments retrospectively for some of the issues, the amendments for those issues would be applied prospectively as of the earliest date practicable. We are currently evaluating the future impact of this standard on our consolidated financial statements. In November 2016, the FASB issued "Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force)." The amendments in this standard require the 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 are to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period amounts shown on the statement of cash flows. The new standard is effective for us on a retrospective basis for annual reporting periods beginning after December 15, 2017, and interim periods therein. Early adoption is permitted, including adoption in an interim period. Our restricted cash balances are nominal and accordingly we do not expect the adoption of this standard to have a material impact on our consolidated financial statements.

Accumulated Comprehensive Margi

Accumulated Comprehensive Margin	3 Months Ended
	Mar. 31, 2017
Accumulated Comprehensive Margin
Accumulated Comprehensive Margin	(F) Accumulated Comprehensive Margin. The table below provides detail of the beginning and ending balance for each classification of other comprehensive margin along with the amount of any reclassification adjustments included in margin for each of the periods presented in the unaudited Consolidated Statements of Patronage Capital and Membership Fees and Accumulated Other Comprehensive (Deficit) Margin. There were no material changes in the nature, timing or amounts of expected (gain) loss reclassified to net margin from the amounts disclosed in our 2016 Form 10-K. Amounts reclassified to net margin in the table below are reflected in "Other income" on our unaudited Consolidated Statements of Revenues and Expenses. Our effective tax rate is zero; therefore, all amounts below are presented net of tax. ​ ​ ​ ​ ​ Accumulated Other Three Months Ended ​ ​ ​ ​ ​ (dollars in thousands) Available-for-sale ​ ​ ​ ​ ​ Balance at December 31, 2015 $ Unrealized gain (Gain) reclassified to net margin ) ​ ​ ​ ​ ​ Balance at March 31, 2016 $ ​ ​ ​ ​ ​ Three Months Ended ​ ​ ​ ​ ​ (dollars in thousands) Available-for-sale ​ ​ ​ ​ ​ Balance at December 31, 2016 $ ) Unrealized loss ) Loss reclassified to net margin ​ ​ ​ ​ ​ Balance at March 31, 2017 $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Contingencies and Regulatory Ma

Contingencies and Regulatory Matters	3 Months Ended
	Mar. 31, 2017
Contingencies and Regulatory Matters
Contingencies and Regulatory Matters	(G) Contingencies and Regulatory Matters. We do not anticipate that the liabilities, if any, for any current proceedings against us will have a material effect on our financial condition or results of operations. However, at this time, the ultimate outcome of any pending or potential litigation cannot be determined. a. Patronage Capital Litigation There have been no material changes to this litigation from the disclosure included in Note 12 "Contingencies and Regulatory Matters" of Notes to Consolidated Financial Statements in our 2016 Form 10-K. b. Environmental Matters As is typical for electric utilities, we are subject to various federal, state and local environmental laws which represent significant future risks and uncertainties. Air emissions, water discharges and water usage are extensively controlled, closely monitored and periodically reported. Handling and disposal requirements govern the manner of transportation, storage and disposal of various types of waste. We are also subject to climate change regulations that impose restrictions on emissions of greenhouse gases, including carbon dioxide, for certain new and modified facilities. In general, these and other types of environmental requirements have become increasingly stringent. Such requirements may substantially increase the cost of electric service, by requiring modifications in the design or operation of existing facilities or the purchase of emission allowances. Failure to comply with these requirements could result in civil and criminal penalties and could include the complete shutdown of individual generating units not in compliance. Certain of our debt instruments require us to comply in all material respects with laws, rules, regulations and orders imposed by applicable governmental authorities, which include current and future environmental laws or regulations. Should we fail to be in compliance with these requirements, it would constitute a default under those debt instruments. We believe that we are in compliance with those environmental regulations currently applicable to our business and operations. Although it is our intent to comply with current and future regulations, we cannot provide assurance that we will always be in compliance. At this time, the ultimate impact of any new and more stringent environmental regulations described above is uncertain and could have an effect on our financial condition, results of operations and cash flows as a result of future additional capital expenditures and increased operations and maintenance costs. Additionally, litigation over environmental issues and claims of various types, including property damage, personal injury, common law nuisance, and citizen enforcement of environmental requirements such as air quality and water standards, has increased generally throughout the United States. In particular, personal injury and other claims for damages caused by alleged exposure to hazardous materials, and common law nuisance claims for injunctive relief, personal injury and property damage allegedly caused by coal combustion residue, greenhouse gas and other emissions have become more frequent.

Restricted Investments

Restricted Investments	3 Months Ended
	Mar. 31, 2017
Restricted Investments
Restricted Investments	(H) Restricted Investments. Restricted investments consist of funds on deposit with the Rural Utilities Service in the Cushion of Credit Account. The restricted investments can only be utilized for future Rural Utilities Service-guaranteed Federal Financing Bank debt service payments; deposits can also be used for debt service payments on direct loans made by the Rural Utilities Service but we no longer have such direct loans. The funds on deposit earn interest at a rate of 5% per annum. At March 31, 2017 and December 31, 2016, we had restricted cash and investments totaling $431,193,000 and $468,179,000, respectively, of which $184,132,000 and $221,122,000, respectively, were classified as long-term. The funds on deposit with the Rural Utilities Service in the Cushion of Credit Account are held by the U.S. Treasury, acting through the Federal Financing Bank.

Regulatory Assets and Liabiliti

Regulatory Assets and Liabilities	3 Months Ended
	Mar. 31, 2017
Regulatory Assets and Liabilities
Regulatory Assets and Liabilities	(I) Regulatory Assets and Liabilities. We apply the accounting guidance for regulated operations. Regulatory assets represent certain costs that are probable of recovery from our members in future revenues through rates under the wholesale power contracts with our members extending through December 31, 2050. Regulatory liabilities represent certain items of income that we are retaining and that will be applied in the future to reduce revenues required to be recovered from our members. The following regulatory assets and liabilities are reflected on the unaudited consolidated balance sheets as of March 31, 2017 and December 31, 2016. a ar​ ​ ​ ​ ​ ​ ​ 2017 2016 (dollars in thousands) ​ ​ ​ ​ ​ ​ ​ ​ Regulatory Assets: Premium and loss on reacquired debt (a) $ $ Amortization on capital leases (b) Outage costs (c) Interest rate swap termination fees (d) Asset retirement obligations—Ashpond and other (l) Depreciation expense (e) Deferred charges related to Vogtle Units No. 3 and No. 4 training costs (f) Interest rate options cost (g) Deferral of effects on net margin—Smith Energy Facility (h) Other regulatory assets (m) ​ ​ ​ ​ ​ ​ ​ ​ Total Regulatory Assets $ $ Regulatory Liabilities: Accumulated retirement costs for other obligations (i) $ $ Deferral of effects on net margin—Hawk Road Energy Facility (h) Major maintenance reserve (j) Amortization on capital leases (b) Deferred debt service adder (k) Asset retirement obligations (l) Other regulatory liabilities (m) ​ ​ ​ ​ ​ ​ ​ ​ Total Regulatory Liabilities $ $ ​ ​ ​ ​ ​ ​ ​ ​ Net Regulatory Assets $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (a) Represents premiums paid, together with unamortized transaction costs related to reacquired debt that are being amortized over the lives of the refunding debt, which range up to 27 years. (b) Represents the difference between expense recognized for rate-making purposes and financial statement purposes related to capital lease payments and the aggregate of the amortization of the asset and interest on the obligation. (c) Consists of both coal-fired maintenance and nuclear refueling outage costs. Coal-fired outage costs are amortized on a straight-line basis to expense over a 24-month period. Nuclear refueling outage costs are amortized on a straight-line basis to expense over the 18 to 24-month operating cycles of each unit. (d) Represents losses on settled interest rate swap arrangements that are being amortized through the end of 2018. (e) Prior to Nuclear Regulatory Commission (NRC) approval of a 20-year license extension for Plant Vogtle, we deferred the difference between Plant Vogtle depreciation expense based on the then 40-year operating license and depreciation expense assuming an expected 20-year license extension. Amortization commenced upon NRC approval of the license extension in 2009 and is being amortized over the remaining life of the plant. (f) Deferred charges related to Vogtle Units No. 3 and No. 4 training and interest related carrying costs of such training. Amortization will commence effective with the commercial operation date of each unit and amortized to expense over the life of the units. (g) Deferral of costs associated with interest rate options purchased to hedge interest rates on certain borrowings related to Vogtle Units No.3 and No.4 construction that will be amortized over the life of the associated debt. (h) Effects on net margin for Smith and Hawk Road Energy Facilities are being amortized over the remaining life of each respective plant. (i) Represents the accrual of retirement costs associated with long-lived assets for which there are no legal obligations to retire the assets. (j) Represents collections for future major maintenance costs; revenues are recognized as major maintenance costs are incurred. (k) Represents collections to fund certain debt payments to be made through the end of 2025 which will be in excess of amounts collected through depreciation expense; the deferred credits will be amortized over the remaining useful life of the plants. (l) Represents difference in timing of recognition of the costs of decommissioning for financial statement purposes and for ratemaking purposes. (m) The amortization period for other regulatory assets range up to 33 years and the amortization period of other regulatory liabilities range up to 10 years.

Member Power Bill Prepayments

Member Power Bill Prepayments	3 Months Ended
	Mar. 31, 2017
Member Power Bill Prepayments
Member Power Bill Prepayments	(J) Member Power Bill Prepayments. We have a power bill prepayment program pursuant to which members can prepay their power bills from us at a discount based on our avoided cost of borrowing. The prepayments are credited against the participating members' power bills in the month(s) agreed upon in advance. The discounts are credited against the power bills and are recorded as a reduction to member revenues. The prepayments are being credited against members' power bills through January 2022, with the majority of the balance scheduled to be credited by the end of 2017.

Debt

Debt	3 Months Ended
	Mar. 31, 2017
Debt
Debt	(K) Debt. a) Department of Energy Loan Guarantee: Pursuant to the loan guarantee program established under Title XVII of the Energy Policy Act of 2005 (the "Title XVII Loan Guarantee Program"), we and the U.S. Department of Energy, acting by and through the Secretary of Energy, entered into a Loan Guarantee Agreement on February 20, 2014 pursuant to which the Department of Energy agreed to guarantee our obligations under the Note Purchase Agreement dated as of February 20, 2014 (the "Note Purchase Agreement"), among us, the FFB and the Department of Energy and two future advance promissory notes, each dated February 20, 2014, made by us to the FFB (the "FFB Notes" and together with the Note Purchase Agreement, the "FFB Credit Facility Documents"). The FFB Credit Facility Documents provide for a multi-advance term loan facility (the "Facility"), under which we may make long-term loan borrowings through the FFB. Proceeds of advances made under the Facility will be used to reimburse us for a portion of certain costs of construction relating to Vogtle Units No. 3 and No. 4 that are eligible for financing under the Title XVII Loan Guarantee Program. Aggregate borrowings under the Facility may not exceed $3,057,069,461, of which $335,471,604 is designated for capitalized interest. Under the Loan Guarantee Agreement, we are obligated to reimburse the Department of Energy in the event the Department of Energy is required to make any payments to the FFB under the guarantee. Our payment obligations to FFB under the FFB Notes and reimbursement obligations to the Department of Energy under its guarantee, but not our covenants to the Department of Energy under the Loan Guarantee Agreement, are secured equally and ratably with all of our other notes and obligations issued under our first mortgage indenture. The final maturity date for each advance is February 20, 2044. Interest is payable quarterly in arrears and principal payments will begin on February 20, 2020. Under both FFB Notes, the interest rates during the applicable interest rate periods will equal the current average yield on U.S. Treasuries of comparable maturity at the beginning of the interest rate period, plus a spread equal to 0.375%. At March 31, 2017, aggregate DOE-guaranteed borrowings totaled $1,692,350,000, including capitalized interest. Advances may be requested under the Facility on a quarterly basis through December 31, 2020. Future advances are subject to satisfaction of customary conditions, including certification of compliance with the requirements of the Title XVII Loan Guarantee Program, accuracy of project-related representations and warranties, delivery of updated project-related information, our continued ownership of our interest in Vogtle Units No. 3 and No. 4 free and clear of any liens except those permitted under the Loan Guarantee Agreement, evidence of compliance with the prevailing wage requirements of the Davis-Bacon Act, as amended, and certification from the Department of Energy's consulting engineer that proceeds of the advance are used to reimburse eligible project costs. The failure by the Contractor to perform its obligations under the EPC Agreement could affect our ability to satisfy the conditions required to receive future advances. Under the Loan Guarantee Agreement, we are subject to customary borrower affirmative and negative covenants and events of default. In addition, we are subject to project-related reporting requirements and other project-specific covenants and events of default. If certain events occur, the Department of Energy may, at its option, (i) elect to suspend or terminate the FFB's commitment to make further advances under the Facility, and may later revoke any such suspension or (ii) trigger a Loan Guarantee Agreement covenant under which we have agreed to repay the outstanding principal amount of all borrowings under the Facility over a period of five years, with level principal amortization. These events include (i) cessation of the construction of Vogtle Units No. 3 and No. 4 for twelve consecutive months, (ii) termination of the EPC Agreement under certain circumstances, (iii) loss of or failure to receive necessary regulatory approvals under certain circumstances, (iv) loss of access to intellectual property rights necessary to construct or operate Vogtle Units No. 3 and No. 4 under certain circumstances, (v) our failure to fund our share of operation and maintenance expenses for Vogtle Units No. 3 and No. 4 for twelve consecutive months, (vi) change of control of Oglethorpe and (vii) certain events of loss or condemnation. If we receive proceeds from an event of condemnation relating to Vogtle Units No. 3 and No. 4, such proceeds must be applied to immediately prepay outstanding borrowings under the Facility. We may also voluntarily prepay outstanding borrowings under the Facility. Under the FFB Credit Facility Documents, any prepayment will be subject to a make-whole premium or discount, as applicable. Upon notice to the Department of Energy from the Co-owners of their intent to terminate the EPC Agreement for convenience, the Department of Energy may elect to continue construction of Vogtle Units No. 3 and No. 4. In such an event, unless we elect to join the Department of Energy in continuing construction, the Department of Energy will have the right, subject to certain conditions including obtaining necessary NRC approvals, to assume our rights and obligations under the principal agreements relating to Vogtle Units No. 3 and No. 4 and to acquire all or a portion of our ownership interest in Vogtle Units No. 3 and No. 4. b) Rural Utilities Service Guaranteed Loans: For the three-month period ended March 31, 2017 we received advances on Rural Utilities Service-guaranteed Federal Financing Bank loans totaling $4,517,000 for general and environmental improvements at existing plants. These advances are secured under our first mortgage indenture. c) Pollution Control Revenue Bonds: In January 2017, we temporarily refinanced $122,600,000 of variable rate pollution control revenue bonds with original maturity dates ranging from 2020 through 2040, through the issuance of commercial paper. The bonds were classified as current debt at December 31, 2016.

Vogtle Units No. 3 and No. 4 Co

Vogtle Units No. 3 and No. 4 Construction Project	3 Months Ended
	Mar. 31, 2017
Vogtle Units No. 3 and No. 4 Construction Project
Vogtle Units No. 3 and No. 4 Construction Project	(L) Vogtle Units No. 3 and No. 4 Construction Project. We, Georgia Power, the Municipal Electric Authority of Georgia, and the City of Dalton, Georgia, acting by and through its Board of Water, Light and Sinking Fund Commissioners, doing business as Dalton Utilities (collectively, the Co-owners) are parties to an Ownership Participation Agreement that, along with other agreements, governs our participation in two additional nuclear units at Plant Vogtle, Units No. 3 and No. 4. The Co-owners appointed Georgia Power to act as agent under this agreement. Our binding ownership interest and proportionate share of the cost to construct these units is 30%. Pursuant to this agreement, Georgia Power has designated Southern Nuclear Operating Company, Inc. as its agent for licensing, engineering, procurement, contract management, construction and pre-operation services. As of March 31, 2017, our current investment in the additional Vogtle units was $3,432,000,000. In 2008, Georgia Power, acting for itself and as agent for the Co-owners, entered into an Engineering, Procurement and Construction Agreement (the EPC Agreement) with Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor). Stone & Webster was subsequently acquired by Westinghouse and changed its name to WECTEC Global Project Services Inc. (WECTEC). Pursuant to the EPC Agreement, the Contractor agreed to design, engineer, procure, construct and test two 1,100 megawatt nuclear units using the Westinghouse AP1000 technology and related facilities at Plant Vogtle. Under the EPC Agreement, the Co-owners agreed to pay a purchase price that is subject to certain price escalations and adjustments, including fixed escalation amounts and certain index-based adjustments, as well as adjustments for change orders and performance bonuses. The EPC Agreement also provides for liquidated damages upon the Contractor's failure to fulfill the schedule and certain performance guarantees, each subject to an aggregate cap of 10% of the contract price, or approximately $920,000,000. In addition, the EPC Agreement provides for limited cost sharing by the Co-owners for increases to Contractor costs under certain conditions. The maximum amount of additional capital costs under this provision attributable to us is $75,000,000. Each Co-owner is severally, and not jointly, liable to the Contractor for its proportionate share, based on its ownership interest, of all amounts owed under the EPC Agreement. In the event of a credit rating downgrade below investment grade of any Co-owner, that Co-owner will be required to provide a letter of credit or other credit enhancement. Under the terms of the EPC Agreement, the Contractor does not have a right to terminate the EPC Agreement for convenience. The Contractor may terminate the EPC Agreement under certain circumstances, including certain Co-owner suspension or delays of work, action by a governmental authority to permanently stop work, certain breaches of the EPC Agreement by the Co-owners, Co-owner insolvency, and certain other events. In the event of an abandonment of work by the Contractor, the maximum liability of the Contractor under the EPC Agreement is increased to 40% of the contract price (approximately $1,100,000,000 based on our ownership interest). The EPC Agreement permits Georgia Power, acting for itself and as agent for the Co-owners, to terminate the EPC Agreement at any time for convenience; provided that the Co-owners will be required to pay certain termination costs. In addition, Georgia Power, acting for itself and as agent for the Co-owners, may terminate the EPC Agreement for certain Contractor breaches, including abandonment of work by the Contractor. Under the Toshiba Guarantee, Toshiba has guaranteed certain payment obligations of the Contractor, including any liability of the Contractor for abandonment of work. However, due to Toshiba's financial situation described below, substantial risk regarding the Co-owners' ability to fully collect under the Toshiba Guarantee exists. In January 2016, Westinghouse delivered to the Co-owners $920,000,000 of letters of credit from financial institutions (Westinghouse Letters of Credit) to secure a portion of the Contractor's potential obligations under the EPC Agreement. The Westinghouse Letters of Credit are subject to annual renewals through June 30, 2020, and require 60 days' written notice to Georgia Power, as agent of the Co-owners, in the event the Westinghouse Letters of Credit will not be renewed. In the event of such notice, Georgia Power, as agent of the Co-owners, would be able to draw on the entire balance of the Westinghouse Letters of Credit. The Westinghouse Letters of Credit remain in place in accordance with the terms of the EPC Agreement. On March 29, 2017, Westinghouse and WECTEC each filed for bankruptcy protection under Chapter 11 of the United States Bankruptcy Code. Georgia Power, acting for itself and as agent for the other Co-owners, entered into an Interim Assessment Agreement with the Contractor and WECTEC Staffing Services LLC (WECTEC Staffing), dated as of March 29, 2017 (the Interim Assessment Agreement), to provide for a continuation of work with respect to Vogtle Units No. 3 and No. 4. The provisions in the Interim Assessment Agreement became effective upon approval of the Interim Assessment Agreement by the bankruptcy court on March 30, 2017. The term of the Interim Assessment Agreement was originally scheduled to expire on April 28, 2017. On April 28, 2017, Georgia Power, acting for itself and as agent for the other Co-owners, entered into an amendment to the Interim Assessment Agreement with the Contractor and WECTEC Staffing solely to extend the term of the Interim Assessment Agreement through the earlier of (i) May 12, 2017 and (ii) termination of the Interim Assessment Agreement by any party upon five business days' notice (the Interim Assessment Period). The Interim Assessment Agreement provides, among other items, that (i) Georgia Power will be obligated to pay, on behalf of the Co-owners, all costs accrued by the Contractor for subcontractors and vendors for services performed or goods provided during the Interim Assessment Period, with these amounts to be paid to the Contractor, except for amounts accrued for Fluor Corporation (Fluor), which will be paid directly to Fluor, (ii) during the Interim Assessment Period, the Contractor shall provide certain engineering, procurement and management services for Vogtle Units No. 3 and No. 4, to the same extent as contemplated by the EPC Agreement, and Georgia Power, on behalf of the Co-owners, will make payments of $5,400,000 per week for these services, (iii) Georgia Power will have the right to make payments, on behalf of the Co-owners, directly to subcontractors and vendors who have accounts past due with the Contractor, (iv) during the Interim Assessment Period, the Contractor will use its commercially reasonable efforts to provide information reasonably requested by Georgia Power as is necessary to continue construction and investigate the completion status of Vogtle Units No. 3 and No. 4, (v) the Contractor will reject or accept the EPC Agreement by the termination of the Interim Assessment Agreement, and (vi) during the Interim Assessment Period, Georgia Power, on behalf of the Co-owners, will not exercise any remedies against Toshiba under Toshiba's guarantee of certain obligations of Westinghouse under the EPC Agreement (the Toshiba Guarantee). Under the Interim Assessment Agreement, all parties expressly reserve all rights and remedies under the EPC Agreement, all related security and collateral, under applicable law. A number of subcontractors to the Contractor, including Fluor Enterprises, Inc., have alleged non-payment by the Contractor for amounts owed for work performed on Vogtle Units No. 3 and No. 4. Georgia Power, acting for itself and as agent for the Co-owners, has taken, and continues to take, action to remove liens filed by these subcontractors through the posting of surety bonds. Georgia Power estimates the aggregate liability for the Co-owners under the Interim Assessment Agreement and the removal of subcontractor liens to be approximately $470,000,000, of which our proportionate share would total approximately $141,000,000. As of March 31, 2017, $245,000,000 of this aggregate liability had been paid or accrued. Georgia Power is evaluating remedies available to the Co-owners for these payments, including draws under the Westinghouse Letters of Credit and enforcement of the Toshiba Guarantee. Georgia Power, as agent for the Co-owners, has begun the process to access a portion of the funds available under the Westinghouse Letters of Credit. In February 2017, the Contractor provided Georgia Power, as agent for the Co-owners, with revised forecasted in-service dates of December 2019 and September 2020 for Vogtle Units No. 3 and No. 4, respectively. However, we and Georgia Power do not believe the revised in-service dates are achievable. Georgia Power, along with the other Co-owners, is undertaking a comprehensive schedule and cost-to-complete assessment, as well as a cancellation cost assessment. It is reasonably possible these assessments result in estimated incremental costs to complete, including Co-owners' costs, that materially exceed the value of the Toshiba Guarantee. We intend to work with Georgia Power and the other Co-owners to determine future actions related to Vogtle Units No. 3 and No. 4. Georgia Power has stated that it is working with the Georgia Public Service Commission in regards to this same determination. Georgia Power, for itself and as agent for the other Co-owners, is also negotiating a new service agreement which would, if necessary, engage the Contractor to provide design, engineering, and procurement services to Southern Nuclear, in the event Southern Nuclear assumes control over construction management of Vogtle Units No. 3 and No. 4. In addition, Georgia Power, on behalf of itself and the other Co-owners, intends to take all actions available to it to enforce its rights related to the EPC Agreement, including enforcing the Toshiba Guarantee, subject to the Interim Assessment Agreement, and accessing the Westinghouse Letters of Credit. On April 11, 2017, Toshiba filed its unaudited financial statements as of and for the nine months ended December 31, 2016, which reflected a negative shareholders' equity balance of $1,900,000,000, with Japanese regulators. Toshiba also announced that further substantial charges may be required in the quarter ended March 31, 2017 in connection with the bankruptcy filing of Westinghouse and WECTEC and that there are material events and conditions that raise substantial doubt about Toshiba's ability to continue as a going concern. The Contractor's bankruptcy filing is expected to have a material impact on the construction cost and schedule of Vogtle Units No. 3 and No. 4 and could have a material impact on our financial condition and results of operations. In addition, an inability or other failure by Toshiba to perform its obligations under the Toshiba Guarantee could have a material impact on the cost to the Co-owners of Vogtle Units No. 3 and No. 4, and, therefore, on our financial condition and results of operations. There have been technical and procedural challenges to the construction and licensing of Vogtle Units No. 3 and No. 4 at the federal and state level and additional challenges may arise as construction proceeds. Processes are in place that are designed to assure compliance with the requirements specified in the Westinghouse Design Control Document and the combined construction and operating licenses, including inspections by Southern Nuclear and the Nuclear Regulatory Commission that occur throughout construction. As a result of such compliance processes, certain license amendment requests have been filed and approved or are pending before the Nuclear Regulatory Commission. Various design and other licensing-based compliance matters, including the timely resolution of Inspections, Tests, Analyses, and Acceptance Criteria and the related approvals by the Nuclear Regulatory Commission, may arise as construction proceeds, which may result in additional license amendments or require other resolution. If any license amendment requests or other licensing-based compliance issues are not resolved in a timely manner, there may be further delays in the project schedule that could result in increased costs. As construction continues, the risk remains that challenges with labor productivity, fabrication, delivery, assembly, and installation of plant systems, structures, and components, or other issues could arise and may further impact project schedule and cost. Our previously estimated owner's and financing costs of approximately $20,000,000 per month in the near term for Vogtle Units No. 3 and No. 4 are being evaluated as part of the comprehensive schedule and cost-to-complete analysis being performed as a result of the Contractor's bankruptcy. The ultimate outcome of these matters cannot be determined at this time.

Fair Value (Tables)

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2017
Fair Value
Schedule of assets and liabilities measured at fair value on a recurring basis	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at Reporting Date Using March 31, Quoted Prices in Significant Other Significant ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Nuclear decommissioning trust funds: Domestic equity $ $ $ — $ — International equity trust — — Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — Mutual funds — — Municipal bonds — — Other — — Long-term investments: International equity trust — — Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — Mutual funds — — Natural gas swaps ) — ) — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at Reporting Date Using December 31, Quoted Prices in Significant Other Significant ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Nuclear decommissioning trust funds: Domestic equity $ $ $ — $ — International equity trust — — Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — Municipal bonds — Other — — Long-term investments: Corporate bonds — — US Treasury and government agency securities — — Agency mortgage and asset backed securities — — International equity trust — — Mutual funds — — Other — — Natural gas swaps ) — ) — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of estimated fair values of long-term debt, including current maturities	The estimated fair values of our long-term debt, including current maturities at March 31, 2017 and December 31, 2016 were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2017 2016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Carrying Fair Carrying Fair ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Long-term debt $ $ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Derivative Instruments (Tables)

Derivative Instruments (Tables)	3 Months Ended
	Mar. 31, 2017
Derivative Instruments
Schedule of volume activity of natural gas derivatives that is expected to settle or mature each year	The following table reflects the volume activity of our natural gas derivatives as of March 31, 2017 that is expected to settle or mature each year. ​ ​ ​ ​ ​ Year Natural Gas Swaps (in millions) ​ ​ ​ ​ ​ 2017 2018 2019 2020 2021 2022 .02 ​ ​ ​ ​ ​ Total ​ ​ ​ ​ ​
Schedule of costs to be amortized and collected in rates over the life of the associated debt	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance Sheet Fair Value ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2017 2016 (dollars in thousands) Not designated as hedge: Assets: Natural gas swaps Other current assets $ $ Natural gas swaps Other deferred charges $ $ Liabilities: Natural gas swaps Other current liabilities $ $ Natural gas swaps Other deferred credits $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of the gross realized gains and (losses) on derivative instruments recognized in margin	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Statement of Three months Location 2017 2016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Not Designated as hedges: Natural Gas Swaps Fuel $ $ Natural Gas Swaps Fuel ) ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of unrealized gains and (losses) on derivative instruments deferred on the balance sheet	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance Sheet 2017 2016 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) Not designated as hedges: Natural gas swaps Regulatory asset $ ) $ ) Natural gas swaps Regulatory liability Interest rate options Regulatory asset — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total not designated as hedges $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of gross amounts of derivatives and their related offset amounts as permitted by their respective master netting agreements and obligations to return cash collateral	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Amounts Gross Net Amounts of ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) March 31, 2017 Natural gas swaps $ $ — $ December 31, 2016 Natural gas swaps $ $ — $ Interest rate options $ $ ) $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Investments in Debt and Equit21

Investments in Debt and Equity Securities (Tables)	3 Months Ended
	Mar. 31, 2017
Investments in Debt and Equity Securities
Summary of available-for-sale securities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) March 31, 2017 Cost Gains Losses Fair ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Equity $ $ $ ) $ Debt ) Other — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Unrealized ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (dollars in thousands) December 31, 2016 Cost Gains Losses Fair ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Equity $ $ $ ) $ Debt ) Other — ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total $ $ $ ) $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Accumulated Comprehensive Mar22

Accumulated Comprehensive Margin (Tables)	3 Months Ended
	Mar. 31, 2017
Accumulated Comprehensive Margin
Schedule of changes in accumulated other comprehensive (deficit) margin	​ ​ ​ ​ ​ Accumulated Other Three Months Ended ​ ​ ​ ​ ​ (dollars in thousands) Available-for-sale ​ ​ ​ ​ ​ Balance at December 31, 2015 $ Unrealized gain (Gain) reclassified to net margin ) ​ ​ ​ ​ ​ Balance at March 31, 2016 $ ​ ​ ​ ​ ​ Three Months Ended ​ ​ ​ ​ ​ (dollars in thousands) Available-for-sale ​ ​ ​ ​ ​ Balance at December 31, 2016 $ ) Unrealized loss ) Loss reclassified to net margin ​ ​ ​ ​ ​ Balance at March 31, 2017 $ ) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Regulatory Assets and Liabili23

Regulatory Assets and Liabilities (Tables)	3 Months Ended
	Mar. 31, 2017
Regulatory Assets and Liabilities
Schedule of regulatory assets and liabilities	​ ​ ​ ​ ​ ​ ​ ​ 2017 2016 (dollars in thousands) ​ ​ ​ ​ ​ ​ ​ ​ Regulatory Assets: Premium and loss on reacquired debt (a) $ $ Amortization on capital leases (b) Outage costs (c) Interest rate swap termination fees (d) Asset retirement obligations—Ashpond and other (l) Depreciation expense (e) Deferred charges related to Vogtle Units No. 3 and No. 4 training costs (f) Interest rate options cost (g) Deferral of effects on net margin—Smith Energy Facility (h) Other regulatory assets (m) ​ ​ ​ ​ ​ ​ ​ ​ Total Regulatory Assets $ $ Regulatory Liabilities: Accumulated retirement costs for other obligations (i) $ $ Deferral of effects on net margin—Hawk Road Energy Facility (h) Major maintenance reserve (j) Amortization on capital leases (b) Deferred debt service adder (k) Asset retirement obligations (l) Other regulatory liabilities (m) ​ ​ ​ ​ ​ ​ ​ ​ Total Regulatory Liabilities $ $ ​ ​ ​ ​ ​ ​ ​ ​ Net Regulatory Assets $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (a) Represents premiums paid, together with unamortized transaction costs related to reacquired debt that are being amortized over the lives of the refunding debt, which range up to 27 years. (b) Represents the difference between expense recognized for rate-making purposes and financial statement purposes related to capital lease payments and the aggregate of the amortization of the asset and interest on the obligation. (c) Consists of both coal-fired maintenance and nuclear refueling outage costs. Coal-fired outage costs are amortized on a straight-line basis to expense over a 24-month period. Nuclear refueling outage costs are amortized on a straight-line basis to expense over the 18 to 24-month operating cycles of each unit. (d) Represents losses on settled interest rate swap arrangements that are being amortized through the end of 2018. (e) Prior to Nuclear Regulatory Commission (NRC) approval of a 20-year license extension for Plant Vogtle, we deferred the difference between Plant Vogtle depreciation expense based on the then 40-year operating license and depreciation expense assuming an expected 20-year license extension. Amortization commenced upon NRC approval of the license extension in 2009 and is being amortized over the remaining life of the plant. (f) Deferred charges related to Vogtle Units No. 3 and No. 4 training and interest related carrying costs of such training. Amortization will commence effective with the commercial operation date of each unit and amortized to expense over the life of the units. (g) Deferral of costs associated with interest rate options purchased to hedge interest rates on certain borrowings related to Vogtle Units No.3 and No.4 construction that will be amortized over the life of the associated debt. (h) Effects on net margin for Smith and Hawk Road Energy Facilities are being amortized over the remaining life of each respective plant. (i) Represents the accrual of retirement costs associated with long-lived assets for which there are no legal obligations to retire the assets. (j) Represents collections for future major maintenance costs; revenues are recognized as major maintenance costs are incurred. (k) Represents collections to fund certain debt payments to be made through the end of 2025 which will be in excess of amounts collected through depreciation expense; the deferred credits will be amortized over the remaining useful life of the plants. (l) Represents difference in timing of recognition of the costs of decommissioning for financial statement purposes and for ratemaking purposes. (m) The amortization period for other regulatory assets range up to 33 years and the amortization period of other regulatory liabilities range up to 10 years.

General (Details)

General (Details)	3 Months Ended
	Mar. 31, 2017item
General
Number of electric distribution cooperative members	38

Fair Value - Asset and liabilit

Fair Value - Asset and liabilities measured at fair value on a recurring basis (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Fair value measurement
Long-term investments	$ 106,444,000	$ 99,874,000
Natural gas swaps
Fair value measurement
Derivative assets	4,968,000	15,090,000
Recurring basis | Natural gas swaps
Fair value measurement
Derivative liabilities	(4,968,000)	(15,090,000)
Recurring basis | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	128,483,000	170,408,000
Recurring basis | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	70,815,000	66,861,000
Long-term investments	16,895,000	15,946,000
Recurring basis | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	67,336,000	60,019,000
Long-term investments	14,145,000	11,853,000
Recurring basis | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	68,026,000	65,725,000
Long-term investments	11,849,000	12,187,000
Recurring basis | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	18,157,000	17,410,000
Long-term investments	1,386,000	1,651,000
Recurring basis | Mutual funds
Fair value measurement
Nuclear decommissioning trust funds	44,172,000
Long-term investments	62,169,000	57,932,000
Recurring basis | Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	313,000	943,000
Recurring basis | Other
Fair value measurement
Nuclear decommissioning trust funds	2,720,000	4,663,000
Long-term investments		305,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic equity
Fair value measurement
Nuclear decommissioning trust funds	128,483,000	170,408,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | US Treasury and government agency securities
Fair value measurement
Nuclear decommissioning trust funds	68,026,000	65,725,000
Long-term investments	11,849,000	12,187,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Fair value measurement
Nuclear decommissioning trust funds	44,172,000
Long-term investments	62,169,000	57,932,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Fair value measurement
Nuclear decommissioning trust funds	2,720,000	4,663,000
Long-term investments		305,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Natural gas swaps
Fair value measurement
Derivative liabilities	(4,968,000)	(15,090,000)
Recurring basis | Significant Other Observable Inputs (Level 2) | International equity trust
Fair value measurement
Nuclear decommissioning trust funds	70,815,000	66,861,000
Long-term investments	16,895,000	15,946,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair value measurement
Nuclear decommissioning trust funds	67,336,000	60,019,000
Long-term investments	14,145,000	11,853,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Agency mortgage and asset backed securities
Fair value measurement
Nuclear decommissioning trust funds	18,157,000	17,410,000
Long-term investments	1,386,000	1,651,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair value measurement
Nuclear decommissioning trust funds	$ 313,000	$ 943,000

Fair Value - Estimated fair val

Fair Value - Estimated fair value of long-term debt (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Carrying Value
Grouping Financial Statement Captions
Long-term debt	$ 8,123,581	$ 8,304,523
Total Fair Value
Grouping Financial Statement Captions
Long-term debt	$ 8,858,128	$ 9,043,029

Derivative instruments - Volume

Derivative instruments - Volume activity of natural gas derivatives (Details) - Natural gas swaps item in Thousands	Mar. 31, 2017USD ($)item	Dec. 31, 2016USD ($)
Gas hedges
Derivative assets | $	$ 4,968,000	$ 15,090,000
Collateral or letters of credit required to be posted with counterparties, if credit-risk-related contingent features were triggered due to credit rating being downgraded below investment grade | $	$ 1,837,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	71,300
2,017
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20,380
2,018
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20,910
2,019
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	14,310
2,020
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	10,990
2,021
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	4,690
2,022
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20

Derivative instruments - Intere

Derivative instruments - Interest rate options (Details) - Interest rate options	3 Months Ended
	Dec. 31, 2011USD ($)item	Mar. 31, 2017USD ($)
Derivative instruments and hedging activities
Number of derivative instruments originally purchased | item	17
Purchased amount of derivative instrument	$ 100,000,000
Notional Dollar Amount	$ 2,200,000,000
Number of nuclear units expected to be financed | item	2
Derivative notional value expired		$ 80,169,000

Derivative instruments - Fair v

Derivative instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Assets:
Fair value of assets	$ 4,968,000	$ 15,090,000
Not designated as hedges | Other current assets
Assets:
Fair value of assets	8,917,000	13,833,000
Not designated as hedges | Other deferred charges
Assets:
Fair value of assets	144,000	3,289,000
Not designated as hedges | Other current liabilities
Liabilities:
Fair Value of liabilities	271,000	54,000
Not designated as hedges | Other deferred credits
Liabilities:
Fair Value of liabilities	$ 3,822,000	$ 1,977,000

Derivative instruments - Realiz

Derivative instruments - Realized and unrealized gains and (losses) on derivative instruments (Details) - Not designated as hedges - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total unrealized gains (losses)	$ 4,968		$ 9,302
Natural gas swaps | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(1,837)		(62)
Natural gas swaps | Regulatory liability
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized gains on derivatives	6,805		15,152
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	840	$ 11
Losses	(744)	(4,228)
Total gains (losses) on derivatives	$ 96	$ (4,217)
Interest rate options | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives			$ (5,788)

Derivative instruments - Master

Derivative instruments - Master netting agreements (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Gross amounts of derivatives and their related offset amounts
Cash Collateral	$ 0	$ 0
Natural gas swaps
Assets:
Gross Amounts of Recognized Assets (Liabilities)	4,968	15,090
Net Amounts of Assets Presented on the Balance Sheet	$ 4,968	15,090
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)		5,788
Gross Amounts offset on the Balance Sheet		$ (5,788)

Investments in Debt and Equit32

Investments in Debt and Equity Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale securities
Available-for-sale securities, gross unrealized losses that were in effect for less than one year (as a percent)	66.00%
Cost	$ 463,868	$ 442,148
Gains	49,349	52,221
Losses	(6,751)	(8,466)
Fair Value	506,466	485,903
Equity
Available-for-sale securities
Cost	248,135	237,317
Gains	47,893	51,054
Losses	(3,936)	(5,041)
Fair Value	292,092	283,330
Debt.
Available-for-sale securities
Cost	213,015	201,492
Gains	1,456	1,167
Losses	(2,815)	(3,423)
Fair Value	211,656	199,236
Other
Available-for-sale securities
Cost	2,718	3,339
Losses		(2)
Fair Value	$ 2,718	$ 3,337

Recently Issued or Adopted Ac33

Recently Issued or Adopted Accounting Pronouncements (Details)	3 Months Ended
	Mar. 31, 2017item
Recently Issued or Adopted Accounting Pronouncements
Number of electric distribution cooperative members	38

Accumulated Comprehensive Mar34

Accumulated Comprehensive Margin (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Comprehensive Margin
Effective income tax rate (as a percent)	0.00%
Accumulated Comprehensive (Deficit) Margin
Balance at the beginning of the period	$ (370)	$ 58
Unrealized gain (loss)	(76)	294
(Gain) / Loss reclassified to net margin	37	(10)
Balance at the end of the period	$ (409)	$ 342

Restricted Investments (Details

Restricted Investments (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Restricted Investments
Guaranteed interest rate on deposit (as a percent)	5.00%	5.00%
Restricted cash and investments	$ 431,193,000	$ 468,179,000
Restricted cash and investments	$ 184,132,000	$ 221,122,000

Regulatory Assets and Liabili36

Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 564,314	$ 545,387
Total Regulatory Liabilities	205,603	197,748
Net Regulatory Assets	358,711	347,639
Accumulated retirement costs for other obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	11,769	9,829
Deferral of effects on net margin- Hawk Road Energy Facility | Hawk Road
Regulatory Assets and Liabilities
Total Regulatory Liabilities	20,011	20,163
Major maintenance reserve
Regulatory Assets and Liabilities
Total Regulatory Liabilities	31,737	28,379
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Liabilities	22,230	23,084
Deferred debt service adder
Regulatory Assets and Liabilities
Total Regulatory Liabilities	88,496	86,082
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Liabilities	21,367	11,766
Other regulatory liabilities
Regulatory Assets and Liabilities
Total Regulatory Liabilities	$ 9,993	18,445
Other regulatory liabilities | Maximum
Regulatory Assets and Liabilities
Amortization Period	10 years
Premium and loss on reacquired debt
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 55,014	55,084
Premium and loss on reacquired debt | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	27 years
Amortization on capital leases
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 32,668	32,274
Outage costs
Regulatory Assets and Liabilities
Total Regulatory Assets	48,611	39,986
Interest rate swap termination fees
Regulatory Assets and Liabilities
Total Regulatory Assets	3,124	3,570
Asset retirement obligations
Regulatory Assets and Liabilities
Total Regulatory Assets	42,357	33,747
Depreciation expense
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 43,735	44,091
Depreciation expense | Plant Vogtle
Regulatory Assets and Liabilities
Operating license expected extension period for Plant Vogtle	20 years
Operating license period	40 years
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs | Vogtle Units Number 3 And Number 4
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 44,797	43,444
Interest rate options cost
Regulatory Assets and Liabilities
Total Regulatory Assets	108,562	107,394
Deferral of effects on net margin - Smith Energy Facility | Smith
Regulatory Assets and Liabilities
Total Regulatory Assets	170,913	172,399
Other regulatory assets
Regulatory Assets and Liabilities
Total Regulatory Assets	$ 14,533	$ 13,398
Other regulatory assets | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	33 years
Coal-fired maintenance outage costs
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 months
Nuclear refueling outage costs | Minimum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	18 months
Nuclear refueling outage costs | Maximum
Regulatory Assets and Liabilities
Amortization period, other regulatory assets	24 months

Debt (Details)

Debt (Details)	3 Months Ended
	Mar. 31, 2017USD ($)	Jan. 31, 2017USD ($)	Feb. 20, 2014USD ($)item
Facility
Debt
Number of future advance promissory notes | item			2
Spread on variable rate (as a percent)	0.375%
Aggregate DOE-guaranteed borrowings	$ 1,692,350,000
Term of credit facility (in years)	5 years
Period in which failing to fund the Company's share of operation and maintenance expenses results in termination of funding advances	12 months
Facility | Maximum
Debt
Aggregate borrowings			$ 3,057,069,461
Capitalized interest			$ 335,471,604
Pollution Control Revenue Bonds
Debt
Principal amount		$ 122,600,000
Rural Utilities Service Guaranteed Loans
Debt
Additional advances	$ 4,517,000

Vogtle Units No. 3 and No. 4 38

Vogtle Units No. 3 and No. 4 Construction Project (Details)	Mar. 29, 2017USD ($)	Jan. 31, 2016USD ($)	Mar. 31, 2017USD ($)item	Dec. 31, 2008USD ($)itemMW	Apr. 11, 2017USD ($)
Vogtle Units Number 3 And Number 4
Vogtle Units No. 3 and No. 4 Construction Project
Current investment			$ 3,432,000,000
Anticipated additional costs per month on failure to meet revised forecasted in-service dates			$ 20,000,000
Vogtle Units Number 3 And Number 4 | Ownership participation agreement
Vogtle Units No. 3 and No. 4 Construction Project
Number of nuclear units | item			2
Ownership interest (as a percent)			30.00%
Vogtle Units Number 3 And Number 4 | Engineering, Procurement and Construction Agreement (the EPC Agreement) | Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor)
Vogtle Units No. 3 and No. 4 Construction Project
Number of nuclear units | item				2
Generating capacity of each nuclear unit | MW				1,100
Vogtle Units Number 3 And Number 4 | Engineering, Procurement and Construction Agreement (the EPC Agreement) | Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor) | In the event of abandonment of work
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages, as a percent of contract price				40.00%
Liquidated damages				$ 1,100,000,000
Vogtle Units Number 3 And Number 4 | Engineering, Procurement and Construction Agreement (the EPC Agreement) | Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor) | Maximum
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages, as a percent of contract price				10.00%
Liquidated damages				$ 920,000,000
Additional capital cost under the provision				$ 75,000,000
Vogtle Units Number 3 And Number 4 | Engineering, Procurement and Construction Agreement (the EPC Agreement) | Westinghouse Electric Company LLC
Vogtle Units No. 3 and No. 4 Construction Project
Written notice period for non-renewal of letter of credit		60 days
Vogtle Units Number 3 And Number 4 | Interim Assessment Agreement
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages	$ 141,000,000
Aggregate liability paid or accrued			$ 245,000,000
Vogtle Units Number 3 And Number 4 | Interim Assessment Agreement | The Contractor and WECTEC Staffing Services LLC
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages	$ 470,000,000
Notice period for termination of agreement, number of business days	5 days
Amount paid for services, per week	$ 5,400,000
Toshiba Corporation
Vogtle Units No. 3 and No. 4 Construction Project
Shareholder's equity negative balance					$ 1,900,000,000
Toshiba Corporation | Vogtle Units Number 3 And Number 4 | Engineering, Procurement and Construction Agreement (the EPC Agreement) | Westinghouse Electric Company LLC
Vogtle Units No. 3 and No. 4 Construction Project
Letters of credit		$ 920,000,000