Summary of significant accounting policies: (Tables)

12 Months Ended

Dec. 31, 2016

Summary of significant accounting policies:

Schedule of changes in accumulated other comprehensive (deficit) margin

Accumulated Other Comprehensive (Deficit) Margin
		(dollars in thousands)
		Available-for-sale Securities
Balance at December 31, 2013	$	(549	)
Unrealized gain		1,180
(Gain) reclassified to net margin		(163	)
Balance at December 31, 2014		468
Unrealized gain		265
(Gain) reclassified to net margin		(675	)
Balance at December 31, 2015		58
Unrealized loss		(234	)
(Gain) reclassified to net margin		(194	)
Balance at December 31, 2016	$	(370	)

Schedule of members whose revenues accounted for 10% or more of total operating revenues

	2016		2015		2014
Jackson EMC	14.3	%	9.7	%	10.4	%
Cobb EMC	13.7	%	13.1	%	13.6	%
Sawnee EMC	10.5	%	10.4	%	9.5	%

Schedule reflecting details of Asset Retirement Obligations included in the consolidated balance sheets

		(dollars in thousands)
		Nuclear		Coal Ash Pond			Other			Total
Balance at December 31, 2015	$	488,458	$	93,622		$	20,150		$	602,230
Liabilities settled		–		(553	)		(707	)		(1,260	)
Accretion		29,107		2,215			1,039			32,361
Change in Cash Flow Estimates		–		61,181			3,539			64,720
Balance at December 31, 2016	$	517,565	$	156,465		$	24,021		$	698,051

		(dollars in thousands)
		Nuclear		Coal Ash Pond		Other			Total
Balance at December 31, 2014	$	369,046	$	42,609	$	20,605		$	432,260
Liabilities settled		–		–		(299	)		(299	)
Accretion		23,231		1,929		948			26,108
Change in Cash Flow Estimates		96,181		49,084		(1,104	)		144,161
Balance at December 31, 2015	$	488,458	$	93,622	$	20,150		$	602,230

Schedule of estimated costs of decommissioning of co-owned nuclear facilities

		(dollars in thousands)
2015 site study		Hatch Unit No. 1		Hatch Unit No. 2		Vogtle Unit No. 1		Vogtle Unit No. 2
Expected start date of decommissioning		2034		2038		2047		2049
Estimated costs based on site study in 2015 dollars:
Radiated structures	$	193,000	$	213,000	$	178,000	$	195,000
Spent fuel management		49,000		47,000		49,000		47,000
Non-radiated structures		16,000		22,000		26,000		33,000
Total estimated site study costs	$	258,000	$	282,000	$	253,000	$	275,000

Schedule of external and internal trust funds by type of investment

	2016
External Trust Funds:
		12.31.15 Cost		Purchases		Net Proceeds(1)			Unrealized Gain(Loss)			12.31.16 FV
Equity	$	198,265	$	46,865	$	(43,395	)	$	38,749		$	240,484
Debt		144,187		347,383		(343,040	)		(1,675	)	$	146,855
Other		187		1,258		(2,754	)		(1	)	$	(1,310	)
Total	$	342,639	$	395,506	$	(389,189	)	$	37,073		$	386,029

Internal Funds:
			12.31.15 Cost		Purchases		Net Proceeds(1)			Unrealized Gain(Loss)			12.31.16 FV
Equity		$	33,513	$	0	$	5,285		$	7,263		$	46,061
Debt			25,539		42,783		(42,115	)		(211	)	$	25,996
Total		$	59,052	$	42,783	$	(36,830	)	$	7,052		$	72,057
(1)	Also included in net proceeds are net realized gains or losses, interest income and dividends, contributions and fees of $12,270,144.

	2015
External Trust Funds:
		12.31.14 Cost		Purchases		Net Proceeds(2)			Unrealized Gain(Loss)			12.31.15 FV
Equity	$	183,527	$	104,417	$	(77,504	)	$	23,321		$	233,761
Debt		130,407		454,072		(450,747	)		(2,128	)	$	131,604
Other		324		79		(1,935	)		(4	)	$	(1,536	)
Total	$	314,258	$	558,568	$	(530,186	)	$	21,189		$	363,829

Internal Funds:
			12.31.14 Cost		Purchases		Net Proceeds(2)			Unrealized Gain(Loss)			12.31.15 FV
Equity		$	19,796	$	0	$	13,717		$	4,537		$	38,050
Debt			22,767		82,554		(79,782	)		(263	)	$	25,276
Total		$	42,563	$	82,554	$	(66,065	)	$	4,274		$	63,326
(2)	Also included in net proceeds are net realized gains or losses, interest income and dividends, contributions and fees of $44,871,375.

Schedule of annual depreciation rates

		Range of Useful Life in years*	2016		2015		2014
Steam production		49-65	2.84	%	1.93	%	1.86	%
Nuclear production		37-60	1.96	%	1.55	%	1.53	%
Hydro production		50	2.00	%	2.00	%	2.00	%
Other production		27-33	2.55	%	2.38	%	2.56	%
Transmission		36	2.75	%	2.75	%	2.75	%
General		3-50	2.00-33.33	%	2.00-33.33	%	2.00-33.33	%
*	Calculated based on the composite depreciation rates in effect for 2016.

Schedule of regulatory assets and liabilities

		(dollars in thousands)
		2016		2015
Regulatory Assets:
Premium and loss on reacquired debt(a)	$	55,084	$	61,916
Amortization on capital leases(b)		32,274		30,253
Outage costs(c)		39,986		42,027
Interest rate swap termination fees(d)		3,570		5,355
Asset Retirement Obligations – Ashpond and other(l)		33,747		1,098
Depreciation expense(e)		44,091		45,514
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs(f)		43,444		37,646
Interest rate options cost(g)		107,394		102,554
Deferral of effects on net margin – Smith Energy Facility(h)		172,399		178,343
Other regulatory assets(m)		13,398		25,548
Total Regulatory Assets		545,387		530,254
Regulatory Liabilities:
Accumulated retirement costs for other obligations(i)	$	9,829	$	8,910
Deferral of effects on net margin – Hawk Road Energy Facility(h)		20,163		20,775
Major maintenance reserve(j)		28,379		22,422
Amortization on capital leases(b)		23,084		26,502
Deferred debt service adder(k)		86,082		76,334
Asset retirement obligations – Nuclear(l)		11,766		8,316
Other regulatory liabilities(m)		18,445		3,708
Total Regulatory Liabilities		197,748		166,967
Net regulatory assets	$	347,639	$	363,287

(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 net loss associated with the change in fair value and expired cost of interest rate options purchased to hedge interest rates on certain borrowings related to Vogtle Units No.3 and No.4 construction. Amortization will commence in February 2020 and will be amortized through February 2044, the life of the DOE-guaranteed loan which is financing a portion of the construction project.
(h)	Effects on net margin for Smith and Hawk Road Energy Facilities were deferred through the end of 2015 and 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 and ashpond remediation for financial statement purposes and for ratemaking purposes.
(m)	The amortization periods for other regulatory assets range up to 33 years and the amortization periods of other regulatory liabilities range up to 10 years.

Schedule of components of other income

		(dollars in thousands)
		2016			2015		2014
Capital credits from associated companies (Note 4)	$	1,679		$	1,859	$	1,986
Net revenue from Georgia Transmission and Georgia System Operations for shared Administrative and General costs		6,553			6,278		4,944
Miscellaneous other		(5,561	)		1,006		(310	)
Total	$	2,671		$	9,143	$	6,620