Summary of significant accounting policies: (Tables)

12 Months Ended

Dec. 31, 2015

Summary of significant accounting policies:

Schedule of changes in accumulated other comprehensive margin (deficit)

Accumulated Other Comprehensive Margin (Deficit)
		(dollars in thousands)
		Available-for-sale Securities
Balance at December 31, 2012	$	903
Unrealized loss		(1,396	)
(Gain) reclassified to net margin		(56	)
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

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

	2015		2014		2013
Cobb EMC	13.1	%	13.6	%	13.2	%
Sawnee EMC	10.4	%	9.5	%	9.5	%
Jackson EMC	9.7	%	10.4	%	10.9	%

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, 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

		(dollars in thousands)
		Nuclear		Coal Ash Pond		Other			Total
Balance at December 31, 2013	$	347,201	$	40,760	$	20,089		$	408,050
Liabilities settled		–		–		(406	)		(406	)
Accretion		21,845		1,849		922			24,616
Balance at December 31, 2014	$	369,046	$	42,609	$	20,605		$	432,260

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 annual depreciation rates as approved by Rural Utilities Service

		Range of Useful Life in years*	2015		2014		2013
Steam production		49-65	1.93	%	1.86	%	1.82	%
Nuclear production		37-60	1.55	%	1.53	%	1.54	%
Hydro production		50	2.00	%	2.00	%	2.00	%
Other production		27-33	2.38	%	2.56	%	2.55	%
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 2015.

Schedule of regulatory assets and (liabilities)

			(dollars in thousands)
			2015		2014
Regulatory Assets:
Premium and loss on reacquired debt(a)		$	61,916	$	71,731
Amortization on capital leases(b)			30,253		27,829
Outage costs(c)			42,027		45,795
Interest rate swap termination fees(d)			5,355		9,345
Depreciation expense(e)			45,514		46,938
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs(f)			37,646		32,501
Interest rate options cost(g)			102,554		98,671
Deferral of effects on net margin – Smith Energy Facility(h)			178,343		128,666
Other regulatory assets(m)			26,646		22,573
Total Regulatory Assets			530,254		484,049
Regulatory Liabilities:
Accumulated retirement costs for other obligations(i)		$	8,910	$	18,559
Deferral of effects on net margin – Hawk Road Energy Facility(h)			20,775		29,867
Major maintenance reserve(j)			22,422		23,427
Amortization on capital leases(b)			26,502		21,693
Deferred debt service adder(k)			76,334		66,754
Asset retirement obligations(l)			8,316		28,870
Other regulatory liabilities(m)			3,708		4,903
Total Regulatory Liabilities			166,967		194,073
Net regulatory assets		$	363,287	$	289,976
(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 28 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 2016 and 2019.
(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 unrealized and realized change in fair value 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 will be 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 expenses; 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 34 years and the amortization period of other regulatory liabilities range up to 11 years.

Schedule of components of other income

		(dollars in thousands)
		2015		2014			2013
Capital credits from associated companies (Note 4)	$	1,859	$	1,986		$	1,954
Net revenue from Georgia Transmission and Georgia System Operations for shared Administrative and General costs		6,278		4,944			4,459
Miscellaneous other		1,006		(310	)		(723	)
Total	$	9,143	$	6,620		$	5,690