Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Schedule of sales to members

Sales to members were as follows:

	(dollars in thousands)
	2019		2018		2017
Capacity revenues	$	942,057	$	927,419	$	912,421
Energy revenues	487,795		551,960		521,409
Total	$	1,429,852	$	1,479,379	$	1,433,830

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

The following table reflects members whose revenues accounted for 10% or more of our total operating revenues in 2019, 2018 or 2017:

	2019		2018		2017
Jackson EMC	14.4	%	14.1	%	14.7	%
Cobb EMC	13.8	%	13.9	%	14.3	%

Schedule reflecting details of asset retirement obligations included in the consolidated balance sheets

The following table reflects the details of the asset retirement obligations included in the consolidated balance sheets for the years 2019 and 2018.

	(dollars in thousands)
	Nuclear		Coal Ash Pond		Other		Total
Balance at December 31, 2018	$	658,956	$	326,248	$	32,359	1,017,563
Liabilities settled	—		(3,380)		(1,158)		(4,538)
Accretion	38,485		10,494		1,494		50,473
Deferred accretion	—		1,860		—		1,860
Change in cash flow estimates	—		(5,958)		11,240		5,282
Balance at December 31, 2019	$	697,441	$	329,264	$	43,935	$	1,070,640

	(dollars in thousands)
	Nuclear		Coal Ash Pond		Other		Total
Balance at December 31, 2017	$	548,574	$	161,755	$	24,668	$	734,997
Liabilities settled	(1,686)		(1,596)		(1,398)		(4,680)
Accretion	32,857		4,238		995		38,090
Change in cash flow estimates	79,211		161,851		8,094		249,156
Balance at December 31, 2018	$	658,956	$	326,248	$	32,359	$	1,017,563

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

Our portion of the estimated costs of decommissioning co-owned nuclear facilities are as follows:

	(dollars in thousands)
2018 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 2018 dollars:
Radiated structures	$	209,000	$	231,000	$	188,000	$	206,000
Spent fuel management	54,000		49,000		55,000		51,000
Non-radiated structures	14,000		19,000		23,000		29,000
Total estimated site study costs	$	277,000	$	299,000	$	266,000	$	286,000

Schedule of external and internal trust funds by type of investment

The following table outlines the fair value of our nuclear decommissioning funds as of December 31, 2019 and December 31, 2018. The funds were invested in a diversified mix of approximately 64% equity and 36% fixed income securities in 2019 and 60% equity and 40% fixed income in 2018.

	2019
External Trust Funds:	(dollars in thousands)
	Cost 12/31/2018		Purchases		Net Proceeds(1)		Unrealized Gain(Loss)		Fair Value 12/31/2019
Equity	$	207,313	$	11,950	$	(6,678)	$	119,263	$	331,848
Debt	166,023		361,844		(353,222)		5,548		180,193
Other	115		544		(1,361)		—		(702)
	$	373,451	$	374,338	$	(361,261)	$	124,811	$	511,339

(1)Also included in net proceeds are net realized gains or losses, interest income, dividends and fees of $13,078,000.

	2019
Internal Funds:	(dollars in thousands)
	Cost 12/31/2018		Purchases		Net Proceeds(1)		Unrealized Gain(Loss)		Fair Value 12/31/2019
Equity	$	44,295	$	—	$	2,767	$	19,578	$	66,640
Debt	38,382		140,997		(135,033)		1,161		45,507
	$	82,677	$	140,997	$	(132,266)	$	20,739	$	112,147

(1)Also included in net proceeds are net realized gains or losses, interest income, dividends, contributions and fees of $8,732,000.

	2018
External Trust Funds:	(dollars in thousands)
	Cost 12/31/2017		Purchases		Net Proceeds(1)		Unrealized Gain(Loss)		Fair Value 12/31/2018
Equity	$	203,622	$	12,186	$	(7,789)	$	49,475	$	257,494
Debt	164,901		445,353		(443,712)		(2,108)		164,434
Other	141		370		(1,621)		—		(1,110)
	$	368,664	$	457,909	$	(453,122)	$	47,367	$	420,818

(1)Also included in net proceeds are net realized gains or losses, interest income, dividends and fees of $4,786,000.

	2018
Internal Funds:	(dollars in thousands)
	Cost 12/31/2017		Purchases		Net Proceeds(1)		Unrealized Gain(Loss)		Fair Value 12/31/2018
Equity	$	43,698	$	—	$	596	$	6,373	$	50,667
Debt	33,540		161,454		(156,611)		(246)		38,137
	$	77,238	$	161,454	$	(156,015)	$	6,127	$	88,804

(1)Also included in net proceeds are net realized gains or losses, interest income, dividends, contributions and fees of $689,000.

Schedule of annual depreciation rates

Annual weighted average depreciation rates in effect in 2019, 2018, and 2017 were as follows:

	Range of Useful Life in years*	2019		2018		2017
Steam production	49-65	2.61	%	2.57	%	2.91	%
Nuclear production	37-60	1.94	%	1.92	%	1.96	%
Hydro production	50	2.00	%	2.00	%	2.00	%
Other production	30-35	2.61	%	2.61	%	2.58	%
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 2019.

Schedule of regulatory assets and liabilities

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 members.

	(dollars in thousands)
	2019		2018
Regulatory Assets:
Premium and loss on reacquired debt(a)	$	40,067	$	46,315
Amortization on financing leases(b)	35,433		34,918
Outage costs(c)	34,367		36,352
Asset retirement obligations –  Ashpond and other(k)	245,932		137,835
Asset retirement obligations –  Nuclear(k)	—		7,031
Depreciation expense(d)	39,820		41,244
Deferred charges related to Vogtle Units No. 3 and No. 4 training costs(e)	53,466		51,549
Interest rate options cost(f)	121,938		116,960
Deferral of effects on net margin – Smith Energy Facility(g)	154,564		160,509
Other regulatory assets(m)	37,925		22,350
Total Regulatory Assets	$	763,512	$	655,063
Regulatory Liabilities:
Accumulated retirement costs for other obligations(h)	$	12,692	$	13,873
Deferral of effects on net margin – Hawk Road Energy Facility(g)	18,485		19,101
Major maintenance reserve(i)	50,144		45,547
Amortization on financing leases(b)	14,256		17,156
Deferred debt service adder(j)	114,453		105,192
Asset retirement obligations – Nuclear(k)	61,516		—
Revenue deferral plan(l)	90,066		15,670
Other regulatory liabilities(m)	2,629		2,459
Total Regulatory Liabilities	$	364,241	$	218,998
Net regulatory assets	$	399,271	$	436,065

(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 24 years.

(b)Represents the difference between expense recognized for rate-making purposes versus financial statement purposes related to finance 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 periods up to 48 months, depending on the operating cycle of each unit. Nuclear refueling outage costs are amortized on a straight-line basis to expense over the 18 or 24-month operating cycles of each unit.

(d)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.

(e)Deferred charges consist of training related costs, including interest and 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.

(f)Deferral of premiums paid to purchase interest rate options used to hedge interest rates on certain borrowings, related carrying costs and other incidentals associated with construction of Vogtle Units No. 3 and No. 4. Amortization will commence when Vogtle Unit 3 goes in-service, which is expected November 2021.

(g)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.

(h)Represents the accrual of retirement costs associated with long-lived assets for which there are no legal obligations to retire the assets.

(i)Represents collections for future major maintenance costs; revenues are recognized as major maintenance costs are incurred.

(j)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.

(k)Represents the difference in the timing of recognition of decommissioning costs for financial statement purposes versus ratemaking purposes, as well as the deferral of unrealized gains and losses of funds set aside for decommissioning.

(l)Deferred revenues under a rate management program that allows for additional collections over a five-year period beginning in 2018. These amounts will be amortized to income and applied to member billings over the subsequent five-year period.

(m)The amortization periods for other regulatory assets range up to 30 years and the amortization periods of other regulatory liabilities range up to 7 years.