Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Regulatory Assets and Liabilities [Table Text Block]

The following table presents a summary of OG&E's regulatory assets and liabilities.

December 31 (In millions)	2020		2019
REGULATORY ASSETS
Current:
SPP cost tracker under recovery (A)	$	7.0	$	—
Generation Capacity Replacement rider under recovery (A)	4.4		3.7
Fuel clause under recoveries	—		39.5
Other (A)	8.4		5.5
Total current regulatory assets	$	19.8	$	48.7
Non-current:
Benefit obligations regulatory asset	$	164.9	$	167.2
Deferred storm expenses	158.8		65.5
Sooner Dry Scrubbers	19.7		20.6
Pension tracker	18.1		2.3
Smart Grid	11.2		18.4
Unamortized loss on reacquired debt	9.7		10.6
Arkansas deferred pension expenses	9.3		8.0
Frontier Plant deferred expenses	6.4		—
COVID-19 impacts	6.4		—
Other	11.1		13.4
Total non-current regulatory assets	$	415.6	$	306.0
REGULATORY LIABILITIES
Current:
Fuel clause over recoveries	$	28.6	$	4.8
Oklahoma demand program rider over recovery (B)	1.5		2.0
Reserve for tax refund and interim surcharge (B)	0.8		12.7
SPP cost tracker over recovery (B)	—		2.6
Other (B)	4.2		6.9
Total current regulatory liabilities	$	35.1	$	29.0
Non-current:
Income taxes refundable to customers, net	$	867.4	$	899.2
Accrued removal obligations, net	316.8		318.5
Other	4.7		5.8
Total non-current regulatory liabilities	$	1,188.9	$	1,223.5

(A)Included in Other Current Assets in the balance sheets.

(B)Included in Other Current Liabilities in the balance sheets.

Components of Benefit Obligation Regulatory Asset [Table Text Block]

The following table presents a summary of the components of the benefit obligations regulatory asset.

December 31 (In millions)	2020		2019
Pension Plan and Restoration of Retirement Income Plan:
Net loss	$	147.3	$	160.5
Postretirement Benefit Plans:
Net loss	26.2		23.3
Prior service cost	(8.6)		(16.6)
Total	$	164.9	$	167.2

Schedule of Jointly Owned Utility Plants [Table Text Block]

The following tables present OG&E's ownership interest in the jointly-owned McClain Plant and the jointly-owned Redbud Plant, and, as disclosed below, only OG&E's ownership interest is reflected in the property, plant and equipment and accumulated depreciation balances in these tables. The owners of the remaining interests in the McClain Plant and the Redbud Plant are responsible for providing their own financing of capital expenditures. Also, only OG&E's proportionate interests of any direct expenses of the McClain Plant and the Redbud Plant, such as fuel, maintenance expense and other operating expenses, are included in the applicable financial statement captions in the statements of income.

December 31, 2020 (In millions)	Percentage Ownership		Total Property, Plant and Equipment		Accumulated Depreciation		Net Property, Plant and Equipment
McClain Plant (A)	77	%	$	257.1	$	96.0	$	161.1
Redbud Plant (A)(B)	51	%	$	531.8	$	181.9	$	349.9

(A)Construction work in progress was $0.1 million and $1.8 million for the McClain and Redbud Plants, respectively.

(B)This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $67.3 million.

December 31, 2019 (In millions)	Percentage Ownership		Total Property, Plant and Equipment		Accumulated Depreciation		Net Property, Plant and Equipment
McClain Plant (A)	77	%	$	254.4	$	83.5	$	170.9
Redbud Plant (A)(B)	51	%	$	529.9	$	159.0	$	370.9

(A)Construction work in progress was $0.2 million and $1.4 million for the McClain and Redbud Plants, respectively.

(B)This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $61.8 million.

Property, Plant and Equipment [Table Text Block]

The following tables present the Registrants' major classes of property, plant and equipment and related accumulated depreciation. 

December 31, 2020 (In millions)	Total Property, Plant and Equipment		Accumulated Depreciation		Net Property, Plant and Equipment
OG&E:
Distribution assets	$	4,809.9	$	1,422.1	$	3,387.8
Electric generation assets (A)	4,932.2		1,713.6		3,218.6
Transmission assets (B)	2,944.6		591.7		2,352.9
Intangible plant	254.1		153.9		100.2
Other property and equipment	495.3		186.3		309.0
OG&E property, plant and equipment	13,436.1		4,067.6		9,368.5
Non-OG&E property, plant and equipment	6.1		—		6.1
Total OGE Energy property, plant and equipment	$	13,442.2	$	4,067.6	$	9,374.6

(A)This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $67.3 million.

(B)This amount includes a plant acquisition adjustment of $3.3 million and accumulated amortization of $0.9 million.

December 31, 2019 (In millions)	Total Property, Plant and Equipment		Accumulated Depreciation		Net Property, Plant and Equipment
OG&E:
Distribution assets	$	4,468.6	$	1,381.1	$	3,087.5
Electric generation assets (A)	4,838.6		1,601.0		3,237.6
Transmission assets (B)	2,901.1		565.5		2,335.6
Intangible plant	225.2		145.4		79.8
Other property and equipment	473.1		175.1		298.0
OG&E property, plant and equipment	12,906.6		3,868.1		9,038.5
Non-OG&E property, plant and equipment	6.1		—		6.1
Total OGE Energy property, plant and equipment	$	12,912.7	$	3,868.1	$	9,044.6

(A)This amount includes a plant acquisition adjustment of $148.3 million and accumulated amortization of $61.8 million.

(B)This amount includes a plant acquisition adjustment of $3.3 million and accumulated amortization of $0.8 million.

Schedule of Change in Asset Retirement Obligation [Table Text Block]

The following table presents changes to OG&E's asset retirement obligations during the years ended December 31, 2020 and 2019.

(In millions)	2020		2019
Balance at January 1	$	73.5	$	83.9
Accretion expense	0.5		1.0
Revisions in estimated cash flows (A)	5.8		(2.4)
Liabilities settled (B)	(0.2)		(9.0)
Balance at December 31	$	79.6	$	73.5

(A)Assumptions changed related to the estimated timing and estimated cost of the removal of asbestos at OG&E's generating facilities.

(B)Asset retirement obligations were settled for asbestos removal at one of OG&E's generating facilities

Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]

The following table presents changes in the components of accumulated other comprehensive income (loss) attributable to OGE Energy during 2019 and 2020. All amounts below are presented net of tax.

	Pension Plan and Restoration of Retirement Income Plan		Postretirement Benefit Plans
(In millions)	Net Gain  (Loss)		Net Gain (Loss)		Prior Service Cost (Credit)		Other Comprehensive Loss from Unconsolidated Affiliates		Total
Balance at December 31, 2018	$	(38.8)	$	4.6	$	5.3	$	—	$	(28.9)
Other comprehensive loss before reclassifications	(8.3)		(0.2)		—		(0.6)		(9.1)
Amounts reclassified from accumulated other comprehensive income (loss)	3.4		(0.2)		(1.7)		—		1.5
Settlement cost	8.6		—		—		—		8.6
Net current period other comprehensive income (loss)	3.7		(0.4)		(1.7)		(0.6)		1.0
Balance at December 31, 2019	(35.1)		4.2		3.6		(0.6)		(27.9)
Other comprehensive income (loss) before reclassifications	(5.1)		(2.4)		—		(0.7)		(8.2)
Amounts reclassified from accumulated other comprehensive income (loss)	3.9		(0.1)		(1.7)		—		2.1
Curtailment cost	—		(0.3)		—		—		(0.3)
Settlement cost	2.2		—		—		—		2.2
Net current period other comprehensive income (loss)	1.0		(2.8)		(1.7)		(0.7)		(4.2)
Balance at December 31, 2020	$	(34.1)	$	1.4	$	1.9	$	(1.3)	$	(32.1)

Reclassification out of Accumulated Other Comprehensive Income [Table Text Block]

The following table presents significant amounts reclassified out of accumulated other comprehensive income (loss) by the respective line items in net income (loss) during the years ended December 31, 2020 and 2019.

Details about Accumulated Other Comprehensive Income (Loss) Components	Amount Reclassified from Accumulated Other Comprehensive Income (Loss)				Affected Line Item in OGE Energy's Statements of Income
	Year Ended December 31,
(In millions)	2020		2019
Amortization of Pension Plan and Restoration of Retirement Income Plan items:
Actuarial losses	$	(5.1)	$	(4.5)	(A)
Settlement cost	(2.9)		(11.3)		(A)
	(8.0)		(15.8)		Income (Loss) Before Taxes
	(1.9)		(3.8)		Income Tax Expense (Benefit)
	$	(6.1)	$	(12.0)	Net Income (Loss)
Amortization of postretirement benefit plans items:
Prior service credit	$	2.3	$	2.3	(A)
Curtailment cost	0.4		—		(A)
Actuarial gains	0.1		0.2		(A)
	2.8		2.5		Income (Loss) Before Taxes
	0.7		0.6		Income Tax Expense (Benefit)
	$	2.1	$	1.9	Net Income (Loss)
Total reclassifications for the period, net of tax	$	(4.0)	$	(10.1)	Net Income (Loss)

(A)These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost (see Note 13 for additional information).