Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liability, Noncurrent		$ 1,188.9	$ 1,223.5
Current Federal Tax Expense (Benefit)		8.4	(6.4)	$ (1.9)
Current State and Local Tax Expense (Benefit)		0.5	5.1	(4.4)
Current Income Tax Expense (Benefit)		8.9	(1.3)	(6.3)
Deferred Federal Income Tax Expense (Benefit)		(105.2)	48.5	74.7
Deferred State and Local Income Tax Expense (Benefit)		(31.1)	(17.4)	3.7
Deferred Income Tax Expense (Benefit)		(136.3)	31.1	78.4
Investment Tax Credit		0.0	0.0	0.1
Income Tax Expense (Benefit)		$ (127.4)	$ 29.8	$ 72.2
Statutory federal tax rate		21.00%	21.00%	21.00%
Federal deferred tax revaluation		0.00%	0.00%	0.40%
Federal renewable energy credit (B)	[1]	(5.00%)	(6.00%)	(5.10%)
Remeasurement of state deferred tax liabilities		0.90%	(0.80%)	(0.40%)
401(k) dividends		(0.40%)	(0.40%)	(0.30%)
State income taxes, net of federal income tax benefit		(1.40%)	(1.20%)	0.40%
Effective Income Tax Rate Reconciliation, Executive Compensation Limitation, Percent		0.20%	0.20%	0.20%
Amortization of net unfunded deferred taxes		(4.40%)	(4.50%)	(2.10%)
Other		0.10%	(0.70%)	0.40%
Effective income tax rate		42.30%	6.40%	14.50%
Accrued liabilities		$ (9.6)	$ (10.7)
Accrued vacation		(2.2)	(2.1)
Uncollectible accounts		(0.7)	(0.4)
Accelerated depreciation and other property related differences		1,721.2	1,656.8
Investment in Enable		302.6	478.2
Regulatory assets		52.3	28.4
Income taxes recoverable from customers, net		(221.8)	(229.9)
Pension Plan		3.9	4.1
Bond redemption-unamortized costs		2.0	2.2
Derivative instruments		1.7	1.6
Federal tax credits		(236.6)	(238.0)
State tax credits		(204.4)	(185.8)
Postretirement medical and life insurance benefits		(22.4)	(23.3)
Net operating losses		(12.0)	(16.6)
Regulatory liabilities		(81.0)	(68.1)
Asset retirement obligations		(20.3)	(19.2)
Deferred federal investment tax credits		(2.7)	(1.8)
Other		(1.4)	0.4
Total deferred income tax liabilities, net		1,268.6	1,375.8
Unrecognized Tax Benefits		21.9	20.7	$ 20.7	$ 20.7
Current year additions		1.2	0.0	$ 0.0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		0.0
Tax Credit Carryforward, Deferred Tax Asset		$ 236.6	$ 238.0
Stock-based compensation		(0.30%)	(1.20%)	0.00%
Effective Income Tax Rate Reconciliation, Tax Contingency, Percent	[2]	31.60%	0.00%	0.00%
Deferred Tax Asset [Member]
Unrecognized Tax Benefits		$ 17.6
State operating loss [Member]
Operating Loss Carryforwards		268.0
Deferred Tax Assets, Operating Loss Carryforwards, State and Local		12.0
Federal tax credits [Member]
Federal tax credits		(236.6)
Tax Credit Carryforward, Amount		236.6
Tax Credit Carryforward, Deferred Tax Asset		236.6
Oklahoma investment tax credits [Member]
State tax credits		(162.3)
Tax Credit Carryforward, Amount		205.6
Oklahoma capital investment board credits [Member]
State tax credits		(12.7)
Tax Credit Carryforward, Amount		12.7
Oklahoma zero emission tax credits [Member]
State tax credits		(29.3)
Tax Credit Carryforward, Amount		37.2
Louisiana inventory credits
State tax credits		(0.1)
Tax Credit Carryforward, Amount		0.2
Research and Development Credits [Member]
Tax Credit Carryforward, Amount		1.2
Og and E [Member]
Regulatory Liability, Noncurrent		1,188.9	$ 1,223.5
Current Federal Tax Expense (Benefit)		(3.8)	(7.9)	$ (12.4)
Current State and Local Tax Expense (Benefit)		(0.6)	4.1	(4.1)
Current Income Tax Expense (Benefit)		(4.4)	(3.8)	(16.5)
Deferred Federal Income Tax Expense (Benefit)		45.7	37.7	53.7
Deferred State and Local Income Tax Expense (Benefit)		(6.6)	(13.8)	2.7
Deferred Income Tax Expense (Benefit)		39.1	23.9	56.4
Investment Tax Credit		0.0	0.0	0.1
Income Tax Expense (Benefit)		$ 34.7	$ 20.1	$ 40.0
Statutory federal tax rate		21.00%	21.00%	21.00%
Federal deferred tax revaluation		0.00%	0.00%	0.00%
Federal renewable energy credit (B)	[1]	(5.40%)	(7.60%)	(6.90%)
Remeasurement of state deferred tax liabilities		0.00%	0.00%	0.00%
401(k) dividends		0.00%	0.00%	0.00%
State income taxes, net of federal income tax benefit		(1.60%)	(1.80%)	(0.20%)
Effective Income Tax Rate Reconciliation, Executive Compensation Limitation, Percent		0.00%	0.00%	0.00%
Amortization of net unfunded deferred taxes		(4.80%)	(5.60%)	(2.90%)
Other		0.10%	(0.60%)	(0.10%)
Effective income tax rate		9.30%	5.40%	10.90%
Accrued liabilities		$ (5.2)	$ (4.3)
Accrued vacation		(1.6)	(1.6)
Uncollectible accounts		(0.7)	(0.4)
Accelerated depreciation and other property related differences		1,721.2	1,656.8
Investment in Enable		0.0	0.0
Regulatory assets		52.3	28.4
Income taxes recoverable from customers, net		(221.8)	(229.9)
Pension Plan		27.4	24.5
Bond redemption-unamortized costs		2.0	2.2
Derivative instruments		0.0	0.0
Federal tax credits		(236.6)	(238.0)
State tax credits		(189.0)	(170.8)
Postretirement medical and life insurance benefits		(15.3)	(16.0)
Net operating losses		(1.4)	(5.7)
Regulatory liabilities		(81.0)	(68.1)
Asset retirement obligations		(20.3)	(19.2)
Deferred federal investment tax credits		(2.7)	(1.8)
Other		(6.5)	(4.7)
Total deferred income tax liabilities, net		1,020.8	951.4
Unrecognized Tax Benefits		17.6	16.4	$ 16.4
Tax Credit Carryforward, Deferred Tax Asset		$ 236.6	$ 238.0
Stock-based compensation		0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Contingency, Percent	[2]	0.00%	0.00%	0.00%
Og and E [Member] | State operating loss [Member]
Operating Loss Carryforwards		$ 21.5
Deferred Tax Assets, Operating Loss Carryforwards, State and Local		1.4
Og and E [Member] | Federal tax credits [Member]
Federal tax credits		(236.6)
Tax Credit Carryforward, Amount		236.6
Tax Credit Carryforward, Deferred Tax Asset		236.6
Og and E [Member] | Oklahoma investment tax credits [Member]
State tax credits		(147.0)
Tax Credit Carryforward, Amount		186.1
Og and E [Member] | Oklahoma capital investment board credits [Member]
State tax credits		(12.7)
Tax Credit Carryforward, Amount		12.7
Og and E [Member] | Oklahoma zero emission tax credits [Member]
State tax credits		(29.3)
Tax Credit Carryforward, Amount		37.2
Og and E [Member] | Louisiana inventory credits
State tax credits		0.0
Tax Credit Carryforward, Amount		$ 0.0

[1] Represents credits primarily associated with the production from OG&E's wind farms.

[2] As further discussed in Note 5, OGE Energy recorded a $780.0 million impairment on its investment in Enable in March 2020, which resulted in a tax benefit being recorded that caused a significant variance to the effective tax rate as compared to the prior year. This variance has been presented in the table as a single line item in order to facilitate comparability of other components of the effective tax rate.