Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Regulatory Liability, Noncurrent		$ 1,270.7	$ 1,283.4
Interim Rate Revenue Reserved		15.4
Increase in Tax Expense from Adjustment		2.1
Current Federal Tax Expense (Benefit)		(1.9)	4.9		$ 0.0
Current State and Local Tax Expense (Benefit)		(4.4)	(4.2)		(5.7)
Current Income Tax Expense (Benefit)		(6.3)	0.7		(5.7)
Deferred Federal Income Tax Expense (Benefit)		74.7	(75.9)		126.0
Deferred State and Local Income Tax Expense (Benefit)		3.7	26.0		28.0
Deferred Income Tax Expense (Benefit)		78.4	(49.9)		154.0
Investment Tax Credit		0.1	(0.1)		(0.2)
Income Tax Expense (Benefit)		$ 72.2	$ (49.3)	[1]	$ 148.1
Statutory federal tax rate		21.00%	35.00%		35.00%
Federal deferred tax revaluation		0.40%	(41.20%)		0.00%
Federal renewable energy credit (A)	[2]	(5.10%)	(4.80%)		(6.80%)
Remeasurement of state deferred tax liabilities		(0.40%)	0.40%		0.90%
401(k) dividends		(0.30%)	(0.50%)		(0.60%)
Federal investment tax credits, net		0.00%	(0.10%)		(0.80%)
State income taxes, net of federal income tax benefit		0.40%	2.00%		1.90%
Effective Income Tax Rate Reconciliation, Executive Compensation Limitation, Percent		0.20%	0.00%		0.00%
Uncertain tax positions		0.00%	0.00%		0.10%
Amortization of net unfunded deferred taxes		(2.10%)	0.70%		0.70%
Other		0.40%	(0.10%)		0.10%
Effective income tax rate		14.50%	(8.60%)		30.50%
Accrued liabilities		$ (12.5)	$ (7.4)
Accrued vacation		(2.3)	(2.1)
Uncollectible accounts		(0.4)	(0.4)
Accelerated depreciation and other property related differences		1,605.3	1,449.6
Investment in Enable		469.9	441.7
Regulatory assets		17.4	18.9
Income taxes recoverable from customers, net		(239.6)	(244.3)
Company Pension Plan		7.6	11.5
Bond redemption-unamortized costs		2.4	2.6
Derivative instruments		1.7	1.6
Federal tax credits		(237.8)	(218.5)
State tax credits		(156.0)	(141.7)
Postretirement medical and life insurance benefits		(23.6)	(25.2)
Net operating losses		(20.2)	(21.1)
Regulatory liabilities		(78.8)	(16.8)
Asset retirement obligations		(21.5)	(19.2)
Deferred federal investment tax credits		(1.8)	(0.5)
Other		1.1	(0.9)
Total deferred income tax liabilities, net		1,310.9	1,227.8
Unrecognized Tax Benefits		20.7	20.7		$ 20.7	$ 20.2
Current year additions		0.0	0.0		0.5
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		0.0
Tax Credit Carryforward, Deferred Tax Asset		237.8	218.5
Increased Regulatory Liabilities due to Income Taxes		7.4
State operating loss [Member]
Operating Loss Carryforwards		451.8
Deferred Tax Assets, Operating Loss Carryforwards, State and Local		$ 20.2
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal tax credits [Member]
Federal tax credits		$ (237.8)
Tax Credit Carryforward, Amount		237.8
Tax Credit Carryforward, Deferred Tax Asset		$ 237.8
Tax Credit Carryforward, Expiration Date		Dec. 31, 2032
Oklahoma investment tax credits [Member]
State tax credits		$ (127.7)
Tax Credit Carryforward, Amount		161.6
Oklahoma capital investment board credits [Member]
State tax credits		(8.9)
Tax Credit Carryforward, Amount		8.9
Oklahoma zero emission tax credits [Member]
State tax credits		(19.4)
Tax Credit Carryforward, Amount		$ 24.1
Tax Credit Carryforward, Expiration Date		Dec. 31, 2020
Og and E [Member]
Unrecognized Tax Benefits		$ 16.4	$ 16.4		$ 13.5
Income taxes refundable to customers, net [Member]
Regulatory Liability, Noncurrent		$ 1,022.0

[1] The Company recorded an income tax benefit of $245.2 million and income tax expense of $10.5 million during the fourth quarter of 2017 due to the Company remeasuring deferred taxes related to the natural gas midstream operations and other operations segments, respectively, as a result of the 2017 Tax Act. See Note 8 for further discussion of the effects of the 2017 Tax Act.

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