Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Federal Tax Expense (Benefit)		$ 2.9	$ (17.5)	$ (43.4)
Current State and Local Tax Expense (Benefit)		(5.3)	(5.3)	(6.3)
Current Income Tax Expense (Benefit)		(2.4)	(22.8)	(49.7)
Deferred Federal Income Tax Expense (Benefit)		99.8	117.0	146.8
Deferred State and Local Income Tax Expense (Benefit)		17.2	11.3	15.4
Deferred Income Tax Expense (Benefit)		117.0	128.3	162.2
Investment Tax Credit		(0.2)	(0.7)	(0.9)
Income Tax Expense (Benefit)		$ 114.4	$ 104.8	$ 111.6
Statutory Federal tax rate		35.00%	35.00%	35.00%
Amortization of net unfunded deferred taxes		0.80%	0.90%	0.80%
Federal investment tax credits, net		(0.90%)	(0.20%)	(0.20%)
State income taxes, net of Federal income tax benefit		1.80%	0.50%	0.80%
Federal renewable energy credit	[1]	(8.20%)	(8.80%)	(9.40%)
Uncertain tax positions		0.10%	0.70%	0.70%
Other		0.10%	(0.10%)	0.00%
Effective income tax rate		28.70%	28.00%	27.70%
Accelerated depreciation and other property related differences		$ 2,101.4	$ 2,012.1
OG&E pension plan		48.0	55.1
Regulatory asset		34.4	32.7
Income taxes refundable to customers, net		24.1	22.0
Bond redemption-unamortized costs		4.3	4.8
Federal tax credits		(209.3)	(174.0)
State tax credits		(102.6)	(98.6)
Postretirement medical and life insurance benefits		(34.3)	(40.8)
Regulatory liabilities		(34.6)	(46.3)
Asset retirement obligations		(24.5)	(22.5)
Net operating losses		(26.1)	(89.7)
Other		(16.7)	(7.1)
Accrued liabilities		(8.0)	(5.9)
Accrued vacation		(2.4)	(2.6)
Deferred Federal investment tax credits		(0.8)	(0.9)
Uncollectible accounts		(0.6)	0.5
Non-Current Deferred Income Tax Liabilities, net		1,752.3	1,637.8
Unrecognized Tax Benefits		20.7	20.2	$ 16.1	$ 12.0
Current year additions		0.5	4.1	4.1
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		0.0
Unrecognized Tax Benefits, Period Increase (Decrease)		0.5
Tax Credit Carryforward, Deferred Tax Asset		209.3	174.0
State operating loss [Member]
Operating Loss Carryforwards		403.1
Deferred Tax Assets, Operating Loss Carryforwards, State and Local		$ 14.8
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal operating loss [Member]
Operating Loss Carryforwards		$ 32.2
Deferred Tax Assets, Operating Loss Carryforwards		$ 11.3
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal tax credits [Member]
Federal tax credits		$ (209.3)
Tax Credit Carryforward, Amount		209.3
Tax Credit Carryforward, Deferred Tax Asset		$ 209.3
Tax Credit Carryforward, Expiration Date		Dec. 31, 2029
Oklahoma investment tax credits [Member]
Federal tax credits		$ (79.1)
Tax Credit Carryforward, Amount		121.7
Tax Credit Carryforward, Deferred Tax Asset		79.1
Oklahoma capital investment board credits [Member]
Federal tax credits		(7.3)
Tax Credit Carryforward, Amount		7.3
Tax Credit Carryforward, Deferred Tax Asset		7.3
Oklahoma zero emission tax credits [Member]
Federal tax credits		(16.2)
Tax Credit Carryforward, Amount		24.1
Tax Credit Carryforward, Deferred Tax Asset		$ 16.2
Tax Credit Carryforward, Expiration Date		Dec. 31, 2020
State and Local Jurisdiction [Member]
Unrecognized Tax Benefits, Period Increase (Decrease)		$ 0.3
OG&E [Member]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		$ 13.5	$ 13.2	$ 10.5

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