Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current Federal Tax Expense (Benefit)		$ (17.5)	$ (43.4)	$ (3.2)
Current State and Local Tax Expense (Benefit)		(5.3)	(6.3)	0.6
Current Income Tax Expense (Benefit)		(22.8)	(49.7)	(2.6)
Deferred Federal Income Tax Expense (Benefit)		117.0	146.8	106.4
Deferred State and Local Income Tax Expense (Benefit)		11.3	15.4	11.7
Deferred Income Tax Expense (Benefit)		128.3	162.2	118.1
Investment Tax Credit		(0.7)	(0.9)	(2.0)
Income Tax Expense (Benefit)		$ 104.8	$ 111.6	$ 113.5
Statutory Federal tax rate		35.00%	35.00%	35.00%
Federal renewable energy credit	[1]	(8.80%)	(9.40%)	(9.20%)
Federal investment tax credits, net		(0.20%)	(0.20%)	(0.50%)
State income taxes, net of Federal income tax benefit		0.50%	0.80%	0.00%
Uncertain tax positions		0.70%	0.70%	2.00%
Amortization of net unfunded deferred taxes		0.90%	0.80%	0.80%
Other		(0.10%)	0.00%	(0.20%)
Effective income tax rate		28.00%	27.70%	27.90%
Net operating losses			$ 153.2
Accrued liabilities		$ (5.9)	(6.0)
Federal tax credits			8.3
Accrued vacation		(2.6)	(2.7)
Uncollectible accounts		(0.5)	(0.6)
Total Current Deferred Income Tax Assets			170.8
Accelerated depreciation and other property related differences		2,012.1	1,931.4
OG&E pension plan		55.1	77.2
Regulatory asset		32.7	24.7
Income taxes refundable to customers, net		22.0	21.7
Bond redemption-unamortized costs		4.8	5.3
Federal tax credits		(174.0)	(137.4)
State tax credits		(98.6)	(91.6)
Net operating losses		(89.7)	(19.7)
Regulatory liabilities		(46.3)	(58.0)
Postretirement medical and life insurance benefits		(40.8)	(40.6)
Asset retirement obligations		(22.5)	(21.4)
Deferred Federal investment tax credits		(0.9)	(0.4)
Other		(7.1)	(4.6)
Non-Current Deferred Income Tax Liabilities, net		1,637.8	1,686.6
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		13.2
Unrecognized Tax Benefits		13.2	10.5	$ 7.8	$ 0.0
Current year additions		2.7	2.7	2.7
Prior year additions		0.0	0.0	$ 5.1
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		0.0
Unrecognized Tax Benefits, Period Increase (Decrease)		4.2
Tax Credit Carryforward, Deferred Tax Asset		174.0	$ 137.4
State operating loss [Member]
Operating Loss Carryforwards		562.6
Deferred Tax Assets, Operating Loss Carryforwards, State and Local		$ 20.7
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal operating loss [Member]
Net operating losses		$ 69.0
Operating Loss Carryforwards		$ 197.3
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal tax credits [Member]
Federal tax credits		$ (174.0)
Tax Credit Carryforward, Amount		174.0
Tax Credit Carryforward, Deferred Tax Asset		$ 174.0
Tax Credit Carryforward, Expiration Date		Dec. 31, 2029
Oklahoma investment tax credits [Member]
Federal tax credits		$ (75.1)
Tax Credit Carryforward, Amount		115.6
Tax Credit Carryforward, Deferred Tax Asset		75.1
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)		$ 2.7

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