Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Current Federal Tax Expense (Benefit)	$ (43.4)		$ (3.2)		$ (9.0)
Current State and Local Tax Expense (Benefit)	(6.3)		0.6		0.3
Current Income Tax Expense (Benefit)	(49.7)		(2.6)		(8.7)
Deferred Federal Income Tax Expense (Benefit)	146.8		106.4		111.4
Deferred State and Local Income Tax Expense (Benefit)	15.4		11.7		(5.9)
Deferred Income Tax Expense (Benefit)	162.2		118.1		105.5
Investment Tax Credit	(0.9)		(2.0)		(2.2)
Income Tax Expense (Benefit)	111.6		113.5		94.6
Statutory Federal tax rate	35.00%		35.00%		35.00%
Amortization of net unfunded deferred taxes	0.80%		0.80%		1.00%
Federal investment tax credits, net	(0.20%)		(0.50%)		(0.60%)
State income taxes, net of Federal income tax benefit	0.80%		0.00%		(0.70%)
Federal renewable energy credit	(9.40%)	[1]	(9.20%)	[1]	(9.40%)	[1]
Uncertain tax positions	0.70%		2.00%		0.00%
Other	0.00%		(0.20%)		(0.10%)
Effective income tax rate	27.70%		27.90%		25.20%
Net operating losses	153.2		168.7
Accrued liabilities	6.0		9.0
Federal tax credits	8.3		8.0
Accrued vacation	2.7		2.8
Uncollectible accounts	0.6		0.7
Total Current Deferred Income Tax Assets	170.8		189.2
Accelerated depreciation and other property related differences	1,931.4		1,747.0
OG&E pension plan	77.2		84.7
Regulatory asset	24.7		26.0
Income taxes refundable to customers, net	21.7		21.9
Bond redemption-unamortized costs	5.3		3.6
Total Non-Current Deferred Income Tax Liabilities	2,060.3		1,883.2
Federal tax credits	(137.4)		(101.9)
State tax credits	(91.6)		(86.1)
Regulatory liabilities	(58.0)		(61.3)
Postretirement medical and life insurance benefits	(40.6)		(45.2)
Asset retirement obligations	(21.4)		(20.8)
Net operating losses	(19.7)		(17.6)
Deferred Federal investment tax credits	(0.4)		(0.7)
Other	(4.6)		(4.4)
Total Non-Current Deferred Income Tax Assets	(373.7)		(338.0)
Non-Current Deferred Income Tax Liabilities, net	1,686.6		1,545.2
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	10.5
Unrecognized Tax Benefits	10.5		7.8		0		0
Current year additions	2.7		2.7		0
Prior year additions	0		5.1		0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	0
Unrecognized Tax Benefits, Period Increase (Decrease)	2.7
State operating loss [Member]
Tax Credit Carryforward, Amount	729.0
Tax Credit Carryforward, Deferred Tax Asset	26.9
Tax Credit Carryforward, Expiration Date	Dec. 31, 2030
Federal operating loss [Member]
Tax Credit Carryforward, Amount	417.4
Tax Credit Carryforward, Deferred Tax Asset	146.0
Tax Credit Carryforward, Expiration Date	Dec. 31, 2030
Federal tax credits [Member]
Tax Credit Carryforward, Amount	145.7
Tax Credit Carryforward, Deferred Tax Asset	145.7
Tax Credit Carryforward, Expiration Date	Dec. 31, 2029
Oklahoma investment tax credits [Member]
Tax Credit Carryforward, Amount	104.7
Tax Credit Carryforward, Deferred Tax Asset	68.1
Oklahoma capital investment board credits [Member]
Tax Credit Carryforward, Amount	7.3
Tax Credit Carryforward, Deferred Tax Asset	7.3
Oklahoma zero emission tax credits [Member]
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)	$ 4.2

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