Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Current Federal Tax Expense (Benefit)	$ (3.2)		$ (9.0)		$ 23.4
Current State and Local Tax Expense (Benefit)	0.6		0.3		(0.5)
Current Income Tax Expense (Benefit)	(2.6)		(8.7)		22.9
Deferred Federal Income Tax Expense (Benefit)	106.4		111.4		98.0
Deferred State and Local Income Tax Expense (Benefit)	11.7		(5.9)		0.3
Deferred Income Tax Expense (Benefit)	118.1		105.5		98.3
Investment Tax Credit	(2.0)		(2.2)		(3.3)
Income Tax Expense (Benefit)	113.5		94.6		117.9
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate	35.00%		35.00%		35.00%
Effective Income Tax Rate Reconciliation Amortization of Net Unfunded Deferred Taxes	0.80%		1.00%		0.90%
Effective Income Tax Rate Reconciliation, Deductions, Medicare Prescription Drug Benefit	0.00%		0.00%		0.40%
Effective Income Tax Rate Reconciliation, Tax Credits, Investment	(0.50%)		(0.60%)		(0.90%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes	0.00%		(0.70%)		0.10%
Effective Income Tax Rate Reconciliation, Tax Credits, Other	(9.20%)	[1]	(9.40%)	[1]	(4.40%)	[1]
Effective Income Tax Rate Reconciliation, Tax Contingency, Percent	2.00%		0.00%		0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.20%)		(0.10%)		(0.20%)
Effective Income Tax Rate, Continuing Operations	27.90%		25.20%		30.90%
Deferred Tax Assets, Operating Loss Carryforwards	168.7		113.8
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	9.0		14.0
Current Deferred Tax Assets, Federal Tax Credits	8.0		6.0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Other	2.8		3.1
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts	0.7		1.0
Deferred Tax Assets, Derivative Instruments	0		0.8
Deferred Tax Assets, Gross, Current	189.2		138.7
Deferred Tax Liabilities, Property, Plant and Equipment	1,747.0		1,648.5
Deferred Tax Liabilities, Prepaid Expenses	84.7		102.2
Deferred Tax Liabilities, Regulatory Assets	26.0		18.8
Deferred Tax Liabilities, Income Taxes Refundable to Customers, Net	21.9		21.2
Deferred Tax Liabilities, Unamortized Bond Redemption Costs	3.6		4.0
Deferred Tax Liabilities, Gross, Noncurrent	1,883.2		1,794.7
Deferred Tax Assets, Tax Credit Carryforwards	(101.9)		(69.5)
Non-Current Deferred Tax Assets, State Tax Credits	(86.1)		(78.1)
Non-Current Deferred Tax Assets, Regulatory Liabilities	(61.3)		(71.4)
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits	(45.2)		(42.1)
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Asset Retirement Obligations	(20.8)		0
Non-Current Deferred Tax Asset, Net Operating Losses	(17.6)		(150.0)
Non-Current Deferred Tax Assets, Deferred Federal Investment Tax Credits	(0.7)		(1.5)
Deferred Tax Assets, Other	(4.4)		(4.3)
Deferred Tax Assets, Gross, Noncurrent	(338.0)		(416.9)
Deferred Tax Liabilities, Net, Noncurrent	1,545.2		1,377.8
Unrecognized Tax Benefits	7.8		0		0		0
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	2.7		0		0
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	5.1		0		0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	0
State operating loss [Member]
Tax Credit Carryforward, Amount	656.3
Tax Credit Carryforward, Deferred Tax Asset	24.2
Tax Credit Carryforward, Expiration Date	Dec. 31, 2030
Federal operating loss [Member]
Tax Credit Carryforward, Amount	463.1
Tax Credit Carryforward, Deferred Tax Asset	162.1
Tax Credit Carryforward, Expiration Date	Dec. 31, 2030
Federal tax credits [Member]
Tax Credit Carryforward, Amount	109.9
Tax Credit Carryforward, Deferred Tax Asset	109.9
Tax Credit Carryforward, Expiration Date	Dec. 31, 2029
Oklahoma investment tax credits [Member]
Tax Credit Carryforward, Amount	96.3
Tax Credit Carryforward, Deferred Tax Asset	62.6
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
FIN 48 reserve, gross [Member]
Income Tax Reconciliation, Tax Contingencies, State and Local	7.8
FIN 48 reserve, net of tax [Member]
Income Tax Reconciliation, Tax Contingencies, State and Local	5.1
Additional FIN 48 Reserve [Member]
Income Tax Reconciliation, Tax Contingencies, State and Local	4.1
Additional FIN 48 reserve, net of tax [Member]
Income Tax Reconciliation, Tax Contingencies, State and Local	2.7
Federal operating loss [Member]
Deferred Tax Assets, Operating Loss Carryforwards	$ 168.7

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