Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended							12 Months Ended		12 Months Ended		12 Months Ended				12 Months Ended		3 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Operating Loss [Member]	Dec. 31, 2030 State operating loss [Member]	Dec. 31, 2012 State operating loss [Member]	Dec. 31, 2030 Federal operating loss [Member]	Dec. 31, 2012 Federal operating loss [Member]	Dec. 31, 2029 Federal tax credits [Member]	Dec. 31, 2012 Federal tax credits [Member]	Dec. 31, 2012 Oklahoma investment tax credits [Member]	Dec. 31, 2012 Oklahoma capital investment board credits [Member]	Dec. 31, 2020 Oklahoma zero emission tax credits [Member]	Dec. 31, 2012 Oklahoma zero emission tax credits [Member]	Mar. 31, 2013 FIN 48 reserve, gross [Member]	Mar. 31, 2013 FIN 48 reserve, net of tax [Member]
Current Federal Tax Expense (Benefit)	$ (9.0)		$ 23.4		$ (11.2)
Current State and Local Tax Expense (Benefit)	0.3		(0.5)		3.4
Current Income Tax Expense (Benefit)	(8.7)		22.9		(7.8)
Deferred Federal Income Tax Expense (Benefit)	111.4		98.0		117.0
Deferred State and Local Income Tax Expense (Benefit)	(5.9)		0.3		5.5
Deferred Income Tax Expense (Benefit)	105.5		98.3		122.5
Investment Tax Credit	(2.2)		(3.3)		(3.7)
Income Tax Expense (Benefit)	94.6		117.9		111.0
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	1.00%		0.90%		1.00%
Effective Income Tax Rate Reconciliation, Deductions, Medicare Prescription Drug Benefit	0.00%		0.40%		2.30%
Effective Income Tax Rate Reconciliation, Tax Credits, Investment	(0.60%)		(0.90%)		(1.10%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes	(0.70%)		0.10%		1.60%
Effective Income Tax Rate Reconciliation, Tax Credits, Other	(9.40%)	[1]	(4.40%)	[1]	(4.70%)	[1]
Effective Income Tax Rate Reconciliation, Other Adjustments	(0.10%)		(0.20%)		(0.10%)
Effective Income Tax Rate, Continuing Operations	25.20%		30.90%		34.00%
Unrecognized Tax Benefits	0		0
Deferred Tax Assets, Operating Loss Carryforwards	113.8		0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	14.0		4.6
Current Deferred Tax Assets, Federal Tax Credits	6.0		0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Other	3.1		3.3
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts	1.0		1.4
Deferred Tax Assets, Derivative Instruments	0.8		0.9
Deferred Tax Assets, Tax Deferred Expense, Other	0		0.1
Deferred Tax Assets, Gross, Current	138.7		10.3
Deferred Tax Liabilities, Property, Plant and Equipment	1,648.5		1,425.1
Deferred Tax Liabilities, Prepaid Pension Costs	102.2		94.5
Deferred Tax Liabilities, Income Taxes Refundable to Customers, Net	21.2		28.0
Deferred Tax Liabilities, Regulatory Assets	18.8		21.2
Deferred Tax Liabilities, Unamortized Bond Redemption Costs	4.0		4.4
Deferred Tax Liabilities, Gross, Noncurrent	1,794.7		1,573.2
Non-Current Deferred Tax Asset, Net Operating Losses	(150.0)		(205.6)
Non-Current Deferred Tax Assets, State Tax Credits	(78.1)		(58.8)
Non-Current Deferred Tax Assets, Regulatory Liabilities	(71.4)		(65.3)
Deferred Tax Assets, Tax Credit Carryforwards	(69.5)		(49.6)
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits	(42.1)		(36.0)
Non-Current Deferred Tax Assets, Deferred Federal Investment Tax Credits	(1.5)		(2.4)
Non-Current Deferred Tax Assets, Derivative Instruments	0		(0.7)
Deferred Tax Assets, Other	(4.3)		(8.8)
Deferred Tax Assets, Gross, Noncurrent	(416.9)		(427.2)
Deferred Tax Liabilities, Net, Noncurrent	1,377.8		1,146.0
Tax Credit Carryforward, Amount							500.7		804.3		669.2		75.5	91.9	7.3		16.2
Tax Credit Carryforward, Deferred Tax Asset									29.6		234.2		75.5	59.7	7.3		11.1
Tax Credit Carryforward, Expiration Date								Dec. 31, 2030		Dec. 31, 2030		Dec. 31, 2029				Dec. 31, 2020
Income Tax Reconciliation, Tax Contingencies, State and Local																		$ 7.8	$ 5.1

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