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)		$ 0.0	$ 0.0	$ 0.0
Current State and Local Tax Expense (Benefit)		(5.2)	(4.5)	4.3
Current Income Tax Expense (Benefit)		(5.2)	(4.5)	4.3
Deferred Federal Income Tax Expense (Benefit)		98.8	160.0	154.4
Deferred State and Local Income Tax Expense (Benefit)		4.5	18.2	(26.4)
Deferred Income Tax Expense (Benefit)		103.3	178.2	128.0
Investment Tax Credit		(0.7)	(0.9)	(2.0)
Income Tax Expense (Benefit)		$ 97.4	$ 172.8	$ 130.3
Statutory Federal tax rate		35.00%	35.00%	35.00%
Amortization of net unfunded deferred taxes		0.90%	0.60%	0.60%
State income taxes, net of Federal income tax benefit		0.10%	1.20%	0.40%
Federal investment tax credits, net		(0.20%)	(0.20%)	(0.40%)
401(k) dividends		(0.70%)	(0.50%)	(0.50%)
Income attributable to noncontrolling interest		0.00%	0.00%	(0.30%)
Federal renewable energy credit	[1]	(8.90%)	(6.70%)	(7.20%)
Uncertain tax positions		0.70%	0.50%	1.50%
Remeasurement of state deferred tax liabilities		(0.80%)	0.40%	(4.10%)
Other		0.30%	0.10%	(0.10%)
Effective income tax rate		26.40%	30.40%	24.90%
Net operating losses			$ 158.4
Accrued liabilities		$ (14.0)	(15.6)
Federal tax credits			12.4
Accrued vacation		(3.2)	(4.4)
Uncollectible accounts		(0.5)	(0.6)
Total Current Deferred Income Tax Assets			191.4
Accelerated depreciation and other property related differences		2,016.0	1,936.8
Investment in Enable Midstream Partners		623.4	641.8
Regulatory asset		32.7	24.7
Income taxes refundable to customers, net		22.0	21.7
Company pension plan		13.7	34.6
Bond redemption-unamortized costs		4.8	5.3
Derivative instruments		1.5	1.8
Federal tax credits		(184.4)	(139.0)
State tax credits		(106.7)	(98.6)
Net operating losses		(94.6)	(19.8)
Postretirement medical and life insurance benefits		(56.2)	(56.4)
Regulatory liabilities		(46.3)	(58.0)
Asset retirement obligations		(22.5)	(21.4)
Deferred Federal investment tax credits		(0.9)	(0.4)
Other		(6.6)	(4.8)
Non-Current Deferred Income Tax Liabilities, net		2,178.2	2,268.3
Unrecognized Tax Benefits		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		184.4	$ 139.0
Employee Service Share-based Compensation, Unrecognized Tax Benefit from Compensation Expense		28.3
State operating loss [Member]
Operating Loss Carryforwards		695.5
Deferred Tax Assets, Operating Loss Carryforwards, State and Local		$ 25.6
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal operating loss [Member]
Operating Loss Carryforwards		$ 197.3
Net operating losses		$ 69.0
Tax Credit Carryforward, Expiration Date		Dec. 31, 2030
Federal tax credits [Member]
Federal tax credits		$ (184.4)
Tax Credit Carryforward, Amount		184.4
Tax Credit Carryforward, Deferred Tax Asset		$ 184.4
Tax Credit Carryforward, Expiration Date		Dec. 31, 2029
Oklahoma investment tax credits [Member]
Federal tax credits		$ (83.0)
Tax Credit Carryforward, Amount		127.8
Tax Credit Carryforward, Deferred Tax Asset		83.0
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		(16.4)
Tax Credit Carryforward, Amount		24.3
Tax Credit Carryforward, Deferred Tax Asset		$ 16.4
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.