Retirement Plans and Postretirement Benefit Plans Postretirement Benefit Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost		$ 22.5	$ 40.2	$ 19.3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 Asset Value, Beginning of Period		46.8
Interest income		0.9
Dividend income		0.6
Unrealized gains		0.2
Realized losses		(0.1)
Claims paid		(3.7)
Level 3 Asset Value, End of Period		44.7	46.8
Postretirement Plan, Expected Future Benefit Payments, Next Twelve Months		14.0
Postretirement Plan, Expected Future Benefit Payments, Year Two		14.1
Postretirement Plan, Expected Future Benefit Payments, Year Three		14.1
Postretirement Plan, Expected Future Benefit Payments in Year Four		14.1
Postretirement Plan, Expected Future Benefit Payments, Year Five		14.1
Postretirement Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter		69.1
Postemployment Benefits Liability		2.4	1.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		225.3	280.9
Service cost		0.8	1.5	3.1
Interest cost		9.5	10.3	11.4
Plan settlements		0.0	0.0
Participants' contributions		3.6	3.4
Actuarial gains (losses)		(7.6)	(55.1)
Benefits paid		(15.7)	(15.7)
Benefit Obligation, Ending		215.9	225.3	280.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		55.3	59.6
Actual return on plans' assets		2.0	(0.5)
Employer contributions		7.9	8.5
Plan settlements		0.0	0.0
Participants' contributions		3.6	3.4
Benefits paid		(15.7)	(15.7)
Fair Value of Plan Assets, Ending		53.1	55.3	59.6
Funded Status of Plan		(162.8)	(170.0)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.8	1.5	3.1
Interest cost		9.5	10.3	11.4
Expected return on plan assets		(2.3)	(2.4)	(2.4)
Amortization of net loss		2.6	13.9	12.3
Amortization of unrecognized prior service cost	[1]	(8.8)	(16.5)	(16.5)
Curtailment		0.0	0.0	0.0
Settlement		0.0	0.0	0.0
Net periodic benefit cost		1.8	6.8	7.9
Amount paid by unconsolidated affiliates		0.2	1.3	1.3
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]	1.6	5.5	6.6
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Capitalized Portion of Net Periodic Benefit Cost		$ 0.8	$ 1.9	$ 2.0
Assumptions Used Calculating Benefit Obligation, Discount Rate		4.20%	4.25%	3.80%
Health Care Cost Trend Rate Assumed for Next Fiscal Year		6.75%	6.10%	7.85%
Ultimate Health Care Cost Trend Rate		4.50%	4.50%	4.48%
Year that Rate Reaches Ultimate Trend Rate		2026	2026	2028
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		4.00%	4.00%	4.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]	$ 46.8
Fair Value of Plan Assets, Ending	[3]	44.7	$ 46.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		7.8
Fair Value of Plan Assets, Ending		8.1	7.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Money market funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.7
Fair Value of Plan Assets, Ending			0.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.3
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		8.5
Fair Value of Plan Assets, Ending		8.4	8.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]	0.0
Fair Value of Plan Assets, Ending	[3]	0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		7.8
Fair Value of Plan Assets, Ending		8.1	7.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Money market funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.7
Fair Value of Plan Assets, Ending			0.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.3
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		46.8
Fair Value of Plan Assets, Ending		44.7	46.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]	46.8
Fair Value of Plan Assets, Ending	[3]	44.7	46.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Money market funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending			0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.0
OKLAHOMA | Other Postretirement Benefit Plans, Defined Benefit [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Additional Postretirement Medical Expense to Meet State Requirements		$ 7.9	$ 5.8	$ 5.2

[1] Unamortized prior service cost is amortized on a straight-line basis over the average remaining service period to the first eligibility age of participants who are expected to receive a benefit and are active at the date of the plan amendment.

[2] In addition to the $22.5 million, $40.2 million and $19.3 million of net periodic benefit cost recognized in 2016, 2015 and 2014, respectively, OG&E recognized the following: •a change in pension expense in 2016, 2015 and 2014 of $9.9 million, $(3.1) million and $11.2 million, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory asset or liability (see Note 1); •an increase in postretirement medical expense in 2016, 2015 and 2014 of $7.9 million, $5.8 million and $5.2 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction which are included in the Pension tracker regulatory asset or liability (see Note 1); and •a deferral of pension expense in 2016 and 2015 of $0.1 million and $1.9 million related to the Arkansas jurisdictional portion of the pension settlement charge of $8.6 million and $21.7 million, respectively.

[3] This category represents a group retiree medical insurance contract which invests in a pool of common stocks, bonds and money market accounts, of which a significant portion is comprised of mortgage-backed securities.