Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match				200.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay				5.00%
Pension Contributions				$ 20.0	$ 0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net Periodic Benefit Cost				11.8	27.4	$ 14.2
Effect of One Percentage Point Increase on Service and Interest Cost Components				0.0	0.0	0.0
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation				0.1	0.1	0.1
Effect of One Percentage Point Decrease on Service and Interest Cost Components				0.0	0.0	0.0
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation				0.5	0.5	0.5
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months							$ 35.3
Defined Benefit Plan, Expected Future Benefit Payments, Year Two							37.0
Defined Benefit Plan, Expected Future Benefit Payments, Year Three							40.3
Defined Benefit Plan, Expected Future Benefit Payments, Year Four							42.3
Defined Benefit Plan, Expected Future Benefit Payments, Year Five							42.7
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter							210.0
Defined Contribution Plan, Cost Recognized				8.8	8.2	8.2
Restoration of Retirement Income Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement				0.4	0.0	0.0
Accumulated Benefit Obligation							3.4	$ 2.2
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning				2.7	2.8
Service cost				0.1	0.1	0.0
Interest cost				0.1	0.1	0.1
Plan settlements				1.0	0.0
Participants' contributions				0.0	0.0
Actuarial gains (losses)				2.1	(0.1)
Benefits paid				0.0	(0.2)
Benefit Obligation, Ending		$ 2.7		4.0	2.7	2.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0	0.0
Actual return on plans' assets				0.0	0.0
Employer contributions				1.0	0.2
Plan settlements				1.0	0.0
Participants' contributions				0.0	0.0
Benefits paid				0.0	(0.2)
Fair Value of Plan Assets, Ending		0.0		0.0	0.0	0.0
Funded Status of Plan							(4.0)	(2.7)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost				0.1	0.1	0.0
Interest cost				0.1	0.1	0.1
Expected return on plan assets				0.0	0.0	0.0
Amortization of net loss				0.1	0.1	0.1
Amortization of unrecognized prior service cost	[1]			0.0	0.0	0.1
Settlement				0.4	0.0	0.0
Net Periodic Benefit Cost	[2]			0.7	0.3	0.3
Fair Value of Plan Assets		0.0		0.0	0.0	0.0	0.0	0.0	$ 0.0
Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement		4.2	$ 10.0	0.0	14.2	0.0
Accumulated Benefit Obligation							458.1	470.6
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning				514.4	543.5
Service cost				10.3	10.3	9.1
Interest cost				19.2	19.8	21.4
Plan settlements				0.0	36.5
Participants' contributions				0.0	0.0
Actuarial gains (losses)				1.2	(8.6)
Benefits paid				(44.6)	(14.1)
Benefit Obligation, Ending		514.4		500.5	514.4	543.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				464.2	532.5
Actual return on plans' assets				37.7	(17.7)
Employer contributions				0.0	0.0
Plan settlements				0.0	36.5
Participants' contributions				0.0	0.0
Benefits paid				(44.6)	(14.1)
Fair Value of Plan Assets, Ending		464.2		457.3	464.2	532.5
Funded Status of Plan							$ (43.2)	$ (50.2)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost				10.3	10.3	9.1
Interest cost				19.2	19.8	21.4
Expected return on plan assets				(33.1)	(36.5)	(35.8)
Amortization of net loss				12.4	13.8	11.4
Amortization of unrecognized prior service cost	[1]			0.0	0.5	1.9
Settlement		4.2	$ 10.0	0.0	14.2	0.0
Net Periodic Benefit Cost	[2]			8.8	22.1	8.0
Capitalized Portion of Net Periodic Benefit Cost				$ 2.9	$ 2.6	$ 2.6
Assumptions Used Calculating Benefit Obligation, Discount Rate							4.00%	4.00%	3.80%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				7.50%	7.50%	7.50%
Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase				4.20%	4.20%	4.20%
Fair Value of Plan Assets		464.2		$ 464.2	$ 532.5	$ 532.5	$ 457.3	$ 464.2	$ 532.5
Pension Plans [Member] | OKLAHOMA
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement					12.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Settlement					12.5
Additional Pension Expense to Meet State Requirements				9.9	(3.1)	11.2
Pension Plans [Member] | ARKANSAS
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Plan settlements				(0.1)	(1.9)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan settlements				(0.1)	(1.9)
Pension Plans [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | U.S. common stocks [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				208.2
Fair Value of Plan Assets, Ending		208.2		237.1	208.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		208.2		208.2	208.2		237.1	208.2
Pension Plans [Member] | U.S. common stocks [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				208.2
Fair Value of Plan Assets, Ending		208.2		237.1	208.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		208.2		208.2	208.2		237.1	208.2
Pension Plans [Member] | U.S. common stocks [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	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | U.S. treasury notes and bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure	[3]						0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]			158.9
Fair Value of Plan Assets, Ending	[3]	158.9		122.3	158.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[3]	158.9		158.9	158.9		122.3	158.9
Pension Plans [Member] | U.S. treasury notes and bonds [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	[3]			158.9
Fair Value of Plan Assets, Ending	[3]	158.9		122.3	158.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[3]	158.9		158.9	158.9		122.3	158.9
Pension Plans [Member] | U.S. treasury notes and bonds [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	[3]			0.0
Fair Value of Plan Assets, Ending	[3]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[3]	0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Mortgage-backed securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				14.5
Fair Value of Plan Assets, Ending		14.5		59.2	14.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		14.5		14.5	14.5		59.2	14.5
Pension Plans [Member] | Mortgage-backed securities [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Mortgage-backed securities [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				14.5
Fair Value of Plan Assets, Ending		14.5		59.2	14.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		14.5		14.5	14.5		59.2	14.5
Pension Plans [Member] | Corporate fixed income and other securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending				137.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				137.6			137.6
Pension Plans [Member] | Corporate fixed income and other securities [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Corporate fixed income and other securities [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				140.2
Fair Value of Plan Assets, Ending		140.2		137.6	140.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		140.2		140.2	140.2		137.6	140.2
Pension Plans [Member] | Commingled fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure	[4]						23.8	24.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			24.4
Fair Value of Plan Assets, Ending	[4]	24.4		23.8	24.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	24.4		24.4	24.4		23.8	24.4
Pension Plans [Member] | Commingled fund [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	[4]			0.0
Fair Value of Plan Assets, Ending	[4]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Commingled fund [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	[4]			0.0
Fair Value of Plan Assets, Ending	[4]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Foreign government bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				5.6
Fair Value of Plan Assets, Ending		5.6		5.2	5.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		5.6		5.6	5.6		5.2	5.6
Pension Plans [Member] | Foreign government bonds [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Foreign government bonds [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				5.6
Fair Value of Plan Assets, Ending		5.6		5.2	5.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		5.6		5.6	5.6		5.2	5.6
Pension Plans [Member] | U.S. municipal bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				4.9
Fair Value of Plan Assets, Ending		4.9		1.9	4.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		4.9		4.9	4.9		1.9	4.9
Pension Plans [Member] | U.S. municipal bonds [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | U.S. municipal bonds [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				4.9
Fair Value of Plan Assets, Ending		4.9		1.9	4.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		4.9		4.9	4.9		1.9	4.9
Pension Plans [Member] | Money market funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							2.2	11.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				11.7
Fair Value of Plan Assets, Ending		11.7		2.2	11.7
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		11.7		11.7	11.7		2.2	11.7
Pension Plans [Member] | Money market funds [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Money market funds [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	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Mutual fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				24.3
Fair Value of Plan Assets, Ending		24.3		9.0	24.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		24.3		24.3	24.3		9.0	24.3
Pension Plans [Member] | Mutual fund [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				24.3
Fair Value of Plan Assets, Ending		24.3		9.0	24.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		24.3		24.3	24.3		9.0	24.3
Pension Plans [Member] | Mutual fund [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	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Treasury futures, receivable [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				17.6
Fair Value of Plan Assets, Ending		17.6		10.7	17.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		17.6		17.6	17.6		10.7	17.6
Pension Plans [Member] | Treasury futures, receivable [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Treasury futures, receivable [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				17.6
Fair Value of Plan Assets, Ending		17.6		10.7	17.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		17.6		17.6	17.6		10.7	17.6
Pension Plans [Member] | Treasury futures, payable [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(12.4)
Fair Value of Plan Assets, Ending		(12.4)		(2.3)	(12.4)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(12.4)		(12.4)	(12.4)		(2.3)	(12.4)
Pension Plans [Member] | Treasury futures, payable [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Treasury futures, payable [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				(12.4)
Fair Value of Plan Assets, Ending		(12.4)		(2.3)	(12.4)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(12.4)		(12.4)	(12.4)		(2.3)	(12.4)
Pension Plans [Member] | Cash collateral [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending				0.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				0.3			0.3
Pension Plans [Member] | Cash collateral [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending				0.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				0.3			0.3
Pension Plans [Member] | Cash collateral [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending				0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				0.0			0.0
Pension Plans [Member] | Receivable (foreign currency) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							0.0	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.1
Fair Value of Plan Assets, Ending		0.1		0.2	0.1
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.1		0.1	0.1		0.2	0.1
Pension Plans [Member] | Receivable (foreign currency) [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				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Receivable (foreign currency) [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				0.1
Fair Value of Plan Assets, Ending		0.1		0.2	0.1
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.1		0.1	0.1		0.2	0.1
Pension Plans [Member] | Payable (foreign currency) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure								0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(0.1)
Fair Value of Plan Assets, Ending		(0.1)			(0.1)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(0.1)		(0.1)	(0.1)			(0.1)
Pension Plans [Member] | Payable (foreign currency) [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				0.0
Fair Value of Plan Assets, Ending		0.0			0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0			0.0
Pension Plans [Member] | Payable (foreign currency) [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				(0.1)
Fair Value of Plan Assets, Ending		(0.1)			(0.1)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(0.1)		(0.1)	(0.1)			(0.1)
Pension Plans [Member] | Index fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure								0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				1.8
Fair Value of Plan Assets, Ending		1.8			1.8
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		1.8		1.8	1.8			1.8
Pension Plans [Member] | Index fund [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				1.8
Fair Value of Plan Assets, Ending		1.8			1.8
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		1.8		1.8	1.8			1.8
Pension Plans [Member] | Index fund [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
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0			0.0
Pension Plans [Member] | Interest-bearing cash [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure								0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.4
Fair Value of Plan Assets, Ending		0.4			0.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.4		0.4	0.4			0.4
Pension Plans [Member] | Interest-bearing cash [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				0.4
Fair Value of Plan Assets, Ending		0.4			0.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.4		0.4	0.4			0.4
Pension Plans [Member] | Interest-bearing cash [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
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0			0.0
Pension Plans [Member] | Preferred stock [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure								0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.3
Fair Value of Plan Assets, Ending		0.3			0.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.3		0.3	0.3			0.3
Pension Plans [Member] | Preferred stock [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				0.3
Fair Value of Plan Assets, Ending		0.3			0.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.3		0.3	0.3			0.3
Pension Plans [Member] | Preferred stock [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
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0			0.0
Pension Plans [Member] | Total Plan investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investments, Fair Value Disclosure							26.0	36.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				600.4
Fair Value of Plan Assets, Ending		600.4		607.2	600.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		600.4		600.4	600.4		607.2	600.4
Pension Plans [Member] | Total Plan investments [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				393.9
Fair Value of Plan Assets, Ending		393.9		368.7	393.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		393.9		393.9	393.9		368.7	393.9
Pension Plans [Member] | Total Plan investments [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				170.4
Fair Value of Plan Assets, Ending		170.4		212.5	170.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		170.4		170.4	170.4		212.5	170.4
Pension Plans [Member] | Receivable from broker for securities sold [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	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans [Member] | Interest and dividends receivable [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				3.5
Fair Value of Plan Assets, Ending		3.5		3.0	3.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		3.5		3.5	3.5		3.0	3.5
Pension Plans [Member] | Payable to broker for securities purchased [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(22.2)
Fair Value of Plan Assets, Ending		(22.2)		(14.3)	(22.2)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(22.2)		(22.2)	(22.2)		(14.3)	(22.2)
Pension Plans [Member] | Plan investments attributable to affiliates [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(117.5)
Fair Value of Plan Assets, Ending		(117.5)		(138.6)	(117.5)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(117.5)		(117.5)	(117.5)		(138.6)	(117.5)
Pension Plans [Member] | Total Plan assets [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				464.2
Fair Value of Plan Assets, Ending		464.2		457.3	464.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		464.2		464.2	464.2		457.3	464.2
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement				0.0	0.0	0.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning				176.1	217.6
Service cost				0.6	1.0	2.0
Interest cost				7.4	7.9	8.8
Plan settlements				0.0	0.0
Participants' contributions				2.9	2.6
Actuarial gains (losses)				(7.2)	(40.0)
Benefits paid				(13.4)	(13.0)
Benefit Obligation, Ending		176.1		166.4	176.1	217.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				50.0	54.2
Actual return on plans' assets				1.8	(0.5)
Employer contributions				6.5	6.7
Plan settlements				0.0	0.0
Participants' contributions				2.9	2.6
Benefits paid				(13.4)	(13.0)
Fair Value of Plan Assets, Ending		50.0		47.8	50.0	54.2
Funded Status of Plan							$ (118.6)	$ (126.1)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost				0.6	1.0	2.0
Interest cost				7.4	7.9	8.8
Expected return on plan assets				(2.1)	(2.2)	(2.2)
Amortization of net loss				2.5	11.9	11.0
Amortization of unrecognized prior service cost	[1]			(6.1)	(13.6)	(13.7)
Settlement				0.0	0.0	0.0
Net Periodic Benefit Cost	[2]			2.3	5.0	5.9
Capitalized Portion of Net Periodic Benefit Cost				$ 0.8	$ 1.6	$ 1.8
Assumptions Used Calculating Benefit Obligation, Discount Rate							4.20%	4.25%	3.80%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				4.00%	4.00%	4.00%
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
Fair Value of Plan Assets		50.0		$ 50.0	$ 54.2	$ 54.2	$ 47.8	$ 50.0	$ 54.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Postretirement Medical Expense to Meet State Requirements				7.9	5.8	$ 5.2
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
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				0.3			0.3
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			0.7
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.7		0.7	0.7			0.7
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		8.5		8.4	8.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		8.5		8.5	8.5		8.4	8.5
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
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				0.3			0.3
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			0.7
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.7		0.7	0.7			0.7
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	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		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		46.8		44.7	46.8
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		46.8		46.8	46.8		44.7	46.8
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
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets				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			0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0			0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Total Plan investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				55.3
Fair Value of Plan Assets, Ending		55.3		53.1	55.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		55.3		$ 55.3	55.3		53.1	55.3
Less Than 90% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				50.00%
Projected Benefit Obligation Funded Status Thresholds Equity				50.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
95% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				58.00%
Projected Benefit Obligation Funded Status Thresholds Equity				42.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
100% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				65.00%
Projected Benefit Obligation Funded Status Thresholds Equity				35.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
105% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				73.00%
Projected Benefit Obligation Funded Status Thresholds Equity				27.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
110% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				80.00%
Projected Benefit Obligation Funded Status Thresholds Equity				20.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
115% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				85.00%
Projected Benefit Obligation Funded Status Thresholds Equity				15.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
120% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				90.00%
Projected Benefit Obligation Funded Status Thresholds Equity				10.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
Domestic All-Cap/Large Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				40.00%
Target Plan Asset Allocations Range Minimum				35.00%
Target Plan Asset Allocations Range Maximum				60.00%
Domestic Mid-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				15.00%
Target Plan Asset Allocations Range Minimum				5.00%
Target Plan Asset Allocations Range Maximum				25.00%
Domestic Small-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				25.00%
Target Plan Asset Allocations Range Minimum				5.00%
Target Plan Asset Allocations Range Maximum				30.00%
International Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				20.00%
Target Plan Asset Allocations Range Minimum				10.00%
Target Plan Asset Allocations Range Maximum				30.00%
OG&E [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Contributions				$ 0.0
OG&E [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				50.0
Fair Value of Plan Assets, Ending		50.0		47.8	50.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		$ 50.0		$ 50.0	$ 50.0		$ 47.8	$ 50.0

[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 $11.8 million, $27.4 million and $14.2 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 $0.4 million and $14.2 million, respectively.

[3] This category represents U.S. treasury notes and bonds with a Moody's Investors Services rating of Aaa and Government Agency Bonds with a Moody's Investors Services rating of A1 or higher.

[4] This category represents units of participation in a commingled fund that primarily invested in stocks of international companies and emerging markets.