Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		9 Months Ended		12 Months Ended
		Sep. 30, 2019		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions				$ 20.0		$ 15.0
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		200.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay		5.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions				20.0		15.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
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				0.2		0.2		0.2
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months									$ 44.6
Defined Benefit Plan, Expected Future Benefit Payment, Year Two									44.2
Defined Benefit Plan, Expected Future Benefit Payment, Year Three									42.5
Defined Benefit Plan, Expected Future Benefit Payment, Year Four									42.2
Defined Benefit Plan, Expected Future Benefit Payment, Year Five									42.0
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter									179.8
Defined Contribution Plan, Cost				11.0		9.8		9.7
Postemployment Benefits Liability									1.7		$ 1.7
Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		$ 0.9		0.9
Fair Value of Plan Assets, Ending						0.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.9		0.9		0.9					0.9
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		0.9		0.9
Fair Value of Plan Assets, Ending						0.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.9		0.9		0.9					0.9
Cash [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		0.0
Fair Value of Plan Assets, Ending						0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.0		0.0		0.0					0.0
Restoration of Retirement Income Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant				0.0		0.0
Employer contributions				0.9		0.6
Defined Benefit Plan, Plan Assets, Benefits Paid				0.0		0.0
Defined Benefit Plan, Benefit Obligation, Benefits Paid				0.0		0.0
Settlement				0.5		0.4		0.0
Accumulated Benefit Obligation									4.8		5.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		6.0		6.0		4.2
Service cost				0.2		0.2		0.1
Interest cost				0.2		0.2		0.2
Plan settlements				0.9		0.6
Participants' contributions				0.0		0.0
Actuarial gains (losses)				0.6		2.0
Benefit Obligation, Ending				6.1		6.0		4.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0		0.0		0.0
Actual return on plans' assets				0.0		0.0
Employer contributions				0.9		0.6
Plan settlements				0.9		0.6
Fair Value of Plan Assets, Ending				0.0		0.0		0.0
Funded Status of Plan									(6.1)		(6.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost				0.2		0.2		0.1
Interest cost				0.2		0.2		0.2
Expected return on plan assets				0.0		0.0		0.0
Defined Benefit Plan, Amortization of Gain (Loss)				(0.3)		(0.5)		(0.4)
Amortization of unrecognized prior service cost	[1]			0.0		0.0		0.0
Settlement				0.5		0.4		0.0
Net Periodic Benefit Cost				1.2		1.3		0.7
Amount Attributable to Unconsolidated Affiliates				0.5		1.2		0.0
Fair Value of Plan Assets		0.0		0.0		0.0		0.0	0.0		0.0		$ 0.0
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates				1.7		2.5		0.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement				(0.5)		(0.4)		0.0
Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant				0.0		0.0
Employer contributions				5.0		5.0
Defined Benefit Plan, Plan Assets, Benefits Paid				(12.7)		(12.8)
Defined Benefit Plan, Benefit Obligation, Benefits Paid				(12.7)		(12.8)
Settlement				16.4		19.4		11.7
Accumulated Benefit Obligation									425.8		417.6
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		453.6		453.6		510.6
Service cost				9.0		9.8		10.1
Interest cost				15.6		17.6		19.5
Plan settlements				45.6		52.6
Participants' contributions				0.0		0.0
Actuarial gains (losses)				42.1		(19.0)
Benefit Obligation, Ending				462.0		453.6		510.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		$ 387.6		387.6		477.2
Actual return on plans' assets				64.8		(29.2)
Employer contributions				5.0		5.0
Plan settlements				45.6		52.6
Fair Value of Plan Assets, Ending				399.1		387.6		477.2
Funded Status of Plan									$ (62.9)		$ (66.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost				9.0		9.8		10.1
Interest cost				15.6		17.6		19.5
Expected return on plan assets				(27.6)		(33.1)		(32.8)
Defined Benefit Plan, Amortization of Gain (Loss)				(12.9)		(12.1)		(13.0)
Amortization of unrecognized prior service cost	[1]			0.0		0.0		0.0
Settlement				16.4		19.4		11.7
Net Periodic Benefit Cost				26.3		25.8		21.5
Amount Attributable to Unconsolidated Affiliates				4.5		5.7		0.0
Capitalized Portion of Net Periodic Benefit Cost				$ 3.0		$ 3.2		$ 3.4
Assumptions Used Calculating Benefit Obligation, Discount Rate									3.15%		4.20%		3.60%
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%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase									4.20%		4.20%		4.20%
Fair Value of Plan Assets		$ 387.6		$ 387.6		$ 387.6		$ 477.2	$ 399.1		$ 387.6		$ 477.2
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates				30.8		31.5		21.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement				(16.4)		(19.4)		(11.7)
Pension Plans [Member] | OKLAHOMA
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement	[2]			(17.9)		(22.1)		(13.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement	[2]			(17.9)		(22.1)		(13.2)
Additional Pension Expense to Meet State Requirements	[2]			(16.1)		(14.1)		(2.3)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	[2]			17.9		22.1		13.2
Additional Pension Expense to Meet State Requirements	[2]			(16.1)		(14.1)		(2.3)
Pension Plans [Member] | ARKANSAS
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement	[2]			1.7		2.1		1.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement	[2]			1.7		2.1		1.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	[2]			(1.7)		(2.1)		(1.1)
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.0		0.0		0.0			0.0		0.0
Pension Plans [Member] | Money market funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									7.5	[3]	0.3	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.3		0.3
Fair Value of Plan Assets, Ending				7.5		0.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.3		0.3		0.3			7.5		0.3
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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 Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		169.3		169.3
Fair Value of Plan Assets, Ending				202.0		169.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		169.3		169.3		169.3			202.0		169.3
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		169.3		169.3
Fair Value of Plan Assets, Ending				202.0		169.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		169.3		169.3		169.3			202.0		169.3
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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 Investment									0.0	[3],[5]	0.0	[4],[6]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[6]	137.9		137.9
Fair Value of Plan Assets, Ending				134.8	[5]	137.9	[6]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		137.9	[6]	137.9	[6]	137.9	[6]		134.8	[5]	137.9	[6]
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	[6]	137.9		137.9
Fair Value of Plan Assets, Ending				134.8	[5]	137.9	[6]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		137.9	[6]	137.9	[6]	137.9	[6]		134.8	[5]	137.9	[6]
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	[6]	0.0		0.0
Fair Value of Plan Assets, Ending				0.0	[5]	0.0	[6]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.0	[6]	0.0	[6]	0.0	[6]		0.0	[5]	0.0	[6]
Pension Plans [Member] | Mortgage-backed securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		65.9		65.9
Fair Value of Plan Assets, Ending				45.8		65.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		65.9		65.9		65.9			45.8		65.9
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		65.9		65.9
Fair Value of Plan Assets, Ending				45.8		65.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		65.9		65.9		65.9			45.8		65.9
Pension Plans [Member] | Corporate fixed income and other securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		143.2		143.2
Fair Value of Plan Assets, Ending				130.5		143.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		143.2		143.2		143.2			130.5		143.2
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		143.2		143.2
Fair Value of Plan Assets, Ending				130.5		143.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		143.2		143.2		143.2			130.5		143.2
Pension Plans [Member] | Commingled fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									23.9	[3],[7]	19.7	[4],[8]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[8]	19.7		19.7
Fair Value of Plan Assets, Ending				23.9	[7]	19.7	[8]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		19.7	[8]	19.7	[8]	19.7	[8]		23.9	[7]	19.7	[8]
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	[8]	0.0		0.0
Fair Value of Plan Assets, Ending				0.0	[7]	0.0	[8]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.0	[8]	0.0	[8]	0.0	[8]		0.0	[7]	0.0	[8]
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	[8]	0.0		0.0
Fair Value of Plan Assets, Ending				0.0	[7]	0.0	[8]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.0	[8]	0.0	[8]	0.0	[8]		0.0	[7]	0.0	[8]
Pension Plans [Member] | Foreign government bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		4.4		4.4
Fair Value of Plan Assets, Ending				3.0		4.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		4.4		4.4		4.4			3.0		4.4
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		4.4		4.4
Fair Value of Plan Assets, Ending				3.0		4.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		4.4		4.4		4.4			3.0		4.4
Pension Plans [Member] | U.S. municipal bonds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.6		0.6
Fair Value of Plan Assets, Ending				1.1		0.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.6		0.6		0.6			1.1		0.6
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		0.6		0.6
Fair Value of Plan Assets, Ending				1.1		0.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.6		0.6		0.6			1.1		0.6
Pension Plans [Member] | Mutual fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		8.0		8.0
Fair Value of Plan Assets, Ending				2.4		8.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		8.0		8.0		8.0			2.4		8.0
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		8.0		8.0
Fair Value of Plan Assets, Ending				2.4		8.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		8.0		8.0		8.0			2.4		8.0
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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 Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		27.0		27.0
Fair Value of Plan Assets, Ending				22.9		27.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		27.0		27.0		27.0			22.9		27.0
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		27.0		27.0
Fair Value of Plan Assets, Ending				22.9		27.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		27.0		27.0		27.0			22.9		27.0
Pension Plans [Member] | Treasury futures, payable [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		(20.4)		(20.4)
Fair Value of Plan Assets, Ending				(10.9)		(20.4)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		(20.4)		(20.4)		(20.4)			(10.9)		(20.4)
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		(20.4)		(20.4)
Fair Value of Plan Assets, Ending				(10.9)		(20.4)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		(20.4)		(20.4)		(20.4)			(10.9)		(20.4)
Pension Plans [Member] | Cash collateral [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.7		0.7
Fair Value of Plan Assets, Ending				0.6		0.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.7		0.7		0.7			0.6		0.7
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, Beginning		0.7		0.7
Fair Value of Plan Assets, Ending				0.6		0.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.7		0.7		0.7			0.6		0.7
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, Beginning		0.0		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.0		0.0		0.0			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 Investment									0.0	[3]	0.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.1		0.1
Fair Value of Plan Assets, Ending				0.1		0.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.1		0.1		0.1			0.1		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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		0.1
Fair Value of Plan Assets, Ending				0.1		0.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		0.1		0.1		0.1			0.1		0.1
Pension Plans [Member] | Preferred stock [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment	[3]								0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending				0.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets				0.7					0.7
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, Ending				0.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets				0.7					0.7
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, Ending				0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets				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		3.0		3.0
Fair Value of Plan Assets, Ending				2.4		3.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		3.0		3.0		3.0			2.4		3.0
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		(36.9)		(36.9)
Fair Value of Plan Assets, Ending				(36.5)		(36.9)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		(36.9)		(36.9)		(36.9)			(36.5)		(36.9)
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		(135.2)		(135.2)
Fair Value of Plan Assets, Ending				(131.2)		(135.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		(135.2)		(135.2)		(135.2)			(131.2)		(135.2)
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		387.6		387.6
Fair Value of Plan Assets, Ending				399.1		387.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		387.6		387.6		387.6			399.1		387.6
Pension Plans [Member] | Total Plan investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment									31.4	[3]	20.0	[4]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		556.7		556.7
Fair Value of Plan Assets, Ending				564.4		556.7
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		556.7		556.7		556.7			564.4		556.7
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		315.9		315.9
Fair Value of Plan Assets, Ending				340.5		315.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		315.9		315.9		315.9			340.5		315.9
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		220.8		220.8
Fair Value of Plan Assets, Ending				192.5		220.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		220.8		220.8		220.8			192.5		220.8
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant				3.0		2.9
Employer contributions				4.1		4.8
Defined Benefit Plan, Plan Assets, Benefits Paid				(9.8)		(11.8)
Defined Benefit Plan, Benefit Obligation, Benefits Paid				(9.8)		(11.8)
Settlement				0.0		0.0		0.4
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		104.8		104.8		115.8
Service cost				0.2		0.2		0.4
Interest cost				4.3		4.2		5.6
Plan settlements				0.0		0.0
Participants' contributions				3.0		2.9
Actuarial gains (losses)				2.2		(6.5)
Benefit Obligation, Ending				104.7		104.8		115.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		40.6		40.6		45.2
Actual return on plans' assets				4.0		(0.5)
Employer contributions				4.1		4.8
Plan settlements				0.0		0.0
Fair Value of Plan Assets, Ending				41.9		40.6		45.2
Funded Status of Plan									$ (62.8)		$ (64.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost				0.2		0.2		0.4
Interest cost				4.3		4.2		5.6
Expected return on plan assets				(1.7)		(1.8)		(2.0)
Defined Benefit Plan, Amortization of Gain (Loss)				(2.1)		(3.8)		(1.9)
Amortization of unrecognized prior service cost	[1]			(6.1)		(6.1)		(2.5)
Settlement				0.0		0.0		0.4
Net Periodic Benefit Cost				(1.2)		0.3		3.8
Amount Attributable to Unconsolidated Affiliates				(0.6)		(0.7)		0.0
Capitalized Portion of Net Periodic Benefit Cost				$ 0.1		$ 0.1		$ 1.1
Assumptions Used Calculating Benefit Obligation, Discount Rate									3.25%		4.30%		3.70%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				4.00%		4.00%		4.00%
Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase								4.20%
Health Care Cost Trend Rate Assumed for Next Fiscal Year											7.00%
Fair Value of Plan Assets		40.6		$ 41.9		$ 45.2		$ 45.2	$ 41.9		$ 40.6		$ 45.2
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates				(1.8)		(0.4)		3.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement				0.0		0.0		(0.4)
Other Postretirement Benefit Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements	[2]			1.0		4.4		6.2
Additional Pension Expense to Meet State Requirements	[2]			1.0		4.4		$ 6.2
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		9.8		9.8
Fair Value of Plan Assets, Ending				12.1		9.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		9.8		9.8		9.8			12.1		9.8
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		0.0
Fair Value of Plan Assets, Ending				0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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		36.0		36.0
Fair Value of Plan Assets, Ending				34.8		36.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		36.0		36.0		36.0			34.8		36.0
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, Ending				1.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets				1.2					1.2
Other Postretirement Benefit Plans, Defined Benefit [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, Ending				1.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets				1.2					1.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Money market funds [Member] | Fair Value, Inputs, Level 3 [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 (Credit) [Abstract]
Fair Value of Plan Assets				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		45.8		45.8
Fair Value of Plan Assets, Ending				46.9		45.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		45.8		$ 45.8		45.8			$ 46.9		45.8
Less Than 90% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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 (Credit) [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 (Credit) [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 (Credit) [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 (Credit) [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 (Credit) [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 (Credit) [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 (Credit) [Abstract]
Target Plan Asset Allocations									40.00%
Domestic Mid-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									15.00%
Domestic Small-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									25.00%
International Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									20.00%
OG&E [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions		5.0		$ 5.0		5.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions		5.0		5.0		5.0
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		40.6		40.6
Fair Value of Plan Assets, Ending				41.9		40.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets		$ 40.6		$ 40.6		$ 40.6			$ 41.9		$ 40.6
Minimum [Member] | Domestic All-Cap/Large Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									35.00%
Minimum [Member] | Domestic Mid-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									5.00%
Minimum [Member] | Domestic Small-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									5.00%
Minimum [Member] | International Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									10.00%
Maximum [Member] | Domestic All-Cap/Large Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									60.00%
Maximum [Member] | Domestic Mid-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									25.00%
Maximum [Member] | Domestic Small-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									30.00%
Maximum [Member] | International Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Target Plan Asset Allocations									30.00%

[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] Included in the pension regulatory asset or liability in each jurisdiction, as indicated in the regulatory assets and liabilities table in Note 1.

[3] (A)GAAP allows the measurement of certain investments that do not have a readily determinable fair value at the net asset value. These investments do not consider the observability of inputs; therefore, they are not included within the fair value hierarchy.

[4] GAAP allows the measurement of certain investments that do not have a readily determinable fair value at the net asset value. These investments do not consider the observability of inputs; therefore, they are not included within the fair value hierarchy.

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

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

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

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