Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2021	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
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%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions		$ 40.0	$ 20.0		$ 20.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Ultimate Health Care Cost Trend Rate			4.50%
Expected Future Benefit Payments, Next Twelve Months			$ 164.2
Expected Future Benefit Payments, Year Two			43.3
Expected Future Benefit Payments, Year Three			42.5
Expected Future Benefit Payments, Year Four			44.2
Expected Future Benefit Payments, Year Five			41.1
Expected Future Benefit Payments, Five Fiscal Years Thereafter			203.6
Defined Contribution Plan, Cost			18.2		14.4		$ 13.2
Postemployment Benefits Liability			2.2		2.1
Og and E [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions			10.0		5.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Expected Future Benefit Payments, Next Twelve Months			123.9
Expected Future Benefit Payments, Year Two			33.1
Expected Future Benefit Payments, Year Three			31.9
Expected Future Benefit Payments, Year Four			32.3
Expected Future Benefit Payments, Year Five			29.3
Expected Future Benefit Payments, Five Fiscal Years Thereafter			141.1
Defined Contribution Plan, Cost			14.3		11.0		9.8
Postemployment Benefits Liability			1.8		1.7
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Benefits Paid			(14.2)		(14.6)
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment			0.0		0.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		654.6	616.1		615.9
Service cost			13.2		12.9		14.9
Interest cost			17.0		20.7		23.8
Plan curtailments			0.0		0.0
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			7.6		0.0
Plan settlements			(42.8)		(83.1)
Actuarial losses			57.7		64.3
Benefit Obligation, Ending			654.6		616.1		615.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		570.3	530.3		522.8
Actual return on plans' assets			77.0		85.2
Employer contributions			20.0		20.0
Plan settlements			(42.8)		(83.1)
Fair Value of Plan Assets, Ending			570.3		530.3		522.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			13.2		12.9		14.9
Interest cost			17.0		20.7		23.8
Expected return on plan assets			(37.6)		(36.1)		(44.1)
Defined Benefit Plan, Amortization of Gain (Loss)			(17.1)		(17.3)		(16.2)
Amortization of unrecognized prior service cost	[1]		0.0		0.0		0.0
Curtailment			0.0		0.0		0.0
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits			7.6		0.0		0.0
Settlement cost			14.1		27.6		25.1
Net periodic benefit cost			31.4		42.4		35.9
Amount paid by unconsolidated affiliates	[2]		2.0		2.9		2.5
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates			29.4		39.5		33.4
Plan settlements			(42.8)		(83.1)
Capitalized Portion of Net Periodic Benefit Cost			$ 3.8		$ 3.6		$ 3.8
Assumptions Used Calculating Benefit Obligation, Discount Rate			2.30%		3.15%		4.20%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets			7.50%		7.50%		7.50%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase			4.20%		4.20%		4.20%
Fair Value of Plan Assets, Beginning		570.3	$ 530.3		$ 522.8
Defined Benefit Plan, Benefit Obligation, Benefits Paid			(14.2)		(14.6)
Accumulated Benefit Obligation			(610.8)		(563.3)
Pension Plans, Defined Benefit [Member] | Og and E [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Benefits Paid			(12.3)		(12.7)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		484.1	462.0		453.6
Service cost			9.2		9.0		$ 9.8
Interest cost			12.6		15.6		17.6
Plan curtailments			0.0		0.0
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			5.1		0.0
Plan settlements			(33.5)		(45.6)
Actuarial losses			41.0		42.1
Benefit Obligation, Ending			484.1		462.0		453.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		420.3	399.1		387.6
Actual return on plans' assets			57.0		64.8
Employer contributions			10.0		5.0
Plan settlements			(33.5)		(45.6)
Fair Value of Plan Assets, Ending			420.3		399.1		387.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			9.2		9.0		9.8
Interest cost			12.6		15.6		17.6
Expected return on plan assets			(27.9)		(27.6)		(33.1)
Defined Benefit Plan, Amortization of Gain (Loss)			(12.1)		(12.9)		(12.1)
Amortization of unrecognized prior service cost			0.0		0.0		0.0
Curtailment			0.0		0.0		0.0
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits			5.1		0.0		0.0
Settlement cost			11.4		16.4		19.4
Net periodic benefit cost			22.5		26.3		25.8
Amount Attributable to Parent	[2]		5.9		4.5		5.7
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates			28.4		30.8		31.5
Plan settlements			(33.5)		(45.6)
Capitalized Portion of Net Periodic Benefit Cost			3.1		3.0		3.2
Fair Value of Plan Assets, Beginning		420.3	399.1		387.6
Defined Benefit Plan, Benefit Obligation, Benefits Paid			(12.3)		(12.7)
Accumulated Benefit Obligation			(454.7)		(425.8)
Pension Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[3]		21.6		17.9		22.1
Additional Pension Expense to Meet State Requirements			(13.8)	[3]	(16.1)		(14.1)
Pension Plans, Defined Benefit [Member] | ARKANSAS
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[3]		2.0		1.7		2.1
Pension 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		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, Beginning		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, Benefits Paid			0.0		0.0
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment			0.0		0.3
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		7.8	10.3		9.6
Service cost			0.8		0.5		0.4
Interest cost			0.2		0.4		0.3
Plan curtailments			0.2		0.0
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			0.0		0.0
Plan settlements			(5.3)		(1.2)
Actuarial losses			1.6		0.7
Benefit Obligation, Ending			7.8		10.3		9.6
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			5.3		1.2
Plan settlements			(5.3)		(1.2)
Fair Value of Plan Assets, Ending			0.0		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			0.8		0.5		0.4
Interest cost			0.2		0.4		0.3
Expected return on plan assets			0.0		0.0		0.0
Defined Benefit Plan, Amortization of Gain (Loss)			(0.5)		(0.5)		(0.7)
Amortization of unrecognized prior service cost	[1]		0.0		0.0		0.1
Curtailment			0.2		0.0		0.0
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits			0.0		0.0		0.0
Settlement cost			2.7		0.5		1.0
Net periodic benefit cost			4.4		1.9		2.5
Amount paid by unconsolidated affiliates	[2]		0.1		0.1		0.1
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates			4.3		1.8		2.4
Plan settlements			(5.3)		(1.2)
Fair Value of Plan Assets, Beginning		0.0	0.0		0.0
Defined Benefit Plan, Benefit Obligation, Benefits Paid			0.0		0.0
Accumulated Benefit Obligation			(6.9)		(8.1)
Restoration of Retirement Income Plan [Member] | Og and E [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Benefits Paid			0.0		0.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		3.0	6.1		6.0
Service cost			0.1		0.2		0.2
Interest cost			0.1		0.2		0.2
Plan curtailments			0.0		0.0
Defined Benefit Plan, Benefit Obligation, Special and Contractual Termination Benefits			0.0		0.0
Plan settlements			(4.5)		(0.9)
Actuarial losses			1.2		0.6
Benefit Obligation, Ending			3.0		6.1		6.0
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			4.5		0.9
Plan settlements			(4.5)		(0.9)
Fair Value of Plan Assets, Ending			0.0		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			0.1		0.2		0.2
Interest cost			0.1		0.2		0.2
Expected return on plan assets			0.0		0.0		0.0
Defined Benefit Plan, Amortization of Gain (Loss)			(0.4)		(0.3)		(0.5)
Amortization of unrecognized prior service cost			0.0		0.0		0.0
Curtailment			0.0		0.0		0.0
Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits			0.0		0.0		0.0
Settlement cost			2.4		0.5		0.4
Net periodic benefit cost			3.0		1.2		1.3
Amount Attributable to Parent	[2]		1.3		0.5		1.2
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates			4.3		1.7		2.5
Plan settlements			(4.5)		(0.9)
Fair Value of Plan Assets, Beginning		0.0	0.0		0.0
Defined Benefit Plan, Benefit Obligation, Benefits Paid			0.0		0.0
Accumulated Benefit Obligation			(2.9)		(4.8)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Benefits Paid			(11.0)		(12.1)
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment			0.0		0.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		144.5	136.5		135.8
Service cost			0.2		0.2		0.3
Interest cost			4.2		5.6		5.4
Plan curtailments			4.0		0.0
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant			(3.3)		(4.1)
Actuarial losses			7.3		2.9
Benefit Obligation, Ending			144.5		136.5		135.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		47.6	47.0		45.3
Actual return on plans' assets			1.2		4.6
Employer contributions			7.1		5.1
Fair Value of Plan Assets, Ending			47.6		47.0		45.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			0.2		0.2		0.3
Interest cost			4.2		5.6		5.4
Expected return on plan assets			(1.8)		(1.9)		(2.0)
Defined Benefit Plan, Amortization of Gain (Loss)			(2.0)		(2.0)		(3.8)
Amortization of unrecognized prior service cost	[1]		(8.4)		(8.4)		(8.4)
Curtailment			(1.5)		0.0		0.0
Net periodic benefit cost			(2.3)		(2.5)		(0.9)
Amount paid by unconsolidated affiliates	[4]		(0.7)		(0.6)		(0.5)
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates			(1.6)		(1.9)		(0.4)
Capitalized Portion of Net Periodic Benefit Cost			$ 0.2		$ 0.2		$ 0.2
Assumptions Used Calculating Benefit Obligation, Discount Rate			2.45%		3.25%		4.30%
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%
Fair Value of Plan Assets, Beginning		47.6	$ 47.0		$ 45.3
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant			3.3		4.1
Defined Benefit Plan, Benefit Obligation, Benefits Paid			(11.0)		(12.1)
Accumulated Benefit Obligation			(96.9)		(89.5)
Other Postretirement Benefits Plan [Member] | Og and E [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Benefits Paid			(8.6)		(9.8)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		109.5	104.7		104.8
Service cost			0.2		0.2		$ 0.2
Interest cost			3.2		4.3		4.2
Plan curtailments			3.1		0.0
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant			(2.4)		(3.0)
Actuarial losses			4.5		2.2
Benefit Obligation, Ending			109.5		104.7		104.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		42.7	41.9		40.6
Actual return on plans' assets			1.1		4.0
Employer contributions			5.9		4.1
Fair Value of Plan Assets, Ending			42.7		41.9		40.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost			0.2		0.2		0.2
Interest cost			3.2		4.3		4.2
Expected return on plan assets			(1.7)		(1.7)		(1.8)
Defined Benefit Plan, Amortization of Gain (Loss)			(2.1)		(2.1)		(3.8)
Amortization of unrecognized prior service cost	[1]		(6.1)		(6.1)		(6.1)
Curtailment			(1.3)		0.0		0.0
Net periodic benefit cost			(1.0)		(1.2)		0.3
Amount Attributable to Parent	[4]		(0.5)		(0.6)		(0.7)
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates			(1.5)		(1.8)		(0.4)
Capitalized Portion of Net Periodic Benefit Cost			0.1		0.1		0.1
Fair Value of Plan Assets, Beginning		42.7	41.9		40.6
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant			2.4		3.0
Defined Benefit Plan, Benefit Obligation, Benefits Paid			(8.6)		(9.8)
Accumulated Benefit Obligation			(66.8)		(62.8)
Other Postretirement Benefits Plan [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost			1.4		0.0		0.0
Additional Pension Expense to Meet State Requirements	[5]		0.2		1.0		4.4
Other Postretirement Benefits Plan [Member] | ARKANSAS
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost			0.1		0.0		$ 0.0
Other Postretirement Benefits Plan [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		14.2	12.1
Fair Value of Plan Assets, Ending			14.2		12.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		14.2	12.1
Other Postretirement Benefits Plan [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, Beginning		0.0	0.0
Other Postretirement Benefits Plan [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		33.4	34.8
Fair Value of Plan Assets, Ending			33.4		34.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		33.4	34.8
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		47.6	46.9
Fair Value of Plan Assets, Ending			47.6		46.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		47.6	46.9
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member] | Og and E [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		42.7	41.9
Fair Value of Plan Assets, Ending			42.7		41.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		42.7	$ 41.9
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%
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%
Group Retiree Medical Insurance Contract [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		33.4	$ 34.8
Fair Value of Plan Assets, Ending			33.4		34.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		33.4	34.8
Group Retiree Medical Insurance Contract [Member] | Other Postretirement Benefits Plan [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, Beginning		0.0	0.0
Group Retiree Medical Insurance Contract [Member] | Other Postretirement Benefits Plan [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		33.4	34.8
Fair Value of Plan Assets, Ending			33.4		34.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		33.4	34.8
U.S. common stocks [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		252.3	202.0
Fair Value of Plan Assets, Ending			252.3		202.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		252.3	202.0
U.S. common stocks [Member] | Pension 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		252.3	202.0
Fair Value of Plan Assets, Ending			252.3		202.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		252.3	202.0
U.S. common stocks [Member] | Pension 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, Beginning		0.0	0.0
U.S. treasury notes and bonds [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6],[8]	0.0	[7],[9]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[9]	134.3	134.8
Fair Value of Plan Assets, Ending	[9]		134.3		134.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning	[9]	134.3	134.8
U.S. treasury notes and bonds [Member] | Pension 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]	134.3	134.8
Fair Value of Plan Assets, Ending	[9]		134.3		134.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning	[9]	134.3	134.8
U.S. treasury notes and bonds [Member] | Pension 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	[9]	0.0	0.0
Fair Value of Plan Assets, Ending	[9]		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning	[9]	0.0	0.0
Mortgage-backed securities | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		29.3	45.8
Fair Value of Plan Assets, Ending			29.3		45.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		29.3	45.8
Mortgage-backed securities | Pension 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		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, Beginning		0.0	0.0
Mortgage-backed securities | Pension 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		29.3	45.8
Fair Value of Plan Assets, Ending			29.3		45.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		29.3	45.8
Corporate fixed income and other securities [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		116.6	130.5
Fair Value of Plan Assets, Ending			116.6		130.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		116.6	130.5
Corporate fixed income and other securities [Member] | Pension 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		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, Beginning		0.0	0.0
Corporate fixed income and other securities [Member] | Pension 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		116.6	130.5
Fair Value of Plan Assets, Ending			116.6		130.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		116.6	130.5
Commingled fund [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			25.4	[6],[10]	23.9	[7],[11]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[11]	25.4	23.9
Fair Value of Plan Assets, Ending	[11]		25.4		23.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning	[11]	25.4	23.9
Commingled fund [Member] | Pension 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	[11]	0.0	0.0
Fair Value of Plan Assets, Ending	[11]		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning	[11]	0.0	0.0
Commingled fund [Member] | Pension 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	[11]	0.0	0.0
Fair Value of Plan Assets, Ending	[11]		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning	[11]	0.0	0.0
Foreign government bonds [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		4.6	3.0
Fair Value of Plan Assets, Ending			4.6		3.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		4.6	3.0
Foreign government bonds [Member] | Pension 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		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, Beginning		0.0	0.0
Foreign government bonds [Member] | Pension 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		4.6	3.0
Fair Value of Plan Assets, Ending			4.6		3.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		4.6	3.0
Municipal bonds [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		1.8	1.1
Fair Value of Plan Assets, Ending			1.8		1.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		1.8	1.1
Municipal bonds [Member] | Pension 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		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, Beginning		0.0	0.0
Municipal bonds [Member] | Pension 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		1.8	1.1
Fair Value of Plan Assets, Ending			1.8		1.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		1.8	1.1
Money market funds [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			8.8	[6]	2.4	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		8.8	2.4
Fair Value of Plan Assets, Ending			8.8		2.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		8.8	2.4
Money market funds [Member] | Pension 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		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, Beginning		0.0	0.0
Money market funds [Member] | Pension 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, Beginning		0.0	0.0
Mutual fund [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		9.2	7.5
Fair Value of Plan Assets, Ending			9.2		7.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		9.2	7.5
Mutual fund [Member] | Pension 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		9.2	7.5
Fair Value of Plan Assets, Ending			9.2		7.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		9.2	7.5
Mutual fund [Member] | Pension 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, Beginning		0.0	0.0
Treasury futures, receivable [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment	[7]				0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning			22.9
Fair Value of Plan Assets, Ending					22.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning			22.9
Treasury futures, receivable [Member] | Pension 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			0.0
Fair Value of Plan Assets, Ending					0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning			0.0
Treasury futures, receivable [Member] | Pension 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			22.9
Fair Value of Plan Assets, Ending					22.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning			22.9
Treasury futures, payable [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment	[7]				0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning			(10.9)
Fair Value of Plan Assets, Ending					(10.9)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning			(10.9)
Treasury futures, payable [Member] | Pension 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			0.0
Fair Value of Plan Assets, Ending					0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning			0.0
Treasury futures, payable [Member] | Pension 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			(10.9)
Fair Value of Plan Assets, Ending					(10.9)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning			(10.9)
Cash collateral [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.7	0.6
Fair Value of Plan Assets, Ending			0.7		0.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		0.7	0.6
Cash collateral [Member] | Pension 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		0.7	0.6
Fair Value of Plan Assets, Ending			0.7		0.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		0.7	0.6
Cash collateral [Member] | Pension 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, Beginning		0.0	0.0
Receivable (foreign currency) [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0	[7]
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, Beginning		0.1	0.1
Receivable (foreign currency) [Member] | Pension 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		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, Beginning		0.0	0.0
Receivable (foreign currency) [Member] | Pension 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		0.1	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, Beginning		0.1	0.1
Preferred stocks (foreign) [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			0.0	[6]	0.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.6	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, Beginning		0.6	0.7
Preferred stocks (foreign) [Member] | Pension 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		0.6	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, Beginning		0.6	0.7
Preferred stocks (foreign) [Member] | Pension 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, Beginning		0.0	0.0
Total Plan investments [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment			34.2	[6]	26.3	[7]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		583.7	564.4
Fair Value of Plan Assets, Ending			583.7		564.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		583.7	564.4
Total Plan investments [Member] | Pension 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		397.1	345.6
Fair Value of Plan Assets, Ending			397.1		345.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		397.1	345.6
Total Plan investments [Member] | Pension 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		152.4	192.5
Fair Value of Plan Assets, Ending			152.4		192.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		152.4	192.5
Receivable from broker for securities sold [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.2
Fair Value of Plan Assets, Ending			0.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		0.2
Interest and dividends receivable [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		2.2	2.4
Fair Value of Plan Assets, Ending			2.2		2.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		2.2	2.4
Payable to broker for securities purchased [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		(15.8)	(36.5)
Fair Value of Plan Assets, Ending			(15.8)		(36.5)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		(15.8)	(36.5)
Total Plan assets [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		570.3	530.3
Fair Value of Plan Assets, Ending			570.3		530.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		570.3	530.3
Total Plan assets [Member] | Pension Plans, Defined Benefit [Member] | Og and E [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		420.3	399.1
Fair Value of Plan Assets, Ending			420.3		399.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		420.3	399.1
Plan Assets Attributable to Affiliates | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		(150.0)	(131.2)
Fair Value of Plan Assets, Ending			(150.0)		$ (131.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Fair Value of Plan Assets, Beginning		$ (150.0)	$ (131.2)

[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] Amount paid by unconsolidated affiliates" is only applicable to OGE Energy. "Amount allocated from OGE Energy" is only applicable to OG&E.

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

[4] "Amount paid by unconsolidated affiliates" is only applicable to OGE Energy. "Amount allocated from OGE Energy" is only applicable to OG&E.

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

[6] 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.

[7] 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.

[8] 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.

[9] 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.

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

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