Retirement Plans and Postretirement Benefit Plans Postretirement Benefit Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019		Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions		$ 20.0	$ 15.0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Level 3 Asset Value, Beginning of Period		36.0
Interest income		0.8
Dividend income		0.5
Unrealized gains		1.4
Administrative expenses and charges		(0.1)
Claims paid		(3.8)
Level 3 Asset Value, End of Period		34.8	36.0
Postretirement Plan, Expected Future Benefit Payments, Next Twelve Months					$ 11.2
Postretirement Plan, Expected Future Benefit Payments, Year Two					11.2
Postretirement Plan, Expected Future Benefit Payments, Year Three					11.1
Postretirement Plan, Expected Future Benefit Payments in Year Four					9.5
Postretirement Plan, Expected Future Benefit Payments, Year Five					9.4
Postretirement Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter					42.3
Postemployment Benefits Liability					2.1		$ 1.9
Defined Contribution Plan, Cost		$ 14.4	13.2	$ 13.2
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay		5.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		200.00%
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 34.8	36.0		34.8		36.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Claims Paid		3.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Administrative Expenses and Charges		0.1
Unrealized gains		1.4
Interest income		0.8
Dividend income		0.5
Cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.9
Fair Value of Plan Assets, Ending			0.9
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, Beginning		0.9
Fair Value of Plan Assets, Ending			0.9
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, Beginning		0.0
Fair Value of Plan Assets, Ending			0.0
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		615.9	687.5
Service cost		12.9	14.9	15.5
Interest cost		20.7	23.8	26.2
Plan settlements		(83.1)	(73.7)
Actuarial losses (gains)		64.3	(22.0)
Benefit Obligation, Ending		616.1	615.9	687.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		522.8	635.3
Actual return on plans' assets		85.2	(39.2)
Employer contributions		20.0	15.0
Plan settlements		(83.1)	(73.7)
Fair Value of Plan Assets, Ending		530.3	522.8	635.3
Funded Status of Plan					$ (85.8)		$ (93.1)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		12.9	14.9	15.5
Interest cost		20.7	23.8	26.2
Expected return on plan assets		(36.1)	(44.1)	(42.6)
Amortization of unrecognized prior service cost	[1]	0.0	0.0	(0.1)
Settlement cost		27.6	25.1	15.3
Net periodic benefit cost		42.4	35.9	31.7
Amount paid by unconsolidated affiliates		2.9	2.5	4.3
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		39.5	33.4	27.4
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Capitalized Portion of Net Periodic Benefit Cost		$ 3.6	$ 3.8	$ 4.4
Assumptions Used Calculating Benefit Obligation, Discount Rate				3.60%	3.15%		4.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		3.63%	3.73%	4.00%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		7.50%	7.50%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				4.20%	4.20%		4.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.20%	4.20%	4.20%
Defined Benefit Plan, Benefit Obligation, Benefits Paid		$ (14.6)	$ (14.6)
Defined Benefit Plan, Plan Assets, Benefits Paid		14.6	14.6
Accumulated Benefit Obligation					$ 563.3		$ 561.9
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment		0.0	0.0
Pension Plan [Member] | Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[2]	19.7
Fair Value of Plan Assets, Ending	[2]	23.9	19.7
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Alternative Investment					23.9	[3],[4]	19.7	[2],[5]
Pension Plan [Member] | Money market funds [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		7.5	0.3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Alternative Investment					7.5	[3]	0.3	[5]
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[2]	0.0
Fair Value of Plan Assets, Ending	[2]	0.0	0.0
Pension Plan [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		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[2]	0.0
Fair Value of Plan Assets, Ending	[2]	0.0	0.0
Pension Plan [Member] | Fair Value, Inputs, Level 2 [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
Pension 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		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		135.8	149.4
Service cost		0.2	0.3	$ 0.6
Interest cost		5.6	5.4	7.2
Plan settlements		0.0	0.0
Actuarial losses (gains)		2.9	(9.6)
Benefit Obligation, Ending		136.5	135.8	149.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		45.3	50.2
Actual return on plans' assets		4.6	(0.6)
Employer contributions		5.1	5.4
Plan settlements		0.0	0.0
Fair Value of Plan Assets, Ending		47.0	45.3	50.2
Funded Status of Plan					$ (89.5)		$ (90.5)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.2	0.3	0.6
Interest cost		5.6	5.4	7.2
Expected return on plan assets		(1.9)	(2.0)	(2.2)
Amortization of unrecognized prior service cost	[1]	(8.4)	(8.4)	(3.5)
Settlement cost		0.0	0.0	0.6
Net periodic benefit cost		(2.5)	(0.9)	4.7
Amount paid by unconsolidated affiliates		(0.6)	(0.5)	0.3
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		(1.9)	(0.4)	4.4
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Capitalized Portion of Net Periodic Benefit Cost		$ 0.2	$ 0.2	$ 1.2
Assumptions Used Calculating Benefit Obligation, Discount Rate				3.70%	3.25%		4.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		4.30%	3.70%	4.20%
Health Care Cost Trend Rate Assumed for Next Fiscal Year							7.00%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		4.00%	4.00%	4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase				4.20%
Defined Benefit Plan, Benefit Obligation, Benefits Paid		$ (12.1)	$ (13.5)
Defined Benefit Plan, Plan Assets, Benefits Paid		12.1	13.5
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		36.0
Fair Value of Plan Assets, Ending		34.8	36.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		8.9
Fair Value of Plan Assets, Ending		10.9	8.9
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
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
Fair Value of Plan Assets, Ending		12.1	9.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		8.9
Fair Value of Plan Assets, Ending		10.9	8.9
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, Ending		1.2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		36.0
Fair Value of Plan Assets, Ending		34.8	36.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Group Retiree Medical Insurance Contract [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		36.0
Fair Value of Plan Assets, Ending		34.8	36.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Equity Mutual Funds Investment [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning		0.0
Fair Value of Plan Assets, Ending		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Money market funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.0
Other Postretirement Benefit Plans, Defined Benefit [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		45.8
Fair Value of Plan Assets, Ending		46.9	45.8
Other Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning		9.6	8.1
Service cost		0.5	0.4	$ 0.3
Interest cost		0.4	0.3	0.3
Plan settlements		(1.2)	(2.0)
Actuarial losses (gains)		0.7	2.8
Benefit Obligation, Ending		10.3	9.6	8.1
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		1.2	2.0
Plan settlements		(1.2)	(2.0)
Fair Value of Plan Assets, Ending		0.0	0.0	0.0
Funded Status of Plan					$ (10.3)		$ (9.6)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.5	0.4	0.3
Interest cost		0.4	0.3	0.3
Expected return on plan assets		0.0	0.0	0.0
Amortization of unrecognized prior service cost	[1]	0.0	0.1	0.1
Settlement cost		0.5	1.0	0.0
Net periodic benefit cost		1.9	2.5	1.1
Amount paid by unconsolidated affiliates		0.1	0.1	0.0
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		1.8	2.4	1.1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Benefits Paid		0.0	0.0
Defined Benefit Plan, Plan Assets, Benefits Paid		0.0	0.0
Accumulated Benefit Obligation					$ 8.1		$ 7.8
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment		0.3	0.0
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[6]	$ 17.9	$ 22.1	$ 13.2

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

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

[5] 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.

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