Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost				$ 12.8
Defined Benefit Plan, Plan Assets, Contributions by Employer				20.0
Pension Plan [Member]
Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement or Curtailment Gain (Loss), Tax		$ 0.5	$ 0.3	0.6	$ 2.6
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		3.3	3.4	9.8	9.7
Interest cost		4.2	4.6	13.4	15.5
Expected return on plan assets		(9.4)	(9.5)	(28.0)	(27.1)
Amortization of net loss		4.3	4.4	12.7	13.0
Settlement cost		12.1	2.8	12.1	23.4
Total net periodic benefit cost		14.5	5.7	20.0	34.5
Amount Attributable to Unconsolidated Affiliates		0.5	0.5	1.6	2.2
Net periodic benefit cost		14.0	5.2	18.4	32.3
Capitalized portion of net periodic pension benefit cost		0.9	0.9	2.7	2.7
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.2	0.1	0.6	0.3
Interest cost		0.1	0.1	0.2	0.3
Expected return on plan assets		0.0	0.0	0.0	0.0
Amortization of net loss		0.1	0.1	0.4	0.3
Settlement cost		0.4	0.4	0.7	0.4
Total net periodic benefit cost		0.8	0.7	1.9	1.3
Amount Attributable to Unconsolidated Affiliates		0.1	0.1	0.1	0.1
Net periodic benefit cost		0.7	0.6	1.8	1.2
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	0.1	0.2	0.2
Interest cost		1.0	1.2	3.1	4.2
Expected return on plan assets		(0.5)	(0.5)	(1.4)	(1.4)
Amortization of net loss		0.5	0.5	1.5	1.5
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	[1]	(2.1)	(2.1)	(6.3)	(6.3)
Amount Attributable to Unconsolidated Affiliates		(0.2)	(0.2)	(0.6)	(0.5)
Capitalized portion of net periodic pension benefit cost		0.0	0.0	0.1	0.1
Postretirement Benefit Costs
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Total net periodic benefit cost		(1.0)	(0.8)	(2.9)	(1.8)
Net periodic benefit cost		(0.8)	(0.6)	(2.3)	(1.3)
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[2]	10.8	2.3	11.1	14.0
Additional Pension Expense to Meet State Requirements	[2]	2.4	1.5	5.7	(0.2)
OKLAHOMA | Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements	[3]	0.4	0.1	1.2	0.9
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[2]	$ 1.0	$ 0.2	$ 1.0	$ 1.3

[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] Included in the Pension tracker, as presented in the regulatory assets and liabilities table in Note 1.