Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 0.1	$ 0.0
Interest cost		0.0	0.1
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.1	0.1
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Settlement cost		0.0	0.0
Net periodic benefit cost		0.2	0.2
Amount Attributable to Unconsolidated Affiliates		0.2	0.1
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		2.6	2.4
Interest cost		3.4	4.3
Expected return on plan assets		(7.1)	(6.6)
Amortization of net loss		2.8	2.8
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Settlement cost		0.0	10.9
Net periodic benefit cost		1.7	13.8
Amount Attributable to Unconsolidated Affiliates		0.5	2.0
Capitalized Portion of Net Periodic Benefit Cost		0.9	0.8
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0	0.0
Interest cost		0.8	1.0
Expected return on plan assets		(0.4)	(0.4)
Amortization of net loss		0.7	0.7
Amortization of unrecognized prior service cost	[1]	(1.5)	(1.5)
Settlement cost		0.0	0.0
Net periodic benefit cost		(0.4)	(0.2)
Amount Attributable to Unconsolidated Affiliates		(0.2)	(0.2)
Capitalized Portion of Net Periodic Benefit Cost		0.0	0.1
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates		(0.6)	(0.4)
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[2]	0.0	(11.2)
Additional Pension Expense to Meet State Requirements	[2]	1.7	(1.0)
OKLAHOMA | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements	[3]	0.2	0.3
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[2]	0.0	1.0
Og and E [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		0.4	0.3
Og and E [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost		$ 2.2	$ 15.8

[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.