Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Pension Plan [Member]
Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement or Curtailment Gain (Loss), Tax		$ 0.0	$ 2.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		3.7	3.5
Interest cost		4.6	5.7
Expected return on plan assets		(9.4)	(8.7)
Amortization of net loss		3.8	3.8
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	[1]	0.0	0.0
Settlement cost		0.0	19.7
Total net periodic benefit cost		2.7	24.0
Amount Attributable to Unconsolidated Affiliates		0.5	0.9
Net periodic benefit cost		(2.2)	(23.1)
Capitalized portion of net periodic pension benefit cost		1.0	1.0
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.2	0.1
Interest cost		0.1	0.1
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.1	0.1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	[1]	0.0	0.0
Settlement cost		0.0	0.0
Total net periodic benefit cost		0.4	0.3
Amount Attributable to Unconsolidated Affiliates		0.0	0.0
Net periodic benefit cost		(0.4)	(0.3)
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	0.1
Interest cost		1.1	1.4
Expected return on plan assets		(0.5)	(0.5)
Amortization of net loss		0.6	0.6
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	[1]	(2.1)	(2.1)
Settlement cost		0.0	0.0
Total net periodic benefit cost		(0.8)	(0.5)
Amount Attributable to Unconsolidated Affiliates		(0.2)	(0.2)
Net periodic benefit cost		0.6	0.3
Capitalized portion of net periodic pension benefit cost		0.0	0.1
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 | Postretirement Benefit 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

[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.