Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Plan Assets, Contributions by Employer				$ 20.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost				23.8
Additional Pension Expense to Meet State Requirements	[1]	$ 1.5	$ 0.0	(0.2)	$ 7.8
Pension Plan [Member]
Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement or Curtailment Gain (Loss), Tax		0.3	1.1	2.6	1.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		3.4	3.6	9.7	11.1
Interest cost		4.6	6.1	15.5	17.8
Expected return on plan assets		(9.5)	(10.7)	(27.1)	(33.0)
Amortization of net loss		4.4	3.9	13.0	12.2
Amortization of unrecognized prior service cost (A)	[2]	0.0	0.0	0.0	0.0
Settlement cost		2.8	12.2	23.4	12.2
Total net periodic benefit cost		5.7	15.1	34.5	20.3
Less: Amount paid by unconsolidated affiliates		0.5	0.7	2.2	1.9
Net periodic benefit cost		(5.2)	(14.4)	(32.3)	(18.4)
Capitalized portion of net periodic pension benefit cost		0.9	0.9	2.7	2.8
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	0.1	0.3	0.3
Interest cost		0.1	0.1	0.3	0.3
Expected return on plan assets		0.0	0.0	0.0	0.0
Amortization of net loss		0.1	0.1	0.3	0.5
Amortization of unrecognized prior service cost (A)	[2]	0.0	0.1	0.0	0.1
Settlement cost		0.4	0.6	0.4	0.6
Total net periodic benefit cost		0.7	1.0	1.3	1.8
Less: Amount paid by unconsolidated affiliates		0.1	0.1	0.1	0.1
Net periodic benefit cost		0.6	0.9	1.2	1.7
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	0.0	0.2	0.2
Interest cost		1.2	1.5	4.2	4.1
Expected return on plan assets		(0.5)	(0.5)	(1.4)	(1.5)
Amortization of net loss		0.5	1.0	1.5	2.9
Amortization of unrecognized prior service cost (A)	[3]	(2.1)	(2.1)	(6.3)	(6.3)
Total net periodic benefit cost		(0.8)	(0.1)	(1.8)	(0.6)
Less: Amount paid by unconsolidated affiliates		(0.2)	(0.2)	(0.5)	(0.4)
Net periodic benefit cost		0.6	(0.1)	1.3	0.2
Capitalized portion of net periodic pension benefit cost		0.0	0.0	0.1	0.1
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[1]	(2.3)	(10.5)	(14.0)	(10.5)
OKLAHOMA | Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements	[4]	0.1	0.1	0.9	4.3
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost	[1]	$ 0.2	$ 1.0	$ 1.3	$ 1.0

[1] Included in the Pension tracker regulatory liability for the Oklahoma jurisdiction and in the deferred pension expenses regulatory asset for the Arkansas jurisdiction (see Note 1).

[2] 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.

[3] 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.

[4] Included in the Pension tracker regulatory liability (see Note 1).