Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost						$ 20.6
Net periodic benefit cost		$ (4.3)		$ (2.4)		(27.7)		$ (4.8)
Additional Pension Expense to Meet State Requirements						1.7		7.8
Pension Plan [Member]
Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement or Curtailment Gain (Loss), Tax		0.1		0.0		2.3		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		2.8	[1]	3.4	[1]	6.3	[2]	7.5	[2]
Interest cost		5.2	[1]	5.8	[1]	10.9	[2]	11.7	[2]
Expected return on plan assets		(8.9)	[1]	(11.0)	[1]	(17.6)	[2]	(22.3)	[2]
Amortization of net loss		4.8	[1]	4.4	[1]	8.6	[2]	8.3	[2]
Settlement cost		0.9	[1]	0.0	[1]	20.6	[2]	0.0	[2]
Total net periodic benefit cost		4.8	[1]	2.6	[1]	28.8	[2]	5.2	[2]
Less: Amount paid by unconsolidated affiliates		0.8	[1]	0.7	[1]	1.7	[2]	1.2	[2]
Net periodic benefit cost		(4.0)	[1]	(1.9)	[1]	(27.1)	[2]	(4.0)	[2]
Capitalized portion of net periodic pension benefit cost		0.8		0.9		1.8		1.9
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	[1]	0.1	[1]	0.2	[2]	0.2	[2]
Interest cost		0.1	[1]	0.1	[1]	0.2	[2]	0.2	[2]
Expected return on plan assets		0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Amortization of net loss		0.1	[1]	0.3	[1]	0.2	[2]	0.4	[2]
Settlement cost		0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Total net periodic benefit cost		0.3	[1]	0.5	[1]	0.6	[2]	0.8	[2]
Less: Amount paid by unconsolidated affiliates		0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Net periodic benefit cost		(0.3)	[1]	(0.5)	[1]	(0.6)	[2]	(0.8)	[2]
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0	[3]	0.1	[3]	0.1	[4]	0.2	[4]
Interest cost		1.4	[3]	1.3	[3]	2.8	[4]	2.6	[4]
Expected return on plan assets		(0.4)	[3]	(0.5)	[3]	(0.9)	[4]	(1.0)	[4]
Amortization of net loss		0.4	[3]	0.9	[3]	1.0	[4]	1.9	[4]
Amortization of unrecognized prior service cost (A)	[5]	(2.1)	[3]	(2.1)	[3]	(4.2)	[4]	(4.2)	[4]
Total net periodic benefit cost		(0.7)	[3]	(0.3)	[3]	(1.2)	[4]	(0.5)	[4]
Less: Amount paid by unconsolidated affiliates		(0.1)	[3]	(0.1)	[3]	(0.3)	[4]	(0.2)	[4]
Net periodic benefit cost		0.6	[3]	0.2	[3]	0.9	[4]	0.3	[4]
Capitalized portion of net periodic pension benefit cost		0.0		0.1		0.1		0.1
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost						(11.7)
Additional Pension Expense to Meet State Requirements		0.7		3.8
OKLAHOMA | Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		$ 0.5		$ 2.2		0.8		$ 4.3
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost						$ (1.1)

[1] In addition to the $4.3 million and $2.4 million of net periodic benefit cost recognized during the three months ended June 30, 2019 and 2018, respectively, the Company recognized a decrease of pension expense of $0.7 million and an increase of $3.8 million during the three months ended June 30, 2019 and 2018, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1)

[2] In addition to the $27.7 million and $4.8 million of net periodic benefit cost recognized during the six months ended June 30, 2019 and 2018, respectively, the Company recognized the following: • a decrease of pension expense of $1.7 million and an increase of $7.8 million during the six months ended June 30, 2019 and 2018, respectively, to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1); • a deferral of pension expense during the six months ended June 30, 2019 of $11.7 million related to the pension settlement charge of $20.6 million in accordance with the Oklahoma Pension tracker regulatory liability (see Note 1); and • a deferral of pension expense during the six months ended June 30, 2019 of $1.1 million related to the Arkansas jurisdictional portion of the pension settlement charge of $20.6 million (see Note 1)

[3] In addition to the $0.6 million and $0.2 million of net periodic benefit income recognized during the three months ended June 30, 2019 and 2018, respectively, the Company recognized an increase in postretirement medical expense during the three months ended June 30, 2019 and 2018 of $0.5 million and $2.2 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1).

[4] In addition to the $0.9 million and $0.3 million of net periodic benefit income recognized during the six months ended June 30, 2019 and 2018, respectively, the Company recognized an increase in postretirement medical expense during the six months ended June 30, 2019 and 2018 of $0.8 million and $4.3 million, respectively, to maintain the allowable amount to be recovered for postretirement medical expense in the Oklahoma jurisdiction, which are included in the Pension tracker regulatory liability (see Note 1).

[5] 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.