Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended				12 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017		Dec. 31, 2013
Defined Benefit Plan, Plan Assets, Contributions by Employer						$ 15.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost		$ (12.8)				(12.8)
Net periodic benefit cost		(15.3)		$ (3.2)		(20.1)		$ (10.2)
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		3.6	[1]	3.9	[1]	11.1	[2]	11.6	[2]
Interest cost		6.1	[1]	6.5	[1]	17.8	[2]	19.6	[2]
Expected return on plan assets		10.7	[1]	10.7	[1]	33.0	[2]	32.0	[2]
Amortization of net loss		3.9	[1]	4.4	[1]	12.2	[2]	13.1	[2]
Amortization of unrecognized prior service cost (A)	[3]	0.0	[1]	0.0	[1]	0.0	[2]	0.0	[2]
Settlement cost		12.2	[1]	0.0	[1]	12.2	[2]	0.0	[2]	$ (22.4)
Total net periodic benefit cost		15.1	[1]	4.1	[1]	20.3	[2]	12.3	[2]
Less: Amount paid by unconsolidated affiliates		0.7	[1]	1.2	[1]	1.9	[2]	2.9	[2]
Net periodic benefit cost		(14.4)	[1]	(2.9)	[1]	(18.4)	[2]	(9.4)	[2]
Capitalized portion of net periodic pension benefit cost		0.9		1.0		2.8		3.3
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.1	[1]	0.0	[1]	0.3	[2]	0.2	[2]
Interest cost		0.1	[1]	0.1	[1]	0.3	[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]	0.1	[1]	0.5	[2]	0.3	[2]
Amortization of unrecognized prior service cost (A)	[3]	0.1	[1]	0.1	[1]	0.1	[2]	0.1	[2]
Settlement cost		0.6	[1]	0.0	[1]	0.6	[2]	0.0	[2]
Total net periodic benefit cost		1.0	[1]	0.3	[1]	1.8	[2]	0.8	[2]
Less: Amount paid by unconsolidated affiliates		0.1	[1]	0.0	[1]	0.1	[2]	0.0	[2]
Net periodic benefit cost		(0.9)	[1]	(0.3)	[1]	(1.7)	[2]	(0.8)	[2]
Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0.0	[4]	0.1	[4]	0.2	[5]	0.5	[5]
Interest cost		1.5	[4]	1.6	[4]	4.1	[5]	5.9	[5]
Expected return on plan assets		0.5	[4]	0.6	[4]	1.5	[5]	1.7	[5]
Amortization of net loss		1.0	[4]	0.5	[4]	2.9	[5]	1.3	[5]
Amortization of unrecognized prior service cost (A)	[6]	(2.1)	[4]	(1.4)	[4]	(6.3)	[5]	(1.4)	[5]
Settlement cost		0.0	[4]	0.6	[4]	0.0	[5]	0.6	[5]
Total net periodic benefit cost		(0.1)	[4]	0.8	[4]	(0.6)	[5]	5.2	[5]
Less: Amount paid by unconsolidated affiliates		(0.2)	[4]	(0.1)	[4]	(0.4)	[5]	0.5	[5]
Net periodic benefit cost		(0.1)	[4]	(0.9)	[4]	0.2	[5]	(4.7)	[5]
Capitalized portion of net periodic pension benefit cost		0.0		0.1		0.1		1.3
OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements				2.7		7.8		8.5
OKLAHOMA | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost		(10.5)				(10.5)
OKLAHOMA | Postretirement Benefit Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Additional Pension Expense to Meet State Requirements		0.1		$ 1.9		4.3		3.9
ARKANSAS | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Settlement cost		$ (1.0)				$ (1.0)		$ (2.3)

[1] In addition to the $15.3 million and $3.2 million of net periodic benefit cost recognized during the three months ended September 30, 2018 and 2017, respectively, the Company recognized the following:•a deferral of pension expense during the three months ended September 30, 2018 of $10.5 million related to the pension settlement charge of $12.8 million in accordance with the Oklahoma Pension tracker regulatory liability (see Note 1);•a deferral of pension expense during the three months ended September 30, 2018 of $1.0 million related to the Arkansas jurisdictional portion of the pension settlement charge of $12.8 million; and•an increase in pension expense during the three months ended September 30, 2017 of $2.7 million to maintain the allowable amount to be recovered for pension expense in the Oklahoma jurisdiction, which is included in the Pension tracker regulatory liability (see Note 1).

[2] In addition to the $20.1 million and $10.2 million of net periodic benefit cost recognized during the nine months ended September 30, 2018 and 2017, respectively, the Company recognized the following:•an increase in pension expense during the nine months ended September 30, 2018 and 2017 of $7.8 million and $8.5 million, 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 nine months ended September 30, 2018 of $10.5 million related to the pension settlement charge of $12.8 million in accordance with the Oklahoma Pension tracker regulatory liability (see Note 1);•a deferral of pension expense during the nine months ended September 30, 2018 of $1.0 million related to the Arkansas jurisdictional portion of the pension settlement charge of $12.8 million; and•a deferral of pension expense during the nine months ended September 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.

[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] In addition to the $0.1 million and $0.9 million of net periodic benefit cost recognized during the three months ended September 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the three months ended September 30, 2018 and 2017 of $0.1 million and $1.9 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] In addition to the $0.2 million of net periodic benefit income and $4.7 million of net periodic benefit cost recognized during the nine months ended September 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the nine months ended September 30, 2018 and 2017 of $4.3 million and $3.9 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).

[6] 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.