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Retirement Plans and Postretirement Benefit Plans (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2013
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]          
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates $ 2.4 $ 3.5 $ 4.8 $ 7.0  
Pension Plan [Member]          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]          
Service cost 3.4 [1] 3.5 [1] 7.5 [2] 7.7 [2]  
Interest cost 5.8 [1] 6.6 [1] 11.7 [2] 13.1 [2]  
Expected return on plan assets 11.0 [1] 10.6 [1] 22.3 [2] 21.3 [2]  
Defined Benefit Plan, Amortization of Gain (Loss) 4.4 [1] 4.7 [1] 8.3 [2] 8.7 [2]  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 2.6 [1] 4.2 [1] 5.2 [2] 8.2 [2]  
Less: Amount paid by unconsolidated affiliates 0.7 [1] 0.9 [1] 1.2 [2] 1.7 [2]  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement         $ (22.4)
Capitalized Portion of Net Periodic Benefit Cost 0.9 1.2 1.9 2.3  
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates 1.9 [1] 3.3 [1] 4.0 [2] 6.5 [2]  
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] 0.0 [1] 0.2 [2] 0.1 [2]  
Expected return on plan assets 0.0 [1] 0.0 [1] 0.0 [2] 0.0 [2]  
Defined Benefit Plan, Amortization of Gain (Loss) 0.3 [1] 0.1 [1] 0.4 [2] 0.2 [2]  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 0.5 [1] 0.2 [1] 0.8 [2] 0.5 [2]  
Less: Amount paid by unconsolidated affiliates 0.0 [1] 0.0 [1] 0.0 [2] 0.0 [2]  
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates 0.5 [1] 0.2 [1] 0.8 [2] 0.5 [2]  
Postretirement Benefit Plan [Member]          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]          
Service cost 0.1 [3] 0.2 [3] 0.2 [4] 0.4 [4]  
Interest cost 1.3 [3] 2.1 [3] 2.6 [4] 4.3 [4]  
Expected return on plan assets 0.5 [3] 0.5 [3] 1.0 [4] 1.1 [4]  
Defined Benefit Plan, Amortization of Gain (Loss) 0.9 [3] 0.2 [3] 1.9 [4] 0.8 [4]  
Amortization of unrecognized prior service cost [5] (2.1) [3] 0.0 [3] (4.2) [4] 0.0 [4]  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) (0.3) [3] 2.0 [3] (0.5) [4] 4.4 [4]  
Less: Amount paid by unconsolidated affiliates (0.1) [3] 0.2 [3] (0.2) [4] 0.6 [4]  
Capitalized Portion of Net Periodic Benefit Cost 0.1 0.5 0.1 1.2  
Defined Benefit Plan, Net Periodic Benefit Cost, Net of Unconsolidated Affiliates (0.2) [3] 1.8 [3] (0.3) [4] 3.8 [4]  
OKLAHOMA          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]          
Additional Pension Expense to Meet State Requirements 3.8 2.9 7.8 5.8  
OKLAHOMA | Postretirement Benefit Plan [Member]          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]          
Additional Pension Expense to Meet State Requirements $ 2.2 1.0 $ 4.3 2.1  
ARKANSAS | Pension Plan [Member]          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement   $ (2.3)   $ (2.3)  
[1] In addition to the $2.4 million and $3.5 million of net periodic benefit cost recognized during the three months ended June 30, 2018 and 2017, respectively, the Company recognized the following:•an increase in pension expense during the three months ended June 30, 2018 and 2017 of $3.8 million and $2.9 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); and•a deferral of pension expense during the three months ended June 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.
[2] In addition to the $4.8 million and $7.0 million of net periodic benefit cost recognized during the six months ended June 30, 2018 and 2017, respectively, the Company recognized the following:•an increase in pension expense during the six months ended June 30, 2018 and 2017 of $7.8 million and $5.8 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); and•a deferral of pension expense during the six months ended June 30, 2017 of $2.3 million related to the Arkansas jurisdictional portion of the pension settlement charge of $22.4 million in 2013.
[3] In addition to the $0.2 million of net periodic benefit income and $1.8 million of net periodic benefit cost recognized during the three months ended June 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the three months ended June 30, 2018 and 2017 of $2.2 million and $1.0 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 $0.3 million of net periodic benefit income and $3.8 million of net periodic benefit cost recognized during the six months ended June 30, 2018 and 2017, respectively, the Company recognized an increase in postretirement medical expense in the six months ended June 30, 2018 and 2017 of $4.3 million and $2.1 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.