Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost		$ 4.0	$ 4.6
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		4.4	4.5
Interest cost		6.6	6.4
Expected return on plan assets		(10.5)	(11.8)
Amortization of net loss		4.2	4.3
Amortization of unrecognized prior service cost	[1]	0.0	0.1
Net periodic benefit cost		4.7	3.5
Amount Attributable to Unconsolidated Affiliates		1.3	1.1
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]	3.4	2.4
Capitalized Portion of Net Periodic Benefit Cost		1.2	0.8
Pension Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements		2.3	3.0
Restoration of Retirement Income Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.1	0.4
Interest cost		0.1	0.2
Expected return on plan assets		0.0	0.0
Amortization of net loss		0.2	0.1
Amortization of unrecognized prior service cost	[1]	0.0	0.0
Net periodic benefit cost		0.4	0.7
Amount Attributable to Unconsolidated Affiliates		0.0	0.0
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]	0.4	0.7
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost		0.3	0.5
Interest cost		2.3	2.6
Expected return on plan assets		(0.6)	(0.6)
Amortization of net loss		0.5	3.4
Amortization of unrecognized prior service cost	[1]	(2.2)	(4.1)
Net periodic benefit cost		0.3	1.8
Amount Attributable to Unconsolidated Affiliates		0.1	0.3
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]	0.2	1.5
Capitalized Portion of Net Periodic Benefit Cost		0.1	0.5
Other Postretirement Benefit Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Postretirement Medical Expense to Meet State Requirements		$ 2.0	$ 1.4

[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] In addition to the $4.0 million and $4.6 million of net periodic benefit cost recognized during the three months ended March 31, 2016 and 2015, respectively, the Company recognized the following: •an increase in pension expense in the three months ended March 31, 2016 and 2015 of $2.3 million and $3.0 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 •an increase in postretirement medical expense in the three months ended March 31, 2016 and 2015 of $2.0 million and $1.4 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).