Retirement Plans and Postretirement Benefit Plans (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match				200.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay				5.00%
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions				$ 0.0	$ 0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Curtailment					(0.2)
Net periodic benefit cost				40.2	19.3	$ 60.0
Effect of One Percentage Point Increase on Service and Interest Cost Components				0.0	0.0	0.0
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation				0.2	0.1	0.1
Effect of One Percentage Point Decrease on Service and Interest Cost Components				0.1	0.1	0.1
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation				0.7	0.7	0.6
Expected Future Benefit Payments, Next Twelve Months							$ 66.2
Expected Future Benefit Payments, Year Two							50.6
Expected Future Benefit Payments, Year Three							52.2
Expected Future Benefit Payments, Year Four							54.7
Expected Future Benefit Payments, Year Five							58.9
Expected Future Benefit Payments, Five Fiscal Years Thereafter							275.7
Defined Contribution Plan, Cost Recognized				$ 11.6	15.2	14.2
Current Fiscal Year End Date				--12-31
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning				$ 280.9	258.2
Service cost				1.5	3.1	4.3
Actual return on plans' assets				10.3	11.4	10.3
Plan curtailments				0.0	(0.6)
Plan settlements				0.0	0.0
Participants' contributions				3.4	3.4
Actuarial gains (losses)				(55.1)	19.5
Benefits paid				(15.7)	(14.1)
Benefit Obligation, Ending		$ 225.3		225.3	280.9	258.2
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				59.6	61.4
Actual return on plans' assets				(0.5)	1.8
Employer contributions				8.5	7.1
Participants' contributions				3.4	3.4
Benefits paid				(15.7)	(14.1)
Fair Value of Plan Assets, Ending		55.3		55.3	59.6	61.4
Funded Status of Plan							$ (170.0)	$ (221.3)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost				1.5	3.1	4.3
Interest cost				10.3	11.4	10.3
Expected return on plan assets				(2.4)	(2.4)	(2.5)
Amortization of net loss				13.9	12.3	21.5
Amortization of unrecognized prior service cost	[1]			(16.5)	(16.5)	(16.5)
Curtailment				0.0	0.0	0.0
Settlement				0.0	0.0	0.0
Net periodic benefit cost				6.8	7.9	17.1
Amount Attributable to Unconsolidated Affiliates				1.3	1.3	1.5
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]			5.5	6.6	15.6
Capitalized Portion of Net Periodic Benefit Cost				$ 1.9	$ 2.0	$ 4.5
Assumptions Used Calculating Benefit Obligation, Discount Rate							4.25%	3.80%	4.60%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				4.00%	4.00%	4.00%
Health Care Cost Trend Rate Assumed for Next Fiscal Year				6.10%	7.85%	8.35%
Ultimate Health Care Cost Trend Rate				4.50%	4.48%	4.48%
Year that Rate Reaches Ultimate Trend Rate							2026	2028	2028
Fair Value of Plan Assets		55.3		$ 59.6	$ 61.4	$ 61.4	$ 55.3	$ 59.6	$ 61.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Postretirement Medical Expense to Meet State Requirements				5.8	5.2	0.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				8.6
Fair Value of Plan Assets, Ending		8.5		8.5	8.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		8.5		8.6	8.6		8.5	8.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				51.0
Fair Value of Plan Assets, Ending		46.8		46.8	51.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		46.8		51.0	51.0		46.8	51.0
Restoration of Retirement Income Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated Benefit Obligation							24.6	18.7
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning				19.7	14.0
Service cost				1.3	1.1	1.2
Actual return on plans' assets				0.7	0.6	0.5
Plan curtailments				0.0	0.0
Plan settlements				0.0	0.0
Participants' contributions				0.0	0.0
Actuarial gains (losses)				4.0	4.1
Benefits paid				(0.6)	(0.1)
Benefit Obligation, Ending		25.1		25.1	19.7	14.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0	0.0
Actual return on plans' assets				0.0	0.0
Employer contributions				0.6	0.1
Participants' contributions				0.0	0.0
Benefits paid				(0.6)	(0.1)
Fair Value of Plan Assets, Ending		0.0		0.0	0.0	0.0
Funded Status of Plan							(25.1)	(19.7)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost				1.3	1.1	1.2
Interest cost				0.7	0.6	0.5
Expected return on plan assets				0.0	0.0	0.0
Amortization of net loss				0.6	0.2	0.4
Amortization of unrecognized prior service cost	[1]			0.1	0.2	0.3
Curtailment				0.0	0.0	0.0
Settlement				0.0	0.0	0.0
Net periodic benefit cost				2.7	2.1	2.4
Amount Attributable to Unconsolidated Affiliates				0.1	0.1	0.1
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]			2.6	2.0	2.3
Fair Value of Plan Assets		0.0		0.0	0.0	0.0	0.0	0.0	$ 0.0
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated Benefit Obligation							610.9	688.4
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation, Beginning				725.0	658.1
Service cost				16.1	15.3	19.0
Actual return on plans' assets				26.1	28.1	26.7
Plan curtailments				0.0	(0.7)
Plan settlements				(60.7)	0.0
Participants' contributions				0.0	0.0
Actuarial gains (losses)				(11.3)	79.3
Benefits paid				(15.2)	(55.1)
Benefit Obligation, Ending		680.0		680.0	725.0	658.1
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				679.8	654.9
Actual return on plans' assets				(22.2)	80.0
Employer contributions				0.0	0.0
Participants' contributions				0.0	0.0
Benefits paid				(15.2)	(55.1)
Fair Value of Plan Assets, Ending		581.7		581.7	679.8	654.9
Funded Status of Plan							$ (98.3)	$ (45.2)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost				16.1	15.3	19.0
Interest cost				26.1	28.1	26.7
Expected return on plan assets				(46.0)	(45.3)	(48.4)
Amortization of net loss				18.0	14.3	26.5
Amortization of unrecognized prior service cost	[1]			0.4	1.7	1.8
Curtailment				0.0	(0.2)	0.0
Settlement		5.5	$ 16.2	21.7	0.0	22.4
Net periodic benefit cost				36.3	13.9	48.0
Amount Attributable to Unconsolidated Affiliates				4.2	3.2	5.9
Net Periodic Benefit Cost, Net of Unconsolidated Affiliates	[2]			32.1	10.7	42.1
Capitalized Portion of Net Periodic Benefit Cost				$ 5.0	$ 3.4	$ 6.4
Assumptions Used Calculating Benefit Obligation, Discount Rate							4.00%	3.80%	4.60%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets				7.50%	7.50%	8.00%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				4.20%	4.20%	4.20%
Fair Value of Plan Assets		581.7		$ 679.8	$ 654.9	$ 654.9	$ 581.7	$ 679.8	$ 654.9
Pension Plans, Defined Benefit [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Settlement				14.0		17.0
Additional Pension Expense to Meet State Requirements				12.8	11.2	$ 5.8
Pension Plans, Defined Benefit [Member] | ARKANSAS
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Settlement				1.9
Pension Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | U.S. common stocks [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				201.4
Fair Value of Plan Assets, Ending		189.0		189.0	201.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		189.0		201.4	201.4		189.0	201.4
Pension Plans, Defined Benefit [Member] | U.S. common stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				201.4
Fair Value of Plan Assets, Ending		189.0		189.0	201.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		189.0		201.4	201.4		189.0	201.4
Pension Plans, Defined Benefit [Member] | U.S. common stocks [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Foreign common stocks [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				31.3
Fair Value of Plan Assets, Ending		19.1		19.1	31.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		19.1		31.3	31.3		19.1	31.3
Pension Plans, Defined Benefit [Member] | Foreign common stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				31.3
Fair Value of Plan Assets, Ending		19.1		19.1	31.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		19.1		31.3	31.3		19.1	31.3
Pension Plans, Defined Benefit [Member] | Foreign common stocks [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | U.S. treasury notes and bonds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]			203.2
Fair Value of Plan Assets, Ending	[3]	158.9		158.9	203.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[3]	158.9		203.2	203.2		158.9	203.2
Pension Plans, Defined Benefit [Member] | U.S. treasury notes and bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]			203.2
Fair Value of Plan Assets, Ending	[3]	158.9		158.9	203.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[3]	158.9		203.2	203.2		158.9	203.2
Pension Plans, Defined Benefit [Member] | U.S. treasury notes and bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[3]			0.0
Fair Value of Plan Assets, Ending	[3]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[3]	0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				20.6
Fair Value of Plan Assets, Ending		2.2		2.2	20.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		2.2		20.6	20.6		2.2	20.6
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				20.6
Fair Value of Plan Assets, Ending		2.2		2.2	20.6
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		2.2		20.6	20.6		2.2	20.6
Pension Plans, Defined Benefit [Member] | Corporate fixed income and other securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			167.1
Fair Value of Plan Assets, Ending	[4]	140.2		140.2	167.1
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	140.2		167.1	167.1		140.2	167.1
Pension Plans, Defined Benefit [Member] | Corporate fixed income and other securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			0.0
Fair Value of Plan Assets, Ending	[4]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Corporate fixed income and other securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			167.1
Fair Value of Plan Assets, Ending	[4]	140.2		140.2	167.1
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	140.2		167.1	167.1		140.2	167.1
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			19.3
Fair Value of Plan Assets, Ending	[4]	12.3		12.3	19.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	12.3		19.3	19.3		12.3	19.3
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			0.0
Fair Value of Plan Assets, Ending	[4]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Mortgage-backed securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[4]			19.3
Fair Value of Plan Assets, Ending	[4]	12.3		12.3	19.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[4]	12.3		19.3	19.3		12.3	19.3
Pension Plans, Defined Benefit [Member] | Commingled fund [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[5]			25.1
Fair Value of Plan Assets, Ending	[5]	24.4		24.4	25.1
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[5]	24.4		25.1	25.1		24.4	25.1
Pension Plans, Defined Benefit [Member] | Commingled fund [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[5]			0.0
Fair Value of Plan Assets, Ending	[5]	0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[5]	0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Commingled fund [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[5]			25.1
Fair Value of Plan Assets, Ending	[5]	24.4		24.4	25.1
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[5]	24.4		25.1	25.1		24.4	25.1
Pension Plans, Defined Benefit [Member] | Common/collective trust [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[6]			29.9
Fair Value of Plan Assets, Ending	[6]				29.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[6]			29.9	29.9			29.9
Pension Plans, Defined Benefit [Member] | Common/collective trust [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[6]			0.0
Fair Value of Plan Assets, Ending	[6]				0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[6]			0.0	0.0			0.0
Pension Plans, Defined Benefit [Member] | Common/collective trust [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning	[6]			29.9
Fair Value of Plan Assets, Ending	[6]				29.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets	[6]			29.9	29.9			29.9
Pension Plans, Defined Benefit [Member] | Other [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		6.9		6.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		6.9		6.9			6.9
Pension Plans, Defined Benefit [Member] | Other [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0			0.0
Pension Plans, Defined Benefit [Member] | Other [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		6.9		6.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		6.9		6.9			6.9
Pension Plans, Defined Benefit [Member] | Foreign government bonds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				7.2
Fair Value of Plan Assets, Ending		5.6		5.6	7.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		5.6		7.2	7.2		5.6	7.2
Pension Plans, Defined Benefit [Member] | Foreign government bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Foreign government bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				7.2
Fair Value of Plan Assets, Ending		5.6		5.6	7.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		5.6		7.2	7.2		5.6	7.2
Pension Plans, Defined Benefit [Member] | U.S. municipal bonds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				3.5
Fair Value of Plan Assets, Ending		4.9		4.9	3.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		4.9		3.5	3.5		4.9	3.5
Pension Plans, Defined Benefit [Member] | U.S. municipal bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | U.S. municipal bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				3.5
Fair Value of Plan Assets, Ending		4.9		4.9	3.5
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		4.9		3.5	3.5		4.9	3.5
Pension Plans, Defined Benefit [Member] | Interest-bearing cash [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.2
Fair Value of Plan Assets, Ending		11.5		11.5	0.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		11.5		0.2	0.2		11.5	0.2
Pension Plans, Defined Benefit [Member] | Interest-bearing cash [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.2
Fair Value of Plan Assets, Ending		11.5		11.5	0.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		11.5		0.2	0.2		11.5	0.2
Pension Plans, Defined Benefit [Member] | Interest-bearing cash [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Money Market Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		11.7		11.7
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		11.7		11.7			11.7
Pension Plans, Defined Benefit [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		0.0		0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0			0.0
Pension Plans, Defined Benefit [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Ending		11.7		11.7
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		11.7		11.7			11.7
Pension Plans, Defined Benefit [Member] | Preferred stocks (foreign) [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				1.2
Fair Value of Plan Assets, Ending		0.3		0.3	1.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.3		1.2	1.2		0.3	1.2
Pension Plans, Defined Benefit [Member] | Preferred stocks (foreign) [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				1.2
Fair Value of Plan Assets, Ending		0.3		0.3	1.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.3		1.2	1.2		0.3	1.2
Pension Plans, Defined Benefit [Member] | Preferred stocks (foreign) [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Receivable (foreign currency) [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				11.3
Fair Value of Plan Assets, Ending		0.1		0.1	11.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.1		11.3	11.3		0.1	11.3
Pension Plans, Defined Benefit [Member] | Receivable (foreign currency) [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Receivable (foreign currency) [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				11.3
Fair Value of Plan Assets, Ending		0.1		0.1	11.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.1		11.3	11.3		0.1	11.3
Pension Plans, Defined Benefit [Member] | Payable (foreign currency) [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(15.6)
Fair Value of Plan Assets, Ending		(0.1)		(0.1)	(15.6)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(0.1)		(15.6)	(15.6)		(0.1)	(15.6)
Pension Plans, Defined Benefit [Member] | Payable (foreign currency) [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				0.0
Fair Value of Plan Assets, Ending		0.0		0.0	0.0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		0.0	0.0		0.0	0.0
Pension Plans, Defined Benefit [Member] | Payable (foreign currency) [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(15.6)
Fair Value of Plan Assets, Ending		(0.1)		(0.1)	(15.6)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(0.1)		(15.6)	(15.6)		(0.1)	(15.6)
Pension Plans, Defined Benefit [Member] | Total Plan investments [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				705.7
Fair Value of Plan Assets, Ending		587.0		587.0	705.7
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		587.0		705.7	705.7		587.0	705.7
Pension Plans, Defined Benefit [Member] | Total Plan investments [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				437.3
Fair Value of Plan Assets, Ending		378.8		378.8	437.3
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		378.8		437.3	437.3		378.8	437.3
Pension Plans, Defined Benefit [Member] | Total Plan investments [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				268.4
Fair Value of Plan Assets, Ending		208.2		208.2	268.4
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		208.2		268.4	268.4		208.2	268.4
Pension Plans, Defined Benefit [Member] | Receivable from broker for securities sold [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				3.2
Fair Value of Plan Assets, Ending		0.0		0.0	3.2
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		0.0		3.2	3.2		0.0	3.2
Pension Plans, Defined Benefit [Member] | Interest and dividends receivable [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				3.9
Fair Value of Plan Assets, Ending		3.5		3.5	3.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		3.5		3.9	3.9		3.5	3.9
Pension Plans, Defined Benefit [Member] | Payable to broker for securities purchased [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				(33.0)
Fair Value of Plan Assets, Ending		(8.8)		(8.8)	(33.0)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		(8.8)		(33.0)	(33.0)		(8.8)	(33.0)
Pension Plans, Defined Benefit [Member] | Total Plan assets [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets, Beginning				679.8
Fair Value of Plan Assets, Ending		581.7		581.7	679.8
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Fair Value of Plan Assets		$ 581.7		$ 679.8	$ 679.8		$ 581.7	$ 679.8
Less Than 90% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				50.00%
Projected Benefit Obligation Funded Status Thresholds Equity				50.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
95% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				58.00%
Projected Benefit Obligation Funded Status Thresholds Equity				42.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
100% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				65.00%
Projected Benefit Obligation Funded Status Thresholds Equity				35.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
105% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				73.00%
Projected Benefit Obligation Funded Status Thresholds Equity				27.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
110% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				80.00%
Projected Benefit Obligation Funded Status Thresholds Equity				20.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
115% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				85.00%
Projected Benefit Obligation Funded Status Thresholds Equity				15.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
120% [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Projected Benefit Obligation Funded Status Thresholds Fixed Income				90.00%
Projected Benefit Obligation Funded Status Thresholds Equity				10.00%
Projected Benefit Obligation Funded Status Thresholds				100.00%
Domestic All-Cap/Large Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				50.00%
Target Plan Asset Allocations Range Minimum				50.00%
Target Plan Asset Allocations Range Maximum				60.00%
Domestic Mid-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				15.00%
Target Plan Asset Allocations Range Minimum				5.00%
Target Plan Asset Allocations Range Maximum				25.00%
Domestic Small-Cap Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				15.00%
Target Plan Asset Allocations Range Minimum				5.00%
Target Plan Asset Allocations Range Maximum				25.00%
International Equity [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Target Plan Asset Allocations				20.00%
Target Plan Asset Allocations Range Minimum				10.00%
Target Plan Asset Allocations Range Maximum				30.00%

[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 $40.2 million, $19.3 million and $60.0 million of net periodic benefit cost recognized in 2015, 2014 and 2013, respectively, the Company recognized the following: •an increase in pension expense in 2015, 2014 and 2013 of $12.8 million, $11.2 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 asset or liability (see Note 1); •an increase in postretirement medical expense in 2015, 2014 and 2013 of $5.8 million, $5.2 million and $0.6 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 asset or liability (see Note 1); •a deferral of pension expense in 2015 and 2013 of $14.0 million and $17.0 million related to pension settlement charges of $21.7 million and $22.4 million, respectively, in accordance with the Oklahoma pension tracker regulatory liability (see Note 1); and •a deferral of pension expense in 2015 of $1.9 million related to the Arkansas jurisdictional portion of the pension settlement charge of $21.7 million.

[3] This category represents U.S. treasury notes and bonds with a Moody's Investors Services rating of Aaa and Government Agency Bonds with a Moody's Investors Services rating of A1 or higher.

[4] This category primarily represents U.S. corporate bonds with an investment grade rating at or above Baa3 or BBB- by Moody's Investors Services, Standard & Poor's Ratings Services or Fitch Ratings.

[5] This category represents units of participation in a commingled fund that primarily invested in stocks of international companies and emerging markets.

[6] This category represents units of participation in an investment pool which primarily invests in foreign or domestic bonds, debentures, mortgages, equipment or other trust certificates, notes, obligations issued or guaranteed by the U.S. Government or its agencies, bank certificates of deposit, bankers' acceptances and repurchase agreements, high grade commercial paper and other instruments with money market characteristics with a fixed or variable interest rate. There are no restrictions on redemptions in the common/collective trust.