Retirement Plans and Postretirement Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension Plan Funding [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 35.0
Pension Contributions	35.0		50.0
Expected Future Benefit Payments, Next Twelve Months	75.1
Expected Future Benefit Payments, Year Two	94.5
Expected Future Benefit Payments, Year Three	84.7
Expected Future Benefit Payments, Year Four	77.2
Expected Future Benefit Payments, Year Five	71.5
Expected Future Benefit Payments, Five Fiscal Years Thereafter	295.3
Fair Value of Plan Assets	59.6		61.0
Effect of One Percentage Point Increase on Service and Interest Cost Components	0		0		3.1
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	0.1		0.1		0.7
Effect of One Percentage Point Decrease on Service and Interest Cost Components	0.1		0.1		2.5
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	0.9		0.6		1.6
Postretirement Plan, Expected Future Benefit Payments, Next Twelve Months	15.4
Postretirement Plan, Expected Future Benefit Payments, Year Two	16.3
Postretirement Plan, Expected Future Benefit Payments, Year Three	17.0
Postretirement Plan, Expected Future Benefit Payments in Year Four	17.6
Postretirement Plan, Expected Future Benefit Payments, Year Five	18.1
Postretirement Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	94.9
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Health Care Cost Trend Rate Assumed for Next Fiscal Year	8.55%
Postemployment Benefits Liability	2.6		2.4
Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	6.3		6.7
Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Fair Value, Inputs, Level 3 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	53.3		54.3
U.S. common stocks [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	232.2		179.7
U.S. common stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	232.2		179.7
U.S. common stocks [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Foreign common stocks [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	39.9		59.5
Foreign common stocks [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	39.9		59.5
Foreign common stocks [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
U.S. treasury notes and bonds [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	138.6	[1]	118.8	[1]
U.S. treasury notes and bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	138.6	[1]	118.8	[1]
U.S. treasury notes and bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0	[1]	0	[1]
Mortgage-backed securities
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	55.8		72.0
Mortgage-backed securities | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Mortgage-backed securities | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	55.8		72.0
Other securities [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets			1.0
Other securities [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets			0
Other securities [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets			1.0
Corporate fixed income and other securities [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	98.4	[2]	95.3	[2]
Corporate fixed income and other securities [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0	[2]	0	[2]
Corporate fixed income and other securities [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	98.4	[2]	95.3	[2]
Mortgage-backed securities [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	13.5	[2]	17.2	[2]
Mortgage-backed securities [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0	[2]	0	[2]
Mortgage-backed securities [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	13.5	[2]	17.2	[2]
Commingled fund [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	34.9	[3]	38.5	[4]
Commingled fund [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0	[3]	0	[4]
Commingled fund [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	34.9	[3]	38.5	[4]
Common/collective trust [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	25.6	[5]	29.6	[5]
Common/collective trust [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0	[5]	0	[5]
Common/collective trust [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	25.6	[5]	29.6	[5]
Foreign government bonds [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	3.9		2.9
Foreign government bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Foreign government bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	3.9		2.9
U.S. municipal bonds [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.8		1.7
U.S. municipal bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
U.S. municipal bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.8		1.7
Interest-bearing cash [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.2		2.1
Interest-bearing cash [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.2		2.1
Interest-bearing cash [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Preferred stocks (foreign) [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets			0.6
Preferred stocks (foreign) [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets			0.6
Preferred stocks (foreign) [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets			0
Receivable (foreign currency) [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.4		4.1
Receivable (foreign currency) [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Receivable (foreign currency) [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.4		4.1
Payable (foreign currency) [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	(0.4)		(4.1)
Payable (foreign currency) [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Payable (foreign currency) [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	(0.4)		(4.1)
Total Plan investments [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	643.8		618.9
Total Plan investments [Member] | Fair Value, Inputs, Level 1 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	410.9		360.7
Total Plan investments [Member] | Fair Value, Inputs, Level 2 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	232.9		258.2
Receivable from broker for securities sold [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0.8		4.8
Interest and dividends receivable [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	2.8		3.1
Payable to broker for securities purchased [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	(21.4)		(37.0)
Total Plan assets [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	626.0		589.8
Pension Plan [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	626.0		589.8		574.0
Accumulated Benefit Obligation	705.2		656.1
Benefit Obligation	(747.1)		(697.7)		(640.9)
Plan amendments	0		0
Participants' contributions	0		0
Medicare subsidies received	0		0
Actuarial gains (losses)	(61.4)		(48.3)
Benefits paid	60.0		42.4
Actual return on plans' assets	61.2		8.2
Employer contributions	35.0		50.0
Funded Status of Plan	(121.1)		(107.9)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	17.9		17.6		16.7
Interest cost	30.1		33.3		31.8
Expected return on plan assets	(46.0)		(45.5)		(42.4)
Amortization of transition obligations	0		0		0
Amortization of net loss	23.8		19.2		21.3
Amortization of unrecognized prior service cost	2.2	[6]	2.4	[6]	2.4	[6]
Settlement	0		0		0
Net periodic benefit cost	28.0	[7]	27.0	[7]	29.8	[7]
Assumptions Used Calculating Benefit Obligation, Discount Rate	3.70%		4.50%		5.30%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.00%		8.00%		8.50%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	4.20%		4.40%		4.40%
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0
Restoration of Retirement Income Plan [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	0		0		0
Accumulated Benefit Obligation	12.7		11.9
Benefit Obligation	(14.5)		(13.3)		(10.8)
Plan amendments	0		0
Participants' contributions	0		0
Medicare subsidies received	0		0
Actuarial gains (losses)	(1.8)		(1.0)
Benefits paid	2.2		0.1
Actual return on plans' assets	0		0
Employer contributions	2.2		0.1
Funded Status of Plan	(14.5)		(13.3)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	1.0		1.0		0.9
Interest cost	0.6		0.6		0.5
Expected return on plan assets	0		0		0
Amortization of transition obligations	0		0		0
Amortization of net loss	0.4		0.4		0.3
Amortization of unrecognized prior service cost	0.7	[6]	0.7	[6]	0.7	[6]
Settlement	0.9		0		0
Net periodic benefit cost	3.6	[7]	2.7	[7]	2.4	[7]
Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Net periodic benefit cost	51.4		45.1		61.4
Capitalized Portion of Net Periodic Benefit Cost	6.5		6.1		6.5
Pension Plan [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Pension Expense to Meet State Requirements	8.3		10.8		8.1
Postretirement Benefit Plan [Member]
Pension Plan Funding [Abstract]
Fair Value of Plan Assets	59.6		61.0		58.5
Benefit Obligation	(301.0)		(280.6)		(337.1)
Plan amendments	0		91.4
Participants' contributions	3.5		8.1
Medicare subsidies received	(0.5)		(2.0)
Actuarial gains (losses)	(12.9)		(25.7)
Benefits paid	12.5		16.9
Actual return on plans' assets	4.5		2.7
Employer contributions	2.6		6.6
Funded Status of Plan	(241.4)		(219.6)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	4.1		3.5		4.3
Interest cost	11.9		12.5		17.0
Expected return on plan assets	(3.0)		(5.1)		(6.9)
Amortization of transition obligations	2.7		2.7		2.7
Amortization of net loss	20.6		18.3		12.1
Amortization of unrecognized prior service cost	(16.5)	[6]	(16.5)	[6]	0	[6]
Settlement	0		0		0
Net periodic benefit cost	19.8	[7]	15.4	[7]	29.2	[7]
Capitalized Portion of Net Periodic Benefit Cost	5.5		3.8		6.5
Assumptions Used Calculating Benefit Obligation, Discount Rate	3.60%		4.50%		5.30%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	4.00%		6.50%		8.50%
Health Care Cost Trend Rate Assumed for Next Fiscal Year	8.55%		8.75%		8.99%
Ultimate Health Care Cost Trend Rate	4.48%		4.48%		5.00%
Year that Rate Reaches Ultimate Trend Rate	2028		2028		2020
Postretirement Benefit Plan [Member] | OKLAHOMA
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Additional Postretirement Medical Expense to Meet State Requirements	0.8		3.5
401K [Member]
Pension Plan Funding [Abstract]
Employer contributions	$ 13.4		$ 12.3		$ 11.4
Less Than 90% [Member]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [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]
Pension Plan Funding [Abstract]
Target Plan Asset Allocations	20.00%
Target Plan Asset Allocations Range Minimum	10.00%
Target Plan Asset Allocations Range Maximum	30.00%

[1] 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.

[2] 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.

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

[4] This category represents units of participation in a commingled fund that primarily invest in stocks and bonds of U.S. companies.

[5] 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.

[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.

[7] In addition to the $51.4 million, $45.1 million and $61.4 million and of net periodic benefit cost recognized in 2012, 2011 and 2010, respectively, the Company recognized the following: •an increase in pension expense in 2012, 2011 and 2010 of $8.3 million, $10.8 million and $8.1 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 2012 and 2011 of $0.8 million and $3.5 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).