Provision for Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Current:
Federal	$ (4.0)		$ 2.2		$ 7.9
State	18.9		16.0		11.9
Foreign	0.2		0		1.0
Total current	15.1		18.2		20.8
Deferred:
Federal	11.5		5.3		4.8
State	0.8		2.9		(0.3)
Foreign	(0.2)		(0.3)		0
Total deferred	12.1		7.9		4.5
Total provision for income taxes	27.2		26.1		25.3
Reconciliation of the provision for income taxes [Abstract]
Pre-Tax Net Book Income (NBI)	2,115.5		1,409.8		1,165.6
Texas Margin Tax	19.1	[1]	18.3	[1]	10.1	[1]
State income taxes (net of federal benefit)	0.5		0.4		1.3
Federal income taxes computed by applying the federal statutory rate to NBI of corporate entities	5.0		8.0		8.3
Valuation allowance	(0.2)		0		(1.7)
Expiration of tax net operating loss	0.2		0		1.7
Other permanent differences	2.6		(0.6)		5.6
Total provision for income taxes	27.2		26.1		25.3
Effective income tax rate (in hundredths)	1.30%		1.90%		2.20%
Deferred tax assets:
Net operating loss carryovers	17.8	[2]	23.4	[2]
Employee benefit plans	3.0		3.1
Deferred revenue	1.5		1.2
Equity investment in partnerships	0.9		0.9
AROs	0.1		0.1
Accruals	1.6		1.4
Total deferred tax assets	24.9		30.1
Valuation allowance	2.0	[3]	2.2	[3]
Net deferred tax assets	22.9		27.9
Less: Deferred tax liabilities:
Property, plant and equipment	112.1		103.9
Total deferred tax liabilities	112.1		103.9
Total net deferred tax liabilities	89.2		76.0
Current portion of total net deferred tax assets	2.0		2.0
Long-term portion of total net deferred tax liabilities	91.2		78.0
Operating loss carryforwards expiration period	years between 2012 and 2028
Expected tax benefit	$ 46.5

[1] Although the Texas Margin Tax is not considered a state income tax, it has the characteristics of an income tax since it is determined by applying a tax rate to a base that considers our Texas-sourced revenues and expenses.

[2] These losses expire in various years between 2012 and 2028 and are subject to limitations on their utilization.

[3] We record a valuation allowance to reduce our deferred tax assets to the amount of future benefit that is more likely than not to be realized.