Provision for Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Current:
Federal	$ 18.9		$ (4.0)		$ 2.2
State	28.9		18.9		16.0
Foreign	1.2		0.2		0
Total current	49.0		15.1		18.2
Deferred:
Federal	(64.7)		11.5		5.3
State	(1.4)		0.8		2.9
Foreign	(0.1)		(0.2)		(0.3)
Total deferred	(66.2)		12.1		7.9
Total provision for (benefit from) income taxes	(17.2)		27.2		26.1
Reconciliation of the provision for income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")	2,410.8		2,115.5		1,409.8
Texas Margin Tax	23.5	[1]	19.1	[1]	18.3	[1]
State income taxes (net of federal benefit)	5.3		0.5		0.4
Federal income taxes computed by applying the federal statutory rate to NBI of corporate entities	(1.6)		5.0		8.0
Valuation allowance	(2.0)		(0.2)		0
Expiration of tax net operating loss	2.4		0.2		0
Tax gain on conversion of corporate subsidiaries into limited liability companies	(45.3)		0		0
Other permanent differences	0.5		2.6		(0.6)
Total provision for (benefit from) income taxes	(17.2)		27.2		26.1
Effective income tax rate (in hundredths)	(0.70%)		1.30%		1.90%
Deferred tax assets:
Net operating loss carryovers	0.2	[2]	17.8	[2]
Employee benefit plans	0.1		3.0
Deferred revenue	0		1.5
Equity investment in partnerships	0		0.9
AROs	0		0.1
Accruals	1.5		1.6
Total deferred tax assets	1.8		24.9
Valuation allowance	0	[3]	2.0	[3]
Net deferred tax assets	1.8		22.9
Less: Deferred tax liabilities:
Property, plant and equipment	23.7		112.1
Equity investment in partnerships	0.6		0
Total deferred tax liabilities	24.3		112.1
Total net deferred tax liabilities	22.5		89.2
Current portion of total net deferred tax assets	0		2.0
Long-term portion of total net deferred tax liabilities	$ 22.5		$ 91.2
Operating loss carryforwards expiration period	between 2013 and 2028

[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 2013 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.