Provision for Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Current:
Federal	$ 2.2		$ (0.5)		$ 18.9
State	13.4		19.3		28.9
Foreign	1.4		0.8		1.2
Total current	17.0		19.6		49.0
Deferred:
Federal	2.2		(0.5)		(64.7)
State	3.5		38.9		(1.4)
Foreign	0.4		(0.5)		(0.1)
Total deferred	6.1		37.9		(66.2)
Total provision for (benefit from) income taxes	23.1		57.5		(17.2)
Reconciliation of the provision for income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")	2,856.6		2,664.6		2,410.8
Texas Margin Tax	17.5	[1]	58.3	[1]	23.5	[1]
State income taxes (net of federal benefit)	0.2		(0.1)		5.3
Federal income taxes computed by applying the federal statutory rate to NBI of corporate entities	1.5		(1.4)		(1.6)
Valuation allowance			0		(2.0)
Expiration of tax net operating loss	0		0.1		2.4
Tax gain on conversion of corporate subsidiaries into limited liability companies	0		0		(45.3)
Other permanent differences	3.9		0.6		0.5
Total provision for (benefit from) income taxes	23.1		57.5		(17.2)
Effective income tax rate (in hundredths)	0.80%		2.20%		(0.70%)
Deferred tax assets:
Net operating loss carryovers	0.3	[2]	0.1	[2]
Employee benefit plans	0.3		0.2
Accruals	1.5		2.0
Total deferred tax assets	2.1		2.3
Less: Deferred tax liabilities:
Property, plant and equipment	64.4		59.8
Equity investment in partnerships	4.1		2.9
Total deferred tax liabilities	68.5		62.7
Total net deferred tax liabilities	66.4		60.4
Current portion of total net deferred tax assets	0.2		0.4
Long-term portion of total net deferred tax liabilities	66.6		60.8
Income tax expense due to changes in Texas Margin Tax			$ 19.6

[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 2015 and 2028 and are subject to limitations on their utilization.