Provision for Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
Federal		$ 0.9	$ 2.2	$ (0.5)
State		15.5	13.4	19.3
Foreign		1.7	1.4	0.8
Total current		18.1	17.0	19.6
Deferred:
Federal		(1.4)	2.2	(0.5)
State		(19.2)	3.5	38.9
Foreign		0.0	0.4	(0.5)
Total deferred		(20.6)	6.1	37.9
Total provision for (benefit from) income taxes		(2.5)	23.1	57.5
Reconciliation of the provision for (benefit from) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")		2,555.9	2,856.6	2,664.6
Texas Margin Tax	[1]	(3.7)	17.5	58.3
State income taxes (net of federal benefit)		0.7	0.2	(0.1)
Federal income taxes computed by applying the federal statutory rate to NBI of corporate entities		1.1	1.5	(1.4)
Expiration of tax net operating loss		0.0	0.0	0.1
Other permanent differences		(0.6)	3.9	0.6
Total provision for (benefit from) income taxes		$ (2.5)	$ 23.1	$ 57.5
Effective income tax rate (in hundredths)		(0.10%)	0.80%	2.20%
Deferred tax assets:
Net operating loss carryovers	[2]	$ 0.2	$ 0.3
Accruals		1.6	1.8
Total deferred tax assets		1.8	2.1
Less: Deferred tax liabilities:
Property, plant and equipment		44.9	64.4
Equity investment in partnerships		2.7	4.1
Total deferred tax liabilities		47.6	68.5
Total net deferred tax liabilities		45.8	66.4
Current portion of total net deferred tax assets		0.3	0.2
Long-term portion of total net deferred tax liabilities		$ 46.1	$ 66.6

[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. During 2015, certain legislative changes were enacted to the Texas Margin Tax, which reduced the tax rate for business entities that operate within the state.

[2] These losses expire in various years between 2016 and 2033 and are subject to limitations on their utilization.