Provision for Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal		$ 0.1	$ (0.5)	$ 0.9
State		18.5	16.7	15.5
Foreign		1.0	0.6	1.7
Total current		19.6	16.8	18.1
Deferred:
Federal		(1.8)	1.1	(1.4)
State		7.9	5.2	(19.2)
Foreign		0.0	0.3	0.0
Total deferred		6.1	6.6	(20.6)
Total provision for (benefit from) income taxes		25.7	23.4	(2.5)
Reconciliation of the provision for (benefit from) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")		2,881.3	2,576.4	2,555.9
Texas Margin Tax	[1]	26.4	22.1	(3.7)
State income taxes (net of federal benefit)		0.5	0.2	0.7
Federal income taxes computed by applying the federal statutory rate to NBI of corporate entities		0.1	0.8	1.1
Other permanent differences		(1.3)	0.3	(0.6)
Total provision for (benefit from) income taxes		$ 25.7	$ 23.4	$ (2.5)
Effective income tax rate		0.90%	0.90%	(0.10%)
Deferred tax assets:
Net operating loss carryovers	[2]	$ 0.2	$ 0.2
Accruals		1.4	1.6
Total deferred tax assets		1.6	1.8
Less: Deferred tax liabilities:
Property, plant and equipment		58.0	50.5
Equity investment in partnerships		2.1	3.7
Total deferred tax liabilities		60.1	54.2
Total net deferred tax liabilities		$ 58.5	$ 52.4

[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 2018 and 2033 and are subject to limitations on their utilization.