Provision for Income Taxes (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal		$ 5.3	$ 0.1	$ (0.5)
State		33.1	18.5	16.7
Foreign		0.5	1.0	0.6
Total current		38.9	19.6	16.8
Deferred:
Federal		(0.3)	(1.8)	1.1
State		21.7	7.9	5.2
Foreign		0.0	0.0	0.3
Total deferred		21.4	6.1	6.6
Total provision for (benefit from) income taxes		60.3	25.7	23.4
Reconciliation of the provision for (benefit from) income taxes [Abstract]
Pre-Tax Net Book Income ("NBI")		4,298.8	2,881.3	2,576.4
Texas Margin Tax	[1]	54.8	26.4	22.1
State income taxes (net of federal benefit)		0.2	0.5	0.2
Federal income taxes computed by applying the federal statutory rate to NBI of corporate entities		2.1	0.1	0.8
Other permanent differences		3.2	(1.3)	0.3
Total provision for (benefit from) income taxes		$ 60.3	$ 25.7	$ 23.4
Effective income tax rate		1.40%	0.90%	0.90%
Deferred tax assets:
Net operating loss carryovers	[2]	$ 0.1	$ 0.2
Accruals		2.6	1.4
Total deferred tax assets		2.7	1.6
Less: Deferred tax liabilities:
Property, plant and equipment		80.8	58.0
Equity investment in partnerships		2.3	2.1
Total deferred tax liabilities		83.1	60.1
Total net deferred tax liabilities		$ 80.4	$ 58.5

[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 2019 and 2033 and are subject to limitations on their utilization.