CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Cash flows from operating activities
Net income	[1]	$ 391.8		$ 377.5	[2],[3]	$ 374.0	[2],[3]
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion		45.0		43.1	[2],[3]	37.9	[2],[3]
Loss from disposition of fixed assets		0.1		0.2	[2],[3]	0.3	[2],[3]
Non-cash interest expense		0.4		2.7	[2]	0.0	[2]
Allowance oil reduction to net realizable value		0.3		0.0	[2]	2.7	[2]
Undistributed equity earnings		(6.5)		4.1	[2]	(4.3)	[2]
Changes in operating assets and liabilities
Accounts receivable		(7.4)		0.3	[2]	3.4	[2]
Allowance oil		(3.4)		(4.0)	[2]	(4.7)	[2]
Prepaid expenses and other assets		(6.8)		(0.1)	[2]	(0.7)	[2]
Accounts payable		8.3		0.3	[2]	(7.7)	[2]
Deferred revenue and other unearned income		6.0		7.9	[2]	(14.2)	[2]
Accrued liabilities		4.6		(2.8)	[2]	9.1	[2]
Net cash provided by operating activities		432.4		429.2	[2]	395.8	[2]
Cash flows from investing activities
Capital expenditures		(58.0)		(45.8)	[2]	(51.7)	[2]
Acquisitions from Parent		(420.5)		(172.8)	[2]	(179.8)	[2]
Third party acquisitions		(37.9)		(42.0)	[2]	0.0	[2]
Capital contribution on acquisition		(12.0)		0.0	[2]	0.0	[2]
Purchase price adjustment		0.3		0.0	[2]	0.0	[2]
Return of investment		18.2		15.8	[2]	12.5	[2]
April 2017 Divestiture		0.8		0.0	[2]	0.0	[2]
Payment of pre-IPO distributions from investments to SPLC		0.0		0.0	[2]	(11.9)	[2]
Net cash used in investing activities		(509.1)		(244.8)	[2]	(230.9)	[2]
Cash flows from financing activities
Net proceeds from private equity placement		0.0		0.0	[2]	297.4	[2]
Net proceeds from public offerings		277.9		818.1	[2]	296.8	[2]
Borrowing under credit facilities		1,693.1		638.7	[2]	458.2	[2]
Contributions from general partner		5.8		9.8	[2]	6.1	[2]
Repayment of credit facilities		(533.1)		(410.0)	[2]	0.0	[2]
Capital distributions to general partner		(1,034.5)		(896.3)	[2]	(1,002.1)	[2]
Distributions to noncontrolling interest		(18.9)		(30.2)	[2]	(73.2)	[2]
Distributions to unitholders and general partner		(267.8)		(179.9)	[2]	(96.5)	[2]
Other contributions from Parent		18.2		14.4	[2]	40.4	[2]
Credit facility issuance costs		(2.4)		(0.6)	[2]	(0.3)	[2]
Net distributions to Parent		(65.6)		(120.0)	[2]	(148.1)	[2]
Proceeds from April 2017 Divestiture		20.2		0.0	[2]	0.0	[2]
Other		(0.6)		(0.1)	[2]	0.0	[2]
Net cash provided by (used in) financing activities		92.3		(156.1)	[2]	(221.3)	[2]
Net increase (decrease) in cash and cash equivalents		15.6		28.3	[2]	(56.4)	[2]
Cash at beginning of the year	[2]	122.1	[4]	93.8		150.2
Cash at end of the year		137.7		122.1	[2],[4]	93.8	[2]
Non-cash investing activity
Increase in asset retirement obligation		0.0		(1.0)	[2]	(0.2)	[2]
Non-cash investing and financing transactions
Change in accrued capital expenditures		(0.1)		(1.1)	[2]	(6.5)	[2]
Contribution of fixed assets from Parent		0.0		0.0	[2]	0.4	[2]
Net assets not contributed to the Partnership		(5.1)		0.0	[2]	(6.4)	[2]
Commencement of capital lease		0.0		0.0	[2]	22.8	[2]
Other non-cash contributions from Parent		0.0		0.2	[2]	1.8	[2]
Other non-cash capital distributions to general partner		0.0		(7.1)	[2]	0.0	[2]
Other non-cash contribution from general partner		$ 0.0		$ 7.1	[2]	$ 0.0	[2]

[1] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[2] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[3] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[4] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.