Acquisitions and Divestitures - Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Cash flows from operating activities
Net income	$ 68.7	[1]	$ 70.9	[2]	$ 143.7	[1],[3],[4]	$ 142.7	[2],[5]
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion	9.6	[1]	8.9	[2]	19.1	[1],[4]	18.0	[2],[5]
Non-cash interest expense					0.1	[4]	0.1	[5]
Undistributed equity earnings					(1.5)	[4]	1.0	[5]
Changes in operating assets and liabilities
Accounts receivable					(14.2)	[4]	2.9	[5]
Allowance oil					0.7	[4]	0.4	[5]
Prepaid expenses					1.8	[4]	4.5	[5]
Accounts payable					5.2	[4]	(4.5)	[5]
Deferred revenue					10.4	[4]	(1.7)	[5]
Accrued liabilities					9.7	[4]	4.1	[5]
Net cash provided by operating activities					175.3	[4]	167.5	[5]
Cash flows from investing activities
Capital expenditures					(20.9)	[4]	(19.7)	[5]
Acquisitions					(210.6)	[4]	(93.7)	[5]
Return of investment					8.4	[4]	8.0	[5]
Net cash used in investing activities					(221.9)	[4]	(105.4)	[5]
Cash flows from financing activities
Net proceeds from public offerings					2.9	[4]	818.1	[5]
Borrowing under credit facility					580.0	[4]	296.7	[5]
Contributions from general partner					0.1	[4]	9.8	[5]
Repayment of credit facilities					0.0	[4]	(410.0)	[5]
Capital distributions to general partner					(419.4)	[4]	(599.2)	[5]
Distributions to noncontrolling interest					(6.6)	[4]	(14.4)	[5]
Distributions to unitholders and general partner					(122.2)	[4]	(77.0)	[5]
Net distributions to Parent					(6.3)	[4]	(10.9)	[5]
Other contributions from Parent					12.4	[4]	0.4	[5]
Net cash provided by financing activities					60.1	[4]	13.5	[5]
Net increase in cash and cash equivalents					13.5	[4]	75.6	[5]
Cash and cash equivalents at beginning of the period					121.9	[4],[6]	93.0	[5]
Cash and cash equivalents at end of the period	$ 135.4	[4]	168.6	[5]	135.4	[4]	168.6	[5]
Change in accrued capital expenditures					2.7	[4]	(4.4)	[5]
Other non-cash contributions from Parent					1.1	[4]	0.1	[5]
Other non-cash capital distributions to general partner					0.0	[4]	(7.1)	[5]
Other non-cash contribution from general partner					0.0	[4]	7.1	[5]
Previously Reported
Cash flows from operating activities
Net income			66.3				134.3
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion			5.8				11.7
Non-cash interest expense							0.1
Undistributed equity earnings							1.0
Changes in operating assets and liabilities
Accounts receivable							4.3
Allowance oil							(0.8)
Prepaid expenses							3.2
Accounts payable							(3.5)
Deferred revenue							(1.7)
Accrued liabilities							2.1
Net cash provided by operating activities							150.7
Cash flows from investing activities
Capital expenditures							(13.8)
Acquisitions							(93.7)
Return of investment							8.0
Net cash used in investing activities							(99.5)
Cash flows from financing activities
Net proceeds from public offerings							818.1
Borrowing under credit facility							296.7
Contributions from general partner							9.8
Repayment of credit facilities							(410.0)
Capital distributions to general partner							(599.2)
Distributions to noncontrolling interest							(14.4)
Distributions to unitholders and general partner							(77.0)
Net distributions to Parent							0.0
Other contributions from Parent							0.4
Net cash provided by financing activities							24.4
Net increase in cash and cash equivalents							75.6
Cash and cash equivalents at beginning of the period					121.9		93.0
Cash and cash equivalents at end of the period			168.6				168.6
Change in accrued capital expenditures							(0.6)
Other non-cash contributions from Parent							0.1
Other non-cash capital distributions to general partner							(7.1)
Other non-cash contribution from general partner							7.1
Adjustment
Cash flows from operating activities
Net income			4.6				8.4
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion			3.1				6.3
Non-cash interest expense							0.0
Undistributed equity earnings							0.0
Changes in operating assets and liabilities
Accounts receivable							(1.4)
Allowance oil							1.2
Prepaid expenses							1.3
Accounts payable							(1.0)
Deferred revenue							0.0
Accrued liabilities							2.0
Net cash provided by operating activities							16.8
Cash flows from investing activities
Capital expenditures							(5.9)
Acquisitions							0.0
Return of investment							0.0
Net cash used in investing activities							(5.9)
Cash flows from financing activities
Net proceeds from public offerings							0.0
Borrowing under credit facility							0.0
Contributions from general partner							0.0
Repayment of credit facilities							0.0
Capital distributions to general partner							0.0
Distributions to noncontrolling interest							0.0
Distributions to unitholders and general partner							0.0
Net distributions to Parent							(10.9)
Other contributions from Parent							0.0
Net cash provided by financing activities							(10.9)
Net increase in cash and cash equivalents							0.0
Cash and cash equivalents at beginning of the period					$ 0.0		0.0
Cash and cash equivalents at end of the period			$ 0.0				0.0
Change in accrued capital expenditures							(3.8)
Other non-cash contributions from Parent							0.0
Other non-cash capital distributions to general partner							0.0
Other non-cash contribution from general partner							$ 0.0

[1] The 2017 financial information reflects adjustments for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations from January 1, 2017 through May 9, 2017.

[2] Prior period financial information has been retrospectively adjusted for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations.

[3] Prior period financial information has been retrospectively adjusted for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations.

[4] The 2017 financial information reflects adjustments for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations from January 1, 2017 through May 9, 2017.

[5] Prior period financial information has been retrospectively adjusted for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations.

[6] Prior period financial information has been retrospectively adjusted for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations.