Acquisitions and Divestitures - Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Cash flows from operating activities
Net income	$ 74.5		$ 61.8	[1]	$ 218.2	[2],[3],[4]	$ 204.5	[1],[5]
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion	8.9		9.1	[1]	28.0	[3],[4]	27.1	[1],[5]
Non-cash interest expense					0.3	[4]	0.2	[5]
Undistributed equity earnings					(4.1)	[4]	2.7	[5]
Changes in operating assets and liabilities
Accounts receivable					(0.9)	[4]	7.3	[5]
Allowance oil					(1.7)	[4]	(3.8)	[5]
Prepaid expenses					4.4	[4]	5.0	[5]
Accounts payable					3.6	[4]	(2.3)	[5]
Deferred revenue					14.0	[4]	(0.4)	[5]
Accrued liabilities					13.9	[4]	8.2	[5]
Net cash provided by operating activities					276.0	[4]	248.5	[5]
Cash flows from investing activities
Capital expenditures					(35.5)	[4]	(28.8)	[5]
Acquisitions					(200.7)	[4]	(120.0)	[5]
Return of investment					12.3	[4]	9.6	[5]
Net cash used in investing activities					(222.7)	[4]	(139.2)	[5]
Cash flows from financing activities
Net proceeds from public offerings					277.9	[4]	818.1	[5]
Borrowing under credit facility					580.0	[4]	296.7	[5]
Contributions from general partner					5.8	[4]	9.8	[5]
Repayment of credit facilities					(265.0)	[4]	(410.0)	[5]
Capital distributions to general partner					(429.3)	[4]	(599.2)	[5]
Distributions to noncontrolling interest					(8.6)	[4]	(17.1)	[5]
Distributions to unitholders and general partner					(190.4)	[4]	(126.0)	[5]
Net distributions to Parent					(6.3)	[4]	(16.9)	[5]
Other contributions from Parent					13.6	[4]	3.1	[5]
Net cash used in financing activities					(3.3)	[4]	(41.5)	[5]
Net increase in cash and cash equivalents					50.0	[4]	67.8	[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	$ 171.9	[4]	160.8	[5]	171.9	[4]	160.8	[5]
Change in accrued capital expenditures					1.5	[4]	(5.3)	[5]
Other non-cash contributions from Parent					1.5	[4]	0.3	[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]
Other non-cash credit facilities issuance costs					0.0	[4]	(0.6)	[5]
Previously Reported
Cash flows from operating activities
Net income			58.8				193.1
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion			6.0				17.7
Non-cash interest expense							0.2
Undistributed equity earnings							2.7
Changes in operating assets and liabilities
Accounts receivable							6.6
Allowance oil							(3.6)
Prepaid expenses							4.3
Accounts payable							(1.6)
Deferred revenue							(0.4)
Accrued liabilities							5.1
Net cash provided by operating activities							224.1
Cash flows from investing activities
Capital expenditures							(21.3)
Acquisitions							(120.0)
Return of investment							9.6
Net cash used in investing activities							(131.7)
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							(17.1)
Distributions to unitholders and general partner							(126.0)
Net distributions to Parent							0.0
Other contributions from Parent							3.1
Net cash used in financing activities							(24.6)
Net increase in cash and cash equivalents							67.8
Cash and cash equivalents at beginning of the period					121.9		93.0
Cash and cash equivalents at end of the period			160.8				160.8
Change in accrued capital expenditures							(1.1)
Other non-cash contributions from Parent							0.3
Other non-cash capital distributions to general partner							(7.1)
Other non-cash contribution from general partner							7.1
Other non-cash credit facilities issuance costs							(0.6)
Adjustment
Cash flows from operating activities
Net income			3.0				11.4
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion			3.1				9.4
Non-cash interest expense							0.0
Undistributed equity earnings							0.0
Changes in operating assets and liabilities
Accounts receivable							0.7
Allowance oil							(0.2)
Prepaid expenses							0.7
Accounts payable							(0.7)
Deferred revenue							0.0
Accrued liabilities							3.1
Net cash provided by operating activities							24.4
Cash flows from investing activities
Capital expenditures							(7.5)
Acquisitions							0.0
Return of investment							0.0
Net cash used in investing activities							(7.5)
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							(16.9)
Other contributions from Parent							0.0
Net cash used in financing activities							(16.9)
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							(4.2)
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
Other non-cash credit facilities issuance costs							$ 0.0

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

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

[3] The financial information for the nine months ended September 30, 2017 reflects adjustments for the acquisition of the Shell Delta, Na Kika and Refinery Gas Pipeline Operations from January 1, 2017 through May 9, 2017.

[4] The financial information for the nine months ended September 30, 2017 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.