Acquisitions - Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015		Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net income	$ 58.8	$ 90.6	[1]	$ 193.1	$ 200.4	[1],[2]
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion	6.0	5.4	[1]	17.7	16.0	[2]
Allowance oil reduction to net realizable value				0.0	1.0	[2]
Changes in operating assets and liabilities
Accounts receivable				6.6	6.8	[2]
Allowance oil				(3.6)	(2.5)	[2]
Prepaid expenses				4.3	2.5	[2]
Accounts payable				(1.6)	(5.6)	[2]
Deferred revenue				(0.4)	(7.3)	[2]
Accrued liabilities				5.1	12.5	[2]
Net cash provided by operating activities				224.1	223.8	[2]
Cash flows from investing activities
Capital expenditures				(21.3)	(11.2)	[2]
Acquisitions				(120.0)	(85.9)	[2]
Return of investment				9.6	4.5	[2]
Payment of pre-IPO distributions from investments to SPLC				0.0	(11.9)	[2]
Net cash used in investing activities				(131.7)	(104.5)	[2]
Cash flows from financing activities
Borrowing under credit facility				296.7	420.8	[2]
Net proceeds from private placement				0.0	297.7	[2]
Contributions from general partner				9.8	6.1	[2]
Capital distributions to general partner				(599.2)	(712.1)	[2]
Distributions to noncontrolling interest				(17.1)	(55.9)	[2]
Distributions to unitholders and general partner				(126.0)	(66.2)	[2]
Credit facilities issuance costs				0.0	(0.3)	[2]
Net distribution to Parent				0.0	(42.1)	[2]
Net cash used in financing activities				(24.6)	(152.0)	[2]
Net increase (decrease) in cash and cash equivalents				67.8	(32.7)	[2]
Cash and cash equivalents at beginning of the period				93.0	150.2	[2]
Cash and cash equivalents at end of the period	$ 160.8	117.5	[2]	160.8	117.5	[2]
Change in accrued capital expenditures				(1.1)	(0.4)	[2]
Contribution of fixed assets from Parent				$ 0.0	0.4	[2]
Pecten
Cash flows from operating activities
Net income		90.6			200.4
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion		5.4			16.0
Allowance oil reduction to net realizable value					1.0
Changes in operating assets and liabilities
Accounts receivable					6.8
Allowance oil					(2.5)
Prepaid expenses					2.5
Accounts payable					(5.6)
Deferred revenue					(7.3)
Accrued liabilities					12.5
Net cash provided by operating activities					223.8
Cash flows from investing activities
Capital expenditures					(11.2)
Acquisitions					(85.9)
Return of investment					4.5
Payment of pre-IPO distributions from investments to SPLC					(11.9)
Net cash used in investing activities					(104.5)
Cash flows from financing activities
Borrowing under credit facility					420.8
Net proceeds from private placement					297.7
Contributions from general partner					6.1
Capital distributions to general partner					(712.1)
Distributions to noncontrolling interest					(55.9)
Distributions to unitholders and general partner					(66.2)
Credit facilities issuance costs					(0.3)
Net distribution to Parent					(42.1)
Net cash used in financing activities					(152.0)
Net increase (decrease) in cash and cash equivalents					(32.7)
Cash and cash equivalents at beginning of the period					150.2
Cash and cash equivalents at end of the period		117.5			117.5
Change in accrued capital expenditures					(0.4)
Contribution of fixed assets from Parent					0.4
Shell Midstream Partners, L.P. | Pecten
Cash flows from operating activities
Net income		74.4			161.1
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion		3.5			10.4
Allowance oil reduction to net realizable value					0.9
Changes in operating assets and liabilities
Accounts receivable					6.6
Allowance oil					(2.1)
Prepaid expenses					2.5
Accounts payable					(5.2)
Deferred revenue					(7.3)
Accrued liabilities					11.2
Net cash provided by operating activities					178.1
Cash flows from investing activities
Capital expenditures					(7.6)
Acquisitions					(85.9)
Return of investment					4.5
Payment of pre-IPO distributions from investments to SPLC					(11.9)
Net cash used in investing activities					(100.9)
Cash flows from financing activities
Borrowing under credit facility					420.8
Net proceeds from private placement					297.7
Contributions from general partner					6.1
Capital distributions to general partner					(712.1)
Distributions to noncontrolling interest					(55.9)
Distributions to unitholders and general partner					(66.2)
Credit facilities issuance costs					(0.3)
Net distribution to Parent					0.0
Net cash used in financing activities					(109.9)
Net increase (decrease) in cash and cash equivalents					(32.7)
Cash and cash equivalents at beginning of the period					150.2
Cash and cash equivalents at end of the period		117.5			117.5
Change in accrued capital expenditures					(0.4)
Contribution of fixed assets from Parent					0.4
Shell Auger and Lockport Operations | Pecten
Cash flows from operating activities
Net income		16.2			39.3
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion		1.9			5.6
Allowance oil reduction to net realizable value					0.1
Changes in operating assets and liabilities
Accounts receivable					0.2
Allowance oil					(0.4)
Prepaid expenses					0.0
Accounts payable					(0.4)
Deferred revenue					0.0
Accrued liabilities					1.3
Net cash provided by operating activities					45.7
Cash flows from investing activities
Capital expenditures					(3.6)
Acquisitions					0.0
Return of investment					0.0
Payment of pre-IPO distributions from investments to SPLC					0.0
Net cash used in investing activities					(3.6)
Cash flows from financing activities
Borrowing under credit facility					0.0
Net proceeds from private placement					0.0
Contributions from general partner					0.0
Capital distributions to general partner					0.0
Distributions to noncontrolling interest					0.0
Distributions to unitholders and general partner					0.0
Credit facilities issuance costs					0.0
Net distribution to Parent					(42.1)
Net cash used in financing activities					(42.1)
Net increase (decrease) in cash and cash equivalents					0.0
Cash and cash equivalents at beginning of the period					0.0
Cash and cash equivalents at end of the period		$ 0.0			0.0
Change in accrued capital expenditures					0.0
Contribution of fixed assets from Parent					$ 0.0

[1] (1) The 2015 financial information has been retrospectively adjusted for the acquisition of the Shell Auger and Lockport Operations.

[2] (1) The 2015 financial information has been retrospectively adjusted for the acquisition of the Shell Auger and Lockport Operations.