Equity - Public Offering (Details) - USD ($) $ / shares in Units, $ in Millions						3 Months Ended			9 Months Ended
		Sep. 15, 2017	Jun. 09, 2016	May 23, 2016	Mar. 29, 2016	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2016	Sep. 30, 2017		Sep. 30, 2016	[3]
Class of Stock [Line Items]
Units issued in connection with public offering (in shares)									5,275,510
Net proceeds from public offering	[1]								$ 283.7
Contribution from general partner for additional shares issued		$ 2.8
Aggregate percentage of general partner interest									2.00%
Consideration for units issued to our general partner									$ 5.8	[2]	$ 9.8
Shell Treasury Center West Inc | Five Year Revolver
Class of Stock [Line Items]
Debt instrument term		5 years			5 years	5 years		5 years	5 years
Shell Treasury Center West Inc | Five Year Revolver | May 2016 Acquisition
Class of Stock [Line Items]
Debt instrument term				5 years
Shell Treasury Center West Inc | 364 Day Revolver
Class of Stock [Line Items]
Debt instrument term					364 days			364 days	364 days
Common Units | May 2016 Acquisition
Class of Stock [Line Items]
Units issued in connection with public offering (in shares)				10,500,000
Net proceeds from public offering				$ 345.8
General Partner
Class of Stock [Line Items]
Units issued in connection with public offering (in shares)		105,510	32,143		258,163				105,510
Aggregate percentage of general partner interest		2.00%	2.00%		2.00%	2.00%	2.00%	2.00%	2.00%
Consideration for units issued to our general partner			$ 1.1		$ 8.2	$ 2.9	$ 0.1	$ 0.5
General Partner | May 2016 Acquisition
Class of Stock [Line Items]
Units issued in connection with public offering (in shares)				214,285
Aggregate percentage of general partner interest				2.00%
Consideration for units issued to our general partner				$ 7.1
General Public | Common Units
Class of Stock [Line Items]
Units issued in connection with public offering (in shares)		5,170,000		1,575,000	12,650,000				5,170,000
Net proceeds from public offering		$ 135.2	51.8		$ 395.1
Gross proceeds from public offering			$ 52.4		$ 401.6
Common units per share (in USD per share)			$ 33.25		$ 31.75	$ 26.96	$ 31.51	$ 34.00	$ 26.96
Underwriter fees			$ 0.5		$ 6.3
Transaction fees			$ 0.1		$ 0.2	$ 0.4	$ 0.1	$ 0.1
General Public | Common Units | May 2016 Acquisition
Class of Stock [Line Items]
Units issued in connection with public offering (in shares)				10,500,000
Net proceeds from public offering				$ 345.8
Gross proceeds from public offering				$ 349.1
Common units per share (in USD per share)				$ 33.25
Underwriter fees				$ 2.9
Transaction fees				$ 0.4

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

[2] 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.

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