Equity - At-the-Market Program (Details) - USD ($)					3 Months Ended			9 Months Ended
	Sep. 15, 2017	Jun. 09, 2016	Mar. 29, 2016	Mar. 02, 2016	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2016	Sep. 30, 2017		Sep. 30, 2016	[2]
Class of Stock [Line Items]
Issuance of common units, maximum proceeds				$ 300,000,000
Units issued under the ATM Program (in shares)								5,402,985
Consideration for units issued to our general partner								$ 5,800,000	[1]	$ 9,800,000
Aggregate percentage of general partner interest								2.00%
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 | 364 Day Revolver
Class of Stock [Line Items]
Debt instrument term			364 days				364 days	364 days
General Partner
Class of Stock [Line Items]
Units issued under the ATM Program (in shares)					106,122	1,938	15,307	108,060
Consideration for units issued to our general partner		$ 1,100,000	$ 8,200,000		$ 2,900,000	$ 100,000	$ 500,000
Aggregate percentage of general partner interest	2.00%	2.00%	2.00%		2.00%	2.00%	2.00%	2.00%
General Public | Common Units
Class of Stock [Line Items]
Units issued under the ATM Program (in shares)					5,200,000	94,925	750,000	5,294,925
Net proceeds from the ATM Program					$ 139,800,000	$ 2,900,000	$ 25,400,000
Gross proceeds from the ATM Program					$ 140,200,000	$ 3,000,000	$ 25,500,000
Common units per share (in USD per share)		$ 33.25	$ 31.75		$ 26.96	$ 31.51	$ 34.00	$ 26.96
Transaction fees		$ 100,000	$ 200,000		$ 400,000	$ 100,000	$ 100,000

[1] 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.

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