Acquisitions and Divestiture - 2015 Acquisitions (Details) - USD ($) $ in Millions								3 Months Ended	12 Months Ended
	Sep. 15, 2017	Jun. 09, 2016	May 18, 2016	Mar. 29, 2016	Nov. 17, 2015	Jul. 01, 2015	May 18, 2015	Sep. 30, 2017	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Business Acquisition [Line Items]
Net proceeds from private equity placement									$ 0.0		$ 0.0	[1]	$ 297.4	[1]
Contributions from general partner	$ 2.8								$ 5.8	[2]	$ 16.9	[2]	$ 6.1	[2]
Units issued in connection with public offering (in units)									5,275,510		25,229,591
November 2015 Acquisition
Business Acquisition [Line Items]
Ownership interest (in percentage)					100.00%
Consideration transferred					$ 390.0
Consideration, cash on hand					49.4
Transaction costs					0.3
Business combination capital distribution to general partner					290.0
July 2015 Acquisition
Business Acquisition [Line Items]
Consideration transferred						$ 350.0
Ownership interest (in percentage)						36.00%
Net assets acquired under common control						$ 30.5
Contribution in excess of book value of net assets acquired						319.5
Zydeco Pipeline Company LLC (“Zydeco”)
Business Acquisition [Line Items]
Additional minority interest ownership percentage by parent acquired							19.50%
Colonial
Business Acquisition [Line Items]
Additional minority interest ownership percentage by parent acquired							1.388%
May 2015 Acquisition
Business Acquisition [Line Items]
Consideration transferred							$ 448.0
Net proceeds from private equity placement							297.4
Consideration, cash on hand			$ 80.0				70.8
Transaction costs							0.5
Contribution in excess of book value of net assets acquired							392.6
Revolving Credit Facility | November 2015 Acquisition
Business Acquisition [Line Items]
Consideration, cash on hand					$ 37.4
365 Day Revolver | November 2015 Acquisition
Business Acquisition [Line Items]
Line of credit facility, expiration period									364 days
365 Day Revolver | July 2015 Acquisition
Business Acquisition [Line Items]
Line of credit facility, expiration period									365 days
Borrowings under credit facilities						$ 100.0
Five Year Revolver | July 2015 Acquisition
Business Acquisition [Line Items]
Line of credit facility, expiration period						5 years			5 years
Borrowings under credit facilities						$ 250.0
General Partner
Business Acquisition [Line Items]
Units issued in connection with public offering (in units)	105,510	32,143		258,163	187,755				105,510		504,591
General Partner | November 2015 Acquisition
Business Acquisition [Line Items]
Contributions from general partner					$ 6.1
Units issued in connection with public offering (in units)					187,755
Common Units
Business Acquisition [Line Items]
Net proceeds from private equity placement							$ 297.4	$ 139.8
Units issued in connection with public offering (in units)					9,200,000
Common Units | November 2015 Acquisition
Business Acquisition [Line Items]
Net proceeds from private equity placement					$ 297.4

[1] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[2] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.