Segments - Total Assets for Reportable Segments (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total assets		$ 3,371.7	$ 3,277.7
Interest and Other
Segment Reporting Information [Line Items]
Total assets		59.3	67.0
Gathering Opco | Operating Segments
Segment Reporting Information [Line Items]
Total assets		1,862.6	1,844.9
Processing and Storage | Operating Segments
Segment Reporting Information [Line Items]
Total assets	[1]	1,153.7	1,055.1
Terminaling and Export | Operating Segments
Segment Reporting Information [Line Items]
Total assets		$ 296.1	$ 310.7

[1] Includes investment in equity investees of $108.2 million as of September 30, 2020 and $107.8 as of December 31, 2019.