Segments - Total Assets for Reportable Segments (Detail) - USD ($) $ in Millions		Jun. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total assets		$ 3,386.4	$ 3,374.5
Interest and Other
Segment Reporting Information [Line Items]
Total assets		75.6	55.7
Gathering Opco | Operating Segments
Segment Reporting Information [Line Items]
Total assets		1,869.3	1,856.2
Processing and Storage | Operating Segments
Segment Reporting Information [Line Items]
Total assets	[1]	1,153.6	1,170.3
Terminaling and Export | Operating Segments
Segment Reporting Information [Line Items]
Total assets		$ 287.9	$ 292.3

[1] Includes investment in equity investees of $102.5 million as of June 30, 2021 and $108.4 million as of December 31, 2020.