Related Party Transactions - Summary of Revenues From Contracts With Customers on Disaggregated Basis (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 294.8	$ 269.8	$ 583.6	$ 560.4
Other income				0.2
Total revenues	294.8	269.8	583.6	560.6
Oil and Gas Gathering Services
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	133.4	113.1	262.1	230.5
Processing and Storage Services
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	105.3	86.8	208.8	183.6
Terminaling and Export Services
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	34.9	46.6	67.5	98.8
Water Gathering and Disposal Services
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 21.2	$ 23.3	$ 45.2	$ 47.5