Property, Plant and Equipment - Components of Property, Plant and Equipment (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 4,114.8	$ 3,905.0
Accumulated depreciation	(1,011.5)	(894.9)
Property, plant and equipment, net	3,103.3	3,010.1
Logistics Facilities and Railcars
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 385.9	385.5
Logistics Facilities and Railcars | Minimum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	20 years
Logistics Facilities and Railcars | Maximum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	25 years
Storage Facilities
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 19.5	19.5
Storage Facilities | Minimum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	20 years
Storage Facilities | Maximum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	25 years
Other
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 20.9	13.0
Other | Minimum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	20 years
Other | Maximum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	25 years
Construction-In-Progress
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 208.7	149.3
Gathering Assets | Pipelines
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	22 years
Total property, plant and equipment, at cost	$ 1,449.7	1,352.7
Gathering Assets | Compressors, Pumping Stations and Terminals
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 770.8	725.9
Gathering Assets | Compressors, Pumping Stations and Terminals | Minimum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	22 years
Gathering Assets | Compressors, Pumping Stations and Terminals | Maximum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	25 years
Gas Plant Assets | Pipelines, Pipes and Valves
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 460.0	460.0
Gas Plant Assets | Pipelines, Pipes and Valves | Minimum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	22 years
Gas Plant Assets | Pipelines, Pipes and Valves | Maximum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	25 years
Gas Plant Assets | Equipment
Property Plant And Equipment [Line Items]
Total property, plant and equipment, at cost	$ 428.3	428.2
Gas Plant Assets | Equipment | Minimum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	12 years
Gas Plant Assets | Equipment | Maximum
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	30 years
Gas Plant Assets | Buildings
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	35 years
Total property, plant and equipment, at cost	$ 182.3	182.3
Gas Plant Assets | Processing and Fractionation Facilities
Property Plant And Equipment [Line Items]
Property, plant and equipment, Estimated useful life	25 years
Total property, plant and equipment, at cost	$ 188.7	$ 188.6