Significant Accounting Policies - Components of Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment	$ 10,515.1	$ 10,499.9
Accumulated depreciation	(3,863.0)	(3,418.6)
Property and equipment, net of accumulated depreciation	6,652.1	7,081.3
Transmission assets
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,410.5	1,376.5
Transmission assets | Minimum
Property, Plant and Equipment [Line Items]
Useful life	20 years
Transmission assets | Maximum
Property, Plant and Equipment [Line Items]
Useful life	25 years
Gathering systems
Property, Plant and Equipment [Line Items]
Property and equipment	$ 4,782.9	4,856.5
Gathering systems | Minimum
Property, Plant and Equipment [Line Items]
Useful life	20 years
Gathering systems | Maximum
Property, Plant and Equipment [Line Items]
Useful life	25 years
Gas processing plants
Property, Plant and Equipment [Line Items]
Property and equipment	$ 4,082.1	3,862.2
Gas processing plants | Minimum
Property, Plant and Equipment [Line Items]
Useful life	20 years
Gas processing plants | Maximum
Property, Plant and Equipment [Line Items]
Useful life	25 years
Other property and equipment
Property, Plant and Equipment [Line Items]
Property and equipment	$ 161.0	188.0
Other property and equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Other property and equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful life	25 years
Construction in process
Property, Plant and Equipment [Line Items]
Property and equipment	$ 78.6	$ 216.7