Significant Accounting Policies - Components of Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Environmental remediation expense	$ 0.0	$ 0.0	$ 0.0
Property and equipment	10,499.9	9,814.1
Accumulated depreciation	(3,418.6)	(2,967.4)
Property and equipment, net of accumulated depreciation	7,081.3	6,846.7
Transmission assets
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,376.5	1,329.4
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	$ 4,856.5	4,410.5
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	$ 3,862.2	3,590.5
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	$ 188.0	171.7
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	15 years
Construction in process
Property, Plant and Equipment [Line Items]
Property and equipment	$ 216.7	$ 312.0