Significant Accounting Policies - Components of Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Environmental remediation expense	$ 0.0	$ 0.0	$ 0.0
Property and equipment	9,814.1	9,120.0
Accumulated depreciation	(2,967.4)	(2,533.0)
Property and equipment, net of accumulated depreciation	6,846.7	6,587.0
Transmission assets
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,329.4	1,338.7
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	$ 4,410.5	4,040.9
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	$ 3,590.5	3,401.8
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	$ 171.7	157.8
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	$ 312.0	$ 180.8