Significant Accounting Policies - Components of Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 9,120.0	$ 8,380.8
Accumulated depreciation	(2,533.0)	(2,124.1)
Property and equipment, net of accumulated depreciation	6,587.0	6,256.7
Transmission assets
Property, Plant and Equipment [Line Items]
Property and equipment	$ 1,338.7	1,191.7
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	$ 4,040.9	3,530.9
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	$ 3,401.8	3,163.0
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	$ 157.8	149.5
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	$ 180.8	$ 345.7