Significant Accounting Policies (PPE) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) $ / shares
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 8,380.8	$ 7,424.4
Accumulated depreciation	(2,124.1)	(1,757.6)
Property, plant and equipment, net	6,256.7	5,666.8
Depreciation expense	386.9	331.3	$ 247.8
Assets disposed	106.6
Gain (Loss) on Disposition of Property Plant Equipment	(13.2)	(1.2)	0.1
Impaired Assets to be Disposed of by Method Other than Sale, Carrying Value of Asset		5.1
Proceeds from Insurance Settlement, Investing Activities	0.3	2.9
Proceeds from Sale of Property, Plant, and Equipment	93.1	1.0	0.1
Gain on Business Interruption Insurance Recovery		2.4
Depreciation, Depletion and Amortization, Nonproduction	503.9	387.3	284.3
NPTL
Property, Plant and Equipment [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment	(13.4)
Proceeds from Sale of Property, Plant, and Equipment	$ 84.6
Number of miles of natural gas | item	140
Depreciation Method
Property, Plant and Equipment [Line Items]
Change in Accounting Estimate, Financial Effect			$ 29.4
Change in Accounting Estimate, Financial Effect (in dollars per share) | $ / shares			$ 0.18
EnLink Midstream Partners, LP
Property, Plant and Equipment [Line Items]
Tangible Asset Impairment Charges		12.1
Transmission Assets
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 1,191.7	1,285.1
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, Plant and Equipment	$ 3,530.9	2,999.2
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, Plant and Equipment	$ 3,163.0	2,673.7
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, Plant and Equipment	$ 149.5	135.9
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 Progress
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	$ 345.7	$ 330.5