Business Combinations, Purchase Price Allocation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Mar. 29, 2018	Dec. 31, 2017	Apr. 30, 2017	Dec. 31, 2016
Liabilities assumed in business combination:
Goodwill	$ 5,745.2		$ 5,745.2		$ 5,745.2
Delaware Basin Gas Processing LLC [Member]
Consideration:
Purchase price for remaining 50% equity interest in Delaware Processing	154.5
Fair value of our 50% equity interest in Delaware Processing held before the acquisition	146.4
Total consideration	$ 300.9
Assets acquired in business combination:
Current assets, including cash		$ 10.8
Property, plant, and equipment		200.0
Total assets acquired		303.3
Liabilities assumed in business combination:
Current liabilities		(1.8)
Long-term liabilities		(0.6)
Total liabilities assumed		(2.4)
Total identifiable net assets		300.9
Goodwill		0.0
Description of Business Combination:
Cash acquired		3.9
Delaware Basin Gas Processing LLC [Member] | Contract-based intangibles [Member]
Assets acquired in business combination:
Intangible assets		82.6
Delaware Basin Gas Processing LLC [Member] | Customer relationship intangibles [Member]
Assets acquired in business combination:
Intangible assets		$ 9.9
Azure Midstream Partners, L.P. [Member]
Assets acquired in business combination:
Current assets, including cash				$ 3.1
Property, plant, and equipment				193.1
Total assets acquired				196.2
Liabilities assumed in business combination:
Current liabilities				(1.4)
Long-term liabilities				(3.4)
Total liabilities assumed				(4.8)
Total identifiable net assets				$ 191.4