Business Combinations, Purchase Price Allocation (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Mar. 29, 2018	Dec. 31, 2017
Liabilities assumed in business combination:
Goodwill	$ 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:
Other current assets, including cash of $3.9 million		$ 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 assets acquired less liabilities assumed		300.9
Total consideration given for ownership interests	$ 300.9
Goodwill		0.0
Description of Business Combination:
Cash acquired		3.9
Delaware Basin Gas Processing LLC [Member] | Customer relationship intangibles [Member]
Assets acquired in business combination:
Intangible assets		9.9
Delaware Basin Gas Processing LLC [Member] | Contract-based intangibles [Member]
Assets acquired in business combination:
Intangible assets		$ 82.6