Segment Information - Other Financial Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Other financial information [Abstract]
Total assets		$ 17,569.2		$ 16,938.2
Goodwill		46.6		$ 46.6
Capital expenditures		905.6	$ 558.0
Operating Segments [Member]
Other financial information [Abstract]
Total assets	[1]	17,569.2	14,753.2
Goodwill		46.6	256.6
Operating Segments [Member] | Gathering and Processing [Member]
Other financial information [Abstract]
Total assets	[1]	11,713.9	10,908.5
Goodwill		46.6	256.6
Capital expenditures		417.8	273.2
Operating Segments [Member] | Logistics and Marketing [Member]
Other financial information [Abstract]
Total assets	[1]	5,644.9	3,595.1
Capital expenditures		470.9	251.0
Operating Segments [Member] | Other Segment [Member]
Other financial information [Abstract]
Total assets	[1]	87.9	103.0
Operating Segments [Member] | Corporate and Eliminations [Member]
Other financial information [Abstract]
Total assets	[1]	122.5	146.6
Capital expenditures		$ 16.9	$ 33.8

[1] Assets in the Corporate and Eliminations column primarily include tax-related assets, cash, prepaids and debt issuance costs for our revolving credit facilities.