Segment Information - Revenues and Operating Margin (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues:
Revenues	$ 2,299.4	$ 2,455.6
Operating margin	383.2	341.4
Sales of Commodities [Member]
Revenues:
Revenues	1,976.5	2,173.7
Fees from Midstream Services [Member]
Revenues:
Revenues	322.9	281.9
Gathering and Processing [Member]
Revenues:
Revenues	1,272.2	1,294.9
Operating margin	229.0	220.8
Logistics and Marketing [Member]
Revenues:
Revenues	1,893.7	2,109.5
Operating margin	152.1	138.4
Other Segment [Member]
Revenues:
Revenues	2.1	(17.8)
Operating margin	2.1	(17.8)
Corporate and Eliminations [Member]
Revenues:
Revenues	(868.6)	(931.0)
Operating Segments [Member]
Revenues:
Revenues	2,299.4	2,455.6
Operating Segments [Member] | Sales of Commodities [Member]
Revenues:
Revenues	1,976.5	2,173.7
Operating Segments [Member] | Fees from Midstream Services [Member]
Revenues:
Revenues	322.9	281.9
Operating Segments [Member] | Gathering and Processing [Member]
Revenues:
Revenues	447.5	426.5
Operating Segments [Member] | Gathering and Processing [Member] | Sales of Commodities [Member]
Revenues:
Revenues	247.6	265.2
Operating Segments [Member] | Gathering and Processing [Member] | Fees from Midstream Services [Member]
Revenues:
Revenues	199.9	161.3
Operating Segments [Member] | Logistics and Marketing [Member]
Revenues:
Revenues	1,849.8	2,046.9
Operating Segments [Member] | Logistics and Marketing [Member] | Sales of Commodities [Member]
Revenues:
Revenues	1,726.8	1,926.3
Operating Segments [Member] | Logistics and Marketing [Member] | Fees from Midstream Services [Member]
Revenues:
Revenues	123.0	120.6
Operating Segments [Member] | Other Segment [Member]
Revenues:
Revenues	2.1	(17.8)
Operating Segments [Member] | Other Segment [Member] | Sales of Commodities [Member]
Revenues:
Revenues	2.1	(17.8)
Intersegment Eliminations [Member] | Gathering and Processing [Member]
Revenues:
Revenues	824.7	868.4
Intersegment Eliminations [Member] | Gathering and Processing [Member] | Sales of Commodities [Member]
Revenues:
Revenues	822.8	866.5
Intersegment Eliminations [Member] | Gathering and Processing [Member] | Fees from Midstream Services [Member]
Revenues:
Revenues	1.9	1.9
Intersegment Eliminations [Member] | Logistics and Marketing [Member]
Revenues:
Revenues	43.9	62.6
Intersegment Eliminations [Member] | Logistics and Marketing [Member] | Sales of Commodities [Member]
Revenues:
Revenues	38.4	55.7
Intersegment Eliminations [Member] | Logistics and Marketing [Member] | Fees from Midstream Services [Member]
Revenues:
Revenues	5.5	6.9
Intersegment Eliminations [Member] | Corporate and Eliminations [Member]
Revenues:
Revenues	(868.6)	(931.0)
Intersegment Eliminations [Member] | Corporate and Eliminations [Member] | Sales of Commodities [Member]
Revenues:
Revenues	(861.2)	(922.2)
Intersegment Eliminations [Member] | Corporate and Eliminations [Member] | Fees from Midstream Services [Member]
Revenues:
Revenues	$ (7.4)	$ (8.8)