Segments (Summary Of Segment Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 3,654.1	$ 3,880.9	$ 2,520.5
Intersegment Revenues	0.0	0.0	0.0
Costs of product/services sold	3,129.4	3,374.7	1,925.1
Operations and maintenance	125.8	136.0	158.1
General and administrative	88.1	96.5	88.2
Loss on long-lived assets, net	(28.6)	(65.6)	(65.6)
Goodwill impairment	0.0	(38.8)	(162.6)
Loss on contingent consideration	0.0	(57.0)	0.0
Earnings from unconsolidated affiliates, net	53.3	47.8	31.5
Other income, net	0.4	1.3	0.5
EBITDA	335.9	161.4	152.9
Goodwill	138.6	147.6
Assets	4,294.5	4,284.9
Payments to Acquire Property, Plant, and Equipment	305.5	188.4	100.7
CMLP
Segment Reporting Information [Line Items]
Operating revenues	3,654.1	3,880.9	2,520.5
Intersegment Revenues	0.0	0.0	0.0
Costs of product/services sold	3,129.4	3,374.7	1,925.1
Operations and maintenance	125.8	136.0	155.0
General and administrative	83.5	93.1	85.6
Loss on long-lived assets, net	(28.6)	(65.6)	(65.6)
Goodwill impairment	0.0	(38.8)	(162.6)
Loss on contingent consideration	0.0	(57.0)	0.0
Earnings from unconsolidated affiliates, net	53.3	47.8	31.5
Other income, net		0.8
EBITDA	340.1	164.3	158.1
Goodwill	138.6	147.6
Assets	4,462.0	4,466.3
Payments to Acquire Property, Plant, and Equipment	305.5	188.4	100.7
Gathering systems and pipelines and related assets
Segment Reporting Information [Line Items]
Operating revenues	946.7	1,688.2	1,118.8
Intersegment Revenues	192.4	134.5	108.6
Costs of product/services sold	767.0	1,480.8	917.0
Operations and maintenance	71.7	68.4	77.0
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(3.0)	(14.4)	(2.0)
Goodwill impairment		0.0	(8.6)
Loss on contingent consideration		0.0
Earnings from unconsolidated affiliates, net	22.5	18.9	20.3
Other income, net	0.0	0.8	0.0
EBITDA	319.9	278.8	243.1
Goodwill	45.9	45.9
Assets	2,633.4	2,474.1
Payments to Acquire Property, Plant, and Equipment	294.7	162.7	76.6
Gathering systems and pipelines and related assets | CMLP
Segment Reporting Information [Line Items]
Operating revenues	946.7	1,688.2	1,118.8
Intersegment Revenues	192.4	134.5	108.6
Costs of product/services sold	767.0	1,480.8	917.0
Operations and maintenance	71.7	68.4	77.0
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(3.0)	(14.4)	(2.0)
Goodwill impairment		0.0	(8.6)
Loss on contingent consideration		0.0
Earnings from unconsolidated affiliates, net	22.5	18.9	20.3
Other income, net		0.8
EBITDA	319.9	278.8	243.1
Goodwill	45.9	45.9
Assets	2,807.1	2,662.0
Payments to Acquire Property, Plant, and Equipment	294.7	162.7	76.6
Storage and Transportation
Segment Reporting Information [Line Items]
Operating revenues	17.1	37.2	165.3
Intersegment Revenues	10.5	6.7	4.2
Costs of product/services sold	0.2	0.3	5.1
Operations and maintenance	3.3	4.2	21.4
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	0.0	0.0	(32.2)
Goodwill impairment		0.0	(44.9)
Loss on contingent consideration		(57.0)
Earnings from unconsolidated affiliates, net	30.8	28.9	11.2
Other income, net	0.0	0.0	0.0
EBITDA	54.9	11.3	77.1
Goodwill	0.0	0.0
Assets	1,004.4	1,040.6
Payments to Acquire Property, Plant, and Equipment	0.6	1.3	3.3
Storage and Transportation | CMLP
Segment Reporting Information [Line Items]
Operating revenues	17.1	37.2	165.3
Intersegment Revenues	10.5	6.7	4.2
Costs of product/services sold	0.2	0.3	5.1
Operations and maintenance	3.3	4.2	18.3
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	0.0	0.0	(32.2)
Goodwill impairment		0.0	(44.9)
Loss on contingent consideration		(57.0)
Earnings from unconsolidated affiliates, net	30.8	28.9	11.2
Other income, net		0.0
EBITDA	54.9	11.3	80.2
Goodwill	0.0	0.0
Assets	1,004.4	1,040.6
Payments to Acquire Property, Plant, and Equipment	0.6	1.3	3.3
Marketing Supply and Logistics
Segment Reporting Information [Line Items]
Operating revenues	2,690.3	2,155.5	1,236.4
Intersegment Revenues	(202.9)	(141.2)	(112.8)
Costs of product/services sold	2,362.2	1,893.6	1,003.0
Operations and maintenance	50.8	63.4	59.7
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(27.3)	(48.2)	(31.4)
Goodwill impairment		(38.8)	(109.1)
Loss on contingent consideration		0.0
Earnings from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net	0.0	0.0	0.0
EBITDA	47.1	(29.7)	(79.6)
Goodwill	92.7	101.7
Assets	612.5	757.1
Payments to Acquire Property, Plant, and Equipment	5.6	17.7	19.1
Marketing Supply and Logistics | CMLP
Segment Reporting Information [Line Items]
Operating revenues	2,690.3	2,155.5	1,236.4
Intersegment Revenues	(202.9)	(141.2)	(112.8)
Costs of product/services sold	2,362.2	1,893.6	1,003.0
Operations and maintenance	50.8	63.4	59.7
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(27.3)	(48.2)	(31.4)
Goodwill impairment		(38.8)	(109.1)
Loss on contingent consideration		0.0
Earnings from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net		0.0
EBITDA	47.1	(29.7)	(79.6)
Goodwill	92.7	101.7
Assets	612.5	757.1
Payments to Acquire Property, Plant, and Equipment	5.6	17.7	19.1
Corporate Segment
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0
Intersegment Revenues	0.0	0.0	0.0
Costs of product/services sold	0.0	0.0	0.0
Operations and maintenance	0.0	0.0	0.0
General and administrative	88.1	96.5	88.2
Loss on long-lived assets, net	1.7	(3.0)	0.0
Goodwill impairment		0.0	0.0
Loss on contingent consideration		0.0
Earnings from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net	0.4	0.5	0.5
EBITDA	(86.0)	(99.0)	(87.7)
Goodwill	0.0	0.0
Assets	44.2	13.1
Payments to Acquire Property, Plant, and Equipment	4.6	6.7	1.7
Corporate Segment | CMLP
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0
Intersegment Revenues	0.0	0.0	0.0
Costs of product/services sold	0.0	0.0	0.0
Operations and maintenance	0.0	0.0	0.0
General and administrative	83.5	93.1	85.6
Loss on long-lived assets, net	1.7	(3.0)	0.0
Goodwill impairment		0.0	0.0
Loss on contingent consideration		0.0
Earnings from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net		0.0
EBITDA	(81.8)	(96.1)	(85.6)
Goodwill	0.0	0.0
Assets	38.0	6.6
Payments to Acquire Property, Plant, and Equipment	$ 4.6	$ 6.7	$ 1.7