Segments (Summary Of Segment Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 3,880.9	$ 2,520.5	$ 2,632.8
Intersegment Revenues	0.0	0.0	0.0
Costs of product/services sold	3,374.7	1,925.1	1,883.5
Operations and maintenance	136.0	158.1	190.2
General and administrative	96.5	88.2	116.3
Loss on long-lived assets, net	(65.6)	(65.6)	(821.2)
Goodwill impairment	(38.8)	(162.6)	(1,406.3)
Loss on contingent consideration	(57.0)	0.0	0.0
Earnings (loss) from unconsolidated affiliates, net	47.8	31.5	(60.8)
Other income, net	1.3	0.5	0.6
EBITDA	161.4	152.9	(1,844.9)
Goodwill	147.6	199.0
Assets	4,284.9	4,448.9
Payments to Acquire Property, Plant, and Equipment	188.4	100.7	182.7
CMLP
Segment Reporting Information [Line Items]
Operating revenues	3,880.9	2,520.5	2,632.8
Intersegment Revenues	0.0	0.0	0.0
Costs of product/services sold	3,374.7	1,925.1	1,883.5
Operations and maintenance	136.0	155.0	188.7
General and administrative	93.1	85.6	105.6
Loss on long-lived assets, net	(65.6)	(65.6)	(227.8)
Goodwill impairment	(38.8)	(162.6)	(1,149.1)
Loss on contingent consideration	(57.0)	0.0	0.0
Earnings (loss) from unconsolidated affiliates, net	47.8	31.5	(60.8)
Other income, net	0.8
EBITDA	164.3	158.1	(982.7)
Goodwill	147.6	199.0	1,085.5
Assets	4,466.3	4,640.6
Payments to Acquire Property, Plant, and Equipment	188.4	100.7	182.7
Gathering systems and pipelines and related assets
Segment Reporting Information [Line Items]
Operating revenues	1,688.2	1,118.8	1,381.0
Intersegment Revenues	134.5	108.6	66.7
Costs of product/services sold	1,480.8	917.0	1,103.9
Operations and maintenance	68.4	77.0	89.0
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(14.4)	(2.0)	(787.3)
Goodwill impairment	0.0	(8.6)	(329.7)
Loss on contingent consideration	0.0
Earnings (loss) from unconsolidated affiliates, net	18.9	20.3	(43.4)
Other income, net	0.8	0.0	0.0
EBITDA	278.8	243.1	(905.6)
Goodwill	45.9	45.9
Assets	2,474.1	2,359.7
Payments to Acquire Property, Plant, and Equipment	162.7	76.6	132.7
Gathering systems and pipelines and related assets | CMLP
Segment Reporting Information [Line Items]
Operating revenues	1,688.2	1,118.8	1,381.0
Intersegment Revenues	134.5	108.6	66.7
Costs of product/services sold	1,480.8	917.0	1,103.9
Operations and maintenance	68.4	77.0	89.0
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(14.4)	(2.0)	(194.1)
Goodwill impairment	0.0	(8.6)	(72.5)
Loss on contingent consideration	0.0
Earnings (loss) from unconsolidated affiliates, net	18.9	20.3	(43.4)
Other income, net	0.8
EBITDA	278.8	243.1	(55.2)
Goodwill	45.9	45.9
Assets	2,662.0	2,561.9
Payments to Acquire Property, Plant, and Equipment	162.7	76.6	132.7
Storage and Transportation
Segment Reporting Information [Line Items]
Operating revenues	37.2	165.3	266.3
Intersegment Revenues	6.7	4.2	0.0
Costs of product/services sold	0.3	5.1	20.1
Operations and maintenance	4.2	21.4	31.7
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	0.0	(32.2)	(1.6)
Goodwill impairment	0.0	(44.9)	(623.4)
Loss on contingent consideration	(57.0)
Earnings (loss) from unconsolidated affiliates, net	28.9	11.2	(17.4)
Other income, net	0.0	0.0	0.0
EBITDA	11.3	77.1	(427.9)
Goodwill	0.0	0.0
Assets	1,040.6	1,094.6
Payments to Acquire Property, Plant, and Equipment	1.3	3.3	26.4
Storage and Transportation | CMLP
Segment Reporting Information [Line Items]
Operating revenues	37.2	165.3	266.3
Intersegment Revenues	6.7	4.2	0.0
Costs of product/services sold	0.3	5.1	20.1
Operations and maintenance	4.2	18.3	30.2
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	0.0	(32.2)	(1.4)
Goodwill impairment	0.0	(44.9)	(623.4)
Loss on contingent consideration	(57.0)
Earnings (loss) from unconsolidated affiliates, net	28.9	11.2	(17.4)
Other income, net	0.0
EBITDA	11.3	80.2	(426.2)
Goodwill	0.0	0.0
Assets	1,040.6	1,094.6
Payments to Acquire Property, Plant, and Equipment	1.3	3.3	26.4
Marketing Supply and Logistics
Segment Reporting Information [Line Items]
Operating revenues	2,155.5	1,236.4	985.5
Intersegment Revenues	(141.2)	(112.8)	(66.7)
Costs of product/services sold	1,893.6	1,003.0	759.5
Operations and maintenance	63.4	59.7	69.5
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(48.2)	(31.4)	(32.3)
Goodwill impairment	(38.8)	(109.1)	(453.2)
Loss on contingent consideration	0.0
Earnings (loss) from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net	0.0	0.0	0.0
EBITDA	(29.7)	(79.6)	(395.7)
Goodwill	101.7	153.1
Assets	757.1	972.2
Payments to Acquire Property, Plant, and Equipment	17.7	19.1	22.8
Marketing Supply and Logistics | CMLP
Segment Reporting Information [Line Items]
Operating revenues	2,155.5	1,236.4	985.5
Intersegment Revenues	(141.2)	(112.8)	(66.7)
Costs of product/services sold	1,893.6	1,003.0	759.5
Operations and maintenance	63.4	59.7	69.5
General and administrative	0.0	0.0	0.0
Loss on long-lived assets, net	(48.2)	(31.4)	(32.3)
Goodwill impairment	(38.8)	(109.1)	(453.2)
Loss on contingent consideration	0.0
Earnings (loss) from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net	0.0
EBITDA	(29.7)	(79.6)	(395.7)
Goodwill	101.7	153.1
Assets	757.1	972.2
Payments to Acquire Property, Plant, and Equipment	17.7	19.1	22.8
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	96.5	88.2	116.3
Loss on long-lived assets, net	(3.0)	0.0	0.0
Goodwill impairment	0.0	0.0	0.0
Loss on contingent consideration	0.0
Earnings (loss) from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net	0.5	0.5	0.6
EBITDA	(99.0)	(87.7)	(115.7)
Goodwill	0.0	0.0
Assets	13.1	22.4
Payments to Acquire Property, Plant, and Equipment	6.7	1.7	0.8
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	93.1	85.6	105.6
Loss on long-lived assets, net	(3.0)	0.0	0.0
Goodwill impairment	0.0	0.0	0.0
Loss on contingent consideration	0.0
Earnings (loss) from unconsolidated affiliates, net	0.0	0.0	0.0
Other income, net	0.0
EBITDA	(96.1)	(85.6)	(105.6)
Goodwill	0.0	0.0
Assets	6.6	11.9
Payments to Acquire Property, Plant, and Equipment	$ 6.7	$ 1.7	$ 0.8