Segments (Reconciliation of Net Income (Loss) to EBITDA) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net income (loss)		$ 67.0	$ (166.6)	$ (192.1)
Interest and debt expense, net		(99.2)	(99.4)	(125.1)
(Gain) loss on modification/extinguishment of debt	$ (10.0)	0.9	37.7	(10.0)
(Provision) benefit for income taxes		0.1	(0.8)	0.3
Depreciation, amortization and accretion		168.7	191.7	229.6
EBITDA		335.9	161.4	152.9
CMLP
Segment Reporting Information [Line Items]
Net income (loss)		58.6	(175.5)	(197.5)
Interest and debt expense, net		(99.2)	(99.4)	(125.1)
(Gain) loss on modification/extinguishment of debt		0.9	37.7	(10.0)
(Provision) benefit for income taxes		0.0	0.0	0.0
Depreciation, amortization and accretion		181.4	202.7	240.5
EBITDA		$ 340.1	$ 164.3	$ 158.1