Certain Balance Sheet Information (Accrued Expenses and Other Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses and Other Liabilities [Line Items]
Accrued expenses	$ 56.6	$ 46.5
Accrued expenses and other liabilities	105.9	90.5
Accrued property taxes	4.8	4.2
Accrued natural gas purchases	0.0	4.9
Income tax payable	0.3	1.6
Interest payable	20.3	22.8
Accrued additions to property, plant and equipment	22.3	1.7
Capital leases	1.0	1.3
Deferred revenue	0.6	7.5
Total accrued expenses and other liabilities	105.9	90.5
CMLP
Accrued Expenses and Other Liabilities [Line Items]
Accrued expenses	55.5	45.1
Accrued expenses and other liabilities	104.7	87.9
Accrued property taxes	4.8	4.2
Accrued natural gas purchases	0.0	4.9
Income tax payable	0.3	0.4
Interest payable	20.3	22.8
Accrued additions to property, plant and equipment	22.2	1.7
Capital leases	1.0	1.3
Deferred revenue	0.6	7.5
Total accrued expenses and other liabilities	$ 104.7	$ 87.9