Certain Balance Sheet Information (Accrued Expenses and Other Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accrued Expenses and Other Liabilities [Line Items]
Accrued expenses	$ 46.9	$ 46.4
Accrued expenses and other liabilities	90.5	105.6
Accrued property taxes	4.2	4.8
Accrued natural gas purchases	4.9	1.5
Tax payable	1.2	0.5
Interest payable	22.8	26.2
Accrued additions to property, plant and equipment	1.7	10.4
Capital leases	1.3	1.6
Deferred revenue	7.5	14.2
Total accrued expenses and other liabilities	90.5	105.6
CMLP
Accrued Expenses and Other Liabilities [Line Items]
Accrued expenses	45.5	44.1
Accrued expenses and other liabilities	87.9	103.3
Accrued property taxes	4.2	4.8
Accrued natural gas purchases	4.9	1.5
Tax payable	0.0	0.5
Interest payable	22.8	26.2
Accrued additions to property, plant and equipment	1.7	10.4
Capital leases	1.3	1.6
Deferred revenue	7.5	14.2
Total accrued expenses and other liabilities	$ 87.9	$ 103.3