Leases (Investment in Direct Financing Lease) (Details) $ in Millions	Dec. 31, 2017 USD ($) mi	Dec. 31, 2016 USD ($)
Direct Financing Lease [Line Items]
Direct Financing Lease, Minimum Payments to be Received	$ 19.2	$ 23.3
Direct Financing Lease, Unearned Income	4.1	4.8
Direct Financing Lease, Net Investment	15.1	18.5
Direct Financing Leases, Future Minimum Payments Receivable, Next Twelve Months	1.7
Direct Financing Leases, Future Minimum Payments, Receivable in Two Years	1.7
Direct Financing Leases, Future Minimum Payments Due in Three Years	1.7
Direct Financing Leases, Future Minimum Payments, Receivable in Four Years	1.7
Direct Financing Leases, Future Minimum Payments Due in Five Years	1.7
Other Current Assets [Member]
Direct Financing Lease [Line Items]
Direct Financing Lease, Net Investment	1.1	3.4
Other Noncurrent Assets [Member]
Direct Financing Lease [Line Items]
Direct Financing Lease, Net Investment	$ 14.0	$ 15.1
Texas and New Mexico Pipeline (Leased) [Member]
Direct Financing Lease [Line Items]
Pipeline, Length | mi	40