Cash, Cash Equivalents and Restricted Cash (Schedule of Restricted Cash) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	$ 30.2	$ 16.3
Restricted cash included in other long-term assets	6.2	19.7
Other financial assurance [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	0.2	0.2
Restricted cash included in other long-term assets	0.3	0.3
ARO financial assurance [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	16.3	16.1
Restricted cash included in other long-term assets	0.0	0.0
Workers compensation financial assurance [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	0.0	0.0
Restricted cash included in other long-term assets	5.9	5.9
Lease turnover financial assurance [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash included in other current assets	13.7	0.0
Restricted cash included in other long-term assets	$ 0.0	$ 13.5