Cash, Cash Equivalents and Restricted Cash (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Restricted cash included in other current assets	$ 30.2	$ 16.3
Accrued lease turnover costs	1.6	1.8
Restricted cash included in other long-term assets	6.2	19.7
ARO financial assurance [Member]
Restricted cash included in other current assets	16.3	16.1
Restricted cash included in other long-term assets	0.0	0.0
Lease turnover financial assurance [Member]
Restricted cash included in other current assets	13.7	0.0
Restricted cash included in other long-term assets	$ 0.0	$ 13.5