Cash, Cash Equivalents and Restricted Cash (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Restricted cash included in other current assets	$ 30.3	$ 16.3
Accrued lease turnover costs	1.6	0.8
Restricted cash included in other long-term assets	6.3	19.7
ARO financial assurance [Member]
Restricted cash included in other current assets	16.3	16.1
Lease turnover financial assurance [Member]
Restricted cash included in other current assets	13.8
Restricted cash included in other long-term assets	13.8	13.5
Workers compensation financial assurance [Member]
Restricted cash included in other long-term assets	$ 6.0	$ 5.9