Balance Sheet Accounts - Schedule of Other Noncurrent Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Conditional asset retirement obligations	$ 44.9	$ 44.0
Pension benefits, non-qualified	17.2	17.6
Deferred compensation	29.3	29.4
Deferred revenue	12.4	12.7
Postretirement medical and life insurance benefits	31.4	32.7
Competitive improvement program obligations	13.7	18.4
Uncertain income tax positions	4.9	2.8
Other	20.9	13.5
Other noncurrent liabilities	$ 174.7	$ 171.1