Supplemental Financial Statement Information - Other Non-operating Income (Expense) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Nonoperating Income (Expense) [Abstract]
Rent and royalty income		$ 3.1	$ 3.5	$ 1.4
Net gains on property and investments		1.3	0.5	1.0
Income (loss) from equity method investments		(1.0)	0.0	0.0
Gain on joint venture deconsolidation	$ 15.9	15.9	0.0	0.0
Other		1.2	0.0	0.0
Other income, net		$ 20.5	$ 4.0	$ 2.4