Supplemental Financial Statement Information - Other Non-operating Income (Expense) (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Rent, royalty income and other income					$ 0.9	$ 2.9	$ 3.1
Gains from disposal of property, plant and equipment, net					2.9	90.7	1.3
Income (loss) from equity method investments					(7.0)	(10.7)	(1.0)
Loss on sale of businesses, net					0.0	(1.9)	0.0
Gain on joint venture deconsolidation					0.0	0.0	15.9
Non-cash joint venture impairment charge	$ (2.4)	$ (11.4)			(2.4)	(11.4)	0.0
Revisions of estimates					0.2	0.0
Other					0.1	0.1	1.2
Other income, net					(1.2)	69.7	20.5
Gain from sale of certain oil and gas rights			$ 62.4	$ 29.3		91.7
Gain on asset sales, net					2.5	89.8	0.0
Buyer of Formerly-Owned Business
Disaggregation of Revenue [Line Items]
Revisions of estimates					$ 4.3	$ 0.0	$ 0.0