Other Events - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended	18 Months Ended
	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2019
Other Commercial Events [Line Items]
Restructuring charges		$ 8.1	$ 0.7	$ 31.1	$ 1.3
Interest in unincorporated joint venture project		50.00%		50.00%
Contribution to construction and development of joint venture				$ 45.0
Provision for North Goonyella equipment loss		$ 0.0	0.0	0.0	24.7	$ 66.4
Insurance recovery related to North Goonyella equipment losses		0.0	0.0	0.0	125.0
North Goonyella Mine
Other Commercial Events [Line Items]
Provision for North Goonyella equipment loss					24.7		$ 149.6
Containment and idling costs		$ 3.8	$ 29.3	25.2	94.6
Insurance recovery related to North Goonyella equipment losses	$ 125.0				$ 125.0
Deductible amount	$ 50.0
North Goonyella Mine | Mine Carrying Value
Other Commercial Events [Line Items]
Costs related to North Goonyella events				$ 300.0