Other Events (Details) $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2020 employees	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Jun. 18, 2019
Other Commercial Events [Line Items]
Restructuring Charges		$ 6.5	$ 0.2
Provision for North Goonyella equipment loss		0.0	24.7	$ 66.4
Carrying value of longwall		4,607.8			$ 4,679.1
Insurance recovery related to North Goonyella equipment losses		0.0	125.0
Proceeds from Sale of Property, Plant, and Equipment		10.5	11.0
Net gain on disposals		$ 8.1	1.5
Interest in unincorporated joint venture project		50.00%
At-risk carrying value		$ 2,100.0
Other US Mining Operations [Member]
Other Commercial Events [Line Items]
Asset impairment charges		0.0
North Goonyella Mine
Other Commercial Events [Line Items]
Costs Related to North Goonyella Events		10.1	36.9
Containment and idling costs			9.0
Insurance recovery related to North Goonyella equipment losses		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
Wildcat Hills Underground [Member]
Other Commercial Events [Line Items]
Asset impairment charges		$ 0.0	$ 0.0
Scenario, Forecast
Other Commercial Events [Line Items]
Number of employees resulting from workforce reduction | employees	250
PRB Colorado Joint Venture [Member]
Other Commercial Events [Line Items]
Joint Venture Ownership Percentage by Parent						66.50%
Joint Venture Ownership Percentage by Noncontrolling Owners						33.50%