Other Events (Details) T in Millions, $ in Millions, $ in Millions		1 Months Ended	3 Months Ended		6 Months Ended				9 Months Ended	12 Months Ended
	Apr. 30, 2019 USD ($)	Jun. 30, 2018 USD ($) T	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 AUD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 18, 2019	Sep. 30, 2018 employees	Feb. 06, 2018
Other Commercial Events [Line Items]
Joint Venture Ownership Percentage by Parent											66.50%
Joint Venture Ownership Percentage by Noncontrolling Owners											33.50%
Provision for North Goonyella equipment loss			$ 0.0	$ 0.0		$ 24.7	$ 0.0			$ 66.4
Carrying value of longwall			4,974.8			4,974.8			$ 4,974.8	5,207.0
Insurance recovery related to North Goonyella equipment losses			0.0	0.0		125.0	0.0
Proceeds from Sale of Property, Plant, and Equipment						15.8	52.6
Net gain on disposals			$ 0.2	$ (1.6)		$ 1.7	29.0
Interest in unincorporated joint venture project			50.00%			50.00%			50.00%
Asset impairment charges			$ 0.0			$ 0.0	0.0
At-risk carrying value			248.4			248.4			$ 248.4
DTA and PBGC
Other Commercial Events [Line Items]
Equity Method Investment, Ownership Percentage													50.00%
Red Mountain Joint Venture
Other Commercial Events [Line Items]
Proceeds from Sale of Property, Plant, and Equipment							20.0
Net gain on disposals							7.1
Queensland’s Bowen Basin
Other Commercial Events [Line Items]
Proceeds from Sale of Property, Plant, and Equipment								$ 37.0
Net gain on disposals							$ 20.6
Stanmore Coal Limited
Other Commercial Events [Line Items]
Coal Reserves, in tons | T		23.0
Proceeds from Sale of Property, Plant, and Equipment		$ 22.0				10.0
Accounts receivable, net of metallurgical coal reserves			4.0			4.0			4.0
North Goonyella Mine
Other Commercial Events [Line Items]
Number of employees harmed | employees												0
Costs Related to North Goonyella Events										58.0
Provision for North Goonyella equipment loss						24.7				$ 66.4
Containment and idling costs			28.4			65.3
Carrying value of longwall			$ 30.0			30.0			30.0
Insurance recovery related to North Goonyella equipment losses						125.0
Deductible Amount						$ 50.0
North Goonyella Mine | Yancoal Technology Development Pty Ltd
Other Commercial Events [Line Items]
Payments to exercise option to acquire equipment	$ 54.2
North Goonyella Mine | Other
Other Commercial Events [Line Items]
Provision for North Goonyella equipment loss									17.2
North Goonyella Mine | Leased Equipment
Other Commercial Events [Line Items]
Provision for North Goonyella equipment loss									50.7
North Goonyella Mine | Owned Equipment
Other Commercial Events [Line Items]
Provision for North Goonyella equipment loss									$ 23.2
Scenario, Forecast | North Goonyella Mine | Leased Equipment
Other Commercial Events [Line Items]
Costs Related to North Goonyella Events					$ 5.0
Scenario, Forecast | North Goonyella Mine | Mine Carrying Value
Other Commercial Events [Line Items]
Costs Related to North Goonyella Events					$ 290.0