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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	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) employees	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($) employees	Sep. 30, 2018 AUD ($) employees	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 18, 2019	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	$ 49.3		$ 24.7	$ 49.3			$ 66.4
Carrying value of longwall			4,899.2			4,899.2			$ 4,899.2	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						27.6	69.0
Net gain on disposals			$ 1.1	20.8		$ 2.8	49.8
Interest in unincorporated joint venture project			50.00%			50.00%			50.00%
Asset impairment charges			$ 20.0	0.0		$ 20.0	0.0
At-risk carrying value			358.0			358.0			$ 358.0
DTA and PBGC
Other Commercial Events [Line Items]
Equity Method Investment, Ownership Percentage												50.00%
Red Mountain Joint Venture [Member]
Other Commercial Events [Line Items]
Proceeds from Sale of Property, Plant, and Equipment							20.0
Net gain on disposals							7.1
Stanmore Coal Limited [Member]
Other Commercial Events [Line Items]
Net gain on disposals				$ 20.5			20.5
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 [Member]
Other Commercial Events [Line Items]
Coal Reserves, in tons \| T		23.0
Proceeds from Sale of Property, Plant, and Equipment		$ 22.0
Other US Mining Operations [Member]
Other Commercial Events [Line Items]
Asset impairment charges						0.0
North Goonyella Mine
Other Commercial Events [Line Items]
Number of employees harmed \| employees				0			0	0
Provision for North Goonyella equipment loss			60.0	$ 49.3		24.7				66.4
Containment and idling costs			29.3	9.0		94.6	$ 9.0			$ 58.0
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
Wildcat Hills Underground [Member]
Other Commercial Events [Line Items]
Asset impairment charges			$ 20.0	$ 0.0		$ 20.0	$ 0.0
Scenario, Forecast \| North Goonyella Mine \| Mine Carrying Value
Other Commercial Events [Line Items]
Costs Related to North Goonyella Events					$ 272.0