Other Events (Details) T in Millions, $ in Millions, $ in Millions			1 Months Ended		3 Months Ended							9 Months Ended	12 Months Ended				15 Months Ended
	Nov. 28, 2017 USD ($) T	Nov. 27, 2017 USD ($)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) T	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Apr. 01, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 AUD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Jun. 18, 2019	Feb. 06, 2018	Nov. 30, 2016 USD ($)
OtherCommercialEvents [Line Items]
Interest in unincorporated joint venture project					50.00%								50.00%				50.00%
Insurance Recoveries						$ 125.0				$ 0.0		$ 0.0	$ 125.0	$ 0.0
Gain on formation of United Wambo JV					$ (48.1)					0.0		0.0	(48.1)	0.0
Provision for North Goonyella equipment loss					58.5	$ 24.7	$ 17.1	$ 49.3		0.0		0.0	83.2	66.4
Restructuring charges	$ 3.0				23.0					0.0		7.6	24.3	1.2
Coal Reserves, in tons | T	345
Reclamation and Mine Shutdown Provision	$ 34.2
Increase (Decrease) in Asset Retirement Obligations										10.2		12.1	6.6	5.7
Proceeds from Sale of Property, Plant, and Equipment										24.3		17.9	30.0	76.4
Gain (Loss) on Disposition of Assets								20.7	$ 30.6	22.8		84.0	2.1	48.2
Springfield Reclamation Liability
OtherCommercialEvents [Line Items]
Gain (Loss) on Sale of Properties										0.0		31.2	0.0	0.0
Gain (Loss) on Disposition of Assets												31.2
Burton Mine
OtherCommercialEvents [Line Items]
Gain (Loss) on Sale of Properties										0.0		52.2	0.0	0.0
Gain (Loss) on Disposition of Assets		$ 52.2
Red Mountain Joint Venture
OtherCommercialEvents [Line Items]
Proceeds from Sale of Property, Plant, and Equipment														20.0
Gain (Loss) on Disposition of Assets														7.1
Queensland’s Bowen Basin
OtherCommercialEvents [Line Items]
Proceeds from Sale of Property, Plant, and Equipment															$ 37
Gain (Loss) on Disposition of Assets														20.6
DTA and PBGC
OtherCommercialEvents [Line Items]
Ownership percentage of equity method investment											37.50%									50.00%
Proceeds from Sale of Property, Plant, and Equipment											$ 20.5
Gain (Loss) on Disposition of Assets										$ 19.7
Metropolitan Collieries Pty Ltd
OtherCommercialEvents [Line Items]
Planned Divestiture, Agreed Upon Proceeds from Divestiture																					$ 200.0
New Mexico/Colorado Mining Tenement
OtherCommercialEvents [Line Items]
Proceeds from Sale of Property, Plant, and Equipment																$ 358.0
Gain (Loss) on Contract Termination												$ 20.0
Stanmore Coal Limited
OtherCommercialEvents [Line Items]
Coal Reserves, in tons | T				23
Proceeds from Sale of Property, Plant, and Equipment				$ 22.0
Gain (Loss) on Disposition of Assets			$ 20.5					$ 20.5
Lenton Joint Venture | Burton Mine
OtherCommercialEvents [Line Items]
Planned Divestiture, Agreed Upon Proceeds from Divestiture		11.7
Increase (Decrease) in Asset Retirement Obligations		40.5
Decrease in Restricted Cash		$ 30.0
Wambo Mine
OtherCommercialEvents [Line Items]
Fair value of interest in joint venture													63.7
Carrying value of net assets													15.6
Powder River Basin Mining
OtherCommercialEvents [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent																			66.50%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners																			33.50%
North Goonyella Mine
OtherCommercialEvents [Line Items]
Costs Related to North Goonyella Events														58.0
Insurance Recoveries													125.0
Containment and idling costs													111.5
Deductible Amount														50.0
North Goonyella Mine | Yancoal Technology Development Pty Ltd
OtherCommercialEvents [Line Items]
Payments to Acquire Machinery and Equipment													54.2
North Goonyella Mine | Leased Equipment
OtherCommercialEvents [Line Items]
Provision for North Goonyella equipment loss														50.7			$ 50.7
North Goonyella Mine | Owned Equipment
OtherCommercialEvents [Line Items]
Provision for North Goonyella equipment loss														$ 45.6
North Goonyella Mine | Longwall development, unrecoverable [Member]
OtherCommercialEvents [Line Items]
Provision for North Goonyella equipment loss													39.7
North Goonyella Mine | Mine Carrying Value
OtherCommercialEvents [Line Items]
Costs Related to North Goonyella Events													300.0
Kayenta Mine
OtherCommercialEvents [Line Items]
Decrease in Restricted Cash													53.5
Receivable for additional consideration on settlement, total					78.5								78.5				78.5
Receivable for additional consideration on settlement					$ 35.4								35.4				$ 35.4
Kayenta Mine | Scenario, Forecast
OtherCommercialEvents [Line Items]
Receivable for additional consideration on settlement																		$ 16.3
Other | North Goonyella Mine
OtherCommercialEvents [Line Items]
Provision for North Goonyella equipment loss													$ 13.6