Management - Labor Relations Labor Relations (Details) T in Millions, $ in Millions, $ in Millions		1 Months Ended		3 Months Ended						9 Months Ended	12 Months Ended
	Nov. 28, 2017 USD ($) T	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) T	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 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 ($)	Dec. 31, 2016 USD ($)	Feb. 06, 2018
Concentration Risk [Line Items]
Entity Number of Employees				7,400								7,400	7,400
Entity Number Of Hourly Employees				5,600								5,600	5,600
Percentage of hourly employees represented by organized labor unions				42.00%								42.00%	42.00%
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions												20.00%	20.00%
Number of US Mines Represented by Unions				2								2	2
Successor
Concentration Risk [Line Items]
Costs Related to North Goonyella Events												$ 58.0
Provision for North Goonyella equipment loss				$ 17.1	$ 49.3					$ 0.0		66.4
Insurance Claim Limit												125.0
Deductible Amount												50.0
Restructuring charges	$ 3.0									7.6		1.2
Coal Reserves, in tons | T	345.0
Proceeds from Sale of Property, Plant, and Equipment										17.9		76.4
Gain (Loss) on Disposition of Assets					20.7	$ 30.6	$ 83.1			$ 84.0		48.2
Successor | Leased Equipment
Concentration Risk [Line Items]
Provision for North Goonyella equipment loss												50.4
Successor | Owned Equipment
Concentration Risk [Line Items]
Provision for North Goonyella equipment loss												16.0
Successor | Stanmore Coal Limited
Concentration Risk [Line Items]
Coal Reserves, in tons | T			23.0
Proceeds from Sale of Property, Plant, and Equipment			$ 22.0									7.0
Gain (Loss) on Disposition of Assets		$ 20.5			$ 20.5
Successor | DTA and PBGC
Concentration Risk [Line Items]
Ownership percentage of equity method investment															50.00%
Successor | Red Mountain Joint Venture
Concentration Risk [Line Items]
Proceeds from Sale of Property, Plant, and Equipment												20.0
Gain (Loss) on Disposition of Assets												7.1
Successor | Queensland’s Bowen Basin
Concentration Risk [Line Items]
Proceeds from Sale of Property, Plant, and Equipment													$ 37.0
Gain (Loss) on Disposition of Assets												$ 20.6
Successor | United Mine Workers of America | Kayenta Mine
Concentration Risk [Line Items]
Percentage of hourly employees represented by organized labor unions				7.00%								7.00%	7.00%
Percentage Of Coal Production Generated By Hourly Employees												4.00%	4.00%
Successor | United Mine Workers of America | Shoal Creek
Concentration Risk [Line Items]
Percentage of hourly employees represented by organized labor unions				9.00%								9.00%	9.00%
Successor | Wambo Operation Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				23.00%								23.00%	23.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation												18.00%	18.00%
Successor | North Goonyella UG Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				8.00%								8.00%	8.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation										5.00%
Successor | Metropolitan Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				12.00%								12.00%	12.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation										6.00%
Successor | Moorvale Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				14.00%								14.00%	14.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation												7.00%	7.00%
Successor | Millenium Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				1.00%								1.00%	1.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation										6.00%
Successor | Coppabella/Moorvale Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				20.00%								20.00%	20.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation												9.00%	9.00%
Successor | Wilpinjong Employees
Concentration Risk [Line Items]
Percentage Of Australian Hourly Employees Under Contract Negotiations				23.00%								23.00%	23.00%
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation										48.00%
Predecessor
Concentration Risk [Line Items]
Provision for North Goonyella equipment loss								$ 0.0						$ 0.0
Restructuring charges								0.0						15.5
Proceeds from Sale of Property, Plant, and Equipment								24.3						144.4
Gain (Loss) on Disposition of Assets								22.8						$ 23.2
Predecessor | DTA and PBGC
Concentration Risk [Line Items]
Proceeds from Sale of Property, Plant, and Equipment									$ 20.5
Ownership percentage of equity method investment									37.50%
Gain (Loss) on Disposition of Assets								$ 19.7
Scenario, Forecast
Concentration Risk [Line Items]
Costs Related to North Goonyella Events											$ 285.0
Scenario, Forecast | Leased Equipment
Concentration Risk [Line Items]
Costs Related to North Goonyella Events											$ 16.0