Summary of Significant Accounting Policies Discussion Textuals (Details) - USD ($)		3 Months Ended			9 Months Ended	12 Months Ended		36 Months Ended
	Nov. 28, 2017	Dec. 31, 2019	Sep. 30, 2019	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Apr. 02, 2017
Summary of Significant Accounting Policies Textuals
Royalty Expense				$ 115,200,000	$ 364,600,000	$ 388,600,000	$ 474,300,000
Minimum Annual Production Of Federal Coal Mining Leases On Original Amount							1.00%
Monthly federal royalties payable from sale using surface mining methods							12.50%
Operating Lease, Liability		$ 82,400,000				$ 82,400,000		$ 82,400,000
Period considered to treat short-term, highly liquid investments as cash equivalents						three months or less
Other-than-temporary impairment losses on equity and cost method investments					0		$ 0	0
Recognition of tax benefits from uncertain tax positions						greater than fifty percent likelihood of being realized upon ultimate settlement
Restructuring charges for voluntary and involuntary workforce reductions	$ 3,000,000.0	23,000,000.0		0	7,600,000	$ 24,300,000	1,200,000
Asset impairment charges related to long-lived assets				30,500,000		261,200,000	0
Foreign currency remeasurement gain (loss)				10,600,000	700,000	(2,700,000)	1,400,000
Foreign currency translation adjustment				5,500,000	1,400,000	200,000	(5,900,000)
Interest Costs Capitalized				0	0	0	0
Equity Method Investment, Other than Temporary Impairment				0	0	9,000,000.0	0
Asset impairment		250,200,000	$ 20,000,000.0	$ 30,500,000	$ 0	270,200,000	$ 0
Monthly federal royalties payable of production using underground mining methods							8.00%
Restructuring Reserve, Current		$ 7,400,000				$ 7,400,000		$ 7,400,000
Equity method investment and financing receivables related to Middlemount
Summary of Significant Accounting Policies Textuals
Ownership percentage of equity method investment		50.00%				50.00%		50.00%
Buildings and improvements
Summary of Significant Accounting Policies Textuals
Estimated useful life						up to 29
Machinery and equipment
Summary of Significant Accounting Policies Textuals
Estimated useful life						1 - 15
Leasehold improvements [Member]
Summary of Significant Accounting Policies Textuals
Estimated useful life						Shorter of Useful Life or Remaining Life of Lease
Maximum | Buildings and improvements
Summary of Significant Accounting Policies Textuals
Estimated useful life							27
Maximum | Machinery and equipment
Summary of Significant Accounting Policies Textuals
Estimated useful life							27
Treasury Stock
Summary of Significant Accounting Policies Textuals
Voting Shares Received Upon Reorganization, Percent									50.00%