Impairment Charges and Mine Closure Costs (Details) - USD ($) $ in Thousands	3 Months Ended						6 Months Ended		9 Months Ended	12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Oct. 01, 2016	Dec. 31, 2017
Impaired Long-Lived Assets Held and Used [Line Items]
Other-than-temporary impairment loss on available-for-sale securities			$ 2,900
Severance costs							$ 7,200
Curtailments							$ 3,600
Asset impairment and mine closure costs	$ 0			$ 500,000	$ 2,100,000	$ 19,100		$ 2,600,000		$ 0
Impairment charge of higher-cost mining complex						5,600
Royalty Agreements
Impaired Long-Lived Assets Held and Used [Line Items]
Prepaid royalties			3,400			$ 12,200
MET
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment and mine closure costs									$ 2,200,000
Other Thermal
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment and mine closure costs									$ 400,000
Kentucky
Impaired Long-Lived Assets Held and Used [Line Items]
Net book value of idled facility			$ 74,100
Longview, Washington
Impaired Long-Lived Assets Held and Used [Line Items]
Other-than-temporary impairment loss on available-for-sale securities		$ 38,000