Impairment Charges and Mine Closure Costs (Details) - USD ($) $ in Millions	3 Months Ended						6 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2015
Impaired Long-Lived Assets Held and Used [Line Items]
Other-than-temporary impairment loss on available-for-sale securities		$ 2.9
Severance costs							$ 7.2
Curtailments							$ 3.6
Asset impairment and mine closure costs			$ 488.9	$ 2,100.0	$ 19.1			$ 2,600.0
Impairment charge of higher-cost mining complex					5.6
Restructuring charges				$ 5.1
Royalty Agreements [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Prepaid royalties		3.4			$ 12.2
Impairment charge						$ 15.4			$ 48.0
APP [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment and mine closure costs									2,200.0
Other Segments [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment and mine closure costs									$ 400.0
Kentucky [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Net book value of idled facility		$ 74.1
Longview, Washington [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Other-than-temporary impairment loss on available-for-sale securities	$ 38.0