Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Property, Plant, Equipment and Mine Development, Net [Line Items]
Land and coal interests	$ 10,637.5	$ 11,021.1
Buildings and improvements	1,536.9	1,569.1
Machinery and equipment	2,341.9	2,685.7
Less: Accumulated depreciation, depletion and amortization	(5,040.9)	(4,698.6)
Total, net	$ 9,475.4	$ 10,577.3