Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Property, Plant, Equipment and Mine Development, Net [Line Items]
Land and coal interests	$ 10,297.7	$ 10,330.8
Buildings and improvements	1,479.3	1,507.6
Machinery and equipment	2,143.8	2,130.2
Less: Accumulated depreciation, depletion and amortization	(5,266.9)	(5,191.9)
Total, net	$ 8,653.9	$ 8,776.7