Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Property, Plant, Equipment and Mine Development, Net [Line Items]
Land and coal interests	$ 10,756.7	$ 10,503.7
Buildings and improvements	1,533.0	1,506.0
Machinery and equipment	2,272.5	2,280.4
Less: Accumulated depreciation, depletion and amortization	(5,346.7)	(5,031.6)
Total, net	$ 9,215.5	$ 9,258.5