Property, Plant, Equipment and Mine Development (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Property, Plant, Equipment and Mine Development, Net [Line Items]
Land and coal interests	$ 10,957.6	$ 11,024.1
Buildings and improvements	1,553.2	1,525.4
Machinery and equipment	2,663.3	2,777.5
Less: Accumulated depreciation, depletion and amortization	(4,473.6)	(4,244.5)
Total, net	$ 10,700.5	$ 11,082.5