Property, Plant, Equipment and Mine Development (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant, Equipment and Mine Development[Line Items]
Land and coal interests	$ 10,330.8	$ 10,503.7
Buildings and improvements	1,507.6	1,506.0
Machinery and equipment	2,130.2	2,280.4
Less: accumulated depreciation, depletion and amortization	(5,191.9)	(5,031.6)
Total, net	8,776.7	9,258.5	$ 10,577.3
Coal reserves	5,500.0	5,700.0
Acquired interest in mineral rights	1,200.0
Coal reserves not subject to depletion	1,600.0	1,700.0
Mining Properties and Mineral Rights [Member]
Property, Plant, Equipment and Mine Development[Line Items]
Mineral rights and advanced royalties	4,400.0	$ 4,600.0
Coal reserves held by fee ownership [Member]
Property, Plant, Equipment and Mine Development[Line Items]
Coal reserves	$ 1,100.0