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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 |  |

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