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Supplemental Coal Data (Unaudited) (Tables) | 12 Months Ended
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Dec. 31, 2016
SUPPLEMENTAL COAL DATA [Abstract] |
Reserves Reported to Other Agencies [Table Text Block] | Supplemental Coal Data (unaudited)
|  | Millions of Tons
|  | For the Year Ended December 31,
|  | 2016 |  | 2015 |  | 2014 |  | 2013 |  | 2012
Proven and probable coal reserves at beginning of period |  | 3,047 |  |  | 3,238 |  |  | 3,032 |  |  | 4,229 |  |  | 4,314 |
Purchased reserves |  | — |  |  | 24 |  |  | — |  |  | 1 |  |  | — |
Reserves sold in place |  | (601 | ) |  | (43 | ) |  | (233 | ) |  | (1,199 | ) |  | (155 | )
Production |  | (26 | ) |  | (29 | ) |  | (32 | ) |  | (55 | ) |  | (55 | )
Revisions and other changes |  | (59 | ) |  | (143 | ) |  | 471 |  |  | 56 |  |  | 125 |
Consolidated proven and probable coal reserves at end of period* (1) |  | 2,361 |  |  | 3,047 |  |  | 3,238 |  |  | 3,032 |  |  | 4,229 |
Proportionate share of proven and probable coal reserves of unconsolidated equity affiliates (excluded from the table above)* |  | — |  |  | — |  |  | 55 |  |  | 57 |  |  | 41 |
______________ * Proven and probable coal reserves are the equivalent of “demonstrated reserves” under the coal resource classification system of the U.S. Geological Survey. Generally, these reserves would be commercially mineable at year-end prices and cost levels, using current technology and mining practices.

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- Definition SUPPLEMENTAL COAL DATA [Abstract] + References No definition available. + Details
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- Definition Tabular disclosure of reserves reported to other federal agencies or authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Industry Guide -Number 2 -Paragraph 2 + Details
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