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Supplemental Coal Data (unaudited) (Tables)
 12 Months Ended
Dec. 31, 2017
Mineral Industries Disclosures [Abstract]  
Schedule of Proven and Probable Reserves Rollforward
Supplemental Coal Data (unaudited)
 
 
Millions of Tons
 
 
For the Year Ended December 31,
 
 
2017
 
2016
 
2015
 
2014
 
2013
Proven and probable coal reserves at beginning of period
 
2,361

 
3,047

 
3,238

 
3,032

 
4,229

Purchased reserves
 
—

 
—

 
24

 
—

 
1

Reserves sold in place
 
(16
)
 
(601
)
 
(43
)
 
(233
)
 
(1,199
)
Production
 
(26
)
 
(26
)
 
(29
)
 
(32
)
 
(55
)
Revisions and other changes
 
(21
)
 
(59
)
 
(143
)
 
471

 
56

Consolidated proven and probable coal reserves at end of period* (1)
 
2,298

 
2,361

 
3,047

 
3,238

 
3,032

______________
* 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.
(1) 143.3 million tons of the Northern Appalachia product are controlled by CCC, a former subsidiary of ParentCo that was sold in December 2013. As of this filing, these tons are still controlled by CCC but are shown in CONSOL Energy's reserves due to a binding agreement that these tons will be released to CONSOL Energy upon the assignment of the underlying lease to CONSOL Energy.