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Supplemental Coal Data (Unaudited) (Details) MMcf in Thousands, $ in Thousands, T in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 USD ($) T MMcf | Dec. 31, 2015 USD ($) T MMcf | Dec. 31, 2014 USD ($) T MMcf | Dec. 31, 2013 USD ($) T | Dec. 31, 2012 USD ($) T | Dec. 31, 2011 T
Reserve Quantities [Line Items] |  |  |  |  |  |
Proved Oil and Gas Property, Successful Effort Method | $ | $ 2,016,916 | $ 1,922,602 |  |  |  |
Reserve Quantities [Roll Forward] |  |  |  |  |  |
Proved Developed and Undeveloped Reserves, Production | MMcf | 394,387 | 328,657 | 235,714 |  |  |
Net change due to revisions in quantity estimates | $ | $ (35,502) | $ 408,006 | $ 151,233 |  |  |
Unproved Oil and Gas Property, Successful Effort Method | $ | $ 1,116,282 | $ 1,421,083 |  |  |  |
Assigned Coal Reserves [Member] |  |  |  |  |  |
Reserve Quantities [Line Items] |  |  |  |  |  |
Coal Reserves | 141 |  |  |  |  |
Steam [Member] |  |  |  |  |  |
Reserve Quantities [Line Items] |  |  |  |  |  |
Proved Undeveloped Reserve (Mass) | 2,274 |  |  |  |  |
Coal Segment [Member] |  |  |  |  |  |
Reserve Quantities [Line Items] |  |  |  |  |  |
Proved Undeveloped Reserve (Mass) | 2,361 | 3,047 | 3,238 | 3,032 | 4,229 | 4,314
Reserve Quantities [Roll Forward] |  |  |  |  |  |
Purchase of minerals in place (Mass) | 0 | 24 | 0 | 1 | 0 |
Sales of minerals in place (Mass) | (601) | (43) | (233) | (1,199) | (155) |
Production of proved reserves (Mass) | (26) | (29) | (32) | (55) | (55) |
Net change due to revisions in quantity estimates | $ | $ (59,000) | $ (143,000) | $ 471,000 | $ 56,000 | $ 125,000 |
metallurgical coal [Member] |  |  |  |  |  |
Reserve Quantities [Line Items] |  |  |  |  |  |
Proved Undeveloped Reserve (Mass) | 87 |  |  |  |  |
Affiliated Entity [Member] |  |  |  |  |  |
Reserve Quantities [Line Items] |  |  |  |  |  |
Proved Undeveloped Reserve (Mass) |  | 0 | 0 | 55 | 57 | 41

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- Definition Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves. + References No definition available. + Details
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