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RELATED-PARTY TRANSACTIONS - WKY CoalPlay and Cavalier Minerals (Details) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Feb. 28, 2015 | Dec. 31, 2014 | Dec. 31, 2020 | Jan. 03, 2019
WKY CoalPlay | February 2015 coal lease - Henderson and Union Counties, Kentucky |  |  |  |
Related Party Transaction |  |  |  |
Initial term of lease | 20 years |  |  |
Percentage of earned royalty on coal sale price | 4.00% |  |  |
Annual minimum royalties | $ 2.1 |  |  |
Period for option to acquire the leased reserves | 3 years |  |  |
Percentage of internal rate of return on purchase price, if leased reserves acquired | 7.00% |  |  |
WKY CoalPlay | December 2014 coal lease - Henderson and Union Counties, Kentucky |  |  |  |
Related Party Transaction |  |  |  |
Initial term of lease |  | 20 years |  |
Percentage of earned royalty on coal sale price |  | 4.00% |  |
Annual minimum royalties |  | $ 3.6 |  |
Period for option to acquire the leased reserves |  | 3 years |  |
Percentage of internal rate of return on purchase price, if leased reserves acquired |  | 7.00% |  |
WKY CoalPlay | December 2014 coal lease - Webster County, Kentucky |  |  |  |
Related Party Transaction |  |  |  |
Initial term of lease |  | 7 years |  |
Percentage of earned royalty on coal sale price |  | 4.00% |  |
Annual minimum royalties |  | $ 2.6 |  |
Period for option to acquire the leased reserves |  | 3 years |  |
Percentage of internal rate of return on purchase price, if leased reserves acquired |  | 7.00% |  |
Accrued payments |  |  | $ 2.6 |
WKY CoalPlay | December 2014 coal lease - Henderson County, Kentucky |  |  |  |
Related Party Transaction |  |  |  |
Annual minimum royalties |  | $ 2.5 |  |
Cavalier Minerals |  |  |  |
Related Party Transaction |  |  |  |
Ownership interest in VIE (as a percent) |  |  | 96.00% |
Cavalier Minerals | Bluegrass Minerals |  |  |  |
Related Party Transaction |  |  |  |
Noncontrolling ownership interest (as a percent) |  |  | 4.00% | 4.00%

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