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RELATED-PARTY TRANSACTIONS - Affiliate Royalty Agreements - SGP (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Tunnel Ridge |  |  |
Related Party Transaction |  |  |
Payments for earned royalties | $ 6.1 | $ 7.2 | $ 6.0
Tunnel Ridge | Joseph W Craft III Foundation | Tunnel Ridge |  |  |
Related Party Transaction |  |  |
Property ownership (as a percent) | 0.50% |  |
SGP | Tunnel Ridge |  |  |
Related Party Transaction |  |  |
Annual minimum royalties | $ 3.0 |  |

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