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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Change in Accounting Estimate (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 9 Months Ended
Jan. 01, 2014
Dec. 31, 2013
Oct. 31, 2015
Jul. 31, 2014
Sep. 30, 2016
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2014
Change in Accounting Estimate                
Net book value           $ 240,706 $ 214,899  
Land improvements | Monarch Casino Black Hawk parking facility                
Change in Accounting Estimate                
Net book value   $ 4,800   $ 2,900       $ 400
Remaining depreciable life   37 years            
Land improvements | Monarch Casino Black Hawk parking facility | Service Life                
Change in Accounting Estimate                
Estimated depreciable life P15M     P18M        
Depreciation expense per month $ 300              
Depreciation expense net of tax per month $ 200              
Depreciation expense     $ 40   $ 266      
Net income         $ (173)      
Basic and diluted earnings per share (in dollars per share)         $ (0.01)