SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Change in Accounting Estimate (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended								12 Months Ended
	Jan. 01, 2014	Dec. 31, 2013	Oct. 31, 2015	Jul. 31, 2014	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Change in Accounting Estimate
Net book value					$ 250,714				$ 214,899				$ 250,714	$ 214,899
Net income					$ 4,397	$ 9,030	$ 7,239	$ 4,872	$ 6,470	$ 7,834	$ 5,695	$ 4,575	$ 25,538	24,574	$ 20,659
Land improvements | Monarch Casino Black Hawk parking facility
Change in Accounting Estimate
Net book value		$ 4,800		$ 2,900												$ 400
Estimated depreciable life		P37Y
Land improvements | Monarch Casino Black Hawk parking facility | Service Life
Change in Accounting Estimate
Estimated depreciable life	P15M			P18M
Depreciation expense per month	$ 300		$ 40
Depreciation expense net of tax per month	$ 200
Depreciation expense														300	1,400
Net income														$ (200)	$ (900)
Basic and diluted earnings per share (in dollars per share)														$ (0.01)	$ 0.05