SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Intangible Assets
Amortization expense attributable to intangible assets	$ 15,100	$ 3,000	$ 3,000
Original Cost	47,283	36,854
Accumulated Amortization	(26,737)	(12,326)
Intangibles, Net	20,546	24,528
Estimated amortization expense attributable to intangible assets
2016	15,300
2017	1,682
2018	1,021
2019	1,021
2020	385
Thereafter	1,137
Noncompete agreements
Intangible Assets
Original Cost	14,729	15,152
Accumulated Amortization	(9,750)	(8,545)
Intangibles, Net	4,979	6,607
Customer contracts
Intangible Assets
Original Cost	31,054	17,859
Accumulated Amortization	(16,959)	(3,599)
Intangibles, Net	14,095	14,260
Mining permits
Intangible Assets
Original Cost	1,500	3,843
Accumulated Amortization	(28)	(182)
Intangibles, Net	$ 1,472	$ 3,661