VARIABLE INTEREST ENTITY (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Variable Interest Entity [Line Items]
Ownership percentage in Constitution (percent)	100.00%
Cash and cash equivalents	$ 17,754	$ 22,936	$ 53,393	$ 14,258
Receivables	187,621	159,916
Other current assets	19,951	16,103
Land and mineral rights	596,448	476,447
Property, plant and equipment, net	879,381	713,116
Contractual third-party reclamation receivables, less current portion	154,816	86,915
Other noncurrent assets	20,837	11,904
Debt	(77,375)	(38,852)
Asset Retirements Obligations	(450,422)	(375,813)
Other liabilities	(21,986)	$ (24,613)
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Cash and cash equivalents	4,349
Receivables	26,866
Other current assets	14,482
Land and mineral rights	108,641
Property, plant and equipment, net	69,743
Contractual third-party reclamation receivables, less current portion	71,336
Other noncurrent assets	8,875
Debt	(114,464)
Trade accounts payable	(22,530)
Other current liabilities	(25,361)
Asset Retirements Obligations	(71,495)
Deferred income taxes	(47,977)
Other liabilities	$ (9,361)