ACQUISITIONS - White Oak - Assets and Liabilities (Details) - USD ($) $ in Thousands	Jul. 31, 2015	Dec. 31, 2015
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Goodwill		$ 136,399
White Oak
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Cash and cash equivalents	$ 3,125
Trade receivables	3,018
Prepaid expenses	3,942
Inventories	7,240
Other current assets	9,456
Property, plant and equipment	299,214
Advance royalties	3,349
Deposits	6,981
Other assets	12,829
Total identifiable assets acquired	349,154
Accounts payable	(31,181)
Accrued expenses	(20,987)
Deferred revenue	(517)
Current maturities, long-term debt	(29,529)
Long-term debt, excluding current maturities	(63,973)
Other long-term liabilities	(12,175)
Asset retirement obligations	(12,484)
Total liabilities assumed	(170,846)
Net identifiable assets acquired	178,308
Goodwill	132,026
Net tangible and intangible assets acquired	310,334
White Oak | Total intangibles acquired
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Net identifiable assets acquired	8,277
White Oak | Customer contracts
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Net identifiable assets acquired	$ 6,777
Weighted-average amortization period	3 years
White Oak | Customer contracts | Below-market contracts
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Accrued expenses	$ (4,702)
Other long-term liabilities	(1,525)
White Oak | Customer contracts | Above-market contracts
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Other current assets	9,333
Other assets	3,671
White Oak | Mining permits
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Other assets	$ 1,500
Weighted-average amortization period	20 years
White Oak | Preliminary
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Cash and cash equivalents	$ 3,125
Trade receivables	3,122
Prepaid expenses	4,364
Inventories	7,240
Other current assets	9,415
Property, plant and equipment	258,798
Advance royalties	3,349
Deposits	6,981
Other assets	5,580
Total identifiable assets acquired	301,974
Accounts payable	(31,399)
Accrued expenses	(18,609)
Deferred revenue	(517)
Current maturities, long-term debt	(29,529)
Long-term debt, excluding current maturities	(64,588)
Other long-term liabilities	(15,175)
Asset retirement obligations	(12,484)
Total liabilities assumed	(172,301)
Net identifiable assets acquired	129,673
Goodwill	157,612
Net tangible and intangible assets acquired	287,285
White Oak | Adjustments
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Trade receivables	(104)
Prepaid expenses	(422)
Other current assets	41
Property, plant and equipment	40,416
Other assets	7,249
Accounts payable	218
Accrued expenses	(2,378)
Long-term debt, excluding current maturities	615
Other long-term liabilities	3,000
Goodwill	$ (25,586)