ACQUISITIONS - White Oak - Assets and Liabilities (Details) - USD ($) $ in Thousands	Jul. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Goodwill		$ 136,399	$ 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 | Above-market contracts
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Other current assets	9,333
White Oak | Total intangibles acquired
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Net tangible and intangible assets acquired	8,277
White Oak | Customer contracts and other, net
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 and other, net | 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 and other, net | Above-market contracts
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Other assets	3,671
White Oak | Permits
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Other assets	$ 1,500
Weighted-average amortization period	20 years