SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Retirement Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Changes in AROs
ARO liability, balance at the beginning of the period	$ 3,663	$ 10,917
Decrease in ARO liability		(3,798)
ARO settlements	(1,040)	(3,456)
Increase in ARO liability	4,200
ARO liability, balance at the end of the period	6,823	3,663
Accrued and other current liabilities
Changes in AROs
ARO liability, balance at the beginning of the period	1,100
ARO liability, balance at the end of the period	1,400	1,100
Other non-current liabilities
Changes in AROs
ARO liability, balance at the beginning of the period	2,600
ARO liability, balance at the end of the period	5,400	$ 2,600
Springfield, Massachusetts
Changes in AROs
Increase in ARO liability	$ 4,200