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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Jan. 01, 2017	Dec. 31, 2016
Recent accounting pronouncements
Valuation allowance	$ 35,829		$ 42,462
Accounting Standards Update 2016-09 \| Adjustment
Recent accounting pronouncements
Reclassification to retained earnings		$ 300
Accounting Standards Update 2016-01
Recent accounting pronouncements
Equity investments under cost method	20,100
Accounting Standards Update 2016-18
Recent accounting pronouncements
Restricted cash	11,900
Accounting Standards Update 2016-15 \| Proforma Adjustment
Recent accounting pronouncements
Cash payments - investing activities	(1,200)
Cash payments - financing activities	1,200
ASU 2018-02 \| Adjustment
Recent accounting pronouncements
Reclassification to retained earnings	800
Valuation allowance	$ 800