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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Risk (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues | Customer concentration | Verizon
Credit concentrations and significant customers
Percentage of concentration risk	10.00%	12.00%	19.00%
Revenues | Customer concentration | AT&T
Credit concentrations and significant customers
Percentage of concentration risk	13.00%	14.00%	17.00%
Accounts receivable | Credit concentration | Verizon
Credit concentrations and significant customers
Percentage of concentration risk		12.00%
Accounts receivable | Credit concentration | AT&T
Credit concentrations and significant customers
Percentage of concentration risk	18.00%	17.00%

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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