REVENUE RECOGNITION AND RECEIVABLES - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Contract Assets and Liabilities
Contract Asset - current	$ 2,413
Change in contract asset - current	65
Contract Asset - current	$ 2,478	$ 2,413
% of change in contract asset - current	3.00%
Contract asset, noncurrent	$ 905
Change in contract asset - noncurrent	5
Contract asset, noncurrent	$ 910	905
% of change in contract asset - noncurrent	1.00%
Contract liability- current	$ (15,044)
Change in contract liabilities - current	(3,500)
Contract liability- current	$ (18,544)	(15,044)
% of change in contract liabilities - current	(23.00%)
Contract liability- noncurrent	$ (5,450)
Change in contract liabilities - noncurrent	3,257
Contract liability- noncurrent	$ (2,193)	(5,450)
% of change in contract liabilities - Noncurrent	60.00%
Net contract liability	$ (17,176)
Change in net contract liability	(173)
Net contract liability	$ (17,349)	(17,176)
% of change in net contract liability	(1.00%)
Revenue recognized related to contract liability	$ 16,900
Amortization of contract assets		2,300
Revenue recognized in the period related to performance obligations that were satisfied or partially satisfied in previous periods	$ 0	$ 0
Retail revenue period for billing postpaid customers in advance	1 month