REDEEMABLE PREFERRED STOCK - Issuances, Reclassifications and Redemptions for each class of Preferred Stock in Permanent Equity (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended		60 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 364,526	$ 278,515	$ 278,114	$ 284,393	$ 279,539	$ 278,195	$ 278,114	$ 278,195
Reclassification of Series A Preferred Stock from temporary equity to permanent equity							34,552	27,434	$ 134,700
Ending balance	$ 375,505	$ 364,526	$ 278,515	$ 286,719	$ 284,393	$ 279,539	$ 375,505	$ 286,719	$ 375,505
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	10,671,562	10,125,641	9,784,067	8,954,508	8,532,056	8,224,254	9,784,067	8,224,254
Beginning balance	$ 284,193	$ 270,546	$ 262,036	$ 241,667	$ 231,144	$ 223,467	$ 262,036	$ 223,467
Ending balance (in shares)	11,265,130	10,671,562	10,125,641	9,446,326	8,954,508	8,532,056	11,265,130	9,446,326	11,265,130
Ending balance	$ 299,323	$ 284,193	$ 270,546	$ 253,683	$ 241,667	$ 231,144	$ 299,323	$ 253,683	$ 299,323
Series A Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of Series A Preferred Stock from temporary equity to permanent equity	13,828	12,481	8,243	10,747	9,739	6,948
Redemption of Series A Preferred Stock	$ (607)	$ (431)	$ (685)	$ (55)	$ (183)	$ (66)
Series A Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	5,253,377	4,715,291	4,377,762	3,560,448	3,138,916	2,837,094	4,377,762	2,837,094
Beginning balance	$ 130,595	$ 117,140	$ 108,729	$ 88,660	$ 78,160	$ 70,633	$ 108,729	$ 70,633
Reclassification of Series A Preferred Stock to permanent equity (in shares)	593,300	556,587	366,991	482,374	427,064	304,274
Reclassification of Series A Preferred Stock from temporary equity to permanent equity	$ 15,132	$ 13,915	$ 9,144	$ 11,786	$ 10,638	$ 7,588
Redemption of Series A Preferred Stock (in shares)	(25,564)	(18,501)	(29,462)	(2,393)	(5,532)	(2,452)
Redemption of Series A Preferred Stock	$ (634)	$ (460)	$ (733)	$ (60)	$ (138)	$ (61)
Ending balance (in shares)	5,821,113	5,253,377	4,715,291	4,040,429	3,560,448	3,138,916	5,821,113	4,040,429	5,821,113
Ending balance	$ 145,093	$ 130,595	$ 117,140	$ 100,386	$ 88,660	$ 78,160	$ 145,093	$ 100,386	$ 145,093
Series D Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Series D Preferred Stock	$ 612	$ 185	$ 96	$ 280	$ 22	$ 145
Series D Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	31,025	23,190	19,145	6,900	5,980	0	19,145	0
Beginning balance	$ 764	$ 572	$ 473	$ 173	$ 150	$ 0	$ 473	$ 0
Issuance of Series D Preferred Stock (in shares)	25,832	7,835	4,045	11,837	920	5,980
Issuance of Series D Preferred Stock	$ 632	$ 192	$ 99	$ 290	$ 23	$ 150
Ending balance (in shares)	56,857	31,025	23,190	18,737	6,900	5,980	56,857	18,737	56,857
Ending balance	$ 1,396	$ 764	$ 572	$ 463	$ 173	$ 150	$ 1,396	$ 463	$ 1,396
Series L Preferred Stock | Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160
Beginning balance	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834
Ending balance (in shares)	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160	5,387,160
Ending balance	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834	$ 152,834