REDEEMABLE CONVERTIBLE PREFERRED STOCK - Outstanding Stock (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) shares
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance	$ 196,437
Conversion of preferred stock to common stock	(196,594)
Ending balance	$ 0
Series G’ Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	1,295,000
Beginning balance	$ 44,672
Accretion of redeemable convertible preferred stock	$ 0
Conversion of preferred stock to common stock (in shares) | shares	(1,295,000)
Conversion of preferred stock to common stock	$ (44,672)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series G Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	346,000
Beginning balance	$ 5,110
Accretion of redeemable convertible preferred stock	$ 108
Conversion of preferred stock to common stock (in shares) | shares	(346,000)
Conversion of preferred stock to common stock	$ (5,218)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series F-R Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	1,199,000
Beginning balance	$ 58,449
Accretion of redeemable convertible preferred stock	$ 38
Conversion of preferred stock to common stock (in shares) | shares	(1,199,000)
Conversion of preferred stock to common stock	$ (58,487)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series E-R Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	795,000
Beginning balance	$ 29,972
Accretion of redeemable convertible preferred stock	$ 1
Conversion of preferred stock to common stock (in shares) | shares	(795,000)
Conversion of preferred stock to common stock	$ (29,973)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series D-R Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	1,396,000
Beginning balance	$ 32,728
Accretion of redeemable convertible preferred stock	$ 7
Conversion of preferred stock to common stock (in shares) | shares	(1,396,000)
Conversion of preferred stock to common stock	$ (32,735)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series C-R Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	1,508,000
Beginning balance	$ 18,366
Accretion of redeemable convertible preferred stock	$ 3
Conversion of preferred stock to common stock (in shares) | shares	(1,508,000)
Conversion of preferred stock to common stock	$ (18,369)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series B-R Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	2,247,000
Beginning balance	$ 5,288
Accretion of redeemable convertible preferred stock	$ 0
Conversion of preferred stock to common stock (in shares) | shares	(2,247,000)
Conversion of preferred stock to common stock	$ (5,288)
Ending balance (in shares) | shares	0
Ending balance	$ 0
Series A-R Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance (in shares) | shares	1,857,000
Beginning balance	$ 1,852
Accretion of redeemable convertible preferred stock	$ 0
Conversion of preferred stock to common stock (in shares) | shares	(1,857,000)
Conversion of preferred stock to common stock	$ (1,852)
Ending balance (in shares) | shares	0
Ending balance	$ 0