STOCKHOLDERS' EQUITY - Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Oct. 01, 2019	Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 29, 2020	Apr. 06, 2017
Restricted stock
Weighted average grant date fair value of the shares granted			$ 5,400
Number of Shares
Unvested outstanding shares at the beginning of the period (in shares)			2,272,893	2,272,893	2,272,893	745,507	1,738,077
Granted (in shares)			4,163,348			2,326,961	2,022,432
Vested (in shares)			(1,611,465)			(537,411)	(2,516,647)
Accelerated vesting (in shares)			(2,724,086)
Forfeited (in shares)			(2,100,690)			(262,164)	(498,355)
Unvested outstanding shares at the end of the period (in shares)						2,272,893	745,507	1,738,077
Weighted Average Grant Date fair Value Per Share
Unvested outstanding shares at the beginning of the period (in dollars per share)			$ 5.80	$ 5.80	$ 5.80	$ 7.05	$ 9.06
Granted (in dollars per share)			1.29			5.47	7.07
Vested (in dollars per share)			4.65			5.89	8.39
Accelerated vesting (in dollars per share)			2.07
Forfeited (in dollars per share)			$ 2.58			6.29	7.41
Unvested outstanding shares at the end of the period (in dollars per share)						$ 5.80	$ 7.05	$ 9.06
Aggregate Intrinsic Value
Aggregate Intrinsic Value at the beginning of the period (in dollars)			$ 3,864	$ 3,864	$ 3,864	$ 5,643	$ 16,234
Aggregate Intrinsic Value at the end of period (in dollars)						3,864	5,643	$ 16,234
Employees and non-employee directors
Number of Shares
Granted (in shares)		0
General and administrative
Stock-based compensation
Allocated share based compensation credit			$ 8,000
Compensation expense recorded		$ 0				$ 15,300	$ 36,800
Stock options
Number of Shares
Outstanding at the beginning of the period (in shares)			7,468,926	7,468,926	7,468,926	6,735,903	5,319,400
Granted (in shares)						1,206,800	1,790,605
Exercised (in shares)						(41,667)
Forfeited (in shares)			(5,182,238)			(432,110)	(374,102)
Cancelled (in shares)			(2,286,688)
Outstanding at the end of the period (in shares)						7,468,926	6,735,903	5,319,400
Weighted Average Exercise Price Per Share
Outstanding at the beginning of the period (in dollars per share)			$ 8.34	$ 8.34	$ 8.34	$ 8.84	$ 9.22
Granted (in dollars per share)						5.65	7.72
Exercised (in dollars per share)						7.75
Forfeited (in dollars per share)			8.38			8.68	8.82
Cancelled (in dollars per share)			$ 8.26
Outstanding at the end of the period (in dollars per share)						$ 8.34	$ 8.84	$ 9.22
Aggregate Intrinsic Value
Outstanding at the beginning of the period						$ 29	$ 631
Exercised						$ 29
Outstanding at the end of the period							$ 29	$ 631
Weighted Average Remaining Contractual Term
Outstanding at the end of the period						8 years 1 month 6 days	8 years 10 months 24 days	9 years 8 months 12 days
2016 Incentive Plan
Stock-based compensation
Incremental reduction to stock-based compensation expense			$ 9,500
2016 Incentive Plan | Common Stock
Stock-based compensation
Aggregate number of shares available										19,000,000
Maximum number of shares that remained reserved for issuance under the Plan						4,900,000
2016 Incentive Plan | Stock options
Stock-based compensation
Share Based Compensation Arrangements by Share Based Payment Award Options Annual Vesting Percentage.					33.33%
Vesting period					3 years
Expiration term					10 years
Weighted average remaining vesting period						10 months 24 days	1 year 2 months 12 days
Weighted average grant date fair value of the shares granted						$ 3,500	$ 7,800
Proceeds from stock options						300
Number of Shares
Granted (in shares)	0	0		0
Restricted stock
Unrecognized compensation expense stock option						$ 5,200	$ 13,000
2016 Incentive Plan | Restricted Stock
Stock-based compensation
Percentage of awards other than options vesting on the annual anniversary date of the grant					33.33%
Vesting period					3 years
Weighted average remaining vesting period						1 year 1 month 6 days	1 year 4 months 24 days
Unrecognized compensation expense other than options						$ 6,100	$ 3,200
Restricted stock
Weighted average grant date fair value of the shares granted						12,700	14,300
Additional Disclosures
Total fair value of shares vested			$ 1,800			$ 2,000	$ 16,100
2016 Incentive Plan | Restricted Stock | Non-employee director
Stock-based compensation
Vesting period					6 months
2020 Incentive Plan | Common Stock | Subsequent event
Stock-based compensation
Aggregate number of shares available									1,500,000