INCOME TAXES - Summary of Federal and State Carryforwards and Credits Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Federal NOLs
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 403,911	$ 36,400
Federal NOLs | 2020
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	13,311
Federal NOLs | 2021
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	9,866
Federal NOLs | 2022
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	24,100
Federal NOLs | 2023
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	24,743
Federal NOLs | 2024
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	26,332
Federal NOLs | 2025 and thereafter
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	242,941
Federal NOLs | Indefinite
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	62,618
State NOLs
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	350,557	$ 31,000
State NOLs | 2020
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
State NOLs | 2021
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
State NOLs | 2022
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
State NOLs | 2023
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
State NOLs | 2024
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
State NOLs | 2025 and thereafter
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	350,557
State NOLs | Indefinite
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
Federal R&D Credit
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	9,073
Federal R&D Credit | 2020
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	334
Federal R&D Credit | 2021
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	334
Federal R&D Credit | 2022
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	483
Federal R&D Credit | 2023
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	322
Federal R&D Credit | 2024
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	213
Federal R&D Credit | 2025 and thereafter
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	7,387
Federal R&D Credit | Indefinite
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
Federal Orphan Drug Credit
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	21,264
Federal Orphan Drug Credit | 2020
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	2,288
Federal Orphan Drug Credit | 2021
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	1,962
Federal Orphan Drug Credit | 2022
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	1,610
Federal Orphan Drug Credit | 2023
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	929
Federal Orphan Drug Credit | 2024
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	663
Federal Orphan Drug Credit | 2025 and thereafter
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	13,813
Federal Orphan Drug Credit | Indefinite
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	9,265
State R&D Credit | 2020
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit | 2021
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit | 2022
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit | 2023
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit | 2024
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit | 2025 and thereafter
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	0
State R&D Credit | Indefinite
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 9,265