ACQUISITIONS - Acquisition Date Fair Value of Consideration Transferred (Details) - Spectrum Pharmaceuticals, Inc. - USD ($) $ / shares in Units, $ in Thousands	Jul. 31, 2023	Mar. 31, 2024	Dec. 31, 2023
Business Acquisition [Line Items]
Assertio shares issued (in shares)	38,013,000
Assertio closing price per share as of the Effective Date (in dollars per share)	$ 5.69
Fair value of Assertio shares issued	$ 216,294
Repayment of Spectrum's long-term debt	32,647
CVRs	3,932	$ 0	$ 0
Total fair value of consideration transferred	$ 252,873
Business combination, contingent consideration liability, number of shares (in shares)	223,397
Business acquisition, contingent consideration, per share (in dollars per share)	$ 0.0176