INTANGIBLE ASSETS - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended				3 Months Ended	12 Months Ended
	Jun. 20, 2016	Nov. 30, 2018	Mar. 31, 2018	Nov. 30, 2017	Sep. 30, 2015	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Intangible assets finite-lived, net							$ 157,200
Payments to date under terms of licensing agreement							15,370	$ 18,974	$ 13,122
Amortization expense							19,707	18,028	17,331
Impairment of intangible assets						$ 25,500	25,500	0	$ 0
Economic Interest - L-UDCA (acquired IPR&D)
Finite-Lived Intangible Assets [Line Items]
Intangible assets with definite lives	$ 25,500
Assets useful life (in years)	17 years
Total purchase price	$ 25,500
Ligand License Agreement
Finite-Lived Intangible Assets [Line Items]
Intangible assets finite-lived, net											$ 2,500
Require to make substantial payments payable upon achievement of milestones											$ 114,100
Payments to date under terms of licensing agreement							$ 7,200
Payment of amendment consideration					$ 1,000
Ligand License Agreement | Minimum
Finite-Lived Intangible Assets [Line Items]
Annual royalty percentage							15.00%
Ligand License Agreement | Maximum
Finite-Lived Intangible Assets [Line Items]
Annual royalty percentage							17.00%
Licensing Agreements
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired			$ 4,600
Product rights
Finite-Lived Intangible Assets [Line Items]
Intangible assets finite-lived, net							$ 43,398	$ 47,636
Assets useful life (in years)							16 years	16 years
Finite-lived intangible asset							$ 67,849	$ 67,849
Product rights | Manchester Pharmaceuticals LLC
Finite-Lived Intangible Assets [Line Items]
Intangible assets with definite lives										$ 67,800
Assets useful life (in years)										16 years
Trade Name
Finite-Lived Intangible Assets [Line Items]
Intangible assets finite-lived, net							$ 0	$ 0
Assets useful life (in years)							1 year	1 year
Finite-lived intangible asset							$ 175	$ 175
Trade Name | Manchester Pharmaceuticals LLC
Finite-Lived Intangible Assets [Line Items]
Intangible assets with definite lives										$ 200
Assets useful life (in years)										1 year
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets finite-lived, net							$ 171	$ 211
Assets useful life (in years)							10 years	10 years
Finite-lived intangible asset							$ 403	$ 403
Customer relationships | Manchester Pharmaceuticals LLC
Finite-Lived Intangible Assets [Line Items]
Intangible assets with definite lives										$ 400
Assets useful life (in years)										10 years
Thiola License Agreement
Finite-Lived Intangible Assets [Line Items]
Assets useful life (in years)										10 years
Automatic renewal periods										1 year
Remaining weighed average period of amortization (in years)							9 years 4 months 24 days
Thiola License Agreement | Mission Pharmacal Company
Finite-Lived Intangible Assets [Line Items]
Annual royalty percentage							20.00%
Mission an up-front license fee							$ 3,000
Guaranteed minimum royalties							2,000
Present value of guaranteed minimum royalties payable							14,300	15,200
Thiola License Agreement | Mission Pharmacal Company | Other Current Liabilities
Finite-Lived Intangible Assets [Line Items]
Guaranteed minimum royalties							2,100	2,100
Thiola License Agreement | Mission Pharmacal Company | Other Noncurrent Liabilities
Finite-Lived Intangible Assets [Line Items]
Guaranteed minimum royalties							$ 12,200	13,100
Thiola License Agreement | Mission Pharmacal Company | Minimum
Finite-Lived Intangible Assets [Line Items]
Discount rate							7.00%
Thiola License Agreement | Mission Pharmacal Company | Maximum
Finite-Lived Intangible Assets [Line Items]
Discount rate							11.00%
Acquired Product Rights L-UDCA
Finite-Lived Intangible Assets [Line Items]
Impairment of intangible assets						$ 25,500
Thiola
Finite-Lived Intangible Assets [Line Items]
Intangible assets finite-lived, net							$ 65,407	$ 55,486
Assets useful life (in years)							15 years	15 years
Licensing agreement, extension term				5 years
Finite-lived intangible asset				$ 5,900			$ 85,824	$ 70,009
Payments for the option to acquire business		$ 300
Payment of guaranteed minimum royalty		$ 100
Minimum royalty, percentage		20.00%
Amortization expense		$ 1,000
Guaranteed minimum liability		$ 700
Increase in intangible assets							$ 15,800
United States | Asklepion Pharmaceuticals LLC
Finite-Lived Intangible Assets [Line Items]
Capitalized economic interest												$ 75,900
International | Asklepion Pharmaceuticals LLC
Finite-Lived Intangible Assets [Line Items]
Capitalized economic interest												$ 7,300