DESCRIPTION OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Description of Operations and Variable Interest Entity (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Description of Operations and Summary of Significant Accounting Policies
Cash balance	$ 278,096				$ 62,417				$ 278,096	$ 62,417
Total net revenue	75,971	$ 65,755	$ 64,107	$ 55,183	79,858	$ 61,680	$ 67,086	$ 52,380	261,016	261,004	$ 217,217
Total operating expenses	2,332	$ 4,962	$ 4,347	$ 3,015	2,638	$ 4,019	$ 4,411	$ 11,685	14,656	22,753	33,613
GSK
Description of Operations and Summary of Significant Accounting Policies
Total net revenue									$ 261,016	261,004	$ 217,217
Long-Acting Beta2 Agonist (LABA) Collaboration | GSK
Description of Operations and Summary of Significant Accounting Policies
Percentage of economic interest in any future payments made under the agreements									15.00%
Long-Acting Beta2 Agonist (LABA) Collaboration | GSK | RELVAR/BREO
Description of Operations and Summary of Significant Accounting Policies
Royalty rate for first level of annual global net sales (as a percent)									15.00%
Annual global sales level used to determine royalty rate									$ 3,000,000
Royalty rate for sales above first level of annual global net sales (as a percent)									5.00%
Long-Acting Beta2 Agonist (LABA) Collaboration | GSK | ANORO | Minimum
Description of Operations and Summary of Significant Accounting Policies
Royalty rate for combination products (as a percent)									6.50%
Long-Acting Beta2 Agonist (LABA) Collaboration | GSK | ANORO | Maximum
Description of Operations and Summary of Significant Accounting Policies
Royalty rate for combination products (as a percent)									10.00%
Long-Acting Beta2 Agonist (LABA) Collaboration | GSK | TRELEGY | Minimum
Description of Operations and Summary of Significant Accounting Policies
Royalty rate for combination products (as a percent)									6.50%
Long-Acting Beta2 Agonist (LABA) Collaboration | GSK | TRELEGY | Maximum
Description of Operations and Summary of Significant Accounting Policies
Royalty rate for combination products (as a percent)									10.00%
TRC
Description of Operations and Summary of Significant Accounting Policies
Cash balance	22,300								$ 22,300
TRC | TRELEGY
Description of Operations and Summary of Significant Accounting Policies
Total net revenue									42,800	13,400
TRC | Collaborative arrangement
Description of Operations and Summary of Significant Accounting Policies
Related-party receivables	$ 14,400				$ 6,400				14,400	$ 6,400
Theravance Biopharma | TRC | Arbitration
Description of Operations and Summary of Significant Accounting Policies
Total operating expenses									$ 3,400