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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Description of Operations and Summary of Significant Accounting Policies
Total net revenue	$ 79,858	$ 61,680	$ 67,086	$ 52,380	$ 69,520	$ 48,643	$ 58,562	$ 40,492	$ 261,004	$ 217,217	$ 133,569
GSK
Description of Operations and Summary of Significant Accounting Policies
Total net revenue									$ 261,004	217,217	$ 133,569
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 | TRELEGY
Description of Operations and Summary of Significant Accounting Policies
Total net revenue									$ 13,400	200
TRC | Collaborative arrangement
Description of Operations and Summary of Significant Accounting Policies
Related-party receivables	$ 6,400				$ 200				$ 6,400	$ 200