Schedule of Revenue from Collaborations and Services (Detail) - 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
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues	$ 16,031	$ 4,469	$ 3,893	$ 3,465	$ 4,530	$ 2,043	$ 2,163	$ 3,009	$ 27,859	$ 11,745	$ 174,758
Research Agreement | United Therapeutics Corporation
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									3,800
Collaboration and License Agreement | United Therapeutics Corporation
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									6,400
Collaborations and services
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									10,583	250	171,965
Collaborations and services | License Agreement | United Therapeutics Corporation
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									6,386
Collaborations and services | Research Agreement | United Therapeutics Corporation
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									3,758
Collaborations and services | Collaboration and License Agreement | Receptor
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									341	$ 250
Collaborations and services | Distribution Agreement | Cipla Ltd
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues									$ 98
Collaborations and services | Collaboration and License Agreement | Sanofi-Aventis Deutschland GmbH
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenues											$ 171,965