Collaboration and License Agreements - Mylan Collaboration Agreement (Details) $ in Millions		9 Months Ended	12 Months Ended
	Jan. 08, 2016 USD ($)	Sep. 30, 2018	Dec. 31, 2018 USD ($) performance_obligation product_candidate	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of additional performance obligations | performance_obligation			4
Mylan Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of product candidates | product_candidate			6
Cash received from milestone payments				$ 60.0
Number of years for ceasing agreement			2 years
Revenue recognized			$ 31.5
Percentage of cost sharing arrangement under collaborative and license agreements		50.00%	50.00%
Milestone payment on collaboration agreement	$ 200.0
Accounting Standards Update 2009 to 2013 | Mylan Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total arrangement consideration	45.0
Portion of revised arrangement consideration allocated to the first initial product license as a result of termination of product	8.2
Portion allocated to second initial product license			$ 7.1
Deferred revenue amortized to research and development revenue			$ 5.7
Minimum | Accounting Standards Update 2009 to 2013 | Mylan Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of revised arrangement consideration allocated to each of the three additional product licenses as a result of termination of product	5.7
Maximum | Accounting Standards Update 2009 to 2013 | Mylan Collaboration Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Portion of revised arrangement consideration allocated to each of the three additional product licenses as a result of termination of product	$ 9.0