Collaborative Arrangements (Details) - USD ($)	1 Months Ended										6 Months Ended	12 Months Ended
	Oct. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Sep. 30, 2015	Aug. 31, 2015	Jul. 31, 2014	Feb. 29, 2012	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development revenues												$ 20,748,000	$ 31,316,000	$ 25,599,000
Deferred revenue				$ 1,710,000								12,292,000	1,710,000
Technology transfer and license agreement | Merck
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from license fees received								$ 5,000,000
Term of milestone agreement								2 years
Supply agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue				700,000								700,000	700,000
Upfront milestone payment				750,000
Supply agreement | Merck
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Term of milestone agreement										5 years
License and service revenue												1,300,000	1,300,000	1,900,000
Deferred revenue				$ 1,000,000								1,300,000	1,000,000
Term of agreement extension										5 years
GlaxoSmithKline (GSK)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from license fees received									$ 6,000,000
Revenue recognized					$ 7,500,000		$ 6,500,000				$ 5,000,000
Minimum milestone receivable									5,750,000
Maximum milestone receivable									$ 38,500,000.0
Term of milestone agreement									3 years
License and service revenue												0	3,000,000	2,000,000
Merck
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Sales revenue, goods												9,000,000	6,000,000	1,600,000
Merck | Technology transfer and license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum milestone receivable								$ 15,000,000
License and service revenue												0	4,000,000	1,000,000
Merck | Supply agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized		$ 300,000
Termination notice period						24 months
Fine chemical customer | Research and development agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized												1,900,000
Deferred revenue												3,100,000
Upfront milestone payment			$ 3,000,000
Licenses revenue												1,300,000
Research and development revenues												3,200,000
Term of collaborative research and development agreement			21 months
Collaborative arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized												0	1,800,000	500,000
Collaborative arrangement | Merck
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and service revenue												3,600,000	$ 3,000,000	$ 1,900,000
Collaborative arrangement | Nestec Ltd. (Nestle Health Sciences)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development revenues												500,000
Deferred revenue												1,100,000
Global Development, Option and License Agreement | Nestec Ltd. (Nestle Health Sciences)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized												7,200,000
Deferred revenue												$ 6,800,000
Progress payment eligible after commencement of Phase 1a clinical trial	$ 4,000,000
Upfront milestone payment	14,000,000
Nestle Health Science exercises alternative option	3,000,000
Global Development, Option and License Agreement | Nestec Ltd. (Nestle Health Sciences) | Sales-based milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Target sales for sales milestone	1,000,000,000
Maximum | Global Development, Option and License Agreement | Nestec Ltd. (Nestle Health Sciences) | Research and development agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront milestone payment	86,000,000
Maximum | Global Development, Option and License Agreement | Nestec Ltd. (Nestle Health Sciences) | Sales-based milestone
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront milestone payment	$ 250,000,000